Open access peer-reviewed chapter

Citrus Biotechnology: Current Innovations and Future Prospects

Written By

Ghulam Mustafa, Muhammad Usman, Faiz Ahmad Joyia and Muhammad Sarwar Khan

Submitted: 28 May 2021 Reviewed: 02 September 2021 Published: 24 September 2021

DOI: 10.5772/intechopen.100258

From the Edited Volume

Citrus - Research, Development and Biotechnology

Edited by Muhammad Sarwar Khan and Iqrar Ahmad Khan

Chapter metrics overview

511 Chapter Downloads

View Full Metrics

Abstract

Citrus is a valuable fruit crop worldwide. It not only provides essential minerals and vitamins but is also of great commercial importance. Conventional research has contributed a lot to the improvement of this fruit plant. Numerous improved varieties have been developed through conventional breeding, mutational breeding, polyploidization and tissue culture yet pathogens continue to emerge at a consistent pace over a wide range of citrus species. Citriculture is vulnerable to various biotic and abiotic stresses which are quite difficult to be controlled through conventional research. Biotechnological intervention including transgenesis, genome editing, and OMICS offers several innovative options to resolve existing issues in this fruit crop. Genetic transformation has been established in many citrus species and transgenic plants have been developed having the ability to tolerate bacterial, viral, and fungal pathogens. Genome editing has also been worked out to develop disease-resistant plants. Likewise, advancement in OMICS has helped to improve citrus fruit through the knowledge of genomics, transcriptomics, proteomics, metabolomics, interactomics, and phenomics. This chapter highlights not only the milestones achieved through conventional research but also briefs about the achievements attained through advanced molecular biology research.

Keywords

  • citriculture
  • conventional research
  • transgenesis
  • genome editing
  • multi-OMICS

1. Introduction

Citrus is one of the most diverse members of the family Rutaceae and is the leading tree fruit crop in the world. Citrus comprises different species of edible fruits like mandarins (Citrus reticulata Blanco), sweet oranges (C. sinensis Osbeck), grapefruit (C. paradisi Macf.), acid limes (C. aurantifolia Swingle), and sweet limes (C. limettioides), lemons (C. limon Burmf.) and their hybrids including tangerines, tangelos, tangors, etc. [1].

Citrus is being widely cultivated in the sub-tropical, tropical, and temperate regions of the world. Global citrus production is 157 million tons per annum from an area of 15 million hectares. About 50% of the area and production of citrus is being contributed by the northern hemisphere of the world. China (28%) and the Mediterranean regions (25%) are the major contributors to global citrus production followed by Brazil (13%). China is leading in grapefruit and mandarin production. Among Mediterranean countries, Spain is leading in global citrus production (6 million tons) including mandarins, oranges, limes, lemons and exports. Brazil is leading in global fresh sweet orange and its juice production. Mexico and India are major lime producers [2]. Pakistan’s share in global citrus production is quite low (1.6%) which includes mandarin and sweet oranges as major species whereas limes, grapefruit, and lemons have less production and are dealt as minor species. The global citrus industry is facing many biotic (Citrus greening, Citrus tristeza virus, sudden death, citrus canker, and Phytophthora) and abiotic stress (salinity, drought, and temperature fluctuations) which have a direct impact on fruit crop production and yield [3].

Advertisement

2. Origin and diversification

Citrus and other genera including Poncirus, Clymenia, Fortunella, Eremocitrus, and Microcitrus belong to tribe Citreae and sub-tribe Citrineae and are considered as true citrus [4]. Classification in citrus has been controversial since ancient times due to vast morphological diversity, interspecific and intergeneric sexual compatibility. However, molecular biology tools have revealed four species including mandarins, citron (C. medica), pummelos (C. grandis Linn.), and wild cultivar of papeda (C. micrantha Wester) as the true parental species that have contributed to the development of other species during the process of evolution [5, 6]. Based on phylogenetic and genomic studies it is revealed that mandarin originated in China, Vietnam, and Japan whereas citron was originated in northeast India and China. Pummelo originated in Indonesia and Malay whereas C. micrantha was originated in the Philippines [6]. Other citrus species including sweet oranges, grapefruit, lime, lemon, sour oranges, and hybrids (tangelos and tangors) have developed from these ancestral species through random hybridization and natural mutation events [7].

Among citrus genetic resource centers, major collections are found in the USA, China, Spain, France, Japan, and Brazil where a large number of wild species, their relatives, old and new varieties, and breeding lines are conserved [8]. In Pakistan, citrus genetic resources are conserved mainly in the field as orchards or germplasm units in Sargodha, Faisalabad, and Sahiwal in different academic and research institutes.

Advertisement

3. Conventional approaches for crop improvement

Citrus breeders have been using different approaches for their improvement including conventional breeding, mutation breeding, polyploidization and in vitro culture tools particularly somatic hybridization which has played an essential role in developing new somatic hybrids. These techniques have contributed towards the selection and development of new potential cultivars and are still being used as important fundamental tools for the development of genetically diverse germplasm which could be further screened and characterized using modern breeding technologies.

3.1 Classical and mutation breeding

Though conventional breeding has limitations in citrus due to its complex reproductive behavior, nucellar embryony, long juvenility, sterility, sexual incompatibility, and endogametic depression [9, 10]. However, still, many hybrids have been developed by conventional breeding and recovered using in vitro tools.

Mutation breeding has played a pivotal role in fruit crop improvement including citrus and has developed several mutants with improved phenotypic and genotypic traits [11]. Spontaneous or induced mutants do not have intellectual property rights (IPR) related issues that have to be faced in the case of conventional breeding and transgenics [12]. Both spontaneous and induced mutations have enhanced genetic diversity in existing varieties and have provided the raw material for making selections for the novel horticultural traits [13]. About 3365 mutant varieties belonging to 170 plant species have been released including citrus and 20 other fruit species [14]. Among continents, Asia is leading with 2052 mutants released followed by Europe (960 mutants). Among countries, China (817), Japan (479), India (341) and the USA (139) are leading in mutant development whereas Pakistan has released 59 mutants in different crops [15]. In citrus, a total of 15 mutants have been released since 1970 including mandarins and clementine (6), sweet oranges (6), grapefruit (2), and lemon (1) [10]. Pakistan has registered a single mutant variety in citrus, a Kinnow mandarin induced mutant having less number of seeds and named it as “NIAB Kinnow” in 2017.

The rate of spontaneous mutations has been much higher in citrus compared with other fruit crops, however, due to random genetic alterations it has been difficult to identify and utilize such mutants [16, 17]. Induced mutations using different irradiation sources including gamma rays (physical mutagens) and various chemical compounds have enhanced the frequency of genetic variability. Physical mutagens or ionizing radiations have been more commonly used for inducing genetic diversity, chromosomal aberrations, and point mutations. About 70% of the mutant varieties have been developed using physical mutagens [18]. In fruit crops, physical mutagens have altered key horticultural traits like seedlessness, precocious bearing, and dwarfism [19, 20, 21]. Other traits include fruit ripening time, fruit skin and flesh color, fruit aroma, self-compatibility, pathogen resistance, and fertility restoration in sterile hybrids. Among physical mutagens, gamma rays have been most used for the development of mutants due to their shorter wavelength and greater penetration [22], however, the ion beam is getting more popular and is being widely used due to its greater efficiency and precision compared with gamma rays [23]. Among chemical mutagens, ethyl methanesulfonate (EMS), diethyl sulfate (DES), ethylenimine (EI), sodium azide (SA) has been most frequently used for reliable and gene-specific mutations. A comprehensive review of the role of mutation breeding in mandarins and lime crop improvement has been discussed [24, 25]. Irradiation and chemical mutagen treatment of seeds and budwood have been commonly used by breeders for inducing variation followed by selection and clonal propagation. Mutation breeding applications have been reported in different fruit crops including papaya, peach, pear, grapes, sweet and sour cherries, banana, plum, almond [26], apple [27], and rough lemon [28]. Natural bud mutants include Washington navel orange, most of the early grapefruit varieties including Marsh, Foster, Shamber, Salustiana sweet orange, and Shamouti orange have originated as bud sports. Now there are several commercial seedless varieties including Daisy SL, Kinnow SL, Fairchild SL, and Tango that have been developed from their seedy parents through mutation breeding and are being commercially cultivated [29]. Other commercial mutants in citrus include sweet orange varieties Jin Cheng [30], Kozan [21], and NIAB Kinnow mandarin [31]. In grapefruit, Rio Red and Star Ruby are two induced mutants that have obtained commercial significance due to their better fruit color and seedlessness, respectively [32]. These are leading grapefruit varieties in Texas, USA. Star Ruby is the leading variety in Turkey, South Africa, Australia, and Spain. Rio Red is the main cultivar in China, India, and Argentine [33]. In Pakistan, Shamber is the main grapefruit variety that needs to be replaced with other potential candidate varieties like Star Ruby, Rio Red, and Flame [10]. Conclusively mutation breeding has shown its enormous potential in citrus crop improvement particularly in economically important horticultural traits. However, it is a slow and long-term process and takes more time to detection of desirable phenotypic variability. Utilization of modern breeding tools including molecular markers, advanced methods for phenotypic screening like Targeting Induced Local Lesions IN Genomes (TILLING) [34], using targeted mutagenesis and genome editing technologies [35] like Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR/Cas9) could enhance the efficiency and cost-effectiveness of variety development having novel traits in citrus.

3.2 Ploidy manipulations

Polyploid organisms have a greater number of chromosomes compared with their diploid progenitors. Breeders have utilized polyploidization for the investigation of inheritance patterns in genes of interest. Polyploids have shown tremendous success in nature due to higher heterozygosity, less inbreeding depression, and more tolerance to biotic and abiotic stress conditions compared with their diploid progenitors [36, 37, 38]. The duplicated genes may evolve new functionalization during evolution [39]. Polyploids have been reported in many fruit crops including grapes, apples, strawberry, and citrus [40, 41], however, the frequency of spontaneous polyploid events is quite low and breeders prefer to induce hyperploidy using different chemicals.

Among chemical mutagens, colchicine is mostly used for the induction of polyploids due to its more reliability, higher efficacy, and cost-effectiveness. Colchicine is an alkaloid derived from Colchicum autumnale (meadow saffron). It is used for inducing chromosome doubling or developing tetraploids by restricting the chromosomal segregation at metaphase in meiosis [42, 43]. Other methods of polyploid induction include interploid hybridization [44], unreduced gamete formation [45, 46], and endosperm culture [47, 48].

Members of the subfamily Aurantioidae including Citrus, Fortunella, and Poncirus are mainly diploid having chromosome number 2n = 18 [49]. The occurrence of spontaneous polyploids in citrus is known since the 1940s [50]. Important spontaneous polyploids include triploid Tahiti lime [51], Triphasia desert lime [52], Clausena excavata [53], tetraploid mandarins [54], sweet oranges [41, 42] and grapefruit [43]. In spontaneous polyploids, triploids and tetraploids are believed to be formed by doubling of chromosomes in nucellar cells and fertilization of the unreduced gametophytes [55, 56]. Polyploids have been induced using colchicine in several citrus species and tetraploids produced have been used for interploid crossing to develop triploid progenies that are usually seedless due to irregular distribution of the chromosomes during cell division particularly gamete formation and formation of unreduced gametes. In interploid crossing, the formation of tetraploids in addition to triploids indicates the predominant formation of the unreduced (2n) gametes which may be formed by the first division restitution (FDR) or second division restitution (SDR) during meiosis. Production of 2n gametes was predominantly via SDR in lemon [44, 45] and monoembryonic Orah mandarin [57]. Higher tetraploid: triploid ratio in the progeny of the interploid hybridization indicates greater production of the 2n megagemtophytes in that cultivar which is promising to produce a greater number of polyploids.

3.3 In vitro culture: somatic hybridization

Plant tissue culture tools offer advantages related to efficient regeneration, propagation, and crop improvement in citrus and other horticultural crops. Endosperm cultures have been used for the development of triploids in citrus [48]. In interploid and wide hybridizations, the progeny may be sterile or have underdeveloped or shriveled seeds with viable embryos. The embryo rescue technique has shortened the breeding cycle and many plants have been recovered from these embryos through in vitro culture in different citrus species [45]. Similarly, micrografting is another tool in which a miniature bud is grafted under aseptic conditions on in vitro raised rootstocks and micrografted plants have been reported in many citrus varieties [58]. Micrografting is also useful for the production of virus-free citrus plants.

Another highly promising and most widely used approach is somatic hybridization which is utilized to overcome sexual incompatibility and to enhance genetic variability by combining nuclear and organelle (chloroplast and mitochondria) genomes followed by their characterization for hybrid confirmation and variability assessment [59]. The organelle genomes are known to encode genes related to photosynthesis and male sterility and new hybrids could be developed having novel genetic recombinations. Somatic hybrids may be developed through electrofusion of plant embryogenic protoplasts predominantly with mesophyll protoplasts. The plant progeny having nuclear origin could be characterized and separated using flow cytometry and molecular markers [60].

Protoplast fusion of distantly related citrus species bypasses the biological barriers and develops allopolyploids that could not be obtained through classical breeding. Somatic hybridization is an important tool and has been widely used in citrus scion and rootstock breeding. The first intergeneric allotetraploid somatic hybrid of Trovita sweet orange and Poncirus trifoliata was reported by Ohgawara et al. [61] followed by several interspecific and intergeneric hybrids in citrus from the USA [62], Japan [63], and other citrus-producing countries. Triploids were also reported from interspecific and intergeneric somatic hybridization of Citrus species, kumquats (Fortunella japonica), and Poncirus trifoliata by protoplast fusion [64]. Fusion of protoplasts from the haploid lines and diploid cultivars may also yield triploids and hundreds of triploids and tetraploids were developed and planted for field evaluations [65]. Polyethylene glycol may also be used to induce regeneration in the fused protoplast cultures as reported in Willow leaf mandarin (embryogenic parent) and Duncan grapefruit and sweet orange (mesophyll parents). The regenerated plants were identified as alloplasmic cybrids [66].

Polyploids developed through somatic hybridization have also shown enhanced tolerance to abiotic and biotic stress conditions. Allotetraploids of cv. FlhorAG1 (FL-4x) developed by somatic hybridization of diploid Poncirus and Citrus showed greater tolerance to cold and higher light conditions compared with parents (diploid) and their tetraploids [36]. Kumquats (Fortunella species) chloroplast have demonstrated higher resistance to canker in diploid kumquats and their tetraploid somatic hybrids developed with other citrus species including grapefruit [37].

Advertisement

4. Innovative approaches/technologies

4.1 Transgenesis

Since the advent of recombinant DNA technology, transgenesis has proved its significance, and 190.4 million hectares of transgenic crops were grown in more than 29 countries in 2019. They have significantly contributed to food security, climate change mitigation, sustainability thus uplifted the lives of 17 million biotech farmers worldwide. The first transgenic plant was developed in the 1980s and was available as commercial food in the 1990s. More than 400 transformation events have been approved so far wherein 356 events have been approved for crop plants, 23 for ornamentals, 22 for fruit plants, and 2 for trees. Hence a wide range of plant species (maize, cotton, canola, papaya, rice, tomato, sweet pepper, squash, popular, petunia, sugarcane, alfalfa, and citrus) have been engineered for various valuable traits i.e. insect resistance, herbicide tolerance [67], abiotic stress tolerance, improved nutritional value, and disease resistance. In addition to nuclear transformation plastid genome has also been targeted and has proved to be of more value as multiple genes can be introduced at a specific target site, the transgene is contained owing to maternal inheritance, and hyperexpression of the transgene, etc. [68, 69].

Citrus is an economically important fruit crop worldwide. It not only provides essential minerals and vitamins but is also of great commercial importance. Conventional research has contributed a lot to the improvement of this fruit yet serious problems are evolving which are difficult to tackle with these conventional approaches [70]. Juvenility, sexual incompatibility, high heterozygosity, apomixes, large plant size, and nucellar polyembryony, and certain other biological limitations hinder the improvement of these plant species through conventional breeding. Genetic manipulation through advanced innovative techniques is a potential approach to improve crop plants as well as fruit species. Though citrus species are recalcitrant to transformation and subsequent rooting, yet consistent efforts by the researchers have resolved these bottlenecks and proficient protocols have been established. Likewise, various transformation methods i.e. Agrobacterium-mediated transformation [71], biolistic transformation [72], and chemically assisted uptake of recombinant DNA by protoplasts [73] have been attempted to introduce genes of agronomic value as well as to strengthen it against bacterial, viral, and fungal pathogens (Figure 1).

Figure 1.

Schematic sketch showing the importance of conventional and advanced innovative approaches for the improvement of different citrus species.

Genetic manipulation of vegetatively propagated crops like citrus is very tricky as the expression of transgenes over a long period during numerous cycles of graft propagation should be stable.

The first attempt to produce transgenic citrus was made in the 1980s wherein protoplast transformation was attempted but it was not successful. The first authentic report was published by Kaneyoshi et al. [74] who reported transforming NPT II and GUS genes into trifoliate orange through Agrobacterium. Epicotyls of the aforementioned citrus species were used to transform with the selectable marker gene as well as reporter gene and more than 25% transformation efficiency was achieved. Likewise, Yao et al. [72] reported the first successful transformation through gene gun. They transformed tangelo (C. reticulata × C. paradisi) embryogenic cells.

Since genetic transformation has successfully been performed in different species and hybrids including Carrizo citrange, Washington naval orange, Poncirus trifoliata, Sour orange, Mexican lime, sweet orange, Citrus reticulata [75], and a valuable rootstock, swingle citrumelo. Similarly, protocols have been optimized for the genetic transformation of different citrus species by using different explant tissues including seeds, embryogenic cells, epicotyls, embryogenic cells, callus, nodal stem segments, and protoplasts. The most responsive explant tissue has been epicotyl from the in vitro germinated seedlings and is preferably used for genetic transformation research. Duncan grapefruit was successfully transformed through Agrobacterium for the first time using epicotyl and confirmation of the transgene (NPTII and GUS) integration was carried in the resultant 25 transgenic plants by histochemical staining, PCR, and Southern blot hybridization. Transgenic grapefruit, sweet orange, and citrange plants were developed using epicotyls as target explant whereas selection was carried out on kanamycin [76]. Epicotyl has also been used for Agrobacterium-mediated transformation of citrange and sweet orange [77]. In addition, callus, as well as suspension cultures derived from different parts of flower and seed, have also been attempted to transform. The transformation efficiency attained, in this case, was lower than 0.5%. Genetic transformation has also been optimized in pomelo (Citrus grandis) and sour orange wherein internodal stem segments were used as explants and a promising transformation efficiency was achieved (91%) [78].

The biolistic transformation has also been performed successfully in tangelo (C. paradisi Macf. x C. reticulata Blanco) using nucellar embryogenic cells raised from the suspension culture and more than 15 stable whereas 600 transient transgenic lines were attained per bombardment. The transformed calli cells showed proficient growth on kanamycin selection medium and positive GUS activity but were not able to regenerate into plants. Calli treatment with 0.3 M osmoticum sorbitol and 0.3 M mannitol appeared to have positive effects for enhanced transformation efficiency for stable and transient transformation. Thin epicotyl segments of germinated seedlings were also targeted through the biolistic gun and more than 93% transformation efficiency was attained for transient transgene integration in C. citrange. The incubation of explant on culture medium before bombardment appeared to have profound effects on transformation efficiency which was further improved [79].

Since transgenic technology is the most reliable intervention having the massive potential to improve the citrus crop. The introduction of alien genes is only possible through this technology so any of the desired traits can be engineered. Recent research indicates that citrus growing farmers are facing severe problems due to biotic and abiotic stresses i.e. salinity, cold, drought, and diseases. Hence, the development of improved citrus varieties is direly needed to get a quality crop. Various citrus species have been engineered with alien genes to combat abiotic stresses including salinity and drought. Expression of HLA2 gene, isolated from yeast imparted salinity tolerance and resultant transgenic plants were able to tolerate a higher level of the salts as compared with non-transformants [80]. PsCBL and PsCIPK derived from Pisum sativum were transformed into Citrus sinensis and Citrus reticulata by targeting calli derived from mature seed. Bacterial strain LBA4404 was used to induce infection in the target calli cells. The putative transformants showed better performance as compared with control plants for salinity and drought tolerance when tested under in vitro conditions [81].

Citrus paradisi was transformed to improve carotenoid content by manipulating the genes involved in carotenoid biosynthesis i.e. phytoene synthase, lycopene-β-cyclase, and phytoene desaturase. The multigene transgenic citrus plants were aimed to supplement human nutrition with vitamin A along with antioxidants. Similarly, fruit juice quality has been attempted to improve in Valencia orange, a valuable variety that is majorly grown for its juice. Degradation of TSPME (thermostable pectin methylesterase) deteriorates the quality of the juice. This TSPME is encoded by the CsPME4 gene. The protoplasts were isolated from embryogenic suspension cultures and transgene was introduced through the PEG mediated transformation method [82] aiming at down-regulation of the CsPME4 resulting in the citrus with improved juice quality.

Citriculture is prone to be infected by a wide range of diseases that are controlled by chemicals, a drastic non-environmentally friendly strategy. Different types of viral, bacterial, and fungal pathogens infect these plants resulting in drastic losses to crop production and quality of the produce. A range of transgenic citrus lines have been developed varying from fully resistant to susceptible to the diseases. Coat protein (p25) from the CTV (Citrus Tristeza Virus) was expressed in Mexican lime and 33% of the transgenic plants were found to be resistant, neither symptoms appeared nor the viral load was detected. Accumulation of siRNA (small interfering RNA) in the transgenic lines resulted in resistant phenotype and plants were able to withstand viral infection [83]. Expression of viral coat protein (part of p23 gene and the 3UTR), in the sense and antisense orientation also delayed viral infection in grapefruit [84].

Phytophthora is a noxious fungal pathogen that has been reported to infect a wide range of citrus species. Among these Phytophthora parasitica and Phytophthora citrophthora cause more severe damage in the citrus orchards and nurseries all over the world [85]. Expression of bo gene (bacterio-opsin) in Rangpur lime rootstock showed an elevated level of tolerance against Phytophthora parasitica infection. It was observed that expression of the aforementioned gene led to induce expression of defense-related proteins; chitinase, salicylic acid, and glucanase. Hence plants with an elevated level of transgene expression showed greater resistance against the oomycetic fungi including Phytophthora parasitica. Transgenic citrus plants expressing the tomato PR5 gene showed an enhanced survival rate in the presence of pathogen (P. citrophthora). Transgenic grapefruit plants were able to better withstand citrus scab infection when transformed with the attE gene encoding for antimicrobial peptide [86].

Transgenic technology has also played a pivotal role to tackle another noxious disease in citrus i.e. Huanglongbing (HLB) which is supposed to be caused by phloem-restricted Gram-negative bacteria; Candidatus Liberibacter americanus, Candidatus Liberibacter asiaticus, and Candidatus Liberibacter africanus [87]. Various genetic strategies have been tested to develop HLB-resistant citrus lines with decreased susceptibility to the pathogen. These include the expression of anti-microbial peptides from a bacterial, fungal, plant, or animal origin and engineering host-pathogen interaction pathways. The expression of antimicrobial proteins under phloem-specific promoters has been an effective strategy to control this phloem-resident pathogen. Overexpression of synthetic cecropin B gene under phloem specific promoter resulted in reduced bacterial population after one year of inoculation and no disease symptoms appeared even after two years of inoculation [88]. Overexpression of modified methionine under double 35S promoter also appeared to have an inhibitory effect on bacterial growth and lowered down the CLas (Candidatus Liberibacter asiaticus) titer in the roots of transgenic Carrizo citrange (rootstock). Further, newly emerging leaves from the rough lemon, grafted on this transgenic rootstock, also had a non-detectable bacterial titer. Expression of AtNPR1 and chimeric proteins (ThioninLBP and Thionin1-D4E1) demonstrated elimination of CLas providing tolerance against HLB infection [89].

Another economically important disease, the citrus canker has also been addressed through transgenesis resulting in enhanced tolerance against Xanthomonas citri. Engineering sweet orange genome with Xa21 gene showed a significant reduction in disease severity upon inoculation in three lines Hamlin, Pera, and Natal. Expression of the Xa21 gene under its promoter appeared to be more effective in disease resistance when expressed in highly susceptible Anliucheng sweet orange [90]. Transgenic Carrizo citrange and sweet orange plants were developed by introducing RpfF from X. fastidiosa which encodes for a quorum-sensing factor and can disrupt bacterial communication by reducing activation of virulence factors, thus enhancing the ability to tolerate pathogen infection. Similarly, the expression of AMP sarcotoxin from flesh fly also enhanced tolerance against X. citri [91].

4.2 Genome editing

Genome editing through CRISPR-Cas9 has emerged as a breakthrough technology for the precise modification and manipulation of targeted genomic DNA. It has extensively been exploited by several research groups [92] and certain successes have been achieved. Three major sequence-specific engineered nucleases that have so far been used for genome editing are CRISPR-Cas (clustered regularly interspaced short palindromic repeats), TALENs (transcription activators such as effector nucleases), and ZFNs (zinc finger nucleases). Among these, the CRISPR-Cas9 editing system has been established in many plant species through gene activation, repression, mutation, and epigenome editing in wheat, rice, maize, tomato, potato, carrot, apple, grape, and citrus. Even a few of the genome-edited crops have been approved for commercial cultivation. Through this technology, field crops as well fruit crops can not only be manipulated for improved agronomic traits but can also be manipulated for improved nutritional value [93].

For citrus, genome editing research is at infancy, yet few successes have been achieved by editing its genome for enhanced resistance against diseases. The CRISPR-Cas9 system was firstly used to target the CsPDS gene in Duncan grapefruit and sweet orange. The target gene was successfully modified through a transient expression method, Xcc-facilitated agroinfiltration [94]. The modified CsPDS sequence was not detectable in the leaves of sweet orange indicating that CRISPR/Cas9 has induced the desired mutation successfully.

Most of the research studies were carried out to target the LOB1 (LATERAL ORGAN BOUNDARIES 1) gene which has been characterized as a citrus susceptibility gene for Xanthomonas citri. The said gene has been explored to be upregulated by TAL (transcription activator-like) effector PthA4 which binds to the EBE (effector binding elements) in the promoter region of LOB1 thus activates expression of this canker-susceptibility gene [95]. Mutation in the single allele of effector binding elements of the LOB1 gene resulted in minor alleviation of canker symptoms in Duncan grapefruit. Anyhow, a mutation in effector binding elements of both of the alleles of LOB1 promoters alleviated canker symptoms to great extent thus showing a high degree of resistance in Wanjincheng orange [96]. Another research group explored that editing the coding region of LOB1 in Duncan grapefruit, through the CRISPR-Cas9 system also provides resistance against X. citri infection.

A marker gene for pathogen triggered immunity (CsWRKY22) was knocked out in Wanjincheng orange and the resultant mutant plant showed a decreased level of susceptibility to the canker disease [97]. In addition to the CRISPR-Cas9 system, another improved genome editing system (CRISPR/Cas12a (Cpf1) has also been used to edit the CsPDS gene in the Duncan grapefruit gene. It appeared to be a more efficient editing system with lower off-target effects thus will prove a great milestone in citrus genome editing [98]. These studies indicate that CRISPR-mediated genome editing can be a promising pathway to generate disease-resistant citrus cultivars [99].

Advertisement

5. Multi-Omics technology: An integrated approach and useful strategy for the improvement of the citrus crop

MultiOMICS including genomics, transcriptomics, proteomics, metabolomics, interactomics, and phenomics approaches have massive potential for citrus improvement just like other crops and fruits. In all disciplines of OMICS, various techniques can be utilized for genome analyses, transcripts, proteins, metabolites, and interactions between different molecules to indicate the molecules which may result in crop improvement.

5.1 Genomics

The field of genomics is a highly applicable part of Omics technologies. It is based on sequencing technologies and the analysis of subsequent genome sequences. Many advanced techniques in genomics for example sequence determination DNA, marker-assisted selection, and transition from marker-assisted selection to genomic selection assist in quick varietal development. Genome sequencing technologies have brought about a revolution in the field of biology. It has also transformed the citrus breeding that helps to understand a relationship between the genetic makeup and response towards various abiotic and biotic stresses like Alternaria brown spot [100].

A specific pathotype of Alternaria alternata (Fr.) Keisel is a disease with heavy losses [101]. It causes necrosis and resultant lesions on the surface of fruits and young leaves. It leads towards defoliation and fruit drop [102]. Thus, exploitation of innate genetic resistance appears to be the most applicable and effective strategy of disease control. Currently, the control is primarily based on the application of 4–10 sprays of toxic and environmentally injurious fungicide per year [103]. Such limitations are compelling farmers to find alternative cultivars with resistant ones [104].

Usually, the female parent transmits the 2n gamete in 2x × 2x citrus crosses [105, 106, 107, 108]. Cuenca et al. [109] recognized ABS resistance locus containing genomic region by using BSA-genome scan combined with HTA based. Trait segregation in crosses between two heterozygous ABS-susceptible or between heterozygous ABS-susceptible parents and resistance was used to confirm the recessive inheritance of the ABS resistance in triploid populations. ABSr locus was first located at 10 cM from a centromere based on segregation of 368 SDR 2n gametes. A genomic region containing several markers with a high probability (> 99%) of association with phenotype variation was identified on chromosome III by performing BSA over 93 triploid hybrids from a Fortune × Willow leaf population. This identified region contains 25 significant SNP markers within an interval of 13.1 cM. The size of the genomic region among these two markers is 15 Mb. Linkage genetic mapping was performed on identified genomic regions by developing new SNP and SSR markers. A 268-diploid mapping population was performed by Cuenca et al. [110] from a heterozygous-susceptible × resistant hybridization. Fine mapping was performed to confirm the location of ABSr locus in a region of 1.1 cM between the markers SNP05/SNP06/SNP07/AT21 (at 0.7 cM) and SNP08 (at 0.4 cM). Another region containing eight genes with NBS-LRR repeats was identified by the SNP08 marker and considered ABS resistance genes.

In citrus plant, molecular markers are linked to some agronomic traits, e.g. SSR markers are linked to Citrus tristeza virus resistance from Poncirus trifoliate, PCR assay for the anthocyanin content of pulp [111], AFLP markers are associated with polyembryony [112] and RAPD markers are associated to dwarfism and fruit acidity [113]. Some other characteristics such as salinity tolerance and nematode resistance are linked to QTLs [114]. The selection of resistant genotype at the early growth stage was improved by the newly developed SNP08 marker. This marker was mapped at 0.4 cM from the ABD resistance gene and it has role in avoiding the selection of susceptible varieties. On the other side of the gene, some new markers were also identified at 0.7 cM from the ABSr locus. Combining these new markers with SNP08, the probability of selection of resistant genotype was increased by 0.0028%. This marker appeared to be very helpful in the selection of resistant and susceptible genotypes and for analyzing the resistant germplasm to configure the ABS genes. So, it is a very valuable tool for the selection of susceptible heterozygous cultivars which may be used as breeding parents allowing manipulation of genetic diversity in citrus and prevents susceptible homozygous genotypes.

About 40 mandarin genotypes (susceptible and resistant) were tested by the SNP08 marker and were used as breeding parents. An ultimate association was observed between response to Alternaria infections and SNP08 marker. Recently SNP08 is used in breeding programs of citrus performed at CIRAD and IVIA for the selection of ABS-resistant citrus genotypes. About 2187 resistant hybrids were selected from 4517 total hybrids rising from 10 different parental combinations by using the SNP08 marker since its development. This analysis was very helpful to prevent the growth of more than 2000 susceptible lines which were removed at the early growth stage after selection so, a lot of time, cost, personnel, and resources were saved.

5.2 Proteomics and metabolomics approaches

Proteomics is the comprehensive analysis of all the proteins found in a cell. It includes the identification of proteins, their location in the cell, their interactions with other proteins and other biological components in the cell, and most importantly post-translational modifications that a protein undergoes in the cell [115]. Metabolites are referred to as the last product of any biological activity in a cell and are found in very small quantities [116]. Metabolites are small molecules including intermediates of various metabolic reactions, signaling molecules, hormones, and other regulatory products found in a cell. Hence, metabolomics is defined as the study of metabolites of a cell [117, 118]. It is estimated that around five thousand metabolites are found in any cell depending upon the physical and chemical complexity of that cell [119].

Huanglongbing (HLB) is considered one of the most devastating citrus diseases that affect not only the production but also the quality of citrus fruit and its juice. Using a combination of proteomics and metabolomics approaches it was found that in symptomatic fruit, the expression of proteins found in the cytoplasm for glycolysis, in mitochondria involved in the tricarboxylic acid (TCA) cycle, and in chloroplasts for the synthesis of amino-acids was downregulated. Similar downregulation was observed for genes involved in terpenoid metabolism for example valencene, limonene, 3-carene, linalool, myrcene, and aterpineol in fruit found on infected trees. Similar phenomena were observed for sucrose and glucose. Hence, the off-flavor found in symptomatic fruits was linked to the downregulation of the above genes and a decrease in the levels of the abovementioned secondary metabolites [120].

In another study, comparative iTRAQ proteomic profiling was carried out using the fruits of sweet orange which was grafted on sensitive and tolerant rootstocks infected by CaLas. The results showed that symptomatic fruit on sensitive rootstock exhibited a greater number of differentially expressed (DE) proteins as compared to the healthy fruit on a similar rootstock. It was also found that the expression level of various defense-related proteins was reduced in symptomatic fruit on sensitive rootstock, particularly the proteins related to the jasmonate biosynthesis, is signaling, protein hydrolysis, and vesicle trafficking. Hence, it was concluded that the down-regulation of these proteins is likely to be linked with the sensitivity of citrus to the CaLas pathogen [121].

5.3 Interactomics and metabolomics and phenomics

Interactomics bears a broad scope as it may cover a complete set of interactions in a cell [122]. It covers every type of interaction among interacting molecules including proteins and other molecules. It is a well-known fact that the Protein–protein interactions are major of all cellular processes [123].

To designate the complete phenotype of a plant, the term phenome is used. Similarly, a phenotype encloses a group of traits that are liable to be distinguished either by utilizing modern science analytical techniques or by a naked eye evaluation. These traits can also be attributed to being an interaction between external factors (environment) and Genotype. David Houle also termed phenomics as the collection of data from varying backgrounds and dimensions in a single entity [124]. Phenomics involves both “extreme phenotyping,” referring to a comprehensive selection of a wide range of valid and correct phenotypes, and “phenome analysis” indicating towards an analysis of specimen and correlation between syndication of genotype and phenotype.

Plant phenomics utilizes screening of large populations to analyze genetic mutations found in the population for a specific trait (drought, salinity, or high-temperature stress tolerance). Various types of imaging techniques are employed in the phenotyping of plants for various growth and developmental processes. The techniques include visible-light imaging [125], Thermographic imaging [126], Hyperspectral imaging, Chlorophyll fluorescence, X-ray, MRI, PET [127].

Using the phenomics approach and tools, we can study the traits regarding plant growth, leaf growth, root growth, and architecture of soil/root interaction, etc. This extensive use of phenomics and its integration into OMICS is the need of the hour to combat food security issues and overcome adverse effects of climate change on crop production.

Advertisement

6. Conclusions

Conventional research has played a pivotal role in the improvement of citrus. Enhanced heterozygosity has helped in the development of genetically diverse germplasm in most of the citrus species and numerous varieties have been released for commercial cultivation. However, with the advent of modern biotechnological tools, the period involved in crop improvement through indirect mutagenesis and polyploidization could be further reduced and enhancing cost-effectiveness. Transgenic technology and OMICS have great potential to improve this fruit crop. MultiOMICS, integrative-OMICS, or panOMICS technologies may result in better crops having better agronomic traits, enhanced yield potential, and less prone to insect pests. It will ultimately lead towards food security and poverty alleviation. Various OMICS technologies have been used for crop improvement, yet their integrated use will further strengthen the application of this robust technology. Still, there are many challenges associated with tolerant varieties which need to be fine-tuned. Moreover, three thousand reports of enhanced drought and salinity tolerance in wheat, sorghum, canola and rice are present but none of them is in use by farmers. A fundamental reason for this is that salinity and drought are complex multigenic traits. So, to induce tolerance in plants every gene needs to be fine-tuned precisely. However, their evaluation in the field is a long way, and distribution at the commercial level is also a hurdle in their production.

References

  1. 1. Swingle WT. A new taxonomic arrangement of the orange subfamily, Aurantioideae. Journal of Washington Academy of Sciences 1938;28(1) 530-533
  2. 2. Food and Agriculture Organization. Crops country data. http://faostat.fao.org/site/339/default.aspx (assessed 20 May 2021)
  3. 3. Mendonca LBP, Badel JL, Zambolim L. Bacterial citrus diseases: Major threats and recent progress. Bacteriol Mycol Open Access. 2017;5(4):340-350
  4. 4. Swingle WT., Reece PC. The botany of Citrus and its wild relatives. In: Reuther W., Weber HJ., Batchelor LD. (eds.) The Citrus Industry Volume 1, History, World Distribution, Botany and Varieties. Berkeley: University of California Press; 1967. p 190-243
  5. 5. Garcia-Lor A, Curk F, Snoussi-Trifa H, Morillon R, Ancillo G, Luro F, Navarro L, Ollitrault P. A nuclear phylogenetic analysis: SNPs, indels and SSRs deliver new insights into the relationships in the ‘true citrus fruit trees’ group (Citrinae, Rutaceae) and the origin of cultivated species. Annals of Botany 2013;111(1) 1-9
  6. 6. Wu GA, Terol J, Ibanez V, López-García A, Pérez-Román E, Borredá C, Domingo C, Tadeo FR, Carbonell-Caballero J, Alonso R, Curk F, Du D, Ollitrault P, Roose ML, Dopazo J, Gmitter FG, Rokhsar DS, Talon M. Genomics of the origin and evolution of Citrus. Nature 2018;554(1) 311-316
  7. 7. Oueslati A, Salhi-Hannachi A, Luro F, Vignes H, Mournet P, Ollitrault P. Genotyping by sequencing reveals the interspecific C. maxima/C. reticulata admixture along the genomes of modern citrus varieties of mandarins, tangors, tangelos, orangelos and grapefruits. PloS One 2017;12(10) e0185618
  8. 8. Krueger R, Navarro L. Citrus germplasm resources. In: Khan IA. (ed.) Citrus Genetics, Breeding, and Biotechnology. Wallingford: CAB International; 2007. pp. 45-140
  9. 9. Grosser JW, Gmitter FG. Use of in vitro technique to facilitate seedless Citrus cultivars development. Proceedings of the XXIII International Horticultural Congress, 27 August-1 September 1990. Firenze, Italy: 1990a
  10. 10. Rana MA, Usman M, Fatima B, Fatima A, Rana IA, Rehman W, Shoukat D. Prospects of mutation breeding in grapefruit (Citrus paradisi Macf.). Journal of Horticultural Science & Technology 2020;3(2) 31-35
  11. 11. Ahloowalia BS., Maluszynski M, Nichterlein K. Global impact of mutation derived varieties. Euphytica 2004;135(1) 187-204
  12. 12. Dobres MS. Barriers to genetically engineered ornamentals: An industry perspective. In: da Silva JAT. (ed.) Floriculture, Ornamental and Plant Biotechnology; Advances and Topical Issues. Isleworth: Global Science Books; 2008. p. 1-14
  13. 13. Kharkwal MC. A brief history of plant mutagenesis. In: Shu QY, Forster BP, Nakagawa H. (eds.) Plant Mutation Breeding and Biotechnology. Wallingford: CAB International; 2012. p 21-30
  14. 14. Jankowicz-Cieslak J., Mba C, Till BJ. Mutagenesis for crop breeding and functional genomics. In: Jankowicz-Cieslak J, Tai T, Kumlehn J, Till B. (eds.) Biotechnologies for Plant Mutation Breeding. Switzerland: Springer Cham; 2017. p 3-18
  15. 15. Mutant variety database. MVD 2021. http://mvd.iaea.org/ (Accessed: 22 May 2021)
  16. 16. Lonnig WE. Mutation breeding, evolution, and the law of recurrent variation. Recent Research and Development in Genetics and Breeding 2005;2(1) 45-70
  17. 17. Ollitrault P, Navarro L. Citrus. In: Fruit breeding 2012 (pp. 623-662). Springer, Boston, MA
  18. 18. Mba C., Afza R, Shu QY. Mutagenic radiations: X-rays, ionizing particles and ultraviolet. In: Shu QY, Forster BP, Nakagawa H. (eds.) Plant Mutation Breeding and Biotechnology. Wallingford: CAB International; 2012. p 83-90
  19. 19. Goldenberg L, Yaniv Y, Porat R, Carmi N. Effects of gamma-irradiation mutagenesis for induction of seedlessness, on the quality of mandarin fruit. Food and Nutrition Sciences. 2014;5 943-952
  20. 20. Lamo K, Bhat DJ, Kour K, Solanki SPS. Mutation studies in fruit crops: A review. International Journal of Current Microbiology and Applied Sciences 2017;6(12) 3620-3633
  21. 21. Cimen B, Yesiloglu T, Incesu M, Yilmaz B. Studies on mutation breeding in citrus: Improving seedless types of ‘Kozan’ common orange by gamma irradiation. Scientia Horticulturae. 2021;278:109857
  22. 22. Amano E. Use of induced mutants in rice breeding in Japan. Plant Mutation Rep 2006;1(1) 21-24
  23. 23. Matsumura A, Nomitzu T, Furukani N, Hayashi K, Minamiyama Y, Hase Y. Ray florets colour and shape mutants induced by 12C5+ ion beam irradiation in chrysanthemum. Scientia Horticulturae 2010;123(4) 558-561
  24. 24. Usman M., Fatima B. Mandarin (Citrus reticulata Blanco) breeding. In: Al-Khayri JM, Jain SM, Johnson DV. (eds.) Advances in Plant Breeding Strategies: Fruits. Switzerland: Springer Cham; 2018. p 465-533
  25. 25. Usman M., Khan MM, Al-Yahyai R, Fatima B. Lime breeding: A way forward. In: Yahia, E.D. (ed.). Achieving Sustainable Cultivation of Tropical Fruits. Cambridge: Burleigh Dodds Science Publishing; 2019b. p 1-45
  26. 26. Predieri S. Mutation induction and tissue culture in improving fruits. Plant Cell Tissue and Organ Culture 2001;64(2) 185-210
  27. 27. Campeanu G, Neaţa G, Darjanschi G, Stan R. Tree and fruit characteristics of various apple genotypes obtained through mutagenesis. Notulae Botanicae Horti Agrobotanici Cluj-Napoca 2010;38(1) 248-251
  28. 28. Saini HK, Gill MIS. Induction of mutation in rough lemon (Citrus jambhiri Lush.) using gamma rays. Journal of Horticultural Sciences 2009;4(1) 41-44
  29. 29. Williams T, Roose ML. Daisy SL, Fairchild SL and Kinnow SL-Three New, Very Low-Seeded, Mid Season Irradiated Selection of W. Murcott Mandarin from the University of California Riverside. In: International Citrus Congress 2008
  30. 30. Zhang YJ, Wang XJ, Wu JX, Chen SY, Chen H, Chai LJ, Yi HL. Comparative transcriptome analyses between a spontaneous late-ripening sweet orange mutant and its wild type suggest the functions of ABA, sucrose and JA during citrus fruit ripening. PLoS One 2014;9(12) e116056
  31. 31. Altaf S, Khan MM, Jaskani MJ, Khan IA, Usman M, Sadia B, Awan FS, Ali A, Khan AI. Morphogenetic characterization of seeded and seedless varieties of Kinnow Mandarin (Citrus reticulata Blanco). Australian Journal of Crop Science 2014;8(11) 1542-1549
  32. 32. Maluszynski M, Nichterlein K, Van-Zanten L, Aloowalia BS. Officially released mutant varieties – The FAO/IAEA database. Mutation Breeding Review 2000;12(1):1-84
  33. 33. Singh A, Naqvi SAMH, Singh S. Citrus Germplasm. Cultivars and Rootstocks. Ludhiana: Kalyani Publishers; 2002
  34. 34. Parry MA, Madgwick PJ, Bayon C, Tearall K, Hernandez-Lopez A, Baudo M, Rakszegi M, Hamada W, Al-Yassin A, Ouabbou H, Labhilili M. Mutation discovery for crop improvement. Journal of Experimental Botany 2009;60(10) 2817-2825
  35. 35. Luo M, Gilbert B, Ayliffe M. Applications of CRISPR/Cas9 technology for targeted mutagenesis, gene replacement and stacking of genes in higher plants. Plant Cell Reports 2016;35(7) 1439-1450
  36. 36. Oustric J, Morillon R, Ollitrault P, Herbette S, Luro F, Froelicher Y, Tur I, Dambier D, Giannettini J, Berti L, érémie Santini J. Somatic hybridization between diploid Poncirus and Citrus improves natural chilling and light stress tolerances compared with equivalent doubled-diploid genotypes. Trees. 2018;32(3) 883-895
  37. 37. Murata MM, Omar AA, Mou Z, Chase CD, Grosser JW, Graham JH. Novel plastid-nuclear genome combinations enhance resistance to citrus canker in cybrid grapefruit. Frontiers in plant science. 2019;7(9) 1858
  38. 38. Khalid MF, Hussain S, Anjum MA, Ali MA, Ahmad S, Ejaz S, Ali S, Usman M, Ehsan ul Haq, Morillon R. Efficient compartmentalization and translocation of toxic minerals lead tolerance in volkamer lemon tetraploids more than diploids under moderate and high salt stress. Fruits 2020; 75(5) 204-215
  39. 39. Soltis DE, Soltis PS. Polyploidy: Origins of species and genome evolution. Trends in Ecology and Evolution 1999;9(1):348-352
  40. 40. Usman M, Saeed T, Khan MM, Fatima B. Occurrence of spontaneous polyploids in Citrus. Horticultural Science (Prague) 2006;33(3) 124-129
  41. 41. Shafieizargar A, Awang Y, Juraimz AS, Othman R. Comparative studies between diploid and tetraploid Dez Orange (Citrus sinensis (L.) Osb.) under salinity stress. Australian Journal of Crop Science 2013;7(10) 1436-1441
  42. 42. Fatima B, Usman M, Khan IA, Khan MS, Khan MM. Identification of citrus polyploids using chromosome count, morphological and SSR markers. Pakistan Journal of Agricultural Sciences 2015;52(1) 107-114
  43. 43. Usman M, Fatima B, Usman M, Rana IA. Morphological and stomatal diversity in colchiploid germplasm of grapefruit. Pakistan Journal of Agricultural Sciences 2021;58(2) 555-560
  44. 44. Rouiss H, Cuenca J, Navarro L, Ollitrault P, Aleza P. Unreduced megagametophyte production in lemon occurs via three meiotic mechanisms, predominantly second-division restitution. Frontiers in Plant Science 2017;12(8) 1211
  45. 45. Xie KD, Xia QM, Peng J, Wu XM, Xie ZZ, Chen CL, Guo WW. Mechanism underlying 2n male and female gamete formation in lemon via cytological and molecular marker analysis. Plant Biotechnology Reports 2019;13(2) 141-149
  46. 46. Xia Q, Wang W, Xie K, Wu X, Deng X, Grosser JW, Guo W. Unreduced megagametophyte formation via second division restitution contributes to tetraploid production in interploidy crosses with ‘Orah’ mandarin (Citrus reticulata). Frontiers of Agricultural Science and Engineering 2017; https://doi.org/10.15302/J-FASE-2021385
  47. 47. Usman M, Fatima B, Gillani KA, Khan MS, Khan MM. Exploitation of potential target tissues to develop polyploids in citrus. Pakistan Journal of Botany 2008;40(4) 1755-1766
  48. 48. Yanagimoto Y, Kaneyoshi J, Yamasaki A, Kitajima A. Estimation of polyploidy of embryos and gametes based on polyploidy of endosperm remaining in the citrus seed. Horticultural Research (Japan). 2018;17(3) 293-302
  49. 49. Froelicher Y, Ollitrault P. Effects of the hormonal balance on Clausena excavata androgenesis. Acta Horticulturae 2000;535 139-146
  50. 50. Barrett HC, Hutchinson DJ. Spontaneous tetraploidy in apomictic seedlings of Citrus. Economic Botany 1978;32 27-45
  51. 51. Bachi O. Observaceous Citologicalem Citrus. I. Numero de cromosomas de algunas especies y variedades. Journal of Agronomy 1940;3(1) 249-258
  52. 52. Esen A, Soost RK. Tetraploid progenies from 2x x 4x crosses of Citrus and their origin. Journal of the American Society for Horticultural Science 1972;97 410-414
  53. 53. Froelicher Y, Luro F, Ollitrault P. Analysis of meiotic behavior of the tetraploid Clausea excavate species by Molecular Marker Segregation Studies. In: 9th ISC Congress. South Africa; 2000. p116
  54. 54. Jaskani MJ, Khan IA, Khan MM. Fruit set, seed development and embryo germination in interploid crosses of citrus. Scientia Horticulturae 2005;107(1) 51-57
  55. 55. Aleza P, Juárez J, Cuenca J, Ollitrault P, Navarro L. Extensive citrus triploid hybrid production by 2x× 4x sexual hybridizations and parent-effect on the length of the juvenile phase. Plant Cell Reports 2012;31(9) 1723-1735
  56. 56. Xie KD, Wang XP, Biswas MK, Liang WJ, Xu Q, Grosser JW, Guo WW. 2n megagametophyte formed via SDR contributes to tetraploidization in polyembryonic ‘Nadorcott’ tangor crossed by citrus allotetraploids. Plant Cell Reports 2014;33(10) 1641-1650
  57. 57. Xia Q, Wang W, Xie K, Wu X, Deng X, Grosser JW, Guo W. Unreduced megagametophyte formation via second division restitution contributes to tetraploid production in interploidy crosses. https://doi.org/10.15302/J-FASE-2021385
  58. 58. Aleza P, Cuenca J, Juárez J, Navarro L, Ollitrault P. Inheritance in doubled-diploid clementine and comparative study with SDR unreduced gametes of diploid clementine. Plant Cell Reports 2016;35(8) 1573-1586
  59. 59. Viloria Z, Grosser JW. Acid citrus fruit improvement via interploid hybridization using allotetraploid somatic hybrid and autotetraploid breeding parents. Journal of the American Society for Horticultural Science. 2005 May 1;130(3) 392-402
  60. 60. Grosser JW, Jiang J, Mourăo Filho FAA, Louzada ES, Baergen K, Chandler JL, Gmitter FG. Somatic hybridization, an integral component of Citrus cultivar improvement: I. Scion improvement. HortScience 1998;33(1) 1057-1059
  61. 61. Ohgawara T, Kobayashi S, Ohgawara E, Uchimaya H, Ishii S. Somatic hybrid plants obtained by protoplasm fusion between Citrus sinensis and Poncirus trifoliata. Theoretical and Applied Genetics 1985;71 1-4
  62. 62. Grosser JW, Mourăo Filho FAA, Gmitter FG, Louzada ES, Jiang J, Baergen K, Quiros A, Cabasson C, Schell JL, Chandler JL. Allotetraploid hybrids between Citrus and seven related genera produced by somatic hybridization. Theoretical and Applied Genetics 1996;92 577-582
  63. 63. Miranda M, Motomura T, Ikeda F, Ohgawara T, Saito W, Endo T, Omura M, Moriguchi T. Somatic hybrids obtained by fusion between Poncirus trifoliata (2x) and Fortunella hindsii (4x) protoplasts. Plant Cell Reports 1997;16(6) 401-405
  64. 64. Guo WW, Deng XX. Intertribal hexaploid somatic hybrid plants regeneration from electrofusion between diploids of Citrus sinensis and its sexually incompatible relative, Clausena lansium. Theoretical and applied genetics. 1999 1;98(3-4) 581-585
  65. 65. Grosser JW, Ollitrault P, Olivares-Fuster O. Somatic hybridization in citrus: an effective tool to facilitate variety improvement. In Vitro Cellular and Developmental Biology-Plant. 2000 1;36(6) 434-449
  66. 66. Cabasson CM, Luro F, Ollitrault P, Grosser J. Non-random inheritance of mitochondrial genomes in Citrus hybrids produced by protoplast fusion. Plant Cell Reports. 2001 20(7) 604-609
  67. 67. Khan MS, Ali S, Iqbal J. Developmental and photosynthetic regulation of Bacillus thuringiensis δ-endotoxin reveals that engineered sugarcane conferring resistance to ‘dead heart’ contains no toxins in cane juice. Molecular Biology Reports. 2011;38: 2359-2369
  68. 68. Mustafa G, Khan MS. Transmission of engineered plastids in sugarcane, a C4 monocotyledonous plant, reveals that sorting of preprogrammed progenitor cells produce heteroplasmy. Plants. 2021;10,26
  69. 69. Bock, R. and Khan, M.S. (2004). Taming plastids for a green future. Trends in Biotechnology 22: 311-318
  70. 70. Sun L, Ke F, Nie Z, Wang P, Xu J. Citrus genetic engineering for disease resistance: Past, present and future. International journal of molecular sciences 2019;20(21):5256
  71. 71. Dominguez A, Guerri J, Cambra M, Navarro L, Moreno P, Pena L. Efficient production of transgenic citrus plants expressing the coat protein gene of citrus tristeza virus. Plant cell reports. 2000; 19(4):427-433
  72. 72. Yao JL, Wu JH, Gleave AP, Morris BA. Transformation of citrus embryogenic cells using particle bombardment and production of transgenic embryos. Plant Science 1996;113(2):175-183
  73. 73. Fleming GH, Olivares-Fuster O, Del-Bosco SF, Grosser JW. An alternative method for the genetic transformation of sweet organce. In Vitro Cellular and Developmental Biology-Plant 2000;36(6):450-455
  74. 74. Kaneyoshi J, Kobayashi S, Nakamura Y, Shigemoto N, Doi Y. A simple and efficient gene transfer system of trifoliate orange (Poncirus trifoliata Raf.). Plant Cell Reports 1994;13(10):541-545
  75. 75. Singh S, Rajam MV. Citrus biotechnology: Achievements, limitations and future directions. Physiology and Molecular Biology of Plants 2009;15(1):3-22
  76. 76. Yang ZN, Ingelbrecht IL, Louzada E, Skaria M, Mirkov TE. Agrobacterium-mediated transformation of the commercially important grapefruit cultivar Rio Red (Citrus paradisi Macf.). Plant Cell Reports 2000;19(12):1203-1211
  77. 77. Yu C, Huang S, Chen C, Deng Z, Ling P, Gmitter FG. Factors affecting Agrobacterium-mediated transformation and regeneration of sweet orange and citrange. Plant Cell, Tissue and Organ Culture 2002;71(2):147-155
  78. 78. Xiao-hong Y, Zhong-hail S and Rui-jian T Optimizing culture system of Ri T-DNA transformed roots for Citrus grandis cv. Changshou Shatian You. Agric. Sci. China 2006; 5: 90-97
  79. 79. Bespalhok Filho JC, Kobayashi AK, Pereira LF, Galvão RM, Vieira LG. Transient gene expression of beta-glucuronidase in citrus thin epicotyl transversal sections using particle bombardment. Brazilian Archives of Biology and Technology 2003;46(1):1-6
  80. 80. Cervera M, Ortega C, Navarro A, Navarro L, Pena L. Generation of transgenic citrus plants with the tolerance-to-salinity gene HAL2 from yeast. The Journal of Horticultural Science and Biotechnology 2000;75(1):26-30
  81. 81. Hasan N, Kamruzzaman M, Islam S, Hoque H, Bhuiyan FH and Prodhan SH. Development of partial abiotic stress tolerant Citrus reticulata Blanco and Citrus sinensis (L.) Osbeck through Agrobacterium mediated transformation method. Journal of Genetic Engineering and Biotechnology 17:14
  82. 82. Guo W, Duan Y, Olivares-Fuster O, Wu Z, Arias CR, Burns JK, Grosser JW. Protoplast transformation and regeneration of transgenic Valencia sweet orange plants containing a juice quality-related pectin methylesterase gene. Plant Cell Reports 2005;24(8):482-486
  83. 83. Fagoaga C, López C, de Mendoza AH, Moreno P, Navarro L, Flores R, Peña L. Post-transcriptional gene silencing of the p23 silencing suppressor of Citrus tristeza virus confers resistance to the virus in transgenic Mexican lime. Plant molecular biology 2006;60(2):153-165
  84. 84. Febres VJ, Lee RF, Moore GA. Transgenic resistance to Citrus tristeza virus in grapefruit. Plant cell reports 2008;27(1):93-104
  85. 85. Boava LP, Cristofani-Yaly M, Mafra VS, Kubo K, Kishi LT, Takita MA, Ribeiro-Alves M, Machado MA. Global gene expression of Poncirus trifoliata, Citrus sunki and their hybrids under infection of Phytophthora parasitica. BMC genomics 2011;12(1):1-3
  86. 86. Mondal SN, Dutt M, Grosser JW, Dewdney MM. Transgenic citrus expressing the antimicrobial gene Attacin E (attE) reduces the susceptibility of ‘Duncan’ grapefruit to the citrus scab caused by Elsinoë fawcettii. European journal of plant pathology 2012;133(2):391-404
  87. 87. Caserta R, Teixeira-Silva NS, Granato LM, Dorta SO, Rodrigues CM, Mitre LK, Yochikawa JT, Fischer ER, Nascimento CA, Souza-Neto RR, Takita MA. Citrus biotechnology: What has been done to improve disease resistance in such an important crop?. Biotechnology Research and Innovation. 2020;3:95-109
  88. 88. Zou X, Jiang X, Xu L, Lei T, Peng A, He Y, Yao L, Chen S. Transgenic citrus expressing synthesized cecropin B genes in the phloem exhibits decreased susceptibility to Huanglongbing. Plant molecular biology 2017;93(4-5):341-353
  89. 89. Robertson CJ, Zhang X, Gowda S, Orbović V, Dawson WO, Mou Z. Overexpression of the Arabidopsis NPR1 protein in citrus confers tolerance to Huanglongbing. Journal of Citrus Pathology. 2018;5(1):1-8
  90. 90. Omar AA, Murata MM, El-Shamy HA, Graham JH, Grosser JW. Enhanced resistance to citrus canker in transgenic mandarin expressing Xa21 from rice. Transgenic research 2018;27(2):179-191
  91. 91. Kobayashi AK, Vieira LG, Bespalhok Filho JC, Leite RP, Pereira LF, Molinari HB, Marques VV. Enhanced resistance to citrus canker in transgenic sweet orange expressing the sarcotoxin IA gene. European Journal of Plant Pathology 2017;149(4):865-873
  92. 92. LeBlanc C, Zhang F, Mendez J, Lozano Y, Chatpar K, Irish VF, Jacob Y. Increased efficiency of targeted mutagenesis by CRISPR/Cas9 in plants using heat stress. The Plant Journal 2018;93(2):377-386
  93. 93. Knott GJ, Doudna JA. CRISPR-Cas guides the future of genetic engineering. Science 2018;361(6405):866-869
  94. 94. Jia H, Wang N. Xcc-facilitated agroinfiltration of citrus leaves: A tool for rapid functional analysis of transgenes in citrus leaves. Plant cell reports 2014;33(12):1993-2001
  95. 95. Hu Y, Zhang J, Jia H, Sosso D, Li T, Frommer WB, Yang B, White FF, Wang N, Jones JB. Lateral organ boundaries 1 is a disease susceptibility gene for citrus bacterial canker disease. Proceedings of the National Academy of Sciences 2014;111(4):E521-E529
  96. 96. Peng A, Chen S, Lei T, Xu L, He Y, Wu L, Yao L, Zou X. Engineering canker-resistant plants through CRISPR/Cas9-targeted editing of the susceptibility gene Cs LOB 1 promoter in citrus. Plant biotechnology journal 2017;15(12):1509-1519
  97. 97. Wang L, Chen S, Peng A, Xie Z, He Y, Zou X. CRISPR/Cas9-mediated editing of CsWRKY22 reduces susceptibility to Xanthomonas citri subsp. citri in Wanjincheng orange (Citrus sinensis (L.) Osbeck). Plant Biotechnology Reports 2019;13(5):501-510
  98. 98. Jia H, Orbović V, Wang N. CRISPR-LbCas12a-mediated modification of citrus. Plant biotechnology journal 2019;17(10):1928-1937
  99. 99. Razzaq A, Saleem F, Kanwal M, Mustafa, G, Yousaf S, Arshad, HMI, Hameed MK, Khan MS and Joyia FA (2019). Modern trends in plant genome editing: An inclusive review of the CRISPR/Cas9 toolbox. International Journal of Molecular Sciences. 20: 4045; doi:10.3390/ijms20164045
  100. 100. Hamblin MT, Buckler ES, Jannink JL. Population genetics of genomics-based crop improvement methods. Trends in Genetics 2011;27(3):98-106
  101. 101. Ajiro N, Miyamoto Y, Masunaka A, Tsuge T, Yamamoto M, Ohtani K, Fukumoto T, Gomi K, Peever TL, Izumi Y, Tada Y. Role of the host-selective ACT-toxin synthesis gene ACTTS2 encoding an enoyl-reductase in pathogenicity of the tangerine pathotype of Alternaria alternata. Phytopathology 2010;100(2):120-126
  102. 102. Peres NA, Timmer LW. Evaluation of the Alter-Rater model for spray timing for control of Alternaria brown spot on Murcott tangor in Brazil. Crop Protection 2006;25(5):454-460
  103. 103. Vicent A, Armengol J, García-Jiménez J. Protectant activity of reduced concentration copper sprays against Alternaria brown spot on ‘Fortune’ mandarin fruit in Spain. Crop Protection 2009;28(1):1-6
  104. 104. Vicent A, Armengol J, García-Jiménez J. Rain fastness and persistence of fungicides for control of Alternaria brown spot of citrus. Plant Disease 2007;91(4):393-399
  105. 105. Luro F, Maddy F, Jacquemond C, Froelicher Y, Morillon R, Rist D, et al. (Eds.), 2004. Identification and evaluation of diplogyny in clementine (Citrus clementina) for use in breeding. In International Society for Horticultural Science (ISHS), Leuven, Belgium
  106. 106. Cuenca J, Froelicher Y, Aleza P, Juárez J, Navarro L, Ollitrault P. Multilocus half-tetrad analysis and centromere mapping in citrus: Evidence of SDR mechanism for 2 n megagametophyte production and partial chiasma interference in mandarin cv ‘Fortune’. Heredity. 2011;107(5):462-470
  107. 107. Cuenca J, Aleza P, Juarez J, García-Lor A, Froelicher Y, Navarro L, Ollitrault P. Maximum-likelihood method identifies meiotic restitution mechanism from heterozygosity transmission of centromeric loci: Application in citrus. Scientific Reports 2015;5(1):1-11
  108. 108. Rouiss H, Cuenca J, Navarro L, Ollitrault P, Aleza P. Tetraploid citrus progenies arising from FDR and SDR unreduced pollen in 4x X 2x hybridizations. Tree Genetics and Genomes 2017;13(1):10
  109. 109. Cuenca J, Aleza P, Vicent A, Brunel D, Ollitrault P, Navarro L. Genetically based location from triploid populations and gene ontology of a 3.3-Mb genome region linked to Alternaria brown spot resistance in citrus reveal clusters of resistance genes. PloS one 2013;8(10):e76755
  110. 110. Cuenca J, Aleza P, Garcia-Lor A, Ollitrault P, Navarro L. Fine mapping for identification of citrus alternaria brown spot candidate resistance genes and development of new SNP markers for marker-assisted selection. Frontiers in plant science 2016; 7:1948
  111. 111. Butelli E, Licciardello C, Zhang Y, Liu J, Mackay S, Bailey P, Reforgiato-Recupero G, Martin C. Retrotransposons control fruit-specific, cold-dependent accumulation of anthocyanins in blood oranges. The Plant Cell 2012;24(3):1242-1255
  112. 112. Wang X, Xu Y, Zhang S, Cao L, Huang Y, Cheng J, Wu G, Tian S, Chen C, Liu Y, Yu H. Genomic analyses of primitive, wild and cultivated citrus provide insights into asexual reproduction. Nature Genetics 2017;49(5):765-772
  113. 113. Fang DQ, Federici CT, Roose ML. Development of molecular markers linked to a gene controlling fruit acidity in citrus. Genome 1997;40(6):841-849
  114. 114. Ben-Hayyim G, Moore GA. Recent advances in breeding citrus for drought and saline stress tolerance. Advances in molecular breeding toward drought and salt tolerant crops. 2007:627-642
  115. 115. Ricroch AE, Bergé JB, Kuntz M. Evaluation of genetically engineered crops using transcriptomic, proteomic, and metabolomic profiling techniques. Plant physiology 2011;155(4):1752-1761
  116. 116. Randhawa HS, Asif M, Pozniak C, Clarke JM, Graf RJ, Fox SL, Humphreys DG, Knox RE, DePauw RM, Singh AK, Cuthbert RD. Application of molecular markers to wheat breeding in C anada. Plant Breeding 2013;132(5):458-471
  117. 117. Wishart TM, Rooney TM, Lamont DJ, Wright AK, Morton AJ, Jackson M, Freeman MR, Gillingwater TH. Combining comparative proteomics and molecular genetics uncovers regulators of synaptic and axonal stability and degeneration in vivo. PLoS Genetics 2012;8(8):e1002936
  118. 118. Aslam B, Basit M, Nisar MA, Khurshid M, Rasool MH. Proteomics: Technologies and their applications. Journal of chromatographic science. 2017 Feb 1;55(2):182-196
  119. 119. Gregersen N, Hansen J, Palmfeldt J. Mitochondrial proteomics—A tool for the study of metabolic disorders. Journal of inherited metabolic disease 2012;35(4):715-726
  120. 120. Yao L, Yu Q, Huang M, Hung W, Grosser J, Chen S, Wang Y, Gmitter FG. Proteomic and metabolomic analyses provide insight into the off-flavour of fruits from citrus trees infected with ‘Candidatus Liberibacter asiaticus’. Horticulture research 2019;6(1):1-3
  121. 121. Yao L, Yu Q, Huang M, Song Z, Grosser J, Chen S, Wang Y, Gmitter Jr FG. Comparative iTRAQ proteomic profiling of sweet orange fruit on sensitive and tolerant rootstocks infected by ‘Candidatus Liberibacter asiaticus’. PloS one 2020;15(2):e0228876
  122. 122. Graves PR, Haystead TA. Molecular biologist’s guide to proteomics. Microbiology and molecular biology reviews. 2002 Mar 1;66(1):39-63
  123. 123. Singh J, Pandey P, James D, Chandrasekhar K, Achary VM, Kaul T, Tripathy BC, Reddy MK. Enhancing C3 photosynthesis: An outlook on feasible interventions for crop improvement. Plant Biotechnology Journal. 2014;12(9):1217-1230
  124. 124. Weckwerth W. Metabolomics in systems biology. Annual review of plant biology 2003;54(1):669-689
  125. 125. Oliver SG, Winson MK, Kell DB, Baganz F. Systematic functional analysis of the yeast genome. Trends in biotechnology 1998;16(9):373-378
  126. 126. Hansen J, Palmfeldt J, Vang S, Corydon TJ, Gregersen N, Bross P. Quantitative proteomics reveals cellular targets of celastrol. PLoS One 2011;6(10):e26634
  127. 127. Griffin JL, Vidal-Puig A. Current challenges in metabolomics for diabetes research: A vital functional genomic tool or just a ploy for gaining funding?. Physiological Genomics 2008;34(1):1-5

Written By

Ghulam Mustafa, Muhammad Usman, Faiz Ahmad Joyia and Muhammad Sarwar Khan

Submitted: 28 May 2021 Reviewed: 02 September 2021 Published: 24 September 2021