Recent Advances in Mechanisms Underlying Defense Responses of Horticultural Crops to Botrytis cinerea

: Horticultural crops are a crucial component of agriculture worldwide and have great economic value. The notorious plant fungal pathogen Botrytis cinerea can cause gray mold disease in over 200 horticultural crops, leading to severe economic losses. Investigating the mechanisms underlying plant defense responses to pathogens is crucial for developing new strategies for effectively controlling plant diseases, and much progress has occurred in the mechanisms underlying defense responses of horticultural crops to B . cinerea mainly due to the completion of genome sequencing and the establishment of efﬁcient tools for functional genomics. In this review, recent progress in mechanisms underlying defense responses and natural products that can enhance the resistance of horticultural crops to B . cinerea are summarized, and future research directions are also discussed.


Introduction
Horticultural crops include fruits, vegetables, and ornamental plants, containing enormous nutritional and ornamental value.Production and consumption of horticultural products are developing rapidly, thereby attracting much attention [1].However, horticultural crops are seriously threatened by various pathogens in the field, transport, storage, and even market [2].B. cinerea, the second most important fungal plant pathogen, can cause gray mold disease in over 200 horticultural crops, such as tomato, strawberry, grape, and rose, resulting in severe economic losses [3,4].Health or environmental risks derived from synthetic fungicides and fungicide resistance are increasingly concerning [5,6].Thus, it is imperative to develop new strategies for effectively controlling gray mold disease.
During pathogen-plant interactions, plants have evolved unique disease resistance mechanisms and strategies to deal with pathogens to reduce the damage [7].With the development of genome sequencing [8], and the establishment of efficient tools for functional genomics [9], there have been numerous important advances in our understanding of the molecular biology of members in plant defense response to pathogens.Pattern-triggered immunity (PTI) is a key component of plant immune responses to B. cinerea [10], and components in plant PTI signaling, transcription factors, and proteins in hormone synthesis and signaling were proven to be important regulators against B. cinerea infection [11].This article reviews the research progress on defense response mechanisms of horticultural crops and natural products that enhance resistance to B. cinerea in recent years, and prospects for future research directions.

Increase in the Production of Reactive Oxygen Species (ROS) and Nitric Oxide (NO)
The so-called reactive oxygen species (ROS) include various forms of reducing and chemically reactive molecules such as superoxide anion, hydrogen peroxide, hydroxyl radical, or hydroperoxyl group.The outbreak of ROS is one of the main immune responses of plants to resist the invasion of pathogens [12].Oxygen acts as a signaling molecule, and the production of reactive oxygen species can serve as a marker for the successful identification of infection and activation of plant defense (Figure 1).The infection of B. cinerea caused changes in the activities of superoxide dismutase (SOD) enzymes, ascorbate peroxidase, flavonoid peroxidase, catalase, and other antioxidant enzymes in apple fruit [13].Furthermore, some other crops also showed varying degrees of change, for instance, the activities of three antioxidant enzymes SOD, peroxidase (POD), and catalase (CAT)) in infected grape berries were improved to different degrees [14], and the content of H 2 O 2 and O 2 − as well as the activity of SOD, POD, and CAT were influenced in infected cucumber [15].These situations indicate that modifying ROS homeostasis by altering the regulatory components of ROS production in plant immunity contributes to engineering or breeding broad-spectrum disease-resistant crops [16].

Increase in the Production of Reactive Oxygen Species (ROS) and Nitric Oxide (NO)
The so-called reactive oxygen species (ROS) include various forms of reducing and chemically reactive molecules such as superoxide anion, hydrogen peroxide, hydroxyl radical, or hydroperoxyl group.The outbreak of ROS is one of the main immune responses of plants to resist the invasion of pathogens [12].Oxygen acts as a signaling molecule, and the production of reactive oxygen species can serve as a marker for the successful identification of infection and activation of plant defense (Figure 1).The infection of B. cinerea caused changes in the activities of superoxide dismutase (SOD) enzymes, ascorbate peroxidase, flavonoid peroxidase, catalase, and other antioxidant enzymes in apple fruit [13].Furthermore, some other crops also showed varying degrees of change, for instance, the activities of three antioxidant enzymes SOD, peroxidase (POD), and catalase (CAT)) in infected grape berries were improved to different degrees [14], and the content of H2O2 and O2 -as well as the activity of SOD, POD, and CAT were influenced in infected cucumber [15].These situations indicate that modifying ROS homeostasis by altering the regulatory components of ROS production in plant immunity contributes to engineering or breeding broad-spectrum disease-resistant crops [16].The activation of MAPK cascade is an important component of PTI signaling.Several major classes of transcription factors (TFs), mainly including WRKYs, ERFs, MYBs, bHLHs, and NACs, were implicated in immune responses to B. cinerea and may be targets of MAPK phosphorylation.These TFs influence the level of plant defense hormones by modulating gene expression.Mediator is an evolutionarily conserved multisubunit complex that regulates the function of RNA merase.Tomato mediator complex SlMED8 and SlMED25 contribute to immune responses to B. cinerea by modulating gene expression.Some other genes in the cytoplasm also affect the metabolites and phytohormones signaling.
Nitric oxide (NO) also plays a key role in different physiological processes of plants (Figure 1).The production of NO usually involves two main pathways: the NR pathway, reducing nitrite to NO; and the oxidation pathway of nitric oxide synthase (NOS) [17].The accumulation of nitric oxide in tomatoes increased resistance to B. cinerea [18,19].Higher NO supply resulted in much metabolism in tomatoes, which led to higher resistance against B. cinerea [20].NO accumulated in tomato chloroplasts [21] and the level of NO influenced the activities of phenylalanine ammonia-lyase (PAL), chitinase (CHI), β-1, 3glucanase (GLU), and polyphenol oxidase (PPO) [19,22,23], while the accumulation of H 2 O 2 decreased [24], which eventually improved the resistance against B. cinerea.In potato leaves, B. cinerea induces massive accumulation of nitric oxide and H 2 O 2 and ultimately induces programmed cell death [25].Generally, NO can induce ROS production and enhance ROS-related enzymatic activities [26,27].

Change in Plant Metabolites
A large number of plant metabolites including primary metabolites and secondary metabolites (Figure 1) are involved in the process of plant disease resistance.Several processes involved in the metabolism of amino acids and carbohydrates were triggered by B. cinerea in grape berries, and a large number of peptides, fatty acids, hydroxy fatty acids, glycerophospholipids, and glycosphingolipids were accumulated [28].Other aromatic compounds, including acetophenones, benzoic acid derivatives, methoxyphenols, and phenolic glycosides, showed increased abundance upon B. cinerea infection [28].The content of vitamin C and phenolic substances, which are two antioxidant metabolites, was increased in the apple fruit epidermis upon B. cinerea infection [13].When the tomato was treated with B. cinerea, glycolysis products were reduced and TCA intermediates as well as short-chain fatty acids caproic acid and its derivatives were increased.Significantly, the content of tryptophan and shikimic acid has changed [29].The glucosinolate biosynthesis pathway is involved in many plant defense responses to pathogens, and a study in roses showed that key metabolites of the glucosinolate biosynthesis pathway, including L-valine, L-isoleucine, and L-leucine, were altered upon B. cinerea infection [30].B. cinerea-infected strawberries were found to show increased content of hexadecanoic acid, octadecanoic acid, sucrose, and β-lyxopyranose [31].Overall, different plant species showed various accumulations of metabolites upon B. cinerea infection [32], and the change in plant metabolites is an important plant response to B. cinerea.

Receptor-like Kinases
The pattern is generally specific to pathogenic microorganisms and pathogen-related molecular patterns (PAMPs) are one of the patterns.The recognition of PAMPs depends on the pattern recognition receptors (PRRs) on the surface of plant cells (Figure 1).Generally, pattern recognition receptors can be divided into receptor-like kinases (RLKs) containing kinase regions and receptor-like proteins (RLPs) which have no kinase region.The receptor for chitin recognition in tomato is LYK [33], and SlLYK4 and SlCERK1 mediate tomato resistance to B. cinerea [34].Meanwhile, TPK1 complexes with SlLYK1 to recognize chitin, which implies that these PRRs may be involved in chitin-induced immunity.Receptorlike kinases SlPSKR1 in tomato conferred resistance to B. cinerea [35].In addition, wallassociated kinase (WAK)/WAK-like (WAKL) is one of the subfamilies of kinase (RLK) receptors, and a total of 68 RcWAK/RcWAKL gene family members were identified in the rose genome, among which RcWAK4 [36], RcWAK8, and RcWAK22 [37] were involved in plant resistance to B. cinerea.

Receptor-like Cytoplasmic Kinases
The receptor-like cytoplasmic kinases (RLCKs)-with no extracellular domain and no transmembrane helix, but only the cytosolic kinase domain-also have an important role in triggering PTI immune responses [38].Typically, RLCKs are phosphorylated by PRRs, which in turn activate their immune functions that induce downstream immune signals [39].Overexpression of receptor-like cytoplasmic kinase SlRIPK confers broad-spectrum disease resistance without a yield penalty in tomato plants.Although few RLCKs have been reported, some results indicate that RLCK has played an important role in defense response to B. cinerea.

Mitogen-Activated Protein Kinase (MAPK) Cascade
MAPK cascade is a conserved signal transduction module that can convert in vitro signals into in vivo signals [40].Mitogen-activated protein kinases (MAPKs) are key signaling regulators of PTI [41].MAPK cascades mainly include three types of protein kinases, including MAPKKK, MAPKK, and MAPK.It is the way to further transmit and expand these signals [40] (Figure 1 and Table 1).In tomato, SlMKK2 and SlMKK4 positively regulate the resistance response, but the specific action pathway is not yet clear [42].Inhibition of SlMPK1/2/3 disrupted defense signaling pathways and enhanced the susceptibility to B. cinerea [43].For instance, SlMAPK3 plays a positive role in the defense response by regulating the accumulation of ROS and the SA/JA pathway [44].In addition, treatment of MAPK inhibitor reduced tomato resistance to B. cinerea and overexpression of LeMAPK1, LeMAPK2, and LeMAPK3 increased tomato resistance to B. cinerea [45], which implies that MAPK is crucial for tomato resistance to B. cinerea.Other crop species also showed similar results, for example, potato StMKK1 improved plant resistance to B. cinerea by regulating PTI responses and salicylic acid-related signaling pathways [46].Strawberry FaMAPK5 and FaMAPK10 regulated plant resistance to B. cinerea mainly by regulating the content of abscisic acid (ABA) and ROS as well as the enzymatic activities of related enzymes [47].Strawberry FaMAPK19 has also participated in plant resistance to B. cinerea, but how it works remains unclear [48].In conclusion, the MAPK cascade also plays an important role in defense response to B. cinerea.

Transcription Factors
Transcription factors (TFs), also known as trans-acting factors, refer to DNA-binding proteins that can specifically interact with cis-acting elements of eukaryotic genes to activate or inhibit gene transcription.TFs play an important regulatory role in disease resistance to B. cinerea in horticultural crops [49] (Figure 1 and Table 1).

WRKY Transcription Factors
WRKY transcription factors are a large family that responds to biological stress.Tomato SlDRW1 has two WRKY domains and belongs to the WRKY family Group I; its expression was induced by B. cinerea and increased plant resistance to B. cinerea [50].Strawberry WRKY11 influences the expression of other TFs such as WRKY1, WRKY70, and MYB1, and then positively regulates plant resistance to B. cinerea [51].Other WRKY transcription factors including WRKY10 in cucumber and grapevine [15], WRKY25 [52] and WRKY50 [53] in strawberry, WRKY31 [54] and WRKY46 in tomato [55], and rose WRKY41 [56] were also reported to improve resistance to B. cinerea, while Lily WRKY39 and WRKY41a [57] have negative roles in plant resistance to B. cinerea.In addition, rose WRKY13 promoted resistance to B. cinerea by enhancing cytokinin content and reducing abscisic acid signaling [58].

MYB Transcription Factors
MYB family transcription factor is one of the largest transcription factors in plants [59].It refers to a class of transcription factors containing the MYB domain.The MYB domain is a peptide segment of about 51-52 amino acids.Similarly, the MYB transcription factor can pass different pathways to regulate disease resistance responses [60].MYB transcription factors are generally involved in plant resistance to B. cinerea by mediating the JA defense pathway, such as rose MYB84 and MYB123, which have opposite functions [61].Tomato SlMYB75 increased the accumulation of JA and activated the JA defense signaling pathway upon B. cinerea infection [62].Tomato SlMYB1 binds to the promoters of lycopene synthesis-related genes (SlLCY1 and SlPSY2) and the pathogen-related gene SlPR5 to regulate lycopene production and plant resistance to B. cinerea [63].VvMYB44 directly activated the transcripts of enzyme-encoding genes involved in phenylpropanoid and sucrose metabolism [64].MYB1, MYB10, MYB44.2, and MYB44.3 in strawberries were also involved in plant resistance to B. cinerea [48].

bHLH Transcription Factors
Basic helix-loop-helix proteins (bHLHs) are also one of the largest transcription factor families and have a basic region and an HLH (helix-loop-helix) region [64,65].SlJIG belongs to the bHLH family and participated in tomato resistance to B. cinerea [66].Rose RcbHLH13, RcbHLH35, RcbHLH41, RcbHLH44, and RcbHLH49 were upregulated after B. cinerea treatment, which indicates that these bHLHs may be engaged in alleviating biotic stress [67].As a subfamily of bHLH TFs, tomato SlMYC2 plays an important role in methyl-jasmonate-induced fruit resistance to B. cinerea [68].

NAC Transcription Factors
NAC transcription factors are a type of transcription factor that is unique to plants, and there are over 100 NAC transcription factors in plants.Tomato SlSRN1 has a typical NAC domain and affects the amount of ROS and resistance to B. cinerea by affecting superoxide dismutase and catalase [69].Moreover, grapevine VvNAC1 is a crucial regulatory component of the plant signaling defense cascade [70].

Proteins in Hormone Synthesis and Signaling
Plant hormones are an important signal transduction pathway in plant disease resistance.Salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) play a major role in this process; at the same time, regulators such as gibberellins (GAs), auxin (AUX), and ABA also participate in this process [58,[71][72][73][74]. Plant hormones can not only interact with each other but can also be involved in multiple resistance reactions [75] (Figure 1).

Jasmonic Acid
JA is generally involved in the resistance response to necrotic pathogens and is a class of endogenous signaling molecules of hydroxyl lipids [76].JA is synthesized in plastids, peroxisomes, and cytoplasm, and enzymes involved in JA synthesis mainly include LOX, AOS, and AOC [77].JA synthesis genes affected crops resistance to B. cinerea [78,79].Tomato SlTD2 is a threonine deaminase that converts threonine to α-ketobutyrate and ammonia as the committed step in isoleucine (Ile) biosynthesis and contributes to JA responses by producing the Ile needed to make the bioactive JA-Ile conjugate [80].SlBBX20 attenuates JA signaling and regulates resistance to B. cinerea in tomato [81].Methyl jasmonic acid and isoleucine jasmonic acid are derivatives of JA and play important roles in plants.COI1 is a receptor for jasmonic acid [82] and normally forms a complex with SCF ubiquitination enzymes, so the changes of these major genes influence the production of JA.JA influences multiple processes including ROS, resistance genes, and disease resistance enzymes.Extensive research has established that ROS is the most notable change in the JA pathway [55,83,84].Extensive research in different plant species and various biological situations has revealed that there might be different expression levels of downstream genes.For example, these PR family genes including PR1, PR2a, PR2b, PR3a, and PR3b were reduced in the tomato slarg2 mutants [85].Overexpression of VvCOI1 in strawberry fruit resulted in upregulation of the expression levels of a series of plant defense-related genes, such as PAL gene, superoxide dismutase (SOD) gene, POD gene, chitinase gene, polyphenol oxidase (PPO) gene, and β-1, 3-glucanase (BG) gene, thus enhancing the resistance to B. cinerea [86].SlARG2 contributed to MeJA-induced defense responses to B. cinerea in tomato fruit by regulating PRs, defensive enzyme activities, and arginine metabolism [85].The mediator complex links transcription factors to the RNA polymerase II transcriptional machinery.Tomato mediator subunit 8 (SlMED8) is an essential component in JA-dependent defense response against B. cinerea [87].JAZ proteins are repressors of the JA signaling pathway and have conserved Jas and ZIM/TIFY domains.SlVQ15 could interact with JAZ1 and positively affect the defense against B. cinerea [54].The Jas domain binds the COI1 receptor and MYC2 transcription factor, and the ZIM domain binds other JAZ proteins and NINJA proteins [88].When plant endogenous JA increased, the COI1-JA-JAZ ternary complex formed and then induced the degradation of JAZ protein [89], and then higher JA content brings resistance to B. cinerea [81].

Salicylic Acid
SA generally plays a role mainly in the infection of biotrophic pathogens.There are generally two pathways for SA synthesis, including the phenylalanine ammonia lyase (PAL) pathway and isochorismate synthase (ICS) pathway [89][90][91].The expressions of SA-biosynthetic genes EDS1 (Enhanced Disease Susceptibility 1), PAD4 (Phytoalexin Deficient 4), and SA marker gene PR1 were increased in tomato upon B. cinerea infection.However, the expression of JA-biosynthetic gene LoxC (lipoxygenase) as well as two JAdependent genes, PI I and PI II (proteinase inhibitors I and II), were decreased upon B. cinerea infection [44].SA induced substantial transcriptional expression [92], including downstream NPR1, PR, and some WRKY genes.NPR1 plays an important role in salicylicacid-induced plant immunity and can activate downstream PR1, PR2, and PR5 [93].The defense pathways regulated by SA and JA are mutually antagonistic.For instance, SA activates the NPR1 gene, which repressed the PI gene activated by JA in tomato [94], and thus, knockdown of tomato SlNPR1 via modulation of ROS homeostasis and JA/ET signaling enhances resistance to B. cinerea [39,85].Rose RcTGA1 interacted with NPR1 to regulate plant resistance to B. cinerea [30].Strawberry FaSnRK1α increased the resistance of fruit to B. cinerea via SA signaling pathway, regulated the expression of six strawberry PR genes, and interacted with FaWRKY33.2[95].

Ethylene
ET is a simple gaseous hormone with biological activity.Plants can regulate their physiological processes through ethylene biosynthesis and signal transduction pathways.The two enzymes of ethylene synthesis are ACS and ACO; ACS catalyzes the formation of ACC from AdoMet, and ACO catalyzes the conversion of ACC to ethylene [96].Ethylene receptors including ETR1, ERS1, ETR2, ERS2, and EIN4 were identified [38,[97][98][99].CTR1 is a negative regulator of ethylene signaling pathway [100], and CTR1 is inactive in the presence of ethylene and active in the absence of ethylene [101].EIN2 is a key regulator of the ethylene signaling pathway and its loss of function results in a complete loss of ethylene response in plants.In the absence of ethylene in plants, CTR1 phosphorylates the C-terminal domain of EIN2, EIN3, EIL1, and EIL2.The box proteins EBF1 and EBF2 are rapidly degraded by the ubiquitinated proteasome pathway.CTR1 fails to phosphorylate EIN2 in the presence of ethylene [102-105].EIN3 is a core factor in the interaction between ethylene and other hormones.Both JAZ and MYC can affect the ethylene response pathway by inhibiting EIN3 [106].Ethylene has been reported to increase resistance to B. cinerea in tomato, but the exact mechanism of action is unknown [71,107,108].ERF is also a plant-specific transcription factor that usually functions in the last step of the ethylene transduction pathway, but ERF has also been reported to function in the JA and SA pathways, such as tomato SlERF2 [109], SlERF3b, and SlERF5 [110], which play a role in methyl jasmonate-induced resistance to B. cinerea.Tomato ERF.A1, ERF.B4, ERF.C3, and ERF.A3 belongs to the B3 Group as well as ERF.C1, C3, C4, C6, D2, D7, ERF.A4, B12, B13, D6, F4, and H9 were necessary for tomato resistance to B. cinerea, but the specific mechanism remains to be clarified [111,112].ERF.A1, Pti5 (also named ERF.G2), Pti6 (also named ERF.C6), and ERF.A4) were increased in tomato response to B. cinerea infection [113], and Pti4 plays a crucial role in the response to B. cinerea infection [114].Grapevine VaERF16, which interacts with VaMYB306, was induced by B. cinerea infection and positively modulated plant resistance to B. cinerea [115].

Other Hormones
Cytokinin (CTK) plays a positive role in the resistance to B. cinerea, while abscisic acid has an opposite role [58].The other hormones have also been reported in crops.When Vitis vinifera was infected with B. cinerea, genes associated with the 'ABA biosynthetic process', 'ABA catabolic process', and 'response to ABA' were highly induced [116].AUX, CTK, GAs, ABA, ET, brassinosteroid, JA, and SA signal transduction in grape berries and kiwifruits have changed to counter the B. cinerea [14].Co-silencing of ABA receptors SlRCAR9, SlRCAR11, SlRCAR12, and SlRCAR13 weakened the ethylene biosynthesis and signaling pathway that enhanced fruit firmness and altered the shelf-life and susceptibility to B. cinerea [117].AcTPR2, which is associated with IAA signaling pathways, confers resistance to B. cinerea [118].All of these examples shed light on the role of phytohormones and their interaction with each other [75].

Other Proteins
In addition to the reported major proteins, other proteins also play a role in the resistance of B. cinerea.Tomato SlDQD/SDH2 plays a key role in flavonoid biosynthesis and fruit resistance to B. cinerea [119].Strawberry MANNOSE-BINDING LECTIN 1 (MBL1) is a member of the G-type lectin family and was involved in plant defense against B. cinerea [120].Loss-of-function tomato phosphoinositide specific phospholipase C2 SlPLC2 lines showed increased resistance to B. cinerea [121,122].SlIMP3 functions as a bifunctional enzyme involved in the biosynthesis of AsA and myoinositol and increases fruit resistance to B. cinerea [123].FaPG1 has a key role in remodeling pectins during strawberry softening and regulating plant resistance to B. cinerea [124].Tomato β-1, 3-GLUCANASE10 (SlBG10) regulates fruit resistance to B. cinerea by modulating callose deposition [125].

Natural Products That Can Enhance the Resistance to B. cinerea in Horticultural Crops
Though genetic improvement by altering the expression of the defense genes is an effective method, it is also urgent to search for natural products that can improve the resistance to B. cinerea.

Plant Natural Products
Natural products from plants or phytohormones and their derivates were proven to enhance the resistance to B. cinerea in horticultural crops (Table 2).Melatonin is used to treat strawberry [140] and tomato [141], which shows great resistance to B. cinerea.In addition, 1-methylcyclopropene (1-MCP) [113,142] and 2, 5-norbornadiene [113] are useful in the ET signaling pathway.Phenylalanine is one of the essential amino acids and it can be used to control gray mold disease [143].Ferulic acid is a phenolic acid commonly found in the cell walls of grains, vegetables, and fruit and it has a positive effect on defense against B. cinerea [19].Piperonylic acid is the inhibitor of the phenylpropanoid pathway, which induces broad-spectrum disease resistance in tomato [144].Moreover, the defense response against necrotrophic fungi is greatly dependent on phytohormones and the crosstalk between them [75].Thus, it reminds us that certain concentrations of phytohormones and their derivates can help to control B. cinerea.6-benzylaminopurine [58,73], zeatin [58], abscisic acid [58], salicylic Acid [74], MeJA [72, [145][146][147], Ile [148], brassinosteroid [146,149], ethylene [113], and 1-aminocyclopropane-1-carboxylic acid [150] were also used for enhancing the resistance to B. cinerea in rose, tomato, and strawberry.

Conclusions and Prospects
This review summarizes recent advances in mechanisms underlying defense responses of horticultural crops to B. cinerea, including physiological responses, change of metabolites, crucial proteins in PTI signaling pathways, TFs, and hormone pathways, which provides significant insights into horticultural crops against B. cinerea.These findings suggest that transcriptional regulation and hormone transduction pathways undoubtedly play important roles in the defense responses of horticultural crops to B. cinerea.However, there are few reports on epigenetic modification, phase separation, and other new regulatory aspects for underlying defense responses of horticultural crops to B. cinerea.Therefore, future research should focus on these new regulatory aspects.
We proposed approaches to improve horticultural crops resistance to B. cinerea by modifying genes including TFs, MAPK proteins, and some other proteins in different processes [154].These abundant gene resources can not only improve plants disease resistance but also plants development and fruit quality [124,125].Elicitors from B. cinerea and some other microbes can induce the resistance of horticultural crops to B. cinerea by inducing obvious plant defense responses [133,152].Additionally, certain concentrations of natural products can also be used to control gray mold disease on horticultural crops.

Figure 1 .
Figure 1.Immune responses against B. cinerea in tomato.Chitin, other pathogen-associated molecular patterns (PAMPs), and phytosulfokin (PSK) are recognized by receptor-like proteins and kinases that serve as PRRs.SlLYK4 interacts with SlCERK1, which recognizes chitin from B. cinerea.SlRIPK influences downstream ROS, while SlPSKR1 recognizes PSK, but how it works remains unclear.The activation of MAPK cascade is an important component of PTI signaling.Several major classes of transcription factors (TFs), mainly including WRKYs, ERFs, MYBs, bHLHs, and NACs, were implicated in immune responses to B. cinerea and may be targets of MAPK phosphorylation.These TFs influence the level of plant defense hormones by modulating gene expression.Mediator is an evolutionarily conserved multisubunit complex that regulates the function of RNA merase.Tomato mediator complex SlMED8 and SlMED25 contribute to immune responses to B.

Figure 1 .
Figure 1.Immune responses against B. cinerea in tomato.Chitin, other pathogen-associated molecular patterns (PAMPs), and phytosulfokin (PSK) are recognized by receptor-like proteins and kinases that serve as PRRs.SlLYK4 interacts with SlCERK1, which recognizes chitin from B. cinerea.SlRIPK influences downstream ROS, while SlPSKR1 recognizes PSK, but how it works remains unclear.The activation of MAPK cascade is an important component of PTI signaling.Several major classes of transcription factors (TFs), mainly including WRKYs, ERFs, MYBs, bHLHs, and NACs, were implicated in immune responses to B. cinerea and may be targets of MAPK phosphorylation.These TFs influence the level of plant defense hormones by modulating gene expression.Mediator is an evolutionarily conserved multisubunit complex that regulates the function of RNA merase.Tomato mediator complex SlMED8 and SlMED25 contribute to immune responses to B. cinerea by modulating gene expression.Some other genes in the cytoplasm also affect the metabolites and phytohormones signaling.

Table 1 .
Proteins and genetic techniques involved in resistance to B. cinerea in different horticultural crops.

Table 2 .
Natural products used to control gray mold in horticultural crops.