Plant disease resistance (R) genes encode proteins that both determine recognition of specific pathogen-derived avirulence (Avr) proteins and initiate signal transduction pathways leading to complex defense responses. Recent developments suggest that recognition specificity of R proteins is determined by either a protein kinase domain or by a region consisting of leucine-rich repeats. Rgenes conferring resistance to bacterial, viral, and fungal pathogens appear to use multiple signaling pathways, some of which involve distinct proteins and others which converge upon common downstream effectors. Manipulation of Rgenes and their signaling pathways by transgenic expression is a promising strategy to improve disease resistance in plants.
Addresses
Boyce Thompson Institute for Plant Research, and Department of Plant Pathology, Cornell University, Tower Road, Ithaca, NY 14853-1801, USA; e-mail: gbm7@cornell.edu
Current Opinion in Plant Biology1999, 2:273–279 http://biomednet.com/elecref/1369526600200273 © Elsevier Science Ltd ISSN 1369-5266
Abbreviations Avr avirulence
HR hypersensitive response
IL-1R interleukin-1 receptor
JA jasmonic acid
LRR leucine-rich repeat
LZ leucine zipper
MAPK mitogen-activated protein kinase
NBS nucleotide binding site
PR pathogenesis-related
R resistance
SA salicylic acid
SIPK salicylic acid induced protein kinase
TIR Toll/Interleukin-1 receptor
WIPK wound-induced protein kinase
Introduction
Disease resistance (R) genes in plants often determine the recognition of specific pathogens that express a corre-sponding avirulence (avr) gene. Over 20 R genes with recognition-specificity for defined avr genes have been isolated from seven plant species, including both monocots and dicots. These genes are effective against bacterial, viral, and fungal pathogens and in one interesting case, the Migene from tomato, against both nematodes and an aphid species [1–3]. There is great diversity in the lifestyles and pathogenic mechanisms of disease-causing organisms and it was, therefore, somewhat surprising that Rgenes were found to encode proteins with certain common motifs. Five classes of Rgenes are now recognized: intracellular protein kinases; receptor-like protein kinases with an extracellular leucine-rich repeat (LRR) domain; intracellu-lar LRR proteins with a nucleotide binding site (NBS) and
a leucine zipper (LZ) motif; intracellular NBS–LRR pro-teins with a region with similarity to the Toll and interleukin-1 receptor (TIR) proteins; and LRR proteins that encode membrane-bound extracellular proteins.
Despite these significant insights into R gene structure, much remains to be elucidated about the molecular mech-anisms by which R proteins recognize pathogens and transduce this information in the plant cell to initiate defense responses. For example, although R proteins were anticipated to play a direct role in recognizing pathogen-derived ligands, this only has been reported for one R protein [4,5]. Similarly, the cloning of a protein kinase Rgene, Pto, and an LRR gene, Prf, from the same disease resistance pathway indicated that members of these two classes might represent different steps in a sig-naling pathway common to other Rgene pathways [6,7]. No clear evidence has emerged, however, to enable the placement of these two classes of proteins in a single path-way. Finally, the shared motifs among R proteins suggested that they might utilize common downstream signaling components to activate similar defense respons-es. Evidence both for and against such convergence has been reported in the past year. In this review, I examine recent papers that are beginning to shed light on these and other central questions regarding the role of Rgenes in initiating disease resistance.
Recognition specificity of R proteins
A hallmark of R genes is their recognition-specificity for corresponding avr genes. Because of the extracellular life style of most plant pathogens, it was expected that R pro-teins would encode extracellular receptor-like propro-teins. This might be the case for some R proteins (e.g., Xa21, Cf proteins); however, in the majority of cases Rgenes appear to encode intracellular proteins. The discovery of bacterial phytopathogenic type III secretion systems that would allow direct injection of pathogen proteins into the plant cell suggested that recognition might occur inside the plant cell. Transient expression of many Avr proteins inside plant cells has confirmed this speculation (e.g., [8]). Do R proteins then serve as primary receptors for Avr pro-teins? If so, what features of the R protein mediate this recognition? Because LRRs are known to play a role in protein–protein interactions there has been much specula-tion that these regions are directly involved in recognispecula-tion of Avr proteins. In support of this view, LRRs have been shown to have considerable variation among members within clustered gene families and there is now extensive genome sequence data to indicate that unequal crossing over, gene conversion, and divergent selection on genes within families has occurred [9–14]. In a few cases, it is known that this variation in LRRs directly correlates
Functional analysis of plant disease resistance genes and their
downstream effectors
with new recognition specificities [15,16••]. In addition, a truncated member of the Xa21 family that contains the LRRs but not the kinase domain still confers resistance to specific avirulent strains of Xanthomonas[17•]. Still, these observations are only indirect evidence for the role of LRRs in Avr protein binding. Recently, the product of the rice blast Rgene Pi-tahas been shown to interact with the AVR-Pita protein in the yeast two-hybrid system (B Valent, personal communication). However, it is unclear whether this binding directly involves an LRR region. Thus, despite a large amount of supporting evidence a role of LRRs in recognition remains unproven.
Evidence for the direct interaction of an R and Avr protein has come from work with the Pto–avrPtosystem. Pto con-fers resistance to Pseudomonas syringaestrains that express the AvrPto protein, and encodes a serine–threonine pro-tein kinase that lacks any obvious receptor-like domain. Nevertheless, two laboratories have demonstrated that Pto interacts specifically with the AvrPto protein in the yeast two-hybrid system and that mutations which disrupt this interaction lead to loss of recognition in the plant cell [4,5]. Pto recognition specificity for AvrPto was initially found to require a small region in the activation loop of the kinase and recent results demonstrate that a threonine at position 204 plays a key role in the interaction with AvrPto [18•]. Interestingly a threonine at this position is conserved in a large number of protein kinases and it has been proposed that phosphorylation of this residue might lead to a confor-mational change of the kinase which allows AvrPto binding. Despite this appealing explanation for recogni-tion specificity in the Pto–AvrPtosystem, many questions remain about how such binding might affect the activity of the Pto kinase and other components of the pathway.
One hypothesis for how the Pto–AvrPto interaction stimu-lates disease resistance derives from the recent report that overexpression of Pto activates a variety of defense responses targeted against diverse pathogens [19••]. The authors speculate that basal Pto kinase activity might maintain a low level of pathogen defense responses even when AvrPto is not present, and that an increase in Pto abundance simply increases these responses [19••]. In a further extension of this model it is possible that interac-tion with AvrPto might increase Pto kinase activity or it might reduce degradation of Pto and thereby increase its abundance in the plant cell under attack by the avirulent pathogen. In this scenario the Prf, an LRR protein that is required for Pto function, would play a role downstream and not in recognition. Another intriguing suggestion has been made that AvrPto protein, acting as a virulence factor, might have evolved to bind and hence disrupt Pto activity and that Prf evolved later to recognize the Pto–AvrPto complex [20]. In this model, formation of the Pto–AvrPto-Prf complex initiates active resistance against the Pseudomonaspathogen [21]. However, there have been no reports to date that Prf interacts with AvrPto, Pto, or the AvrPto–Pto complex. Nevertheless, further speculation on
the basis of this model might suggest that formation of a Pto–AvrPto–Prf complex directs Pto kinase activity to sub-strates involved in resistance to Pseudomonas, or simply stimulates activity towards substrates that Pto phosphory-lates even in the absence of AvrPto. Several proteins have been identified that interact with Pto in the absence of AvrPto and two of these have been found to be phospho-rylated by the kinase ([22–24]; Y Gu and G Martin, unpublished data). Additional biochemical experiments will be required to examine whether AvrPto or Prf affect Pto phosphorylation activity.
In light of the paucity of data about the function of Prf (and other LRR proteins) several other possibilities are still ten-able. Among these are a role for Prf in the transfer of AvrPto into the plant cell, a role for Prf (either alone or in concert with other proteins) in localizing Pto in the plant cell, and a role as a component in downstream signaling leading to activation of resistance against P. syringae and perhaps other pathogens. This latter role is consistent with the recent demonstration that overexpression of Prf in tomato leads to constitutive activation of some defense responses associated with systemic acquired resistance but not to the hypersensitive response (HR) [25•]. A down-stream role for LRR genes is also supported by the recent report that a mutation in the RPS5bacterial LRR Rgene suppressed resistance to several avirulent Pseudomonas strains and to strains of the downy mildew fungus ([26••]; see below).
Localization of R proteins in the plant cell
If R proteins are directly involved in recognition of Avr proteins, then it is possible they are localized to the plasma membrane to intercept the incoming pathogen protein. Unfortunately, there is little information to date about the localization of most R proteins in the plant cell. In the best studied case, using an epitope-tagging strategy, the RPM1 protein (of the LZ–NBS–LRR class) was found to be enriched in the plasma membrane fraction and was remov-able by treatments that release peripherally-associated membrane proteins [27••]. This localization contrasts with that predicted by the analysis of RPM1 structural motifs and points to the need in this field for similar cell biology studies of other R proteins. A plasma-membrane localiza-tion for the RPM1 protein supports a direct role in recognition for RPM1 although interaction of RPM1 with either of its corresponding Avr proteins, AvrB and AvrRpm1, has not been observed (J Dangl, personal com-munication). The lack of a signal peptide or transmembrane domain in RPM1, similar to R proteins in several other classes, suggests that these proteins might be tethered to membranes viaan interaction with other pro-teins. Interestingly, a two-hybrid screen has uncovered an RPM1-interacting protein that is a good candidate for ful-filling such a role (J Dangl, personal communication).proteins are expressed in the plant cell and also after expression of other Avr proteins recognized by different Rgenes that were present in the plant [27••]. This intrigu-ing result suggests that protein degradation pathways might play a significant previously unanticipated role in limiting cellular damage during the HR. Protein degrada-tion mediated by proteosomes is a well-established mechanism to regulate signaling pathways in mammals and its role in plant hormone signal transduction has been reviewed recently [28]. Although proteosomes have not been directly implicated in Rgene signaling, a protein that interacts with the Pto kinase in the yeast two-hybrid sys-tem shares significant similarity with the alpha subunit of the human proteosome [29].
Additional evidence, albeit indirect, for R protein localiza-tion has been reported for the Pto kinase whose amino terminus contains a sequence (MGSKYSK) that shares similarity with a consensus myristylation motif [30]. In mammalian systems myristylation plays important roles in the subcellular localization of various kinases and phos-phatases by targeting them to cellular membranes. An early report indicated that the myristylation motif of the Fen kinase, which is closely related to Pto, is required for its function in a transient assay [31]. However, site-directed mutagenesis of the critical glycine residue in this motif did not affect the ability of the Pto kinase to confer resistance to avrPto-expressing Pseudomonas in stable transgenic plants of tomato [30]. Although Pto was expressed in these experiments viathe constitutive CaMV 35S promoter, nei-ther RNA nor protein blots revealed detectable levels of Pto in the transgenic plants. Thus it seems unlikely that overexpression obscured an effect of loss of the myristyla-tion motif in Pto. Recently, cell fracmyristyla-tionamyristyla-tion studies using a Pto-specific antibody have found that Pto is present in the soluble fraction (X He, G Sessa, and G Martin, unpub-lished data). Thus, it appears likely that Pto is localized to the cytoplasm.
Involvement of R proteins in signal transduction
There have been significant developments in our under-standing of signaling steps that lie downstream of Rgenes although much still remains to be learned. The similarity of R proteins to components of the Drosophila Toll and mammalian IL-1R (interleukin-1 receptor) pathways has been previously noted [32•], and the complexity of these pathways is perhaps a harbinger of what is to be discovered downstream of R proteins. Despite the structural similari-ties between the TIR-like R proteins and Toll/IL-1R, and between Pto and the IL-1R-associated kinase (IRAK) and the Drosophila kinase Pelle, it still remains to be deter-mined if mechanistic similarities exist. It is worth noting that extensive searches of publicly available plant expressed sequence tag databases have not uncovered genes with obvious similarities to mammalian genes encoding MyD88, IL-1R accessory protein, TRAF6, NF-κB and other components downstream of the IL-1 receptor (M D’Ascenzo and G Martin, unpublished data),and it seems likely that plants have evolved several unique proteins to fulfill signaling roles downstream of Rgenes. Nevertheless, as in mammalian systems, a role for reactive oxygen intermediates and nitric oxide in plant defense sig-naling has been established and there is now evidence in plants for other mammalian-like second messengers of nitric oxide signaling including cyclic GMP and cyclic ADP-ribose [33,34]. The role of nitric oxide and other sig-naling molecules is reviewed in another article in this issue (see P Bolwell this issue, pp 287–294). In addition, in com-mon with IL-1R signaling, mitogen-activated protein kinases (MAPKs) have been identified that are activated by R gene pathways ([35•,36•], see below). Here, I will focus on signaling events that are being revealed by the functional analysis of R genes and by recently isolated genes that appear to function downstream of Rgenes.
Role of the NBS–LRR motifs in signaling
With the exception of the Xa21 protein, which contains a protein kinase domain, the primary amino acid sequence of other LRR–NBS R proteins does not suggest an obvious mechanism for the initiation of signal transduction. The most thought-provoking proposition to be put forth [20] about the function of NBS-like R proteins is based on the noted similarity between them and the CED-4 and APAF-1 proteins, which activate proteases involved in mammalian apoptosis [37]. Heterodimerization between CED-4 and Apaf-1 and their respective proteases, CED-3 and caspase-9, results from the interaction between homol-ogous domains present in the amino terminal portions of the proteins [37]. By analogy, it has been proposed that NBS–LRR proteins might rely on their TIR or LZ regions to form heterodimers with downstream proteins [20]. In this model, the LRRs confer recognition specificity, whereas the NBS serves to activate downstream effectors.
Indirect, but significant, evidence that LRRs play a direct role in downstream signaling comes from studies of the R gene RPS5, where a mutation affecting the third LRR was found to suppress resistance conferred by multi-ple R genes [26••]. Interestingly, overexpression of the wild-type RPS5allele in the mutant background failed to completely restore resistance mediated by several of these Rgenes. On the basis of these results, it was proposed that the LRR mutation might increase binding to a pathway component shared by multiple Rgenes and interfere with essential signaling. This is the first compelling evidence that LRRs might perform a downstream signaling func-tion. Curiously, a mutation in another carboxy-terminal LRR specifically affected recognition of the corresponding AvrPphB protein, but not resistance mediated by other R genes [26••]. Thus, the LRR region of RPS5 protein appears to function in both recognition and signaling simi-lar to the protein kinase domain of Pto.
Signaling via the Pto kinase
signal transduction viaprotein phosphorylation. Both Xa21 and Pto are active protein kinases (P Ronald, personal communication; [24]) and, at least for Pto, kinase activity appears to be required for its role in disease resistance. In in vitroexperiments, Pto was shown to undergo intramole-cular autophosphorylation on several sites [24]. By analogy with similar work with mammalian kinases, it is likely that at least some of these phosphorylated sites are important for Pto function in vivo. Identification of the phosphorylat-ed residues is underway and will eventually lead to an examination of their role in disease resistance (G Sessa, M D’Ascenzo, and G Martin, unpublished data).
Pto interacts with several plant proteins in the yeast two-hybrid system and to date two of these, Pti1 (a serine–threonine kinase) and Pti4 (a DNA binding pro-tein) have been found to be specific phosphorylation substrates for Pto ([22]; Y Gu and G Martin, unpublished data). Phosphorylation of Pti1 by Pto occurs on one major site and several minor sites. The major site, interestingly, is the same site that is autophosphorylated by Pti1 [24]. The possible role(s) of these phosphorylated residues in disease resistance is being examined by site-directed mutagenesis and transgenic analysis (G Sessa and G Martin, unpublished data). Previous work has indicated that Pti1 is involved in development of the HR [22]. However, Pti1is a member of a gene family and antisense (loss-of-function) experiments to examine its role have been inconclusive (J Zhou, personal communication). Thus, one of the best approaches to determine a role for Pti1 might be the development of constitutive-active pro-teins by altering key phosphorylation sites.
The Pti4/5/6 proteins resemble the ethylene response ele-ment binding proteins (EREBPs) originally isolated from tobacco [23]. Like the EREBPs, Pti4/5/6 bind the GCC box ciselement that is present in the promoter of many patho-genesis-related (PR) genes and other genes that are often expressed in plants upon exposure to ethylene or pathogens. Thus, the identification of the Pti4/5/6genes defines the first direct link between an R gene and expression of defense-related genes. As expected for transcription factors, each of the Pti4/5/6 proteins contains putative nuclear-localization sequences and the proteins are localized to the nucleus (Y-T Loh, Y Gu, and G Martin, unpublished data). Using purified proteins, Pti4 has been found to be phosphorylated in vitroby Pto on at least four sites. Experiments are under-way to examine the possible role of these phosphorylation events in nuclear localization, binding to the GCC box, or in transactivation of target gene expression (Y Gu and G Martin, unpublished data). Although the Pti4/5/6 proteins share sequence similarity they also contain highly divergent regions [23]. In addition, the Pti4/5/6genes are regulated dif-ferently in response to ethylene, jasmonic acid (JA), salicylic acid (SA), wounding, and pathogens (Y Gu and G Martin, unpublished data; KV Thara and J Zhou, personal commu-nication). Thus, it seems possible that their proteins serve to integrate different signals that induce common target genes.
Characterization of the Pti1/4/5/6genes suggests that mul-tiple pathways diverge directly from the Pto kinase. Because Pti1overexpression accelerates the HR, we have proposed that one pathway leads to the HR and perhaps to other defense responses [22]. Additional, distinct pathways appear to be mediated by the interaction of Pto with the putative transcription factors Pti4/5/6 [23]. The Pti1/4/5/6 genes were identified by a yeast two-hybrid screen and to date no mutations in these genes have been identified in tomato. Thus additional evidence for their role in disease resistance is being sought by transgenic analyses and bio-chemical approaches (G Sessa, Y Gu and G Martin, unpublished data).
Restricted conservation of signaling pathways
In light of the shared motifs among R proteins from very diverse plant species it might be expected that down-stream signaling pathways contain similar, or at least compatible, components. If this is the case, then Rgenes isolated from one species might be used to engineer new disease resistance specificities into other economically important species. Rgenes isolated from tomato have been previously shown to function in tobacco and vice versa [38,39]. Recently, the Cf-9 gene from tomato has been shown to function in both potato and tobacco [40]. However, there have been no reports of Arabidopsis Rgenes functioning in Solanaceous species or vice versa, or any other successful transfers outside of a single plant fam-ily. Whether this restricted cross-taxa function is due to the absence or incompatibility of other essential recognition or signaling components remains to be determined.Signaling components acting downstream of
R
genes
led to a significant development in our understanding of downstream pathways mediated by these genes.
Arabidopsisplants with mutations at the ndr1and eds1loci are affected in resistance to both bacterial and fungal pathogens mediated by several different R genes [47••]. Mutations at the eds1 locus lead to loss of resistance to Pseudomonas mediated by the R gene RPS4 and to Peronospora parasiticamediated by the RPP2/4/5/21 genes. Mutations at the ndr1 locus suppress resistance to Pseudomonas mediated by the Rgenes RPS2, RPM1, and RPS5 [47••]. In addition, resistance to P. parasitica con-ferred by the RPP4 and RPP5 genes is partially compromised in ndr1 mutants suggesting some level of cross-talk involving both NDR1 and EDS1. The signifi-cance of these observations is that the Rgenes suppressed by the eds1 mutation are of the TIR–NBS–LRR class, whereas those suppressed by ndr1 are of the LZ–NBS–LRR class. This exciting development thus defines two distinct pathways that lie downstream of these different Rgene classes and provide important new tools to understand the role of the TIR and LZ domains in defense signaling. The NDR1gene was cloned previously to these developments and found to encode a novel pro-tein whose features indicate it might be associated with a membrane; its role remains unclear [48].
Isolation of the EDS1 gene by transposon tagging was reported recently [49••]. EDS1encodes a protein whose amino terminus has similarity to eukaryotic lipases, although the role of this possible activity in disease resis-tance must still be resolved. EDS1 is expressed in healthy tissue and its transcript abundance increases 2–3 fold in response to SA or an avirulent bacterial pathogen. Interestingly, however, exogenous application of JA, which is derived from fatty acid intermediates, does not induce EDS1and is not able to ameliorate the loss of dis-ease resistance in the eds1 mutant [49••]. Despite the lack of response to JA, it remains possible that the EDS1 protein plays a role in modifying an intermediate involved in JA production. Alternatively, EDS1 might participate in another unknown signaling pathway involving lipid intermediates. An understanding of the role of the EDS1 protein will require biochemical analy-sis of its function, determination of its substrates, and perhaps identification of additional proteins that are involved in its activity.
Role of MAPKs in
R
gene signaling
A key role for protein phosphorylation in Rgene signaling is clearly implicated by the isolation of the Pto, Xa21, and Pti1 kinases. However, there are relatively few studies that have examined protein phosphorylation events further downstream of Rgenes. Two recent studies now provide compelling evidence for the activation of MAPKs by Rgene pathways and, in addition, suggest that these kinas-es are convergence points for many different defense responses in plants [35•,36•].
In tobacco, two previously identified MAPKs, a 48 kD sal-icylic acid induced protein kinase (SIPK) and a 44 kD wound-induced protein kinase (WIPK), were found to be activated during infection by tobacco mosaic virus [35•]). Activation of WIPK occurred only in tobacco plants con-taining the tobacco mosaic virus Rgene Nindicating that WIPK might play a role in gene-for-gene mediated defense against virus infection. Activation of WIPK required both serine–threonine and tyrosine phosphoryla-tion and also an increase in its mRNA and protein levels. Although many defense responses mediated by the Ngene are dependent on SA, the activation of WIPK was not affected in plants lacking this signaling molecule [35•].
Additional evidence that SIPK and WIPK lie downstream of Rgenes comes from studies with the Cf-9R gene and its corresponding gene Avr9. Treatment of tobacco suspen-sion cells expressing Cf-9with the Avr9 peptide resulted in rapid activation of both SIPK and WIPK [36•]. Similarly to the observations with the Ngene system, the WIPK tran-script was found to increase in an Avr9-dependent fashion in Cf-9-containing tobacco cells. Activation of WIPK and SIPK was not perturbed by inhibitors of active oxygen species synthesis. Thus, similarly to the SA-independent pathway seen with the Ngene system, the activation of WIPK and SIPK by Cf-9appears to be viaa parallel path-way that is independent of at least some other signaling pathways downstream of Cf-9. Because WIPK and SIPK are known to be activated by many stimuli including non-specific elicitors, wounding, and SA, these reports indicate that some R gene pathways converge on components shared with general stress-responsive pathways. Despite the clear involvement of the Nand Cf-9genes in activating MAPKs, it remains to be determined whether these kinas-es are directly involved in disease rkinas-esistance and, if so, what role they play.
Potential for manipulation of
R
gene signaling
components
One rationale for studying Rgenes and their downstream signaling components is that it might be possible to manip-ulate expression of these genes in economically important plant species in order to improve disease resistance. Although the field is still very much in its infancy, there already have been several promising reports to indicate such a strategy is feasible. Overexpression of Prfin tomato by integration of several copies of the gene under control of its own promoter led to increased levels of SA and PR genes and to increased resistance to three bacterial and one viral pathogen [25•]. Unlike plant mutants that lead to constitutive systemic acquired resistance, the overexpres-sion of Prfcaused no detrimental effects on plant growth or fruit production.
The transgenic lines displayed increased resistance to a Pseudomonasstrain lacking avrPto, to virulent Xanthomonas strains, and to the fungal pathogen Cladosporium fulvum. By using a different approach, Ptohas been shown to also limit the systemic spread of potato virus X in tomato if the virus is engineered to express the avrPto gene [50]. Thus defense responses activated by Pto are effective against bacterial, viral, and fungal pathogens. It remains to be determined if broad-spectrum resistance mediated by Pto (or Prf) will be effective under field conditions.
Overexpression of signaling components that lie down-stream of Rgenes is another possible strategy to increase disease resistance. In the first successful example of this approach, expression of the NPR1gene in Arabidopsiswas manipulated to increase the level of its protein to 1.5- to 3-fold more than in wild-type plants [51•]. Plants with moderately increased levels of NPR1 protein exhibited significant increases in resistance to Pseudomonas and Peronospora pathogens. Defense-related genes in NPR1 -overexpressing plants were found to be expressed more strongly but not more rapidly during pathogen infection [51•]. Manipulation of downstream components such as NPR1potentially allows activation of only certain defense pathways. This might avoid agronomic problems associat-ed with constitutive activation of some Rgene-mediated pathways such as those leading to the HR and could lead to a better understanding of the contributions of specific pathways to disease resistance.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest ••of outstanding interest
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25. Oldroyd GED, Staskawicz BJ: Genetically engineered broad
• spectrum disease resistance in tomato.Proc Natl Acad Sci USA 1998, 95:10300-10305.
The first report that overexpression of an LRR/NBS-like gene can activate various defense responses and confer broadened resistance. Because Prf-overexpressing plants did not exhibit an HR (unlike overexpressed Pto [19••]), the authors suggest that engineering components potentially down-stream of Rgenes might cause fewer detrimental effects to the plant.
26. Warren RF, Henk A, Mowery P, Holub E, Innes RW: A mutation
•• within the leucine-rich repeat domain of the Arabidopsisdisease resistance gene RPS5partially suppresses multiple bacterial and downy mildew resistance genes.Plant Cell1998, 10:1439-1452. A well-written paper that reports the cloning and functional analysis of the NBS–LRR-like gene RPS5. Careful plant pathology work and genetic analy-sis suggested that LRRs play a role in both recognition and signaling. Represents a first significant insight into the possible function of LRRs.
27. Boyes DC, Nam J, Dangl JL: The Arabidopsis thaliana RPM1
•• disease resistance gene product is a peripheral plasma membrane protein that is degraded coincident with the hypersensitive response.Proc Natl Acad Sci USA1998,
95:15849-15854.
Groundbreaking research on the localization of an LZ–NBS–LRR protein. The observation that RPM1 is rapidly degraded upon pathogen recognition suggests a mechanism for the plant cell to tightly regulate the HR and high-lights the need for additional cell biology research in the disease resistance field.
28. del Pozo JC, Estelle M: Function of the ubiquitin-proteosome pathway in auxin response.Trends Plant Sci1999, 4:107-112. 29. Zhou J, Tang X, Frederick R, Martin G: Pathogen recognition and
signal transduction by the Pto kinase.J Plant Sci Res1998,
111:353-356.
30. Loh YT, Zhou J, Martin GB: The myristylation motif of Pto is not required for disease resistance.Mol Plant–Microbe Interact1998,
11:572-576.
31. Rommens CMT, Salmeron JM, Baulcombe DC, Staskawicz BJ: Use of a gene expression system based on potato virus X to rapidly identify and characterize a tomato Ptohomolog that controls fenthion sensitivity.Plant Cell1995, 7:249-257.
32. O’Neill LA, Greene C: Signal transduction pathways activated by
• the IL-1 receptor family: ancient signaling machinery in mammals, insects, and plants.J Leukoc Biol1998, 63:650-657.
An excellent review from a ‘mammalian’ perspective about the conservation of TIR-like signaling components among divergent taxa.
33. Delledonne M, Xia Y, Dixon RA, Lamb C: Nitric oxide functions as a signal in plant disease resistance.Nature1998, 394:585-588. 34. Durner J, Wendehenne D, Klessig DF: Defense gene induction in
tobacco by nitric oxide, cyclic GMP, and cyclic ADP-ribose.Proc Natl Acad Sci USA1998, 95:10328-10333.
35. Zhang S, Klessig DF: Resistance gene N-mediatedde novo
• synthesis and activation of a tobacco mitogen-activated protein kinase by tobacco mosaic virus infection.Proc Natl Acad Sci USA 1998, 95:7433-7438.
The first report that MAPKs are activated by an Rgene-mediated signaling pathway. Because the corresponding MAPK genes are cloned, this study opens the way to elucidating their possible role in disease resistance.
36. Romeis T, Piedras P, Zhang S, Klessig DF, Hirt H, Jones JD: Rapid avr
• 9- and Cf-9-dependent activation of MAP kinases in tobacco cell cultures and leaves: convergence of resistance gene, elicitor, wound, and salicylate responses.Plant Cell1999, 11:273-288. Nicely extends the work reported in [35•] to show that multiple pathways
including one initiated by the Cf-9gene converge upon two MAPKs. The Cf-9pathway which activates the MAPKs is distinct from one initiating produc-tion of active oxygen species thus supporting the observaproduc-tion that parallel pathways lie downstream of Rgenes.
37. Chinnaiyan AM, Chaudhary D, O’Rourke K, Koonin EV, Dixit VM: Role of CED-4 in the activation of CED-3.Nature1997, 388:728-729. 38. Thilmony RT, Chen Z, Bressan RA, Martin GB: Expression of the
tomato Ptogene in tobacco enhances resistance to
Pseudomonas syringaepv. tabaci expressing avrPto.Plant Cell 1995, 7:1529-1536.
39. Whitham S, McCormick S, Baker B: The Ngene of tobacco confers resistance to tobacco mosaic virus in transgenic tomato.Proc Natl Acad Sci USA1996, 93:8776-8781.
40. Hammond-Kosack KE, Tang S, Harrison K, Jones JD: The tomato
Cf-9disease resistance gene functions in tobacco and potato to confer responsiveness to the fungal avirulence gene product Avr 9.Plant Cell1998, 10:1251-1266.
41. Innes RW: Genetic dissection of R gene signal transduction pathways.Curr Opin Plant Biol1998, 1:299-304.
42. Ryals J, Weymann K, Lawton K, Friedrich L, Ellis D, Steiner HY, Johnson J, Delaney TP, Jesse T, Vos P, Uknes S: The Arabidopsis
NIM1 protein shows homology to the mammalian transcription factor inhibitor I kappa B.Plant Cell1997, 9:425-439.
43. Cao H, Glazebrook J, Clarke JD, Volko S, Dong X: The Arabidopsis NPR1gene that controls systemic acquired resistance encodes a novel protein containing ankyrin repeats.Cell1997, 88:57-63. 44. Volko SM, Boller T, Ausubel FM: Isolation of new Arabidopsis
mutants with enhanced disease susceptibility to Pseudomonas syringaeby direct screening.Genetics1998, 149:537-548. 45. Zhou N, Tootle TL, Tsui F, Klessig DF, Glazebrook J: PAD4 functions
upstream from salicylic acid to control defense responses in
Arabidopsis.Plant Cell1998, 10:1021-1030.
46. Yu IC, Parker J, Bent AF: Gene-for-gene disease resistance without the hypersensitive response in Arabidopsis dnd1mutant.Proc Natl Acad Sci USA1998, 95:7819-7824.
47. Aarts N, Metz M, Holub E, Staskawicz BJ, Daniels MJ, Parker JE:
•• Different requirements for EDS1 and NDR1 by disease resistance genes define at least two R gene-mediated signaling pathways in
Arabidopsis.Proc Natl Acad Sci USA1998, 95:10306-10311. Represents a major breakthrough in our understanding of Rgene signaling by demonstrating the existence of distinct pathways downstream of different classes of Rgenes. The eds1and ndr1mutants will be broadly useful for delineating signaling pathways mediated by Arabidopsis Rgenes and other defense-related genes.
48. Century KS, Shapiro AD, Repetti PP, Dahlbeck D, Holub E, Staskawicz BJ: NDR1, a pathogen-induced component required for Arabidopsisdisease resistance.Science1997, 278:1963-1965. 49. Falk A, Feys BJ, Frost LN, Jones JDG, Daniels MJ, Parker JE: EDS1,
•• an essential component of R gene-mediated disease resistance in Arabidopsishas homology to eukaryotic lipases.Proc Natl Acad Sci USA1999, 96:3292-3297.
The isolation of EDS1(and NDR1in 1997 [48]) are major steps towards understanding downstream signaling. However, the uncertain role of either of these proteins reinforces the need for innovative biochemical analysis in the disease resistance field.
50. Tobias CM, Oldroyd GE, Chang JH, Staskawicz BJ: Plants expressing the Ptodisease resistance gene confer resistance to recombinant PVX containing the avirulence gene avrPto.Plant J 1999, 17:41-50.
51. Cao H, Li X, Dong X: Generation of broad-spectrum disease
• resistance by overexpression of an essential regulatory gene in systemic acquired resistance.Proc Natl Acad Sci USA1998,
95:6531-6536.
