Year: 2009
Type: review
Abstract: Plants constantly face a plethora of abiotic and biotic stresses in their natural habitat. Adapting to such changes requires a great degree of phenotypic plasticity that is mainly determined by the plant's genome. We currently do not know how plants are able to integrate the multitude of partly synergistic/partly antagonistic environmental signals that enable them to respond properly under any given condition. What has become apparent, however, is that plants are capable of extensive reprogramming of their transcriptome in a highly dynamic and temporal manner. This regulation in response, leading to adaptive plasticity of plants in highly variable environments, is mainly achieved by enforcement of a network of various transcription factors (TFs). WRKY TFs are a large family of regulatory proteins forming such a network (Eulgem and Somssich, 2007). They are involved in various plant processes but most notably in coping with diverse biotic and abiotic stresses. In this update, we will restrict our attention to the role of WRKY TFs in plant immunity. The WRKY TF superfamily consists of 74 and 109 members in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), respectively (Eulgem and Somssich, 2007; Ross et al., 2007). Members of this family contain at least one conserved DNA-binding region, designated the WRKY domain, comprising the highly conserved WRKYGQK peptide sequence and a zinc finger motif (CX4–7CX22–23HXH/C). This domain generally binds to the DNA element termed the W box (C/TTGACT/C), although alternative binding sites have been identified (Sun et al., 2003; Cai et al., 2008; Ciolkowski et al., 2008; van Verk et al., 2008). WRKY family members are divided into three groups based on the number of WRKY domains and certain features of the zinc finger-like motifs (Eulgem et al., 2000). The NMR solution structure revealed that the C-terminal WRKY domain of Arabidopsis WRKY4 consists of a four-stranded β-sheet, with a zinc-binding pocket formed by the conserved Cys/His residues located at one end of the β-sheet, and the WRKYGQK residues, corresponding to the most N-terminal β-strand (strand β-1), kinked in the middle of the sequence by the Gly residue (Yamasaki et al., 2005). The concave curvature of strand β-1 induced by this kink is predicted to enable this strand to deeply enter the DNA groove and make contact with bases of the W box element. The crystal structure of the extended WRKY domain of Arabidopsis WRKY1 (AtWRKY1-C) revealed that this domain is composed of a globular structure with five β-strands forming an antiparallel β-sheet with an additional novel zinc-binding site at one end (Duan et al., 2007). One should note, however, that no crystal structure information exists of a WRKY domain associated with its DNA-binding site or for a full-length WRKY protein. WRKY factors were generally regarded as being plant specific, but their identification in the protist Giardia lamblia and the slime mold Dictyostelium discoideum imply an earlier origin (Ülker and Somssich, 2004; Pan et al., 2009). They may have evolutionary links with transposons such as Mutator-like elements and could have originated from a BED finger intermediate (an atypical zinc finger DNA-binding domain found both in cellular chromatin boundary element-binding proteins BEAF and DREF and in transposases from animals), although this is controversially debated (Babu et al., 2006; Yamasaki et al., 2008). Duplicated WRKY genes have been maintained in wild and cultivated plant species in the course of selection during domestication and polyploidization (Petitot et al., 2008). They have recently been associated with the viability of interploidy hybrids (Dilkes et al., 2008). Phylogenetic sequence analysis and comparative transcriptomics have revealed that they have retained their functions between monocots and dicots (Mangelsen et al., 2008). The majority of the analyzed WRKY genes respond to pathogen attack and to the endogenous signal molecule salicylic acid (SA; Eulgem and Somssich, 2007). Plant innate immunity is composed of two interconnected branches: (1) PTI, or pathogen-associated molecular pattern (PAMP)-triggered immunity, which is initiated by the recognition of molecular signatures of many pathogens and often activates downstream mitogen-activated protein (MAP) kinase cascades and defense genes; and (2) ETI, or effector-triggered immunity, driven by plant disease resistance proteins (major R gene products) that recognize directly or indirectly specific pathogen-derived effectors (Chisholm et al., 2006). PTI and ETI activate local as well as systemic defense responses (called systemic acquired resistance [SAR]), which are modulated by phytohormones, especially jasmonic acid (JA) and SA (Durrant and Dong, 2004; Bostock, 2005). JA-dependent plant defenses are generally activated by necrotrophic pathogens and chewing insects, whereas SA-dependent defenses are often triggered by biotrophic pathogens. JA and SA signaling usually act antagonistically, but synergism between these two phytohormones has also been observed (Mur et al., 2006). These responses to pathogen attack require large-scale transcriptional reprogramming, including those of TF families such as WRKY genes (Eulgem, 2005; Ryu et al., 2006; Naoumkina et al., 2008). Loss-of-function and gain-of-function studies in Arabidopsis have been pivotal in demonstrating that WRKY factors act in a complex defense response network as both positive and negative regulators (Eulgem and Somssich, 2007). AtWRKY52 (also designated RRS1) is a novel protein comprising structural features of nucleotide binding-Leu-rich repeat-type R gene products and a WRKY domain that confers wide-ranging resistance toward the bacterial wilt Ralstonia solanacearum (Deslandes et al., 2002). The discovery that AtWRKY52 physically interacts with its cognate bacterial effector PopP2 within the plant cell nucleus (Deslandes et al., 2003) helped to stimulate subsequent research clearly demonstrating the importance of nuclear trafficking for plant immunity (Caplan et al., 2008; Liu and Coaker, 2008). AtWRKY70 acts at a convergence point determining the balance between SA- and JA-dependent defense pathways as well as being required for R gene-mediated resistance (Li et al., 2006; Knoth et al., 2007). The indispensability of AtWRKY70 for JA and SA signaling, however, has recently been questioned (Ren et al., 2008). Similarly, AtWRKY33 functions as a positive regulator of resistance toward the necrotrophic fungi Alternaria brassicicola and Botrytis cinerea (Zheng et al., 2006), and AtWRKY53 and AtWRKY70 both positively modulate SAR (Wang et al., 2006). Moreover, SA biosynthesis and expression of NONEXPRESSOR OF PR1 (NPR1), a key central regulator of SA-dependent defenses and SAR, also appear to be regulated by WRKY TFs (Yu et al., 2001). Two closely related WRKY TFs, AtWRKY3 and AtWRKY4, play a positive role in plant resistance toward necrotrophic pathogens, as Atwrky4, Atwrky3, and Atwrky3 wrky4 mutants showed increasing susceptibility toward the fungus B. cinerea, whereas overexpression of AtWRKY4 enhanced susceptibility toward the biotrophic bacterium Pseudomonas syringae (Lai et al., 2008). Many WRKY TFs act as negative regulators of defense signaling, including AtWRKY7, -11, -17, -18, -23, -25, -27, -38, -40, -41, -48, -53, -58, -60, and -62. Showing functional redundancy, Atwrky7 along with Atwrky11 and Atwrky17 mutants were susceptible to virulent P. syringae (Journot-Catalino et al., 2006; Kim et al., 2006). Similarly, AtWRKY38 and AtWRKY62 also contribute negatively to basal resistance toward this bacterial pathogen (Kim et al., 2008). AtWRKY62 expression is induced by SA and JA in a NPR1-dependent manner. How AtWRKY62 alters JA/SA signaling remains unclear, since one study has shown that loss of AtWRKY62 function resulted in enhanced expression of JA-response genes, whereas AtWRKY62 overexpressor lines inhibited JA-response gene expression (Mao et al., 2007), while in a second study, elevated transcript levels of the SA-response gene PR1 were observed in the Atwrky62 mutant, whereas WRKY62 overexpression led to suppression of PR1 (Kim et al., 2008). AtWRKY48 also negatively influences basal resistance toward virulent P. syringae (Xing et al., 2008). Reduced bacterial growth in Atwrky48 mutants was associated with increased induction of PR1, whereas AtWRKY48 overexpressors showed the opposite phenotypes. AtWRKY58 acts downstream of NPR1, negatively regulating SAR (Wang et al., 2006). Recently, knockdown of AtWRKY23 expression was shown to decrease susceptibility toward the parasitic cyst nematode Heterodera schachtii (Grunewald et al., 2008). Mutation in AtWRKY27 resulted in delayed symptom development against R. solanacearum, possibly by affecting nitric oxide signaling, and vascular trafficking (Mukhtar et al., 2008). The closely related WRKY TFs AtWRKY18, -40, and -60 have partly redundant functions in negatively regulating resistance to P. syringae (Xu et al., 2006). Interestingly, Atwrky18 wrky40 double mutants also displayed enhanced resistance to the powdery mildew pathogen Golovonomyces orontii (Shen et al., 2007). In contrast, Atwrky18 wrky40 and Atwrky18 wrky60 double mutants were more susceptible to B. cinerea (Xu et al., 2006), and AtWRKY18 alone appears also to have positive regulatory functions in SAR (Wang et al., 2006). Dual functionality in defense signaling was also observed for AtWRKY53. While Atwrky53 mutants showed delayed symptom development against R. solanacearum, such plants displayed increased susceptibility toward P. syringae (Murray et al., 2007; Hu et al., 2008). Dual functionality was also suggested for AtWRKY41. Arabidopsis plants overexpressing AtWRKY41 showed enhanced resistance toward virulent Pseudomonas but decreased resistance toward Erwinia carotovora (Higashi et al., 2008). However, Atwrky41 mutants did not display a differential phenotype. Intriguingly, expression of AtWRKY41 is specifically suppressed by a compatible strain of P. syringae in an effector-dependent manner. Finally, overexpression of AtWRKY25 resulted in increased disease symptoms to P. syringae infections, possibly by negatively regulating SA-mediated defense responses. However, Atwrky25 mutants supported normal growth of a virulent P. syringae strain (Zheng et al., 2007). Thus, as in the case for AtWRKY41, the in vivo relevance of such findings remains to be critically assessed. An increasing number of studies in other plants, particularly in rice, have strongly confirmed the importance of WRKY TFs in plant defense signaling. The rice genome contains more than 100 WRKY genes, often present in duplicated chromosomal regions, suggesting genome duplications as one of the mechanisms for the expansion of this family in this plant species (Ross et al., 2007; Ramamoorthy et al., 2008). The majority of these genes respond to (a)biotic stresses and various phytohormones (Ryu et al., 2006; Ramamoorthy et al., 2008). Individual WRKY members have been associated with pathogen defense, albeit with the caveat that the majority of such studies have employed strong ecotopic overexpressor lines. For example, overexpression of OsWRKY13 enhances resistance to the bacterial blight Xanthomonas oryzae pv oryzae (Xoo) and the fungal blast Magnaportha grisea. It exerts its function by activating SA-biosynthesis and SA-response genes while suppressing JA signaling (Qiu et al., 2007, 2008a). Similarly, OsWRKY53 overexpressor lines are more resistant to M. grisea and may act as a positive regulator of basal defense (Chujo et al., 2007). Expression of OsWRKY03 and OsWRKY71 is strongly induced by pathogen-mimicking stimuli, and these genes function upstream of OsNH1 (the rice ortholog of NPR1) in defense signaling (Liu et al., 2005, 2007). In the case of OsWRKY71, overexpressor lines display enhanced resistance to virulent Xoo (Liu et al., 2007). Ectopic expression of OsWRKY31 resulted in enhanced resistance to fungal blast, altered lateral root formation, and constitutive expression of two early auxin-response genes (Zhang et al., 2008a). Whether these two phenotypes are functionally linked remains to be determined. Moreover, corresponding OsWRKY31 RNA interference lines showed no altered disease phenotype. Enhanced resistance to M. grisea was observed with OsWRKY45 overexpressor lines but not with plants overexpressing OsWRKY19, -62, and -76 (Shimono et al., 2007). In this case, OsWRKY45 knockdown lines decreased resistance to this fungal blast. OsWRKY45 appears to act in SA signaling independent of NH1. Notably, ecotopic expression of OsWRKY45 in Arabidopsis resulted in plants with enhanced resistance to virulent P. syringae, increased PR1 expression, elevated tolerance to salt and drought stress, but decreased sensitivity toward abscisic acid signaling (Qiu and Yu, 2009). OsWRKY89 overexpression seems to positively contribute to resistance against fungal blast and the white-backed plant hopper Sogatella furcifera by regulating the wax content/deposition on the leaf surface. OsWRKY89 knockdown lines showed reduced wax content and increased susceptibility to M. grisea (Wang et al., 2007). Finally, OsWRKY62 was recently shown to be a negative regulator of both PTI and ETI. The rice gene Xa21 confers race-specific resistance to Xoo. Xa21 was shown to bind to OsWRKY62, and overexpression of one splice variant, OsWRKY62-1, compromised basal defense and Xa21-mediated resistance to Xoo and suppressed defense gene activation (Peng et al., 2008). The number of WRKY genes identified in other recently sequenced plant genomes are 66 in papaya (Carica papaya), 104 in poplar (Populus spp.), 68 in sorghum (Sorghum bicolor), and 38 in the moss Physcomitrella patens. Currently, no data exist on the role of these factors in mediating plant immunity. Some isolated studies in other plant species, however, have been reported. Overexpression of grapevine (Vitis vinifera) VvWRKY1 in tobacco (Nicotiana tabacum) rendered plants susceptible toward a variety of fungi (Marchive et al., 2007), whereas ectopic expression of grapevine VvWRKY2 resulted in enhanced resistance to the necrotrophic fungi Alternaria tenuis, B. cinerea, and Pythium (Mzid et al., 2007). Similarly, CaWRKY1 from chili pepper (Capsicum annuum) appears to act as a negative regulator of defense, as virus-induced gene silencing of this gene decreased growth of Xanthomonas, whereas its overexpression resulted in enhanced hypersensitive cell death to P. syringae and Tobacco mosaic virus (Oh et al., 2008). In barley (Hordeum vulgare), MLA confers isolate-specific resistance to the powdery mildew Blumeria graminis. MLA was shown to physically interact in the nucleus with HvWRKY1 and -2, two repressors of PAMP-triggered basal defense, thereby interfering with WRKY repressor functions and leading to resistance against the powdery mildew fungus (Shen et al., 2007). In the native tobacco Nicotiana attenuata, two WRKY genes, NaWRKY3 and -6, were identified that coordinate JA-mediated defense responses to native herbivory. Silencing of NaWRKY3, NaWRKY6, or both rendered plants highly vulnerable to Manduca sexta attack (Skibbe et al., 2008). Finally, elicitor-triggered reprogramming of secondary metabolites in Medicago truncatula seems to involve several WRKY factors: overexpression of four WRKY genes in tobacco demonstrated their regulatory roles in lignin deposition, PR gene expression, and systemic defense responses against Tobacco mosaic virus (Naoumkina et al., 2008). Overall, these findings highlight the importance of WRKY factors in transcriptionally reprogramming plant responses toward different invading pathogens (Supplemental Table S1). While some appear to positively influence the outcome of such plant-pathogen interactions, others actually appear to negatively influence it. This negative influence may be due to active targeting of the WRKY genes/factors, or products under their control, by certain pathogens. Manipulation of WRKY proteins by pathogen effectors may partly the of within the WRKY TF family as a for regulatory of and factors could also enable the and of the plant response during pathogen Some key that to be in WRKY research are as (1) How are the WRKY genes (2) which do they interact during defense signaling and during at specific gene What are the of WRKY factors within the The of research has clearly revealed that WRKY factors a complex and highly interconnected regulatory network (Eulgem and Somssich, 2007). a network to be at several The majority of the Arabidopsis WRKY genes are to and many contain W box elements within their (Eulgem and Somssich, 2007). This that several WRKY genes are under positive or negative by WRKY factors specific mechanisms in showed that a specific of W within the of its temporal expression (Eulgem et al., Moreover, chromatin analysis confirmed in vivo binding of to its as well as to the gene et al., have such a of regulation (Eulgem and Somssich, 2007; et al., 2007). transcript was shown to be on NaWRKY3 expression (Skibbe et al., 2008). Moreover, of related WRKY TFs may also be for their as by and complex of Arabidopsis -40, and -60 in response to P. syringae (Xu et al., 2006). including were identified in a that bind to functionally DNA elements within the rice OsWRKY13 et al., 2008). the AtWRKY53 led to the identification of a kinase kinase kinase Interestingly, was also shown to interact with and to AtWRKY53 et al., 2007). The in vivo relevance of these with to plant defense, however, remains to be In expression of the key defense regulator is by WRKY TFs (Yu et al., 2001). not bind DNA on its but with TFs to modulate SA-dependent genes and SAR (Durrant and Dong, Expression of at least WRKY genes, AtWRKY18, -38, -53, -58, -62, and is on NPR1, suggesting that they may be under (Wang et al., 2006; et al., 2007). In the case of and identified its to be by in an SA-dependent et al., 2006). PTI regulated kinase signaling The motif within several WRKY TFs contains sites for and several WRKY TFs have been shown to be in (Kim and 2004; et al., 2005; Eulgem and Somssich, 2007; et al., 2009). Recently, the of with AtWRKY33 and a within the plant cell nucleus was demonstrated (Qiu et al., virulent P. syringae is thereby and and of to structure and gene Interestingly, AtWRKY38 and were found to interact with a that to transcriptional (Kim et al., 2008). Overexpression of enhanced resistance to P. syringae, whereas the was compromised in These are the opposite phenotypes from studies with AtWRKY38 and -62, of WRKY network regulation in the plant basal defense response (Kim et al., 2008). have as a of regulation of gene Plant are into and interfering are and from the by genes are from in an RNA and may be as and natural of the of the transcriptome revealed that a multitude of in plants et al., 2005; et al., 2007; et al., 2008). These to gene expression in a often RNA RNA or gene The importance of in plant processes related to to (a)biotic stresses is and the endogenous have in regulating plant responses to pathogen attack et al., 2006; and 2007; 2008). induced the expression of several in rice (Liu et al., 2009). for several WRKY factors (Zhang et al., and suggesting regulation of WRKY several gene are highly in W box WRKY TFs in their et al., 2008). of a from our studies on AtWRKY18 and in responses to powdery mildew M. and Somssich, Atwrky18 wrky40 double mutants are resistant to powdery mildew and strongly the expression of suggesting suppression of along with (an gene in the and which are of a regulatory species and response et al., 2005). suppression of the by effector proteins has recently been demonstrated in Arabidopsis et al., 2008). the during reprogramming of defense responses. pathogen may be under the of WRKY at the WRKY may be regulated by RNA with other large TF identification of in vivo downstream of specific WRKY TFs is a highly genomes a of W but the relevance of these sites remains identification was to on a and on ectopic expression of the WRKY gene in of the was a and to be under in vivo and 2008). studies in identified two genes activated et al., Similarly, a gene a key of was as a of AtWRKY33 pathogen (Qiu et al., Recently, information from by we identified two key regulators of plant defense as being of during powdery mildew M. and Somssich, of studies was that the number of genes that could be was the of DNA for to or for are the to WRKY TF binding sites on a and 2008). such the remains both with the of various specific and of the of information and such in vivo be highly dynamic in both temporal and WRKY TFs are regulators of responses by They in regulating defense gene expression at various partly by directly downstream genes, by activating or other TF genes, and by regulating WRKY genes by of and regulatory Moreover, they also appear to interact with key thereby of to the WRKY WRKY factors with in the and kinase cascades key of plant defense signaling. In the majority of appear to be within the associated with transcriptional at genes et al., 2006). one that studies will additional nuclear functions of such kinase chromatin at DNA In the of WRKY TFs in the expression of several while at the their transcription is possibly partly under to the regulatory that be to we to to the of proteins associated with WRKY TFs at specific DNA and such as of isolated chromatin and with are to that identification of TFs and associated proteins in vivo at given may become in the and et al., 2009). The WRKY transcriptional network may the balance to respond and to pathogens but at the to restrict defense responses that be for plant growth and of how WRKY TFs to these functions will be in the by the to specific WRKY TF with on a This will to how WRKY factors influence diverse pathways and cellular the they will also with information on and how pathogens on this network to defenses make of for their specific The are in the of this Table Plant WRKY genes in plant immunity.
Source: PLANT PHYSIOLOGY
Authors: Shree P. Pandey, Imre E. Somssich
Institution: Max Planck Institute for Plant Breeding Research
Language: English
Subfield: Plant Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: bronze
OpenAlex ID: W2108652954