Role of Plant Defence System in Crop Protection against Fusarium Pathogens

Abstract

Fusarium pathogens are causal agents of several crop diseases and produce harmful mycotoxins resulting in crop and yield reduction worldwide. Among crop diseases, Fusarium wilt, Fusarium head blight, and Fusarium root blight are mostly reported diseases in numerous vegetables, crops, and fruits and have posed pressure on current food production and safety. In addition, the production of mycotoxins further aggravates plant health and causes serious health risks in humans and animals through food chain contamination. Different management practices have been enlisted in this chapter to reduce or eradicate Fusarium wilt in different crops. Interestingly, various mechanisms developed by plants have also been highlighted to fight against Fusarium pathogens and limit the growth of mycotoxins. One of defence mechanisms is plant antioxidant mechanisms to reduce oxidative stress by increasing enzymatic and non-enzymatic antioxidants to maintain cellular homeostasis under Fusarium infection. The other defence response is through hormonal signalling to combat fungal pathogens. Different phytohormones such as salicylic acid, ethylene, jasmonate, abscisic acid, cytokinin, auxin, and other plant secondary metabolites play a crucial part in the reduction of Fusarium growth and inhibit mycotoxin production through defence-related genes. Further, the use of different pre-harvest and post-harvest strategies has been elucidated to enhance plant resistance and growth by decreasing fungal pathogenicity and virulence.

1. Introduction

World’s population is growing exponentially and will reach 10 billion by 2050, adding pressure on available food production, its demand, and safety [1]. On the contrary, different environmental factors such as climate change, diseases, pests, and droughts are severely affecting current food supply and production system [2]. Different plant diseases cause crop reduction and yield losses which are major threats to food security globally. These diseases caused by various plant pathogens such as bacteria, fungi, and viruses can pose severe plant damage leading to economic losses [3, 4]. Among plant pathogens, Fusarium species are considered important plant pathogens causing wilting, necrosis, plant cell death, and mycotoxin production [5]. These Fusarium pathogens are responsible for many plant diseases such as head blight of root and shoot, root rot, and vascular wilt of numerous plants such as barley, wheat, grasses, etc. [6, 7]. The presence of mycotoxins in stored food and various diseases caused by Fusarium species in agricultural crops have attracted the attention of global researchers to study more about Fusarium species and different strategies to control or inhibit their growth in the agriculture sector [8]. Therefore, it is necessary to take serious action to control the growth of these pathogens to protect plants from disease development and mycotoxin production.

Fusarium pathogens and their mycotoxins cause huge destruction in the agriculture sector and are responsible for different diseases, yield reduction, and economic losses. Fungal species can infect grain causing cereal fusariosis [9]. The fungal pathogens survive on the post-harvest residues as saprotrophs or some fungi as chlamydospores [10]. The members of Fusarium genus cause Fusarium blight of head and root, death of seedlings, and production of harmful toxins. Cereal crops in flowering stage are more vulnerable to pathogenic infection, especially under humid, warm, enriched dew, and rainfall season [11]. Fusarium species have also been reported to cause changes in storage protein and harm to starch granules in the kernels; eventually the mycotoxin production can further reduce grain quality [12]. Fusarium head blight is the most devastating disease and damages many cereal crops, especially wheat and oat. However, different favorable conditions promote the Fusarium growth and development such as high temperature (above 20°C), high humidity, and pathogen inoculum, and cereal cultivation can cause 50% reduction of grain yield [13, 14]. Hence, it is mandatory to take preventive measures in order to sustain agriculture crops and reduce the negative impacts of these Fusarium pathogens.

Plants are equipped with various plant defence mechanisms to combat these phytopathogens, and these mechanisms include plant physical barriers (structural changes), changes in different metabolic pathways to produce plant secondary metabolites (chemical changes), plant antioxidant system, hormonal signalling, and expression of defence-related genes [15, 16, 17]. Therefore, to prevent Fusarium pathogens and their infestations, several approaches have been used such as development of resistant varieties, genetically modified plants with higher resistance, and use of fungicides with their certain limitations. As plant breeding and genetic modifications required more time and sometimes pathogens become resistant in their hosts. In the case of fungicides, environmental contamination can alarmingly affect soil microflora [18, 19]. It is obvious that Fusarium species cause different crop diseases and produce mycotoxins which impose negative impacts on crop production and global food security. Therefore, urgent measures should be carried out to stop the growth of such pathogenic fungal species. Moreover, efficient and safe control measures are needed to be developed for sustainable agriculture and must be implemented to end hunger, increase food security, and promote sustainable agriculture.

1.1 Fusarium species as plant pathogens

Fusarium genus consists of both pathogenic and nonpathogenic fungal species but still taxonomically complex [20, 21]. Fusarium species, especially pathogenic species, are characterized as crop pathogens and mycotoxin producers. Various Fusarium species have been isolated from various media such as soil and plant material and are known as pathogens, endophytes, ascomycetes, and saprobes [22]. The members of this genus are identified as pathogens in crops such as potato, rice, maize, beans, tomato, banana, mangoes, wheat, and sugar cane [23, 24]. Several environmental factors including climate, humidity, temperature, geographical area, and seasons can influence the growth and development of these pathogens [25]. The most well-known pathogens of this genus are F. oxysporum, F. moniliforme, and F. graminearum which have severely affected many cereal crops, vegetables, and fruits [15, 26]. The diseases caused by these species include seed blight, pokkah boeng, Panama disease, head blight, bakane, vascular wilts, and rots of different plant parts such as stem, crown, root, and ear [8, 27]. Therefore, Fusarium-caused diseases can have negative impacts on crop production, yield, and storage after harvesting and consequently pose devastating effects on food security by reducing their quality and quantity.

These pathogenic species may enter into plant roots from soil or other media such as air, ground, and water. In addition, other factors including insects, injuries caused by new roots, and nematodes can lead to disease development or its symptoms such as chlorosis, necrosis, and wilting (Figure 1) [28]. The direct attack of Fusarium species in the field can cause economic damage which is further worsened due to the production of mycotoxins or allergenic compounds during post-harvest storage. These species produce very toxic secondary metabolites including diacetoxyscirpenol, fumonisins, zearalenone, deoxynivalenol, fusaric acid, and nivalenol which have deleterious effects on humans and animals due to contaminated food and feed, respectively [29, 30]. The food contamination due to mycotoxins is a global concern in several countries because their presence has affected around 25% of the world’s crops [31]. Further, the contamination of food products is also linked with different health risks, i.e. carcinogenesis, neurogenesis, and mutagenesis [32]. Hence, based on the toxic effects on Fusarium species and mycotoxin contamination in food and feed, more accurate and proper precautions should be taken while crop cultivation and food storage to limit their growth and reduce the occurrence of mycotoxins.

Figure 1.

Various effects caused by Fusarium infection and different plant defence responses under Fusarium attack. The use of different strategies to control Fusarium pathogens at pre- and post-harvest stages can reduce pathogenic infection and mycotoxin production.

2. Pathogen perception and plant defence system

Plants have evolved specific and effective defence systems to recognize Fusarium pathogens. Plants perceive several signalling molecules both from Fusarium species and from their own cell walls upon Fusarium contact [33]. However, different plants exhibit different mechanisms of defence against these Fusarium attacks. The first plant barrier is the plant cell wall which Fusarium pathogens encounter, and this barrier determines the strength of the host plant. For instance, some banana plants showed resistance against F. oxysporum Foc TR4 even before fungal colonization [34]. The resistance in these banana varieties is due to the presence of different genes such as polyphenol oxidase, cellulose synthase, and glutathione S-transferase which provide strength to the cell wall during pathogen attack in resistant banana plants [34]. Plants possess different pattern recognition receptors (PRRs) to detect the presence of microbial-associated molecular patterns (MAMPs) or pathogen-associated molecular patterns (PAMPs) and send downstream signals to initiate signalling pathways leading to plant immunity [35]. Hence, the plant defence system is triggered by PRRs followed by the transcription of pathogenesis-related (PR) genes. The production of reactive oxygen species (ROS) and other metabolites are also activated [27]. In addition, the production of endogenous and exogenous elicitors from the cell wall and cuticle provides the basal defence against Fusarium attacks. On the other side, Fusarium species also destroy pectin and secrete different fragments consisting of galacturonic acid which serves as signalling molecules and strengthen plant defence systems [36]. In addition, the main components of signalling pathways are chitin elicitor-binding protein and receptor kinase and their coding genes that recognize chitin oligosaccharides. Further, several defence-related genes such as PR1, PR5, thionins (Thi2.1), and plant defensin (pdf1.2) are induced in plants upon Fusarium infection [37]. So, plants induce various cellular, metabolic, and growth changes upon recognition of Fusarium pathogens and initiate their defence responses through defence-related genes and ROS-induced hypersensitive responses.

3. Host perception and Fusarium virulence

Fusarium species such as F. oxysporum require host plants for infection and virulence. These pathogens recognize a particular host plant and penetrate into its vascular tissues [38]. Interestingly, pathogens avoid the plant defence system during attack or infection. The presence of hydroxyl acids such as dihydroy-C16 and trihydroxy-C18 provides the safe intact of the pathogen to the host due to their unique characteristic [39]. During the infection process, Fusarium species perceive various physio-chemical signals from the host and show responses through various morphogenetic and metabolic changes to enhance Fusarium development [40]. Moreover, hydrolases of the host also act as signals to synthesize pectinases, cutinases, and other hydrolytic enzymes essential for the penetration in the host plant [41]. During the fungal infection, various signalling cascades are triggered in fungal pathogens which are mainly dependent on the perception of signalling protein receptors from the host. For instance, Msb2 and Sho1 proteins are involved in the infection process of F. oxysporum [42]. These proteins contribute to Fusarium virulence by up-regulating different Fmk1-modulated genes encoding various enzymes involved in cell wall synthesis, taking part in cell all responses, and assisting in Fmk1-based virulence. Both these proteins are also responsible for the up-regulation of cell all biogenesis genes such as gas1, chsV, and fks1 [39]. Thus, Fusarium species also deploy various defence-related actions for virulence and disease development by competing against the host defence system.

4. Production of Fusarium mycotoxins during plant-pathogen interaction

Fusarium species produce different toxic chemicals during plant-pathogen interaction that are harmful to humans, animals, and plants. Controlling Fusarium pathogens is hard because of their genetic variability and wide range of hosts [43]. Fusarium species are found as biotrophic, hemibiotrophic, and necrotrophic. Upon host contact, Fusarium acts as biotrophic to colonize host plant and then shifts to necrotrophic to produce mycotoxins and other cellulolytic enzymes to take control of the host plant [29]. Different cereal crops such as oats, wheat, maize, and barley are affected due to the production of mycotoxins by pathogenic Fusarium species. Further, these pathogens and their mycotoxins account for 50% yield loss in tropical fruit crops including pea, tomato, banana, lentils, and pineapple [44]. The presence of Fusarium toxins such as trichothecenes, fumonisins, zearalenone, fusarins, fusaric acid, moniliformin, enniatins, and beauvericins is also considered to cause infection symptoms in plants [29]. Trichothecenes are considered best toxins due to their toxicity to living organisms and are biosynthesized after 7 to 10 enzymatic modifications. The TRI gene cluster is responsible for the biosynthesis [45]. These mycotoxins interfere with protein synthesis by blocking elongation of polypeptide chains, intervening peptidyl transferase, and hindering chain termination [46]. Trichothecenes are further classified on the basis of polysomal breakdown as type I and type II. Type I contains T-2 toxin, fusarenon X, verrucarin A, and nivalenol, while type II trichothecenes are DON, verrucarol, and crotocin [29]. Therefore, it is essential to take serious action against mycotoxin production in several crops and somehow control mycotoxin-producing genes in the pathogens.

Fumonisins are polyketide-derived mycotoxins produced by various pathogens such as F. verticillioides, F. sacchari, F. proliferatum, and F. subglutinans. The FUM gene cluster is involved in the biosynthesis of these mycotoxins [47, 48]. These mycotoxins are also phytotoxic; however, fumonisin B1 has been reported widely to damage crops and other cereals. FB1 contamination results in the degradation of endosperm, disruption of plasma membrane, accumulation of sphingolipid intermediates by inhibiting ceramide synthase enzyme [15]. Maize seeds soaked with fumonisins exhibited around 75% reduction of radical elongation as compared to controls [49]. Zearalenone is involved in high electrolytic leakage, blocking of H+ release resulting in acidification and causing root length in maize and red beet. In addition, this toxin can change the permeability of both tonoplast and plasmalemma [50]. Fusarins are also polyketide-derived compounds produced by F. graminearum, F. culmorum, F. avenaceum, F. fujikuroi, F. poae, and F. oxysporum [51]. Among different types of fusarins A, B, C, and D, fusarin C has been identified as mutagen due to the presence of the C13-14 epoxide ring, while A and D are not mutagens due to lack of this ring [52].

Fusaric acid can also cause phytotoxic effects and result in wilt symptoms such as tomato wilt caused by F. oxysporum. This toxin is not involved in the infection phase but promotes pathogenesis in the second phase (necrotrophic stage) [53]. Fusaric acid can significantly enhance ROS production, reduce the activities of antioxidants, and resultantly lead to plant cell death [54, 55]. Exogenous application of this toxin results in the disruption of cell structure, reduction in photosynthetic rate, and decreased cellular metabolism [56]. Moniliformin is another Fusarium toxin produced by F. proliferatum, F. avenaceum, F. oxysporum, and F. subglutinans. It has been reported to reduce photosynthetic pigments, leaf development, and plant biomass [57]. Similarly, other Fusarium toxins are beauvericins and enniatins which are commonly found in food and feeds [58]. Beauvericins can cause apoptosis and DNA fragmentation due to the disruption of the mitochondrial pathway [59]. There are several environmental factors which affect the synthesis of these Fusarium toxins. These factors include temperature and moisture content, pH, nitrogen source, and plant extracts which determine the growth, infection, development, and mycotoxin production [29]. The production of these mycotoxins is a next step for Fusarium pathogens after their infection or disease symptoms. Most of these mycotoxins are produced during infection or post-harvest storage to contaminate food. For that reason, it is crucial to make progress in this regard to reduce their production and associated toxic effects.

5. How plants deal with Fusarium mycotoxins

Plants employ mechanisms such as chemical modification and compartmentation of Fusarium mycotoxins. UDP glycosyltransferase (UGT) enzyme is involved in the detoxification of deoxynivalenol mycotoxin in Arabidopsis [60]. Similarly, around 6 transcripts showed an effective role for trichothecene detoxification in barley [61]. Many UGTs not only perform function in the detoxification process but also act on secondary metabolites including terpenes, hormones (salicylic acid, cytokinin, auxin), and flavonoids which play regulatory roles in plant defence responses against fungal diseases [62]. Besides, plants also deploy transportation of Fusarium mycotoxins using multidrug transporters and their roles were confirmed by transcriptomic studies. These transporters include ATP-binding cassette (ABC), resistance nodulation-cell division family (RND), multidrug resistance family (MDR), major facilitator, and multidrug and toxic compound extrusion family (MATE) which are responsible for binding with the toxic compounds and transport them outside the cell [29]. The ABC transporters are present in tonoplast, chloroplast, mitochondria, plasma membrane, and peroxisomes performing various functions including transportation of mycotoxins which is carried out by CytP40s oxidation by conjugating glucoses or glucuronide (hydrophilic compounds). Thereafter, these less toxic conjugated mycotoxins are brought to the central vacuole and transported outside by the ABC system [63]. In the case of the MATE family, toxic chemicals such as heavy metals or lethal organic compounds are excreted along with other secondary metabolites, hormones, and acids [64]. Further, the microbial transformation or detoxification occurs in the soil by releasing different enzymes, and therefore, such mycotoxins are not accumulated in the soil. Similarly, zearalenone is transformed into its reduced phase such as α-zearalenol and β-zearalenol [65]. However, more research work is required in the identification and screening of different enzymes involved in the detoxification of mycotoxins.

In addition, genetic modification either by plant breeding or transgenesis can significantly prevent pathogens and their mycotoxins. For instance, rice plants expressing the TRII01 gene exhibited acetylase activity of deoxynivalenol in rice plants [66]. Likewise, transgenic wheat plants expressing UDP HvUGT13248 of barley resulted in the suppression of diseases in wheat spikes caused by F. graminearum and also showed conjugation of deoxynivalenol toxin to deoxynivalenol-3-glucoside [67]. Intriguingly, another research showed the relation between Triticum aestivum Fusarium resistance orphan gene (TaFROG) and deoxynivalenol mycotoxins and displayed that TaFROG enhanced wheat tolerance to the mycotoxins [68]. However, the plant-pathogen interaction and possible plant defence can determine the quality of efficacy of both plant breeding and transgenesis approaches to overcome these Fusarium pathogens and their harmful toxins. Consequently, the use of chemical modification, compartmentation, transporters’ function, and use of transgenic plants with detoxifying genes can significantly lower the chances of Fusarium infection and toxin production.

6. Fusarium wilt and its management

Fusarium species have been reported to cause different plant diseases including chlorosis, premature leaf falling, wilting, browning of vascular system, and necrosis leading to huge reduction in crop yield [69, 70]. Due to a broad range of host specificity, Fusarium species are classified into numerous races and formae specials. Among them, wilt pathogens are F. oxysporum, F. verticillioides, F. graminearum, and F. solani [71]. Based on economic damages, F. oxysporum is the most devastating pathogen which has been documented in more than 150 host plants such as cabbage, tomato, cotton, banana, watermelon, flax, onion, pea, tulip, etc. [72]. F. oxysporum causes virulence in plants due to the releases of small cysteine-rich proteins [73]. It has been reported that F. oxysporum commences infection through roots and then penetrates into xylem vessels leading to wilting upon colonization and restricting water movement in vascular bundles [74]. Further, pathogenesis and invasion into host plants is carried out by pathogen-produced toxic metabolites [75]. Similarly, F. graminearum causes crown rot and Fusarium head blight of cereal crops causing yield losses annually [76]. Fusarium head blight is a well-known fungal disease worldwide leading to economic losses of important crops such as maize, barley, and wheat [77]. This pathogen causes compromised quality of seed due to infestation and food chain contamination due to the presence of mycotoxins affecting human and animal health [78]. As the Fusarium wilt is the major cause of crop reduction reported by various studies, therefore, its management is of significance to lessen the effects of this disease in agriculture.

Fusarium wilt is a major problem globally, and several control strategies have been practiced for these fungal pathogens. The success rate of controlling such pathogens is limited due to the discoveries of new pathogens or their races [79]. Various methods used to control Fusarium wilt include resistant development, biological, cultural, and chemical (fungicides or natural products) [80]. The effectiveness of different fungicides such as thiophanate-methyl, acibenzolar-S-methyl, and prothioconazole have been evaluated and found efficient in reducing wilt symptoms in watermelons grown in the field [81]. Other possible options to reduce fungal growth are soil fumigation using different chemicals such as propylene oxide, methyl isothiocyanate, chloropicrin, and sodium azide. Further, cover cropping, use of agrochemicals, and crop rotation are also employed to avoid fungal infection [76]. Furthermore, different plant growth-promoting bacteria (PGPB) are also used as a biocontrol against these pathogens. For instance, species of Pseudomonas and actinobacteria have shown effective control of Fusarium wilt [82, 83]. Moreover, the use of mixing bacterial cultures (Bacillus subtilis OH 131.1 and Cryptococcus flavescens) exhibited efficient control of Fusarium head blight in wheat [84]. Similarly, another research using the combination of three bacterial strains such as B. amyloliquefaciens, Paenibacillus polymyxa, and B. subtilis reported the highest efficacy rate against F. graminearum in wheat [85]. These results show that combinations of different biological control agents can be used for effective control against Fusarium pathogens.

7. Plant antioxidant system to combat fungal pathogens

Fusarium pathogens induce ROS production in plants which affect various plant processes including growth, development, and other metabolic processes leading to reduced crop production and yield [86]. In order to cope with ROS accumulation under Fusarium attacks, plants have developed vigorous antioxidant systems to detoxify ROS accumulation and promote plant growth [87]. This antioxidant system is composed of enzymatic and non-enzymatic antioxidants to protect plants from harmful effects of ROS overproduction such as superoxide (O₂^.−), hydrogen peroxide (H₂O₂), hydroxyl radical (^.OH), and singlet oxygen (¹O₂) [15]. These antioxidants include superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), polyphenol oxidase (PPO), ascorbate peroxidase (APX), ascorbate (ASA), and glutathione (GSH) and perform scavenging function of ROS production during pathogenic attacks [17, 26].

Fusarium infection was reported in different parts of banana which significantly enhanced production of SOD, POD, APX, PPO, and lipoxygenase under oxidative stress and prevented plant cell death [88]. On the other hand, ROS production is considered essential for hypersensitive response to perform its function properly [88]. Further, tomato plants treated with SA showed higher reduction in plant root and shoot dry weight and exhibited higher accumulation of H₂O₂ and lipid peroxidation under F. oxysporum attack. The antioxidant activities of SOD and glutathione peroxidase (GPX), and ASA were increased in infected plants when treated with SA, MeJ, and Trichoderma harzianum which showed resistance against Fusarium wilt [89]. Furthermore, F. oxysporum induced Fusarium wilt in melons and caused drastic yield loss. The effect of this pathogen was studied in two resistant (Shante-F1 and Khatooni) and two susceptible (Shante-T and Shahabadi) melon cultivars and found that Fusarium infection significantly induced production of SOD, CAT, POX, PAL, PPO, chitinase (CHI), β-1,3-glucanase (GLU), and total phenolics in the roots of resistant varieties [17]. However, no significant difference was observed in fresh weight, dry weight, diameters, height, and biomass among resistant and susceptible cultivars [17]. Similarly, tomato plants also exhibited higher activities of SOD and APX antioxidants under F. oxysporum infection, while glutathione reductase (GR) and dehydroascorbate reductase (DHAR) enzymatic activities were also enhanced when plants were inoculated with arbuscular mycorrhizal fungi such as consortium of Claroideoglomus etunicatum, Funneliformis mosseae, and Rhizophagus intraradices [90]. Hence, activation of antioxidant enzymes under fungal attack clearly suggests the regulator role of these antioxidants against Fusarium pathogens.

Guaiacol is a natural antioxidant and antifungal agent and has been studied against F. graminearum in wheat which inhibited mycelia growth, germination and formation of conidia, and production of mycotoxin deoxynivalenol [91]. Guaiacol (1.838 mM) reduced levels of malondialdehyde (MDA; indicator of lipid peroxidation) and activities of SOD, POD, and CAT ith increasing concentration of guaiacol from 1.6 to 6.4 mM [92]. In addition, Fusarium wilt caused in cucumbers due to F. oxysporum was reduced with silicon treatment and enhanced plant resistance by improving POD, CAT, and APX activities and reducing MDA levels [92]. Moreover, other parameters such as transpiration rate, stomatal conductance, net photosynthetic rate, RuBisco activity were also elevated under silicon treatment on infected plants indicating the important role of silicon against Fusarium wilt [92]. The effect of nitrogen treatments was studied in nine varieties of wheat against F. culmorum, and the activities of different antioxidant enzymes such as APX, CAT, and GR were measured. The low nitrogen levels enhanced antioxidant activities of some enzyme under Fusarium infection while high nitrogen levels significantly reduced antioxidant activities [93]. Additionally, the plant antioxidant system was also activated when three different cyanobacteria such as Anabaena oryzae, Desmonostoc muscorum, and Arthrospira platensis were inoculated with pepper plants (Capsicum annuum L.) infected with F. oxysporum [94]. The results showed that addition of cyanobacteria significantly reduced Fusarium infection, enhanced POD and PPO expressions, and also increased SA, indole-3-Acetic levels, while decreased ABA content in infected plants showing that use of foliar treatment of these bacteria can improve plant growth, metabolic characteristics, and defence responses against Fusarium wilt [94]. Likewise, the activities of SOD, POD, and CAT antioxidants were documented higher in Medicago truncatula plants when treated with Sinorhizobium meliloti and Sinorhizobium medicae, and these bacterial treatments also increased sucrose content and reduced oxidative stress due to fungal pathogen F. oxysporum [95]. Thus, it is confirmed from various reported studies that the plant antioxidant system is activated under Fusarium attack and plays its vital part by detoxifying excess ROS production and improving plant growth and development. Further, the use of different exogenous antioxidants or symbiotic bacteria and fungi can improve plant defence responses with higher activities or levels of antioxidants.

8. Role of secondary metabolites in plant resistance

Various plant secondary metabolites are produced upon pathogenic attack or mycotoxin production. These secondary metabolites are involved not only in killing Fusarium pathogens but also in inhibiting mycotoxin production [86]. For instance, terpenoids and phenylpropanoids have been reported to provide a basic defence line for pathogenic fungi [96]. Other studies also documented the role of ferulic acid, chlorogenic acid along with small amounts of carotenoids, α-tocopherols, and putrescine in Fusarium-infected kernels of maize plants [86, 97]. Further, the production of mycotoxins such as HT-2 and T-2 by F. sporotrichioides and F. langsethiae was significantly decreased upon phenolic acid treatments which also act as plant antioxidant to reduce oxidative stress on pathogenic invasion [98]. Furthermore, plants synthesize ferulic acid upon pathogen recognition which also inhibits the transcription of various trichothecene genes such as TRI5, TRI6, and TRI12. The fumonisin production was also blocked by α-tocopherols; however, carotenoids did not show an effect on the production of mycotoxins [29]. Interestingly, trichothecene-producing F. graminearum strain infected barley and induced production of different secondary metabolites such as cinnamic acid, naringin, sinapyl alcohol, geranyl chalconaringenin, heptadecatrienoic acid, dihydroxy linoleic acid, and dihydroquercetin which ultimately enhanced plant resistance and also produced JA phytohormone [99]. Similarly, graminaceous plants also produce benzoxazinoids in the shikimate pathway and other 20 metabolites which have exhibited resistance against fungal pathogens [100]. It is obvious from different scientific reports that plant secondary metabolites can also enhance plant defence systems by reducing fungal growth and inhibiting toxin-producing genes in the pathogens.

9. Phytohormonal Defence Signalling against Fusarium Pathogens

Plant hormones play a crucial role in defence mechanisms to protect plants from Fusarium pathogens and their mycotoxins. An interconnected network of complex signalling is stimulated downstream of effector-triggered immunity (ETI) and pathogen-triggered immunity (PTI) against pathogens [101]. Further, PTI initiates different cellular responses including ROS production, changes in the ionic flux of the cytosol, induction of mitogen-activated protein kinase, hormonal defence, and other physical barriers such as cell wall thickness [102]. The main plant hormones involved in plant defence signalling against pathogens are abscisic acid (ABA), ethylene (ET), jasmonate (JA), and salicylic acid (SA) playing their defensive roles in several ways depending upon plant type, hormone concentration, and infection stage [103, 104]. Based on plant growth stage, these phytohormones can function in synergistic or antagonistic ways [105]. For instance, SA is responsible for defence against biotrophic pathogens by inducing local and systemic resistance, while ET and JA have been found potent against necrotrophic pathogens. Hence, all these plant hormones are involved in plant defence responses by triggering induced systemic resistance (ISR). Further, other plant hormones such as auxin, cytokinins, brassinosteroids, and melatonin are also implicated in plant growth, development, and defence responses [106]. Therefore, the hormonal crosstalk protects plants from pathogens by employing different defence strategies including signalling networks [107].

SA production is linked with PR expressions in dicot plants such as Arabidopsis and tobacco, while the involvement of SA in monocots has not been fully explained under pathogenic attacks [108]. F. graminearum-infected wheat spikelets exhibited the role of SA with higher expressions of PR1 and PR4, while Pdf1.2, PR1, and PR4 were highly expressed in the case of Arabidopsis infected with F. graminearum elucidating the specific roles of both SA and JA [109]. Methyl salicylate (an SA form) has been found effective in signal transduction of systemic acquired response (SAR) against different pathogens such as bacteria, fungi, and viruses [110]. Further SA was also reported in wheat against F. graminearum, a causal agent of Fusarium head blight [108]. SA foliar application also showed resistance against Fusarium wilt and increased peroxidases and phenylalanine ammonia-lyase activities in beans [111]. Conversely, fungal pathogens avoid SA formation in order to escape from SA-mediated defence response. For instance, a causal agent of corn smut Ustilago maydis releases chorismate mutase (Cmu1) to interfere with SA biosynthesis. The deletion mutants of Cmu1 showed higher SA accumulation exhibiting the role of Cmu1 as an antagonist of SA formation [112]. Similarly, some fungal pathogens are also capable of producing salicylate hydroxylases to degrade SA for rapid fungal growth and infection [113]. The role of SA treatment was also documented in tomato plants against Fusarium wilt caused by F. oxysporum and showed crosstalk with nitric oxide to induce plant immunity by reducing disease incidence [114]. Likewise, F. oxysporum also significantly enhanced SA and terpene production in the roots and leaves of Chrysanthemum morifolium as compared to control plants depicting the involvement of SA in plant defence responses [115]. Hence, these findings suggest that SA either exogenously or endogenously plays an essential role in the plant defence system against Fusarium pathogens, and SA could be used in agriculture to prevent yield losses and enhance crop production limiting fungal pathogens.

JA also plays an important part in signal transduction to induce plant defence responses under biotic stress conditions and is also involved in SAR during plant-pathogen interactions. For instance, JA treatment in rice significantly increased plant resistance against necrotrophic pathogens by activating the phenylpropanoid pathway [116]. The important roles of JA, SA, and ET were studied in tomato plants using gene markers of the hormones against F. oxysporum infection and the results showed that pathogenic infection enhanced gene expressions of both SA and ET indicating their activated defence pathway [117]. NahG mutants exhibited higher susceptibility to Fusarium infection as in these mutants SA is degraded elucidating the central role of SA against pathogens. However, in the case of ET, Never ripe (no ET perception) and ACD (no ET biosynthesis) mutants reduced disease symptoms and fungal growth [117]. These findings show the positive regulation of ET and negative modulation of SA. On the contrary, def1 (JA is compromised) and prosystemin overexpressed (JA is activated) mutants showed no changes under Fusarium attack [117]. Another study also reported the significance of JA and ABA in wheat plants against F. graminearum, and the results displayed that JA significantly reduced fungal growth and Fusarium head blight while ABA facilitated wheat head blight [118]. Similarly, the involvement of JA and ABA was also detected in two rice cultivars (Selenio (resistant) and Dorella (susceptible)) infected with F. fujikuroi (caused bakanae disease) and the outcome showed higher levels of phytoalexins in Selenio cultivar and bakanae disease symptoms were observed while susceptible cultivar (Dorella) exhibited less phytoalexin production and increased ABA and gibberellin while inhibiting JA production [119]. These studies indicate how crosstalk among plant hormones is involved in the induction of plant defence responses against fungal pathogens.

ET is a light gas molecule and is involved in the regulation of PTI to prevent fungal pathogens [120]. The ETR receptor (ETR1) is required by F. oxysporum for disease development. For instance, Arabidopsis etr1 mutants showed less symptoms of Fusarium wilt than control plants and it also increased expressions of SA-encoding genes [121]. ET signalling is responsible for plant defence responses against fungal pathogens, for instance, ET signalling impaired Arabidopsis mutants showed higher resistance for F. graminearum; however, overexpressed mutants with ET signalling elicited Fusarium susceptibility confirming the integral role of ET in Fusarium-plant interactions [122, 123]. Similarly, the etr1-1 mutant enhanced plant resistance against F. oxysporum in Arabidopsis, suggesting that ETR1 is also needed for disease development [121]. The use of F. oxysporum isolate was reported for pathogenic virulence and showed that the expressions of different defence-related genes are dependent on fungal virulence [124]. Further, [125] demonstrated that tobacco plants infected with F. solani increased SA accumulation during initial infection and then enhanced JA production elucidating the involvement of both SA and JA for Fusarium resistance. Another study highlighted the role of SA and JA signalling pathways in Arabidopsisinfected with F. sporotrichioides and showed higher expression of SA-responsive gene PR1 at early stage 24 to 48 h while JA-responsive gene PDF1.2 was highly expressed [126]. The role of NPR1 was assessed in Arabidopsis against F. asiaticum and was found responsible for disease susceptibility while showing resistance in floret assays explaining the dual role of NPR in the SA signalling pathway against Fusarium infection [127]. Moreover, the effects of different hormones were also studied in floral and root tissues of Brachypodium distachyon against F. graminearum infection, a causal agent of Fusarium root and head blight and found that SA remarkably enhanced resistance to Fusarium head blight while showed susceptibility to Fusarium root blight [128]. Conversely, JA and ET increased resistance against Fusarium root blight while exhibiting susceptibility to Fusarium head blight. Interestingly, cytokinin and Auxin showed susceptibility and resistance, respectively, against Fusarium root and head blight [128]. Hence, it is evident that different plant hormones play their specific roles during plant-pathogen interactions to enhance plant defence systems and prevent pathogen attacks.

10. Different strategies to control Fusarium pathogens

Plants employ different strategies to control Fusarium pathogens to promote plant growth and development. These plant strategies are pre-and post-harvest approaches including various physical, chemical, and biological methods. The use of natural products or plant extracts has also been described to limit fungal pathogens.

10.1 Biocontrol using bacterial species

Different species of Streptomyces, Pseudomonas, and Bacillus have been reported as biocontrol agents [129]. Nevertheless, Bacillus species have shown more advantages over other species due to the presence of certain characteristics such as endosperm formation, ubiquity, different pH tolerance, osmotic conditions, temperature, and lack of disease development [71, 130]. Bacillus species develop biofilms with plant roots and enhance plant growth by more nutrient uptake and also induce plant resistance through disintegration of fungal mycelia [131]. The production of siderophores by bacterial species can also play an important role in plant growth such as providing iron to plants and limiting it to fungal pathogens. For instance, B. pumili showed an efficient biocontrol activity against tomato Fusarium wilt through siderophore production [132]. In addition, biocontrol bacteria can secrete certain extracellular enzymes for the breakdown of the cell wall of fungal pathogens [133]. Further, B. subtilis SG6 showed antagonistic interaction with F. graminearum and hindered sporulation of fungal pathogens by lysis of fungal cell wall [134]. Similarly, another research showed an effective control of B. thuringiensis NM101-19 in soybeans against Fusarium infection [135]. Likewise, Fusarium wilt disease was suppressed in tomato plants due to the production of phenyl acetic acid by B. fortis IAGS162, and this chemical also assisted in the rhizosphere colonization of this bacterium [136]. Hence, it is clear from the reported literature that the use of different bacteria can promote plant growth by reducing pathogenic attacks through various mechanisms.

10.2 Biocontrol using fungal species

Non-pathogenic Fusarium species have been reported in the suppression of Fusarium wilt or other diseases in various crops, fruits, and cereals and vegetables by employing different mechanisms [137]. For instance, nonpathogenic F. oxysporum showed resistance against F. oxysporum f. sp. Cubens in bananas acting as plant growth promotion (PGP) [138]. Similarly, Fusarium species exhibited resistance in response to F. culmorum in flax plant through induced systemic response (ISR) [139]. Likewise, F. oxysporum competed for nutrition source and used ISR mechanism to enhance resistance in date palm against F. oxysporum f. sp. Albedinis [140]. Moreover, F. oxysporum isolate (F.o-T5) was found effective in tomato plants against F. solani infection using antagonistic strategy to enhance resistance in plants and promote plant growth [141]. The use of other fungal species such as yeast species have also been assessed and found efficient in controlling Fusarium pathogens. For example, Cryptococcus carnescens (E22) displayed high resistance against Fusarium pathogens such as F. culmorum, F. poae, and F. graminearum by antagonistic mechanisms and inhibited their growth by competing for space and nutrients, siderophore production, and secretion of chitinase and β-1,3-glucanase enzymes for the breakdown of fungal cell wall [142]. Thus, non-pathogenic fungal species can also be used as an effective strategy to reduce Fusarium pathogens by using different mechanisms such as ISR, PGP, and antagonistic approach.

10.3 Use of natural products from medicinal plants

Plant products exhibit antifungal properties and are eco-friendly in nature as these compounds do not persist in the environment for a longer period than other synthetic fungicides [143]. These plant extracts are widely being used in the agriculture sector to prevent fungal pathogens and control plant diseases. The antimicrobial property is based on the presence of certain secondary metabolites such as flavonoids, phenols, glycosides, alkaloids, polyphenols, tannins, and other compounds [144]. Numerous studies have elucidated the roles of plant products to prevent fungal infection of various Fusarium species such as F. proliferatum, F. verticillioides, F. solani, and F. oxysporum in vegetables, fruits, and cereal crops [145]. The leaves of Combretum molle R. Br have been used to control various Fusarium pathogens such as F. oxysporum, F. verticillioides, F. solani, F. graminearum, F. proliferatum, F. equiseti, F. semitectum, F. chlamydosporum, and F. subglutinans [146, 147, 148]. Similarly, antifungal compounds isolated from Artemisia annua L. have extensively been studied and used against F. solani and F. oxysporum [149]. Moreover, various antifungal compounds such as artemetin, dehydrodiconiferyl alcohol, denudatin A, B, Futokadsurin B, C, gallic acid, maslinic acid, penduletin, withaferin A, etc. extracted from different plant parts have shown effective control against many Fusarium pathogens [145]. Subsequently, using natural products or extracts can have a huge impact on fungal reduction by promoting plant growth and resistance.

10.4 Pre- and post-harvest strategies to control Fusarium pathogens

The purpose of these preventive measures is to reduce Fusarium pathogens and their production of mycotoxins. These strategies include agronomic methods such as crop rotation, tillage and fertilization, use of quality seeds, appropriate sowing dates, and weather conditions, resistant crop varieties, and biological control of plant protection [14]. In addition, the use of different chemicals such as fungicides including metaconazole, triazole, and tebuconazole can significantly decrease the chances of pathogen occurrence and their secondary metabolites, i.e. mycotoxins [150]. These pre-harvest methods have shown the impact on the reduction of pathogenic fungi and also limited inhibited mycotoxin production. Similarly, the protection of crops after harvesting is also very important, and certain actions should be taken to prevent fungal diseases and grain contamination by mycotoxins. These methods include physical, biological, and chemical (Figure 1). For instance, physical methods are drying, adequate aeration, sorting, grinding, washing, cleaning grain surface, suitable humidity, adequate temperature for seed storage, use of UV light illumination and adsorbents, and ozonation that have shown efficient reduction on the growth of Fusarium pathogens [14]. Biological methods consist of using microorganisms such as non-pathogenic bacteria and fungi which function as antagonistic to Fusarium pathogens and reduce their growth while limiting mycotoxin production (Sections 11.1 & 11.2). In addition, these microbes compete for nutrition sources and space, as well as trigger ISR against Fusarium species [151]. Some microbes are also capable of degrading or transforming these harmful toxins into less toxic or harmless compounds [152]. Moreover, the chemical methods consist of conversion of toxic chemicals into less harmful chemicals via different chemical reactions. The chemicals used in this method are different bases, acids, oxidants, and reductants. In addition, other chemicals such as ammonification (ammonium hydroxide), use of hydrogen sulfate, and 1% sodium hypochlorite have exhibited remarkable reduction in fungal diseases to protect various agricultural crops [153]. Therefore, before and after harvesting, the use of these strategies in agriculture can significantly increase crop production and yield by reducing contamination chances of cereal grains, fungal infection, and mycotoxin production.

11. Future challenges

Numerous crops are susceptible to Fusarium pathogens resulting in crop and yield reduction globally. The present disease management approaches become limited when plant pathogens acquire resistance against chemicals such as fungicides. In addition, the availability of resistant cultivar against these Fusarium pathogens is also confined which further pose pressure on food security. The identification, selection, and time required for attributes of host plants, fungal pathogens, and development of resistant plant varieties put pressure on available resources. During this process, more resistant fungal pathogens emerge and destroy crops through infection or diseases. The emergence of pathogens is faster than the discovery of new resistance cultivars or other antifungal agents. Conversely, the use of fungicides also causes environmental pollution and disturbs soil micro fauna and flora affecting soil characteristics. Further small RNAs of plants hinder expressions of fungal genes acting as an innate plant defence using an RNAi-dependent mechanism which has shown potential to reduce fungal disease by decreasing fungal virulence and pathogenicity. However, gene silencing of Fusarium species can be an effective approach to control Fusarium pathogens by avoiding chemical costs and hazards associated with them.

12. Conclusion and future perspectives

It is concluded that Fusarium pathogens can drastically reduce crop yield and cause various crop diseases during pre- and post-harvest stages. Plants and Fusarium pathogens use various defence responses against each other for pathogen resistance and host virulence, respectively. It is evident from the literature that plants are equipped with different defence systems such as plant antioxidant system, hormonal signalling, expression of defence-related genes, and production of secondary metabolites which can significantly enhance plant defence responses against fungal pathogens by reducing their growth and mycotoxin production. Further Fusarium wilt is the main devastating disease and can be mitigated using different management approaches. Furthermore, various physical, chemical, and biological methods should be adopted to limit fungal growth and enhance crop yield. A deep and thorough knowledge about Fusarium-plant interactions can provide further solid basis to develop resistance in plants against Fusarium pathogens. Scientific advancement in technology such as development of new crop cultivars and transgenic plants resilient to Fusarium infections could also reduce crop and economic losses worldwide. Further, a deeper knowledge of Fusarium pathogens and its effect on plants can assist in better understanding ecosystems for sustainable agriculture and complete control of Fusarium pathogens.