Salicylic acid: The roles in plant immunity and crosstalk with other hormones

ABSTRACT

Land plants use diverse hormones to coordinate their growth, development and responses against biotic and abiotic stresses. Salicylic acid (SA) is an essential hormone in plant immunity, with its levels and signaling tightly regulated to ensure a balanced immune output. Over the past three decades, molecular genetic analyses performed primarily in Arabidopsis have elucidated the biosynthesis and signal transduction pathways of key plant hormones, including abscisic acid, jasmonic acid, ethylene, auxin, cytokinin, brassinosteroids, and gibberellin. Crosstalk between different hormones has become a major focus in plant biology with the goal of obtaining a full picture of the plant hormone signaling network. This review highlights the roles of SA in plant immunity and summarizes our current understanding of the pairwise interactions of SA with other major plant hormones. The complexity of these interactions is discussed, with the hope of stimulating research to address existing knowledge gaps in hormone crosstalk, particularly in the context of balancing plant growth and defense.

This review highlights the roles of salicylic acid in plant immunity and summarizes our current understanding of the interactions of salicylic acid with other major plant hormones.

INTRODUCTION

Plants encounter a variety of microbes throughout their life cycle and have evolved a complex immune system to defend against microbial pathogens. Transmembrane immune receptors are deployed to detect conserved pathogen‐associated molecular patterns (PAMPs) to activate pattern‐triggered immunity (PTI) in plants (Tang et al., 2017; Huang and Joosten, 2025). In response, pathogens adapted to their host plants can deliver effector proteins to suppress or evade PTI (Dou and Zhou, 2012). Evolution of cytoplasmic nucleotide‐binding leucine‐rich repeat receptors (NLRs) or transmembrane receptor‐like proteins (RLPs) enables plants to sense specific effectors and activate effector‐triggered immunity (ETI) (Kourelis and Van Der Hoorn, 2018; Huang and Joosten, 2025). Both PTI and ETI take place at the infection sites to inhibit pathogen growth and reduce their spread. Subsequently, the activation of local defense mounts a secondary immune response in the distal parts of plants for broad‐spectrum resistance against pathogens, termed systemic acquired resistance (SAR).

Salicylic acid (SA) is a phytohormone playing crucial roles in plant immunity (Peng et al., 2021). During PTI and ETI, SA biosynthesis is activated, leading to elevated SA levels in both local and distal tissues. In Arabidopsis thaliana, ICS1 and PBS3 encode two key enzymes for SA biosynthesis (Figure 1). ICS1 converts chorismate to isochorismate in the chloroplast (Wildermuth et al., 2001; Strawn et al., 2007), while PBS3 conjugates glutamate to isochorismate to produce isochorismate‐9‐glutamate in the cytosol, which then spontaneously decays into SA (Rekhter et al., 2019; Torrens‐Spence et al., 2019). To connect these two reactions, the MATE transporter EDS5 transports the SA precursor isochorismate from the chloroplast to the cytosol (Nawrath et al., 2002; Rekhter et al., 2019). The expression of ICS1, EDS5 and PBS3 is activated by the transcription factors SARD1 and CBP60g and is strongly induced during plant defense (Wildermuth et al., 2001; Nawrath et al., 2002; Nobuta et al., 2007; Zhang et al., 2010; Wang et al., 2011; Sun et al., 2015).

Figure 1.

Roles of salicylic acid (SA) in plant immunity based on findings from Arabidopsis research

During pathogen infection, activation of pattern‐triggered immunity (PTI) and effector‐triggered immunity (ETI) leads to SARD1 and CBP60g‐mediated induction of ICS1, EDS5, and PBS3, which results in increased SA biosynthesis. Salicylic acid not only binds to NPR3/4 to release the repression of defense genes by the NPR3/4 and TGA2/5/6 protein complexes, but also binds to NPR1 to further enhance defense gene transcription. Induction of SA‐responsive genes increases expression of pattern recognition receptors, nucleotide‐binding leucine‐rich repeat receptors (NLRs) and their downstream signaling components, leading to amplification of PTI and ETI. Meanwhile, induction of SARD1 and CBP60g promotes N‐hydroxypipecolic acid (NHP) biosynthesis and results in elevated NHP levels, which further promote SA signaling and systemic acquired resistance (SAR). On the other hand, induction of DMR6 and UGT76B1 by SA accelerates the conversion of SA to its inactive catabolites in a negative feedback loop. The induction of UGT76B1 also inactivates NHP by catalyzing its glycosylation.

In Arabidopsis, SA induces downstream defense gene expression through its receptors NPR1 and NPR3/NPR4 and the NPR‐interacting TGA2/5/6 transcription factors (Figure 1) (Cao et al., 1997; Ryals et al., 1997; Zhang et al., 1999, 2003; Despres et al., 2000; Zhou et al., 2000; Ding et al., 2018). NPR1 acts as a transcription activator of SA‐responsive genes, the activity of which is promoted by elevated SA levels during defense (Fan and Dong, 2002; Rochon et al., 2006). In contrast, NPR3/NPR4 act as transcriptional repressors to prevent expression of defense‐related genes in the absence of pathogen threats (Zhang et al., 2006). Following infection, increased SA binding to NPR3/NPR4 inhibits their activity, thereby releasing repression of defense‐related genes during plant immunity (Ding et al., 2018). The combined effects of NPR1 activation and NPR3/NPR4 de‐repression lead to full SA‐mediated immunity. Notably, while loss of function npr3 npr4 double mutants exhibit enhanced pathogen resistance due to the loss of defense repression (Zhang et al., 2006), the gain‐of‐function npr4‐4D mutant with a point mutation in the SA‐binding pocket is insensitive to SA and more susceptible to pathogens (Ding et al., 2018; Wang et al., 2020).

In addition to SA, two other plant hormones, jasmonic acid (JA) and ethylene (ET), also play important roles in plant immunity. Activation of immunity usually results in growth inhibition by attenuating signaling of plant growth hormones such as auxin, brassinosteroid (BR), cytokinin (CK), and gibberellin (GA). Salicylic acid intertwines with these plant hormones in plant defense and balance of plant growth and immunity as well as coordination of responses to biotic and abiotic stresses. This paper reviews the functions of SA in plant immunity and its crosstalk with other phytohormones. Untangling these interactions is essential for a comprehensive understanding of the plant immune system.

THE ROLE OF SA IN PLANT IMMUNITY

Salicylic acid in PTI

Salicylic acid signaling is indispensable in PTI. In the SA‐insensitive receptor mutants npr1‐1 and npr4‐4D, flg22‐induced protection against the bacterial pathogen Pseudomonas syringae pv. tomato (P.s.t.) DC3000 is compromised (Liu et al., 2020). In addition, induction of defense maker genes PR2 and SARD1 by the non‐pathogenic bacteria P.s.t. DC3000 hrcC, which lacks the type‐III secretion system for delivering effectors into the plant cell, is almost completely blocked, and growth of the bacteria is significantly higher in the SA‐insensitive npr1‐1 npr4‐4D mutant (Ding et al., 2018; Liu et al., 2020). Following exogenous SA treatment, up‐regulation of a large number of PTI signaling components and defense‐related genes may contribute to the amplification of PTI (Figure 1) (Ding et al., 2018). For example, Metacaspase 2 (MC2), one of the strongly induced genes by SA, has recently been shown to promote pattern recognition receptor (PRR)‐mediated defense through its prodomain interaction with BIR1 (Wu et al., 2024), a negative regulator of RLP‐mediated immune signaling (Gao et al., 2009). Mutations in the MC2 prodomain result in reduced induction of defense gene expression and SA accumulation following treatment with the elicitor nlp20 (Wu et al., 2024). Among the genes rapidly induced by SA are a large number of genes encoding PPRs and PTI signaling components such as receptor‐like cytoplasmic kinases, subunits of heterotrimeric G proteins, calcium‐dependent protein kinases, and components of mitogen‐activated protein kinase cascades. Up‐regulation of these PTI signaling components most likely also helps boost PTI.

Salicylic acid in ETI

Sufficient accumulation and perception of SA in plants are essential for ETI. In Arabidopsis transgenic plants expressing salicylate hydroxylase (NahG), which converts SA to catechol, plant immunity against P.s.t. DC3000 avrRpt2 and the avirulent oomycete pathogen Hyaloperonospora arabidopsidis (H. a.) Wela strain is severely compromised (Delaney et al., 1994). Similarly, blocking SA perception results in severely reduced induction of defense‐related genes such as SARD1, PR1, and PR2 by the avirulent strains of P.s.t. DC3000 carrying avrRpt2 or avrRps4 and much higher growth of these bacteria strains in the npr1‐1 npr4‐4D double mutant than in the wild type plants (Liu et al., 2020). RNA sequencing (RNA‐seq) analysis revealed that pre‐treatment with SA strongly induces many genes encoding NLRs or signaling components downstream of NLRs (Ding et al., 2018). This would result in positive feedback amplification of ETI (Figure 1), as overaccumulation of NLRs or their downstream signaling components often leads to activation of immune responses.

SA signaling also negatively impacts cell death during ETI. In the npr1‐1 npr4‐4D double mutant, P.s.t. DC3000 avrRpt2 infection induces stronger cell death than in the wild type plants (Radojičić et al., 2018). Similarly, cell death in the Arabidopsis SA‐deficient mutant eds5‐3 following infection by P.s.t. DC3000 avrRpt2 is enhanced compared to that in the wild type (Radojičić et al., 2018). Consistent with the role of SA in suppressing cell death during ETI, cell death triggered by P.s.t. DC3000 avrRpt2 is almost completely blocked in the npr3 npr4 double mutant with constitutively activated SA signaling (Fu et al., 2012). The mechanism by which SA inhibits ETI‐activated cell death remains to be determined.

Salicylic acid in PTI and ETI crosstalk

It has become clear that PTI and ETI signaling converge and interact with each other to amplify plant immunity (Peng et al., 2018; Ngou et al., 2021; Pruitt et al., 2021; Tian et al., 2021; Yuan et al., 2021). Salicylic acid biosynthesis is triggered following activation of PTI and ETI. Interestingly, initiation of SA biosynthesis in Arabidopsis during PTI relies on signals from Toll‐interleukin receptor (TIR) domain‐containing proteins (Tian et al., 2021). A large number of genes encoding TIR or TIR‐NLR proteins are rapidly upregulated following PAMP perception, leading to activation of TIR signaling and SA biosynthesis. In addition, a large number of genes encoding PRRs, NLRs and signaling components of PTI and ETI are induced by SA. It would be interesting to determine whether activation of SA signaling during PTI and ETI is involved in the mutual potentiation between PTI and ETI.

Salicylic acid in SAR

Salicylic acid has long been recognized as pivotal for SAR. Early studies showed that blocking SA accumulation in Nicotiana tabacum and in Arabidopsis transgenic plants expressing NahG resulted in compromised SAR, suggesting that SA is an essential signaling molecule in SAR (Gaffney et al., 1993; Delaney et al., 1994). Subsequent studies confirmed this by using SA‐deficient mutants of Arabidopsis (Nawrath and Metraux, 1999). Furthermore, perception of SA by NPR1 and NPR3/NPR4 is required for SAR. SAR is completely blocked in Arabidopsis npr1 and npr4‐4D mutant plants (Cao et al., 1994; Liu et al., 2020).

Despite its importance for SAR, SA does not appear to be a long‐distance signal for SAR, as evidenced by the grafting experiments on Nicotiana tabacum (Vernooij et al., 1994). Instead, N‐hydroxypipecolic acid (NHP) has emerged as a strong candidate for the SAR mobile signal. N‐hydroxypipecolic acid levels increase dramatically following pathogen infection (Chen et al., 2018; Hartmann et al., 2018), and Arabidopsis mutants deficient in NHP biosynthesis are unable to mount SAR responses (Song et al., 2004; Mishina and Zeier, 2006; Jing et al., 2011). Salicylic acid plays dual roles in SAR (Figure 1). On one hand, SA stimulates NHP biosynthesis. Pathogen‐induced expression of NHP biosynthesis genes relies on SARD1 and CBP60g, and the induction of SARD1/CBP60g and NHP biosynthesis is dependent on SA signaling (Sun et al., 2015; Liu et al., 2020; Sun et al., 2020). On the other hand, SA functions downstream of NHP to amplify immunity. npr1‐1 and npr4‐4D mutants exhibit impaired disease resistance conferred by overexpression of the NHP biosynthesis gene FMO1 and lost NHP‐induced systemic resistance to H. a. Noco2 (Liu et al., 2020).

In addition, SA signaling plays an important role in maintaining its own homeostasis (Figure 1) (Peng et al., 2021). On one hand, SA promotes its own biosynthesis by inducing the expression of SA biosynthesis genes in a positive feedback loop via SARD1 and CBP60g (Ding et al., 2018), which coordinately regulate the expression of SA biosynthesis genes in Arabidopsis. On the other hand, SA activates expression of genes encoding enzymes converting SA to inactive SA catabolites (Liu et al., 2020), such as DOWNY MILDEW RESISTANT6 (DMR6), an SA 5‐hydroxylase that converts SA to 2,5‐DHBA (Zhang et al., 2017), and UDP‐glucosyltransferase 76b1 (UGT76b1) that converts SA to SA 2‐O‐β‐d‐glucoside (SAG) (Noutoshi et al., 2012). Notably, UGT76b1 also catalyzes the glycosylation of NHP to produce inactive O‐glycosylated NHP (NHP‐OG) (Bauer et al., 2021; Holmes et al., 2021; Mohnike et al., 2021; Pastorczyk‐Szlenkier and Bednarek, 2021), therefore UGT76b1 induction by SA also dampens NHP accumulation (Figure 1). For instance, the npr1‐1 mutant plants showed dramatically decreased NHP‐OG levels and increased NHP accumulation (Liu et al., 2020), highlighting the involvement of SA signaling in regulating NHP homeostasis to balance plant growth and immunity.

Beyond its roles in PTI, ETI, and SAR, the high levels of SA accumulated in the extracellular space are potentially toxic to the growth of certain pathogens and thus contribute to plant defense. For example, addition of SA to medium leads to inhibition of the growth of P.s.t. DC3000 (Wilson et al., 2017). Interestingly, SA was also shown to shape the microbiome in Arabidopsis roots (Lebeis et al., 2015).

CROSSTALK BETWEEN SA AND OTHER PLANT HORMONES

Abscisic acid–SA crosstalk

The plant stress hormone abscisic acid (ABA) acts as a negative regulator of plant immunity. Treatment with ABA enhances the growth of P.s.t. DC3000, which activates ABA biosynthesis to promote its own virulence (de Torres‐Zabala et al., 2007). In Arabidopsis, ABA‐deficient mutants are more resistant to Pseudomonas syringae pv. maculicola ES 4326(P.s.m. ES4326) and H.a. Noco2 (Goritschnig et al., 2008). In contrast, the ABA‐hypersensitive era mutants display enhanced susceptibility to both bacterial and oomycete pathogens and can suppress the constitutive defense responses of the Arabidopsis autoimmune mutant snc1 (Goritschnig et al., 2008). Inhibition of plant defense by ABA is at least in part through its suppression of SA‐mediated immunity (Figure 2A).

Figure 2.

Crosstalk between salicylic acid (SA) and other plant hormones

(A) Abscisic acid (ABA) inhibits SA accumulation by suppressing ICS1 expression, and attenuates SA signaling by reducing the accumulation of NPR1 protein. Cytokinins promote SA‐mediated defense responses by inhibiting ABA accumulation and enhancing TGA3‐mediated PR gene expression. Type‐A‐ response regulator proteins downstream of cytokinin receptors inhibit PR gene expression via a negative feedback loop. Reciprocally, SA can repress cytokinin responses. (B) Jasmonic acid (JA) inhibits SA signaling and the JA analog coronatine (COR) induces the expression of genes encoding transcription factors ANAC019/055/072, which inhibits the expression of ICS1 and activates the expression of BSMT1 and SAGT1 to reduce SA accumulation. Effector‐triggered immunity (ETI) also inhibits SA biosynthesis by repressing ICS1 expression. In turn, SA inhibits induction of PDF1.2 by ethylene (ET) and JA as well as induction of HSL1 by ET. (C) Salicylic acid dampens auxin signaling by repressing the expression of TIR1/AFB and inhibiting the activity of PP2A so as to modulate auxin distribution. Salicylic acid also reduces auxin levels by inhibiting the activity of catalase that prevents sulfenylation of TSB1 by H₂O_2, thereby enhancing biosynthesis of the auxin precursor tryptophan. Reciprocally, perception of auxin leads to suppression of SA‐induced PR1 expression. (D) BIN2, the key negative regulator of brassinosteroid (BR) signaling, promotes PR gene expression by phosphorylating TGA3 in Arabidopsis. BR inhibits BIN2 activity and benzo (1,2,3) thiadiazole‐7‐carbothioic acid S‐methyl ester (BTH)‐induced expression of rice NPR1 and WRKY45. (E) Salicylic acid inhibits gibberellin (GA) responses in rice by reducing both GA levels and the accumulation of its receptor GID1. In Arabidopsis, DELLA proteins interfere with JAZ1 function to enhance JA responses, leading to inhibition of SA accumulation.

Multiple lines of evidence indicate that ABA reduces SA levels in plants. Treatment with ABA dramatically reduces SA accumulation in Arabidopsis during infection by P.s.t. DC3000 (Mohr and Cahill, 2007). ABA treatment suppresses the expression of ICS1 and the up‐regulation of SA levels by the SAR inducer 1,2‐benzisothiazol‐3(2H)‐one1,1‐dioxide (BIT) (Yasuda et al., 2008). Moreover, the ABA biosynthesis mutant aao3 shows higher SA accumulation than the wild type before and after infection by P.s.t. DC3000, which correlates with the increase in ICS1 expression (de Torres Zabala et al., 2009).

In addition to suppressing SA accumulation, ABA negatively regulates SA signaling (Figure 2A). In Arabidopsis, pre‐treatment with ABA results in reduced induction of PR1 expression by SA analog benzo (1,2,3) thiadiazole‐7‐carbothioic acid S‐methyl ester (BTH) (Ding et al., 2016). In contrast, ABA‐deficient mutant plants constitutively express PR1 in a NPR1‐dependent manner (Ding et al., 2016). Abscisic acid‐deficient tomato mutant plants also exhibit increased sensitivity to the induction of PR1a by BTH (Audenaert et al., 2002). In addition, induction of OsNPR1 and OsWRKY45 by SA in rice is strongly reduced upon ABA treatment (Jiang et al., 2010). Overexpression of OsNPR1 or OsWRKY45 overcomes ABA‐induced susceptibility to Magnaporthe (M.) grisea, suggesting that ABA inhibits plant immunity through suppressing SA‐induced defense gene expression in rice (Jiang et al., 2010). Moreover, full activation of OsWRKY45 requires phosphorylation by OsMPK6, and ABA treatment results in de‐phosphorylation of OsWRKY45 and reduced BTH‐induced resistance against M. grisea (Ueno et al., 2015).

Furthermore, ABA negatively regulate NPR1 protein levels (Figure 2A) (Ding et al., 2016). Treatment with ABA promotes proteasome‐mediated degradation of NPR1, leading to reduced NPR1 protein levels. In contrast, the accumulation of NPR1 in ABA‐deficient mutants is significantly higher than that in the wild type. It is worth noting that the phosphomimic S11/15D mutant of NPR1 tends to be less sensitive to ABA‐induced degradation (Ding et al., 2016). Despite the evidence observed, the mechanism by which ABA influences the stability of NPR1 remains to be determined.

Jasmonic acid/ET‐SA crosstalk

Jasmonic acid (JA) has been well‐known to antagonize SA in plant immunity (Figure 2B) (Hou and Tsuda, 2022). Coronatine (COR), a JA analog produced by P. syringae, promotes bacterial virulence by activating JA signaling (Feys et al., 1994; Mittal and Davis, 1995). The Arabidopsis JA‐insensitive mutants, coronatine insensitive 1 (coi1) and jasmonate insensitive 1 (jin1/myc2), display dramatically reduced susceptibility to P. syringae (Kloek et al., 2001; Laurie‐Berry et al., 2006). Compared to the wild type, coi1 mutants have about four‐fold higher free SA 24 h after inoculation with P.s.t. DC3000 and stronger induction of PR1 expression (Kloek et al., 2001). The induction of PR1 and enhanced resistance to P.s.t. DC3000 in coi1 are blocked by expression of NahG or by loss of NPR1 function (Kloek et al., 2001). Similarly, jin1 mutants display much higher PR1 expression than wild type plants after P.s.t. DC3000 infection, and its enhanced resistance to P.s.t. DC3000 is SA‐dependent (Laurie‐Berry et al., 2006). These findings suggest that JA and COR inhibit SA signaling and promote the virulence of P.s.t. DC3000 by inhibiting SA‐mediated immunity.

Three closely related COR‐induced NAC transcription factor genes, ANAC019, ANAC055, and ANAC072, play critical roles in suppressing SA accumulation in Arabidopsis (Zheng et al., 2012). In the anac019/055/072 triple knockout mutant, COR‐triggered susceptibility to P.s.m. ES4326 is significantly reduced, suggesting that the NAC transcription factors contribute to COR‐mediated virulence. Chromatin immunoprecipitation – polymerase chain reaction (ChIP‐PCR) analysis showed that ANAC019 binds to the promoters of ICS1 and BSMT1 (Zheng et al., 2012), the latter of which encodes benzoic acid/salicylic acid carboxyl methyltransferase 1, which is an SA metabolism enzyme catalyzing the methylation of SA to produce methyl salicylate (MeSA) (Chen et al., 2003). In the nac triple mutant, COR‐induced expression of BSMT1 and SAGT (SA glucosyltransferase) is blocked, leading to reduced production of MeSA and SAG (Zheng et al., 2012). These findings suggest that induction of the NAC transcription factor genes by COR reduces free SA levels through promoting the conversion of SA to SAG and MeSA (Figure 2B). Loss of function of the NAC transcription factors also results in elevated expression level of ICS1, potentially as a result of direct repression of ICS1 expression by NAC (Zheng et al., 2012). Another possible cause of the observed elevated ICS1 level may come from up‐regulation of ICS1 by the elevated SA levels, as SA induces ICS1 expression via a positive feedback loop. It would be interesting to analyze whether the NAC transcription factors possess transcriptional repression activities.

Salicylic acid also inhibits the accumulation of JA following P.s.t. DC3000 infection. In Arabidopsis NahG transgenic plants, JA levels are about 25‐fold higher than those of the wild type, accompanied by increased expression of JA‐responsive genes (Spoel et al., 2003). Similarly, levels of jasmonoyl isoleucine (JA‐Ile), the active form of JA, are much higher in the sid2‐1 (a mutant of ICS1) mutant than in the wild type (Yuan et al., 2017). The SA‐binding protein catalase2 (CAT2) was shown to stimulate the activities of the JA biosynthesis enzymes Acyl‐CoA Oxidase 2 (ACX2) and ACX3 in vitro, which is inhibited by SA (Yuan et al., 2017). In the cat2‐1 sid2‐1 double mutants, the elevated JA‐Ile levels are partially suppressed, suggesting that SA inhibits JA accumulation by reducing the stimulation of ACX2/ACX3 activities by CAT2 (Yuan et al., 2017).

In addition to JA accumulation, SA also directly represses JA signaling downstream of the JA receptor COI1 in Arabidopsis (Van der Does et al., 2013). Microarray analysis revealed that 59 JA‐inducible genes including PDF1.2 are suppressed by SA. The GCC‐box motif enriched in the promoters of these genes is sufficient for suppression of JA‐responsive genes by SA. Octadecanoid‐Responsive AP2/ERF59 (ORA59), an AP2/ERF‐type transcription factor that can bind to the GCC‐box motif, is required for activation of PDF1.2 (Van der Does et al., 2013). Its accumulation in plants is strongly reduced following treatment with SA, suggesting that SA may suppress JA‐responsive gene expression by negatively regulating the protein levels of ORA59 (Van der Does et al., 2013).

WRKY70 is another transcription factor involved in the crosstalk between SA and JA. Salicylic acid strongly induces the expression of WRKY70 (Li et al., 2004). Overexpression of WRKY70 not only leads to activation of PR genes and enhanced resistance to biotrophic pathogens, but also results in suppression of JA‐responsive genes and compromised resistance to the necrotrophic pathogen Alternaria (A.) brassicicola (Li et al., 2004). In contrast, silencing or knockout of WRKY70 activates JA‐responsive genes and enhances resistance to A. brassicicola (Li et al., 2006). One interesting observation is that overexpression or silencing of WRKY70 does not affect JA levels (Li et al., 2006). The mechanism of how WRKY70 affects JA signaling remains to be determined.

Ethylene was also shown to negatively regulate SA levels in Arabidopsis (Figure 2B). In the ethylene overproducer 1 mutant, basal SA level is significantly lower than that in the wild type (Li et al., 2018). In addition, loss of function of the partially redundant ET‐responsive transcription factors EIN3 (ET‐insensitive 3) and EIL1 (EIN3‐like 1) results in elevated ICS1 expression and SA accumulation (Chen et al., 2009). Given the ability of EIN3 to bind to the promoter of ICS1, its inhibition of SA biosynthesis is likely achieved by direct repression of ICS1 expression (Chen et al., 2009).

In turn, SA negatively regulates ET responses (Figure 2B). GRX480/ROXY19 is a member of the glutaredoxin family that suppresses EIN3‐induced promoter activity of ORA59 (Ndamukong et al., 2007), a key activator of JA and ET‐responsive defense genes such as PDF1.2 (Van der Does et al., 2013). Salicylic acid can strongly induce the expression of GRX480, thereby dampening ET and JA‐induced gene expression (Ndamukong et al., 2007). As a result, GRX480 overexpression plants exhibit suppressed induction of PDF1.2 by ET and JA (Ndamukong et al., 2007). Furthermore, SA was also shown to antagonize ET in terms of plant development. Exogenous application of SA inhibits ET‐induced hook formation by suppressing expression of the ET‐responsive transcription factor Hookless 1 (HLS1) (Huang et al., 2020a).

Several transcription factors are involved in the crosstalk between SA and JA in rice. OsbHLH6 was shown to activate JA signaling and suppresses SA signaling (Meng et al., 2020). Two transcription factor modules, OsEIL3‐OsWRKY28 and OsEIL3‐OsERF040, were recently reported to exert antagonistic effects on the immunity to the necrotrophic Rhizoctonia solani and the hemibiotrophic M. oryzae by regulating SA and JA pathways (Zhu et al., 2024). OsEIL3 promotes immunity against M. oryzae by activation of the expression of OsERF040 and inhibits immunity against Rhizoctonia solani through repression of OsWRKY28.

Adding to the complexity of SA crosstalk with JA and ET, SA does not always antagonize JA and ET in plant immunity. In Arabidopsis, flg22‐induced resistance against P.s.t. DC3000 and avrRpt2‐triggered immunity are largely intact in single mutants deficient in ET, JA, or SA signaling, yet severely compromised in the dde2 /pad4/sid2/ein2‐quadruple mutant deficient in JA/SA biosynthesis and ET signaling, indicating the collaboration of SA, JA, and ET to boost PTI and ETI (Tsuda et al., 2009). Interestingly, no SA–JA antagonism was observed in poplar (Ullah et al., 2022). Salicylic acid and JA are simultaneously induced by rust infection, and treatment with either SA or JA induces the accumulation of other hormones in poplar leaves. In addition, overaccumulation of SA in transgenic poplar plants leads to increased JA and JA‐Ile levels. Similarly, infection of rice by M. oryzae and Xanthomonas oryzae pv. oryzae (Xoo) leads to increased accumulation of both SA and JA, and treatment of either SA or JA results in enhanced disease resistance against the rice pathogens (Iwai et al., 2007; Yamada et al., 2012; Riemann et al., 2013).

Cooperation of SA and JA in plant defense is further supported by gene expression data. Although SA and JA antagonize each other when simultaneously applied at high concentrations, a transient synergistic enhancement of Arabidopsis PDF1.2 and tobacco PR1a expression was observed when they were applied together at low concentrations (Mur et al., 2006). In addition, there is considerable overlap between genes upregulated by JA and those by SA. Meta‐analysis showed that 363 genes in Arabidopsis are induced by both JA and SA (Zhang et al., 2020). In rice, over half of the genes induced by the SA analog BTH are also induced by JA (Tamaoki et al., 2013).

Auxin‐SA crosstalk

Treatment with high concentrations of SA inhibits plant growth. Arabidopsis mutants with constitutive defense responses and elevated SA levels often display morphologies resembling auxin‐insensitive or auxin‐deficient mutants, suggesting that SA negatively regulates auxin signaling (Figure 2C) (Wang et al., 2007). As shown by the microarray analysis of transcriptome of Arabidopsis plants, application of the SA analog BTH represses expression of genes encoding the auxin receptors TRANSPORTER INHIBITOR RESPONSE 1 (TIR1)/AUXIN SIGNALING F‐BOX (AFB), the auxin importer AUXIN INFLUX CARRIER 1 (AUX1), auxin efflux carrier PIN‐FORMED 7 (PIN7), as well as auxin early responsive Aux/IAA‐family transcriptional repressor genes and SMALL AUXIN UP RNA (SAUR) genes (Wang et al., 2007). Consistent with the transcriptomics data, treatment with 0.5 mmol/L of SA results in reduced induction of the auxin‐responsive DR5::GUS reporter by auxin. Despite these observations, how SA inhibits the expression of auxin signaling genes is still unknown.

Salicylic acid regulates root development by modulating auxin transport (Figure 2C) (Tan et al., 2020). PIN auxin efflux carriers are the substrates of protein phosphatase 2A (PP2A). By directly binding to its A subunits, SA inhibits the phosphatase activity of PP2A and increases PIN2 phosphorylation, thereby altering subcellular distribution of PIN2 and decreasing auxin export activity. Exogenous SA application also triggers the compartmentalization of lipid raft nanodomains (Huang et al., 2019), which constrains the lateral movement of PIN2 and condenses it into protein clusters on the plasma membrane (Ke et al., 2021). This results in the disruption of gravity‐induced asymmetric distribution of auxin and PIN2, impairing the root gravitropic response.

Following infection by P.s.t. DC3000, indole‐3‐acetic acid (IAA) level in the SA‐deficient mutant sid2‐2 is slightly higher than that of the wild type, suggesting that SA may negatively regulate IAA levels (Figure 2C). Salicylic acid has long been known to bind to catalases and inhibit their activity so as to increase H₂O₂ production (Chen et al., 1993; Conrath et al., 1995). It was suggested that SA regulates auxin biosynthesis by inhibiting the H₂O₂‐scavenging activity of catalases, as H₂O₂ treatment leads to sulfenylation of Cys308 of the tryptophan synthetase b subunit 1 (TSB1), a key enzyme for the biosynthesis of the auxin precursor tryptophan, and reduced TSB1 activity (Yuan et al., 2017). Although treatment with high concentrations of SA results in decreased catalase activity and increased H₂O₂ levels in Arabidopsis, it remains to be determined whether SA at a lower physiological level can affect IAA levels by directly inhibiting catalases.

Interestingly, many pathogens produce auxin and use it as a virulence factor. Treatment with synthetic auxin naphthyl acetic acid (NAA) increases the susceptibility of Arabidopsis plants to P.s.m. ES4326 infection, suggesting an inhibitory effect of NAA on plant defense (Wang et al., 2007). Consistently, the auxin‐insensitive auxin resistant 2 (axr2‐1) mutant exhibits enhanced resistance to P.s.m. ES4326. Treatment with NAA reduces SA‐induced PR1 expression in Arabidopsis (Wang et al., 2007). In contrast, increased induction of PR1 is observed in the tir1 afb2 mutant with compromised auxin signaling (Iglesias et al., 2011). These findings suggest that activation of auxin signaling inhibits SA responses, and reciprocally SA inhibits auxin signaling as a way to boost plant immunity (Figure 2C).

Brassinosteroid and SA crosstalk

The roles of BRs in plant disease resistance are complex (Figure 2D). Early study in tobacco has shown that exogenous application of brassinolide (BL) can induce broad‐spectrum disease resistance against tobacco mosaic virus (TMV), P.s. tabaci and fungal pathogen Oidium sp. independent of SA (Nakashita et al., 2003). Similarly, in rice, BR strengthens plant resistance to M. grisea and Xoo (Nakashita et al., 2003). On the other hand, BR treatment has also been shown to attenuate BTH‐induced expression of OsNPR1 and OsWRKY45 (Figure 2D), thus dampening resistance against root oomycete pathogen Pythium graminicola (De Vleesschauwer et al., 2012).

A recent study in Arabidopsis revealed that SA activates the BIN2 kinase, a key negative regulator of BR signaling that phosphorylates TGA3 at Ser33 to enhance its activity (Figure 2D) (Han et al., 2022). In the gain‐of‐function bin2‐1 mutant, PR genes are constitutively activated in an NPR1‐dependent manner. Expression of the phosphomimic S33D mutant of TGA3 also leads to constitutive expression of PR genes in the npr1‐1 mutant background, suggesting that BR can negatively regulate plant defense by inhibiting BIN2 activity (Han et al., 2022). However, another study reported that BL enhances the induction of PR1 and WRKY70, and that BIN2 negatively regulates plant immunity by phosphorylating Ser202 of TGA4 to reduce its stability and interactions with NPR1 (Kim et al., 2022). Additionally, overexpression of the phosphor‐dead S202A mutant of TGA4 was shown to enhance SA responses. One caveat to this study is the use of a C‐terminal yellow fluorescent protein (YFP)‐tagged TGA4, which may not be functional due to potential interference of YFP with TGA4 at its C‐terminus. Therefore, some of the data and conclusions in this study should be interpreted with caution.

In Arabidopsis, SA has been shown to reduce the S‐acylation of the BR signaling protein BR signaling kinase 1 (BSK1) (Liu et al., 2023), a reversible post‐translational modification required for its proper plasma membrane localization and signal transduction. Such S‐acylation and membrane association of BSKs are hampered by the de‐S‐acylase ABAPT11, the levels of which in plants are positively regulated by exogenous SA treatment. Hence, it has been proposed that SA‐induced ABAPT11 attenuates BR signaling by detaching BSKs from the plasma membrane through de‐S‐acylation (Liu et al., 2023). However, given that the induction of ABAPT11 by SA was not significant as shown by previous RNA‐seq datasets (Ding et al., 2018), whether it contributes to SA‐BR crosstalk remains to be clarified.

Cytokinin and SA crosstalk

The roles of cytokinin (CK) in plant–microbe interactions are also complex (Figure 2A). On one hand, CKs are produced by certain bacterial and fungal pathogens, such as M. grisea, and they are utilized to aid them in colonizing host plants and achieving full virulence (Ashby, 2000; Chanclud et al., 2016). In Arabidopsis, treatment with low concentration of CKs results in increased susceptibility to H.a. Noco2, revealing its negative regulatory effect on plant basal resistance (Argueso et al., 2012). Consistent with this, loss of function of type‐A response regulator proteins (ARRs), which regulate the output of CK signaling, results in enhanced resistance against H.a. Noco2 as well as increased expression of genes involved in SA biosynthesis and signaling, whereas overexpression of type‐A ARRs led to increased growth of H.a. Noco2 (Argueso et al., 2012). These observations suggest that activation of CK signaling inhibits plant defense against H.a. Noco2.

On the other hand, a number of studies have showed that CKs can also positively regulate plant disease resistance. For instance, treating Arabidopsis plants with 1 µmol/L of t‐zeatin elevates the expression of ICS1 and PR1 and the resistance against P.s.t. DC3000 (Choi et al., 2010). Perception of CKs by their receptors in plants turns out to be required for basal and t‐zeatin‐induced resistance to this pathogenic bacterium (Choi et al., 2010). Interestingly, not just to P.s.t. DC3000, exogenous application of high concentrations of CKs to Arabidopsis plants also enhances their resistance against H.a. Noco2 (Argueso et al., 2012). Growth of H.a. Noco2 on Arabidopsis CK receptor mutants is significantly higher than that in wild type plants, pointing to a crucial role of CK signaling in basal resistance. In rice, CKs and SA act synergistically to induce the expression of defense marker genes OsPR1b and PBZ1 (Jiang et al., 2013). A recent study on Arabidopsis also showed that induction of CK signaling occurs in the early stages of infection by Ralstonia solancearum, which helps root defense against the pathogen (Alonso‐Díaz et al., 2021).

In general, the mechanisms by which CKs exert their regulatory roles in plant immunity are not fully understood. CKs may stimulate defense gene expression through enhanced SA signaling (Figure 2A). Specifically, CK‐induced PR1 expression and resistance against P.s.t. DC3000 are dependent on the actions of TGA3 and ARR2 (Choi et al., 2010). As TGA3 interacts with ARR2, it is likely that TGA3 recruits ARR2 to the promotor of PR1 gene so as to enhance its expression and strengthen plant resistance to P.s.t. DC3000 (Choi et al., 2010). In tobacco, CKs induce resistance against P.s. pv. tabaci and inhibit the accumulation of ABA during infection (Großkinsky et al., 2014). Such CK‐induced resistance is blocked by treatment with ABA or inhibitor of ABA catabolism. Thus, CK may indirectly promote SA signaling by reducing ABA levels in plants (Figure 2A) (Großkinsky et al., 2014).

Gibberellin and SA crosstalk

Gibberellin (GA) negatively regulates basal disease resistance based on studies in rice (Figure 2E). While treatment with GA3 increases disease susceptibility to X. oryzae pv. Oryzae, GA‐insensitive dwarf 1 (gid1) mutant displays enhanced resistance against M. grisea (Tanaka et al., 2006; Yang et al., 2008; De Vleesschauwer et al., 2016). Knockout of ELONGATED UPPERMOST INTERNODE 1 (Eui) which encodes a GA‐inactivating enzyme also leads to increase in disease susceptibility to X. oryzae pv. Oryzae and M. grisea (Yang et al., 2008). Notably, the rice Eui overexpression lines with enhanced resistance to the above two pathogens exhibit two‐ to threefold lower SA levels than those of wild type (Yang et al., 2008). Consistent with the suppressing effect of GA in plant basal defense, SLENDER 1 (SLR1), the sole DELLA protein suppressing GA responses in rice, promotes BTH‐induced expression of OsWRKY45 and activates disease resistance against X. oryzae pv. Oryzae and M. grisea (De Vleesschauwer et al., 2016). Exogenous application of BTH induces the expression of the GA‐degrading gene GA2 OXIDASE 3 (OsGA2ox3) and represses the expression of the GA biosynthesis gene GIBBERELLIN 20 OXIDASE1 (OsGA20ox1) (De Vleesschauwer et al., 2016). In addition, BTH induces the expression of genes encoding proteins that inhibit GA signaling, while increasing the stability of SLR1 in rice seedlings (De Vleesschauwer et al., 2016). These findings point to another way of SA promoting disease resistance in rice by reducing GA levels and attenuating GA signaling (Figure 2E).

Studies have shown that DELLA proteins in Arabidopsis increase disease susceptibility to hemi‐biotrophic P.s.t. DC3000 while enhancing resistance to necrotrophic A. brassicicola by modulating JA signaling (Navarro et al., 2008). DELLA proteins interact with the JA signaling repressor JA ZIM‐domain 1 (JAZ1), which prevents JAZ1 interaction with MYC2, leading to enhanced expression of JA‐responsive genes (Hou et al., 2010). A quadruple della mutant knocking out four out of five Arabidopsis DELLA genes displays partial insensitivity to MeJA, whereas the gain‐of‐function semi‐dominant GA‐insensitive gai mutant exhibits enhanced response to JA (Navarro et al., 2008; Hou et al., 2010). The accumulation of SA and expression of PR genes following infection by P.s.t. DC3000 are elevated in the quadruple della mutant compared to the wild type, which could be due to the indirect effects of attenuated JA signaling (Figure 2E) (Navarro et al., 2008).

Recently, SA has been shown to inhibit GA‐mediated plant growth in Arabidopsis (Yu et al., 2022). Exogenous application of SA leads to increased degradation of the GA receptor GID1 and accumulation of DELLA proteins. It was proposed that NPR1 acts as an E3 ligase to promote polyubiquitination and degradation of GID1. Another possible scenario is that SA elevates the expression levels of GID1 degradation‐related proteins through the transcriptional activation activity of NPR1. This hypothesis is supported by the observation that the mutation in npr1‐1 disrupting its interaction with TGA transcription factors blocks SA‐induced GID1 degradation.

Future perspectives

Over the last 30 years, SA and its interactions with other plant hormones have been extensively studied. Tremendous progress has been made in SA signaling and its roles in plant immunity. However, classic studies on the crosstalk between SA and other hormones were performed when there was limited understanding of signaling pathways of SA and other hormones and their metabolism. Some of the conclusions from these studies may need to be revisited. A number of questions such as how auxin inhibits SA signaling, how ABA affects the stability of NPR1, and how SA coordinates with ET and JA in the plant immune system awaits further investigation. Addressing these questions in the context of new knowledge on SA and other plant hormones and integrating the data into the network of hormone crosstalk are critical to have a deeper understanding of how plants balance growth and defense.

In addition, previous studies on SA and its crosstalk with other plant hormones were mostly conducted in the model plant Arabidopsis. It is important to note that some of the knowledge from Arabidopsis may not be applicable to other plant species. For example, SA is predominantly synthesized via the ICS pathway in Arabidopsis, but in plant families outside Brassicaceae, the PAL pathway might be primarily used for SA biosynthesis (Huang et al., 2020b). Furthermore, an EAR motif involved in transcriptional repression is present in the Arabidopsis SA receptors NPR3/NPR4 but not in NPR1 (Ding et al., 2018). However, NPR1 orthologs of most plant families, except Brassicaceae, also carry this EAR motif, suggesting that they may not function exactly the same way as the Arabidopsis NPR1 (Jeon et al., 2024). Nowadays, with the development of advanced clustered regularly interspaced short palindromic repeats technologies, mutants affecting hormone biosynthesis and signal transduction can be generated in many non‐model plants. With the availability of such mutants, it is important to evaluate SA signaling and its crosstalk with other hormones in representative plant species outside Brassicaceae. It will not be surprising that different plants may have separate ways to regulate SA signaling and balance its effects with other hormones, as plants are masterfully flexible in evolution and adaptation.

CONFLICTS OF INTEREST

The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS

H.T., L.X., X.L., and Y.Z. wrote the manuscript.

Biographies

Tian, H. , Xu, L. , Li, X. , and Zhang, Y. (2025). Salicylic acid: The roles in plant immunity and crosstalk with other hormones. J. Integr. Plant Biol. 67: 773–785.