Perspective Chapter: An Insight into Abiotic Stresses in Pigeonpea – Effects and Tolerance

Abstract

Cajanus cajan (L.) Millsp. is an adaptable, resilient, and nutrient-dense grain legume with qualities that can help agricultural systems become more sustainable in subtropical and tropical regions. Extremes in moisture, temperature, photoperiod, and mineral-related stressors are the most significant that encounter throughout the life cycle. Different stress slows down the plants’ growth by upsetting its typical physiology and morphology. Inefficient conditions can occur sequentially or simultaneously in environments, and plants have evolved defense mechanisms to continue to grow and survive under less-than-ideal edaphic and climatic factors. Although various genotypes of pigeonpea have been identified that are tolerant to heat, drought, and waterlogging, new empirical evidence reveals that genotypic changes have been detected for all of the abiotic stimuli in this crop. Furthermore, to enhance tolerance, breeding techniques or methods such as marker features, including extensive hybridization, double haploids, tissue culture, somaclonal variants, genetic transformation, and marker-assisted breeding, have been employed to lessen the effect of these stressor. These methods help in the development of enhanced germplasm with abiotic stress tolerance and disease resistance, resulting in higher crop quality and production. This chapter focuses on different abiotic stressors and the methods that have been employed to help pigeonpea to overcome environmental constraints.

1. Introduction

Legumes are well known for their nutritional and health benefits, as well as their contribution to agricultural system sustainability. They are the most important single source of vegetable protein in human diets and cattle feed (forages) [1]. Legumes are frequently used as an intercrop (e.g., paired with cereals) or in crop rotation in farming systems, resulting in a reduction in pests, diseases, and weed populations while increasing overall farm production and income for smallholder farmers. Other than commercial and economic importance, legumes have gotten less attention than cereals in terms of increasing agricultural production. A variety of abiotic stresses are threatening the legume crops [2]. Studies on stress tolerance processes have led to the identification of characters related with tolerance in plants, as well as the molecular regulation of stress-responsive genes. Some of these researches have paved the way for new opportunities to investigate the molecular basis of plant stress responses and find novel features and associated genes for agricultural plant genetic improvement (Table 1) [15].

Cajanus cajan
	Genomics resources	References
Species	Diploid	[3]
Genome size	833.07 Mbp	[4]
Genetic maps	Reference genetic map, six intraspecific maps, one consensus map	[5]
	DArT based maternal and paternal maps	[6]
SNPs array	50 K Affymetrix Axiom	[7]
	Genotyping-by-sequencing (GBS)	[6]
	Restriction site-associated DNA sequencing (RAD)	[8]
Database	PpTFDB, Pipemicrodb	[9]
Number of genes and ESTs	48,680 and 25,640	[3]
Whole genome sequencing	Reference genome sequence	[3, 10]
	WGRS	[11, 12]
Genetic purity testing kits	SSR assay	[13, 14]

Table 1.

List of genetic resources of pigeonpea.

Cajanus cajan also named as Pigeonpea, arhar, tur, red gram, is a major pulse crop of the world’s semi-arid regions and India’s second most important pulse crop after chickpea. It is high in protein (21–28%), carbohydrates, vitamins, fats, and minerals [4]. Pigeonpea has become an important crop in India throughout time, with attempts being undertaken to produce high yielding varieties by conventional breeding and biotechnology approaches [16]. Plants have evolved complex signaling pathways that include receptors, secondary messengers, phytohormones, and signal transducers to detect different stresses and adapt to changing environmental conditions. These inherent processes promote stress signal transduction and the activation of stress-responsive gene expression in order to maintain plant growth and productivity [17].

Pigeonpea breeding has been more difficult than in other dietary legumes due to crop specific features and a very sensitive nature [18]. For more than five decades, low productivity and lack of stability have been major production challenges in this crop. This scenario is caused by abiotic stressors, in addition to genetic and agronomic factors. This dilemma can now be turned around by simultaneously reducing crop losses and increasing crop yielding capabilities [19]. This hardy crop is subjected to a variety of abiotic stresses, including moisture (waterlogging/drought), temperature, photoperiod, and mineral (salinity/acidity) stress (Table 2) [25]. Drought and heat stress, two important abiotic stress elements affecting crop loss and yield, are notable effects of climate change. Drought disrupts the pigeonpeas’ symbiotic association, reducing growth and finally leading to lower crop production [26]. The tension exerted on the northern and north-eastern areas of India where temperature extremes (too low/too high) during the reproductive stage affect the production rate [27].

Abiotic stress	Genotypes	Tolerance mechanism	Reference
Waterlogging	ICPL 84023, Asha	Lenticels development, more root biomass and adventitious root.	[20]
Drought	LRG 30, ICPL 85063, ICPL 332	High RWC, pods/plant and HI.	[21]
Low Temperature	IPA 7-2, Bahar, and MAL 19	Ability to flower and pod setting under low temperature.	[22]
Salinity	C11, ICPL 227, WRP1, GS1 and TS3, UPAS 120 and ICPL 151	Reduced translocation of Na and Cl from root to shoot.	[3]
Aluminum toxicity	IPA 7-10 and T 7, 67 B and GT 101E	Aluminium exclusion.	[23]
Cold	ICPL 87119	Involved in seed germination and metabolism	[24]

Table 2.

Different pigeonpea genotypes tolerant to abiotic stress and their mechanism [18].

Other than temperature and drought, aluminum toxicity in acidic soil is also a constraint for production. Some regions of Haryana and Punjab, where the pigeonpea is affected by waterlogging, soil erosion and salinity pressures [28]. All these factors have a significant impact on productivity, yet few changes have been made in genotypes which are resistant to these abiotic stresses. Hence, the purpose of the present study is to examine the available information on abiotic stresses and discuss approaches to improve pigeonpea resistance to these constraints.

2. Major stresses influencing C. cajan productivity

2.1 Waterlogging

Waterlogging is a major abiotic stress affecting pigeonpea production where annual rainfall is 600–1500 mm [29]. The primary biological consequence of waterlogging is a lack (hypoxia) or full lack (anoxia) of oxygen in the soil, which creates anaerobic conditions and limits plant growth and development, thus seed yield. Oxygen shortage causes electrolyte leakage, exposing the cell to the outside environment, which can lead to lipid and nucleic acid peroxidation and, eventually, death [30]. Previous studies found that proline accumulation, starch content, and effective H₂O₂ detoxification are among the significant biochemical alterations that play a major role in waterlogging resistance in pigeonpea genotypes. During monsoon, this plant is susceptible to phytophthora blight disease due to waterlogging and hence causes yield losses [31].

Another study identified that high nitrogen uptake and development of aerenchyma in ICPL-84023 enabled it to sustain growth under waterlogging [32]. Total reducing sugars, superoxide dismutase, membrane stability index, number of pods per plant, pod dry weight, and seed yield, are some biochemical changes that can be affected by waterlogging [33]. Crossings between tolerant and sensitive lines revealed greater genetic variety than crosses between tolerant lines, implying the possibility of genetic improvement for this crop. Lines derived from crosses involving C. acutifoliushave the unique property of enhancing tolerance under water logging circumstances for resource poor farming communities [29]. It is estimated that agriculture would be impacted globally by these forecasted climate changes. A better understanding of this legume crop resource and their characterization in terms of desirable traits for climate change adaptation are essential for the use of adapted C. cajan genetic resources in strengthening the resilience of future production systems [34].

2.2 Drought

Pigeonpea is a rainfed crop grown during the kharif season. Because of its deep root structure, it is considered a drought-tolerant legume [25]. It can suffer from early and terminal drought stress due to its deep and broad root structure [26]. The germplasm has a wide range of Osmotic adjustment variation (0.2–1.6 MPa), with some accessions reaching 5.0 MPa. Some varieties, such as Bahar, BSMR 853, and ICP 84031, have demonstrated increased osmotic adjustment under drought conditions [35]. In response to drought conditions, relative water content (RWC) of leaves and dehydration tolerance are crucial (Figure 1). Drought resistant breeding should be done under true moisture-deficit conditions using agronomic traits such as pods per plant, seeds per pod, seed size, and seed production per plant. Therefore, physiological interactions, as well as high mean seed yield, should be employed to identify superior genotypes for low-soil-moisture situations [23, 36].

Figure 1.

Abiotic stresses in pigeonpea with their effects.

To understand the molecular mechanism for drought response in pigeonpea, a study has been performed on ICP151, ICPL8755 and ICPL227, where 51 genes were selected using Hidden Markov Model (HMM) to identify protein domain responsible for stress-responsive genes. Ten genes of U-box proteins, H⁺ antiporter proteins, and universal stress proteins were studied out of 51 drought genes (AuspA). These genes offer the way for molecular research into drought resistance [37]. The identified genes can also be validated at the sequence level in various genetic backgrounds to identify the presence of sequence variations for the formation of gene-based markers for crop improvement and the development of breeding lines and hybrids that are more tolerant through genomics-assisted breeding [38, 39]. As drought stress stagnates the food security over the globe, it is important to develop new varieties to achieve a proper amount of yield with maintained quality under such climatic perturbation. Strategies should develop where pigeonpea could enhance physicochemical capability of their cells to continue metabolism at low leaf water status [40].

2.3 Soil salinity

Salt stress is a significant constraint to the productivity of the nutritional rich pigeonpea. India accounts for more than 85% of global production and consumption of this legume crop. Excess Na⁺ accumulation during salt stress interferes with hydrogen bonding and polar interactions, causing protein and nucleic acid structure to be disrupted. Thus, the total soluble protein content of stressed pigeonpea plants was found to be significantly lower [24]. The moisture content and succulence of C. cajan decreased dramatically as salinity increased, indicating a loss of turgor. When subjected to increasing salinity, this crop reduces water content in order to reach low osmotic potential. Salinity was performed to extend the 50% flowering stage by 1–2 weeks while also delaying the peak flowering stage. It increases floral shedding, lowering the effective quantity and weight of pods, and lastly lowering the number of seeds, lowering production [41].

Previous studies observed that the salt tolerance gene, CcCYP, is responsible for upregulated salt tolerance in root, whereas CcCDR was upregulated in shoot [42]. To make this legume crop resilient to salt stress, a better understanding of the molecular networks, in particular epigenetic regulation of gene expression, would be beneficial [23]. The potential of producing salt-tolerant lines of pigeon pea through genetic engineering has not been thoroughly studied. There is only one occurrence where transgenic pigeon pea plants were given salt tolerance through overexpression of the mutant 1-pyrroline-5-carboxylate synthetase gene (P5CSF129A) from Vigna aconitifolia [43]. These lines are notably salt tolerant. The identification of novel molecular targets that can be exploited by transgenic technologies would undoubtedly benefit from genome-wide association studies (GWAS) that uncover gene expression profiles in salt-stressed pigeon pea. It is also possible to use genomics-assisted intensive breeding to find quantitative trait loci and potential markers in salt-tolerant pigeon pea cultivars [44].

2.4 Metal toxicity

Changes in the environment are most likely to have a significant impact on how plants evolve, mostly through interfering with the process through mutations, gene flow, and evolution. Heavy metals are the major environmental changes/pollutants and their toxicity is a growing concern for ecological, evolutionary, nutritional and environmental reasons. These contaminants have a negative impact on the environment, impair agricultural output, and pose serious health risks to living organisms [45]. Metals exerts several effects on legume crop generated by elements such as Cd, Cu, Al, Hg, Pb and As, among others.

Cadmium (Cd), the most dangerous heavy metals because of their great mobility, non-degradability, and toxicity to plants as well as animals [46]. Excessive Cd²⁺ accumulation in plants can result in severe phytotoxicity as well as a variety of physiological, morphological, and biochemical toxic effects on plant attributes such as pigment destruction, photosynthetic and respiration process inhibition, reduced nutrient uptake, overproduction of reactive oxygen species (ROS), enzyme and gene suppression, growth inhibition, and even plant death [47, 48].

Copper (Cu) is a vital element for plants since it helps with several physiological processes such as mitochondrial respiration, photosynthetic electron transport, and cell wall metabolism [49]. However, due to its redox characteristics, it is harmful to plants in large quantities (Figure 2). Excessive amounts impede plant growth, interfere with photosynthetic and respiratory activities, reduce nutrient uptake, target the membrane transport system, and produce excessive amounts of ROS [50]. Copper concentrations in the soil gradually lowered pigeonpea secondary metabolite biosynthesis (phenolics and flavonoids). Under Cu stress, pigeonpea had severe oxidative damage, as evidenced by higher levels of MDA (Malondialdehyde contents), hydrogen peroxide, and electrolyte leakage. Antioxidant enzymes (Superoxide dismutase, Peroxidase dismutase, Catalase and Glutathione peroxidase) and proline content were considerably increased with increasing Cu concentration to reduce oxidative damage [51].

Figure 2.

Toxic effect of different heavy metals on pigeonpea.

Mercury contamination has emerged as a critical modern environmental issue. Its treatment highly reduced seed germination. Mercury chloride was found to be very harmful to seedling growth of legume crops. Plants grown at various levels of cadmium revealed a considerable drop in the length of shoots and roots, yellowing and ultrastructural abnormalities of the leaves, and a significant decrease in the essential oil content [52]. This metal exists in both organic and inorganic forms, and both are extremely dangerous. Its concentration in soil and water is an issue due to the widespread use of mercury-containing chemicals, fungicides, algaecide, paper pulp industries, and agriculture. Mercury released into the near environment may penetrate pigeonpea and other crops that humans eat, affecting human health. Therefore, it is critical to reduce the use of mercury in industries, as well as mercury-containing insecticides and fungicides [53].

Aluminum is the third most prevalent element in the earth’s crust (after oxygen and silicon). The presence of poisonous Al³⁺ cations in acidic soils (pH 5.0) is a major constraint to agricultural productivity worldwide. The excess of Al is a major soil limitation to food and biomass production [54]. The suppression of root extension is the first sign of Al toxicity, which has been postulated to be produced by a variety of mechanisms, including Al interactions with the plasma membrane or the symplast. Aluminum poisoning has a negative impact on root growth and interferes with water and mineral nutrient intake [54, 55]. Pigeonpea plants cultivated in Al-challenged soil have lower nodulation. However, the use of 24-EBL inhibited the effect of Al on nodulation. Rhizobium multiplication and nodule development were reported to be more sensitive aspects of the symbiotic interaction to excess Al. Al poisoning caused a significant decrease in chlorophyll concentration. The use of 24-EBL on C. cajan plants significantly boosted photosynthetic pigments and counteracted the negative effects of Al⁺³ stress [18, 23, 56]. Plants have evolved various strategies to minimize metal-induced damage, including metal exclusion, compartmentalization, chelation, and a wide range of ROS-scavenging mechanisms, including antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione reductase (GR), ascorbate peroxidase (APX), as well as non-enzymatic antioxidants such as phenol [18, 51].

2.5 Temperature

2.5.1 Cold stress

During the winter season in northern India, pigeonpea suffers from low temperature stress (December–January). If the minimum temperature goes below 5°C, stress impacts plant growth, survival, and reproductive capacity [57]. At freezing temperatures, intracellular water condenses into ice, causing cell contraction within the plant, resulting in wilting and plant death [18, 58]. Initial research at IIPR Kanpur also revealed genotypic differences in cold tolerance in pigeonpea. Because knowledge on cold stress and its impact on the pigeonpea crop is scarce, screening a large number of pigeonpea genotypes for low temperature tolerance under controlled temperature conditions is still required to confirm and generate precise genetic information [59].

2.5.2 Heat stress (HS)

Pigeonpea is a grain legume that is resistant to climate change. Though the ideal temperature for produced is 25–35°C, wild relatives grow at temperatures ranging from 18 to 45°C [60]. Heat Stress (HS) is the most serious abiotic threat to all legume crops. It reduces plant biomass build-up, resulting in lower yield, particularly in tropical and subtropical environments [61]. A prior analysis stated that a 1°C increase in maximum temperature during crop season could result in a 20.8% decrease in pigeonpea output. HS causes critical protein complexes to dissociate and the production of Reactive Oxygen Species (ROS) [62]. Plants tend to up-regulate the genes encoding molecular chaperones and signaling molecules in response to HS, thereby regulating a chain of events that lead to HS responses [63].

3. Mitigate the climatic change for C. cajan production

Food production, security and climate change are all interconnected and hence affecting living systems. Long-term changes cause the entire weather pattern to alter, and also increase temperature, unpredictable rainfall, floods and a rise in sea level. India and other developing nations struggle to produce enough food to feed their expanding populations [23, 63]. Legumes, especially pulses make up the majority of the food on an Indian meal platter. Over a few decades, Pigeonpea in India has transitioned from being an orphan crop to a cash crop. Its production as a main crop is constrained by its lengthy maturation period and low yield [64]. This crop can withstand prolonged periods of drought and are well adapted to rain-fed conditions. They require little soil moisture to maintain themselves and generate a respectable amount of yield. However, this legume crop is sensitive to high temperatures and waterlogging. The effects of shifting climatic conditions on arhar that are rainfed are significant [64, 65]. According to reports, pulses are especially susceptible to heat stress during the bloom stage; just a few days of exposure to high temperatures ((30–35°C) can result in significant yield losses due to flower drop or pod damage. The crop’s ability to grow in a larger range of latitudes and altitudes has been constrained by shifting rainfall patterns, rising yearly temperatures, and irregular climatic trends. However, there is no denying that the crop has the potential to support food security, nutrition, forage, and income production [66, 67].

Indian farmers have long waited for early-maturing pigeonpea cultivars that are compatible with their farming practices and produce higher yields with little inputs. The super-early varieties (ICPL 11255, ICPL 20340, and ICPL 20338) that ICRISAT’s pigeonpea breeding team recently created are luring farmers from numerous states, including Maharashtra, Odisha, Karnataka, Telangana, and Andhra Pradesh [https://www.icrisat.org/]. Given their photo- and thermo-insensitivity and capacity to grow in a larger range of latitudes (30°N) and altitude (1250 msl), such as in Uttarakhand, Rajasthan, Odisha, and Punjab, these cultivars have the potential to flourish in varied agro-ecologies. Creating short-lived variants has an added benefit. They may be cultivated with minimal inputs post-rainy season or off-season, giving farmers in dryland areas of India an extra source of income [68].

Gene mining for abiotic stress tolerance, restructuring plant types for climate-vulnerable regions, changing cropping patterns, effective nutrient and water management, seed banks for alternative legume crops, watershed management, and micro-irrigation facilities are some of the better options to address climate change-related issues [42, 67, 69]. Furthermore, crop improvement strategies could be enhanced to mitigate climate changes by developing climate resilient varieties, reducing crop duration, adopting diversification in practices, improving crop specific practices, reducing greenhouse gas emission and use of biofertilizers. Therefore, more effective agronomic techniques have a huge potential to counteract the negative effects of climate change on arhar production. Adopting suggested management measures helps agriculture not only conserve soil and water, but also increases soil organic carbon levels and lessens the effects of climate change [70].

4. Genetic enhancement in abiotic stress tolerance in pigeonpea

Genomics is concerned with the physical integrity of the genome, with the purpose of identifying, diagnosing, and regulating genetic traits throughout the chromosomes. We are now considering certain genetic advances to better understand abiotic stress tolerance in Pigeonpea. Specific trait markers for blooming, fertility, and resistance to sterility mosaic disease. QTL mapping, association mapping for candidate genes, transcriptome assembly, and genome sequencing technologies can be used to identify yield factors [71].

5. Quantitative trait loci mapping

Abiotic stress resistance heredity is a complicated process, QTL mapping, genetic and linkage mapping of genomic regions relevant to tolerance, is the most preferred way of discovering QTL. QTL analysis allows researchers to investigate the genetic structure of a trait. QTLs can discover genomic regions associated with the expression of the characteristic under investigation [72]. Different types of bi-parental populations are used for QTL mapping and the discovery of marker-trait associations. These populations include recombinant inbred lines (RILs), near isogenic lines (NILs), doubled haploids (DHs), multiparent advanced generation inter-cross (MAGIC), nested association mapping (NAM), and association mapping (AM) on wider panels [73]. Chickpea, pigeonpea, lentil (Lens culinaris), and groundnut have successfully used advanced backcross quantitative trait loci (AB-QTL). The resolution for locating novel genes, alleles, and QTLs is improved when this bi-parental mapping population is combined with GBS and GWAS [39, 74]. PEG/water deprivation stress conditions were used to create a collection of ESTs from entire plant tissues of pigeonpea [75]. From pigeonpea plants treated with 10% polyethylene glycol, two subtracted cDNA libraries were created (PEG-6000). Among the many ESTs found, three stress-responsive genes, CcHyPRP [76], CcCDR [77], and CcCYP [75], demonstrated extraordinary resistance to different abiotic stimuli in transgenic Arabidopsis.

6. Molecular marker resources

With the advent of genomic research, new opportunities for genetic enhancement of complex traits like salinity and drought endurance have emerged. In comparison to traditional breeding, a combination of genomic approaches and molecular marker resources can significantly speed up the identification of individual-specific genes in breeding populations [78]. This explains the evolution of genetic marker technology from gel or hybridization approaches (DArT, SFP’s) to sequence-based SSR and SNP markers. Diversity arrays technology (DArT), a hybridization based highly parallel genotyping protocol, has generated thousands of polymorphic loci in pigeonpea that were used for genetic diversity analysis and linkage mapping [6]. SNPs helps in the identification of haplotypes, and blocking such haplotypes would act as markers for the identification of relevant attributes utilizing allele mining approaches [79]. The 50 K Rice SNP50 array was developed for Illumina Infinium platform and has thousands of genome-wide SNPs with genic regions responsible for different genic regions. Further, this array was successfully used for variety verification and trait introgression. The 50 K Rice SNP50 chip plays an important role in both functional and genomics studies and molecular breeding [80, 81]. Similar analysis has been performed using a 62 K SNP array in pigeonpea germplasm. Incorporation of 746 disease resistance and defense response genes in the array with average 10 SNPs per gene will be useful for pathologists and breeders in identifying genes for abiotic stress resistance in pigeonpea [82, 83].

Utilizing diverse genomics resources and enhanced genotyping platforms, molecular breeding techniques like MAS (marker-assisted selection), MABC (marker-assisted backcross breeding), GS, and multivariate adaptive regression splines (MARS) allow for the effective use of legume crop genetic resources that contain important alleles and genes. For instance, four molecular markers (ICCM0249, TAA170, GA24, and STMS11) have been transmitted by MABC for the creation of chickpea types that can tolerate drought [84, 85, 86, 87]. Such markers have been developed by understanding the genome-wide sequence variations and are effectively utilized for allele mining, characterizing germplasm for genetic improvement and genetic mapping of important agronomics traits.

7. Genome-wide association studies (GWAS)

The identification of candidate gene(s)/QTLs for complex characteristics is significantly assisted by GWAS. GWAS methods have been used to find small and minor genetic changes linked to a variety of biotic and abiotic stresses as well as crop agronomic traits [88, 89, 90]. GWAS analyzes the entire genome for QTLs and requires for genome-wide markers. Through the GWAS method, different QTLs were also discovered for several abiotic stress-tolerant genes. The genetic resources and gene(s)/QTLs for morphological, quality, and biotic and abiotic stressors have recently been enriched in pigeonpea [89, 90]. Through MAS, the yield traits as well as the detected QTLs/gene(s), such as pod borer and Phytophthora stem blight resistance genes, have been successfully introgressed into the cultivated varieties of pigeonpea [91]. To speed up genetic gain, two high-density Affymetrix Axiom genotyping chips have recently been created. 103 lines were studied using a 56 K Cajanus SNP chip to examine genetic diversity. The SNPs lack haplotype information and are distributed at random [40].

A 62 K genic-SNP chip called “CcSNPnks” has recently been created using the resequencing of 45 different genotypes. Additionally, the ‘CcSNPnks’ chip array will be helpful for gene-based association studies and high-resolution mapping of yield-related QTLs. With the use of these high throughput genotyping arrays, many samples may be genotyped quickly, and the analysis of the primary genotyping data is also relatively simple [92]. In pigeonpea from diverse sets of wild and cultivated genetic backgrounds, this led to the discovery of the most effective genomic loci (genes) associated with abiotic and biotic stress related genes [4, 83].

8. Transcriptomics profiling

Transcriptomic tools scan, provide gene-expression and protein expression levels in real time, making them important in plant improvement in this advanced era. The development of next-generation sequencing technology has made it possible and reliable to sequence plant species [93]. Furthermore, transcriptomics technologies help to understand gene and protein levels. According to the findings of several research, not all genes are turned on or off at the same time; hence, the metabolism adopts a complex phenotype that cannot be determined by genotype [38, 94]. As of December 26th, 2014, 25,577 ESTs for pigeonpea were discovered at NCBI (National Centre for Biotechnology Information). CcTAv1 transcriptome assembly contigs were created with 1, 27, 754 TUS (Tentative Unique Sequences) and were then upgraded with Illumina GAIIX by 454 platforms to construct CcTav2 transcriptome assembly contigs with four data groups and 21, 434 transcriptome assembly contigs (TAC’s) [95, 96, 97]. The expression of WRKY genes in two different genotypes was examined in leaf and root tissue in response to drought and salt stress [98].

Furthermore, Comparative transcriptome analysis and biochemical tests showed that Cajanus species’ responses to heat stress varied widely. The most thermotolerant of the examined species was C. scarabaeoides, followed by C. cajanifolius, C. cajan, and C. acutifolius. When under heat stress, a significant number of genes have been studied that undergo alternative splicing in a species-specific pattern. Chlorophyll content, electrolyte leakage assay, histochemical assay, and gene expression profiling analysis all demonstrated that C. scarabaeoides possesses adaptive traits for heat stress tolerance [61]. It would help breeders find promising candidate genes and appropriate features for creating and boosting legume crop productivity under abiotic challenges [24]. In depth analysis of the transcriptomics would be definitely fascinating for better perception of pigeonpea.

9. Conclusion and future prospects

Food production will face severe hurdles in the near future due to a gradual drop in soil water and an increase in temperature. Drought and high temperature tolerant crops, such as pigeonpea, may be a viable option for ensuring food security. Efforts should be made to define the genetic resources of pigeonpea at both the phenotypic and molecular levels in order to uncover genetic variations that can be leveraged to generate improved cultivars. To achieve a consistent rise in pigeonpea productivity, existing breeding efficiency must be improved.

In order to focus on trait associated marker study, new methodologies such as transcriptome assembly, MAGIC, and NAM populations were developed. It is feasible to introduce genes from wild species to commercially farmed types using cutting-edge advanced backcross-QTL techniques. In the future, efforts should be made to concentrate on phenotypic approaches that are affordable, high throughput, and effective. Innovative breeding designs that are supported by relevant genomic technology will be critical in modernizing breeding programmes. Current genetic advances in pigeonpea for resistance to abiotic stress will also considerably benefit hybrid breeding. Furthermore, intense attempts are being made using in vitro techniques to find complicated abiotic stress features, foreign gene introgression facilitated by embryo rescue, and quick fixation of stress tolerant recombinants via doubled haploid breeding. These procedures, together with more efficient screening methods, demand special attention in the coming days to make pigeonpea farming an attractive, profitable, and feasible option for the world’s pulse farmers.

Acknowledgments

NS acknowledges the Department of Science and Technology; Government of India for the DST INSPIRE Faculty Award (DST/INSPIRE/04/2018/003674).

Conflicts of interest

We have no conflicts of interest to disclose.

Author contributions

NS: conceived the study, edit the manuscript. M: contributed to the writing and editing of the manuscript. Both authors contributed to the writing, editing, and approved the manuscript.