AefR, a TetR Family Transcriptional Repressor, Regulates Several Auxin Responses in Pseudomonas syringae Strain PtoDC3000.

The plant hormone indole-3-acetic acid (IAA), also known as auxin, plays important roles in plant growth and development, as well as in several plant-microbe interactions. IAA also acts as a microbial signal and in many bacteria regulates metabolism, stress responses, and virulence. In the bacterial plant pathogen Pseudomonas syringae pv. tomato strain DC3000 (PtoDC3000), exposure to IAA results in large-scale transcriptional reprogramming, including the differential expression of several known virulence genes. However, how PtoDC3000 senses and responds to IAA and what aspects of its biology are regulated by IAA is not understood. To investigate the mechanisms involved in perceiving and responding to IAA, we carried out a genetic screen for mutants with altered responses to IAA. One group of mutants of particular interest carried disruptions in the aefR gene encoding a TetR family transcriptional regulator. Gene expression analysis confirmed that the aefR mutants have altered responses to IAA. Thus, AefR is the first demonstrated auxin response regulator in PtoDC3000. We also investigated several aspects of PtoDC3000 biology that are regulated by both AefR and IAA, including antibiotic resistance, motility, and virulence. The observation that the aefR mutant has altered virulence on Arabidopsis, suggests that the sector of the IAA response regulated by aefR is important during pathogenesis. Our findings also provide evidence that AefR plays a role in coordinating changes in gene expression during the transition from early to late stages of infection. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Identification of Indole-3-Acetic Acid-Regulated Genes in Pseudomonas syringae pv. tomato Strain DC3000.

The auxin indole-3-acetic acid (IAA) is a plant hormone that not only regulates plant growth and development but also plays important roles in plant-microbe interactions. We previously reported that IAA alters expression of several virulence-related genes in the plant pathogen Pseudomonas syringae pv. tomato strain DC3000 (PtoDC3000). To learn more about the impact of IAA on regulation of PtoDC3000 gene expression, we performed a global transcriptomic analysis of bacteria grown in culture, in the presence or absence of exogenous IAA. We observed that IAA repressed expression of genes involved in the type III secretion (T3S) system and motility and promoted expression of several known and putative transcriptional regulators. Several of these regulators are orthologs of factors known to regulate stress responses and accordingly expression of several stress response-related genes was also upregulated by IAA. Similar trends in expression for several genes were also observed by quantitative reverse transcription PCR. Using an Arabidopsis thaliana auxin receptor mutant that accumulates elevated auxin, we found that many of the P. syringae genes regulated by IAA in vitro were also regulated by auxin in planta. Collectively the data indicate that IAA modulates many aspects of PtoDC3000 biology, presumably to promote both virulence and survival under stressful conditions, including those encountered in or on plant leaves. IMPORTANCE Indole-3-acetic acid (IAA), a form of the plant hormone auxin, is used by many plant-associated bacteria as a cue to sense the plant environment. Previously, we showed that IAA can promote disease in interactions between the plant pathogen Pseudomonas syringae strain PtoDC000 and one of its hosts, Arabidopsis thaliana. However, the mechanisms by which IAA impacts the biology of PtoDC3000 and promotes disease are not well understood. Here, we demonstrate that IAA is a signal molecule that regulates gene expression in PtoDC3000. The presence of exogenous IAA affects expression of over 700 genes in the bacteria, including genes involved in type III secretion and genes involved in stress response. This work offers insight into the roles of auxin-promoting pathogenesis.

Jasmonate Hypersensitive 3 negatively regulates both jasmonate and ethylene-mediated responses in Arabidopsis.

Jasmonate (JA) is an important hormone involved in regulating diverse responses to environmental factors as well as growth and development, and its signalling is influenced by other hormones such as ethylene (ET). However, our understanding of the regulatory relationship between the JA and ET signalling pathways is limited. In this study, we isolated an Arabidopsis JA-hypersensitive mutant, jah3 (jasmonate hypersensitive3)-1. Map-based cloning revealed that the JAH3 gene corresponds to At4g16535. JAH3 encodes a protein of unknown function whose amino acid sequence has similarity to leukocyte receptor cluster-like protein. The mutation in jah3-1 is caused by a single nucleotide change from A to T at position 220 of 759 bp. Using CRISPR-Cas9, we generated a second allele, jah3-2, that encodes a truncated protein. Both of these loss-of-function alleles resulted in hypersensitivity to JA, ET-induced root growth inhibition, and accelerated dark-induced senescence. Double mutant analyses employing coronatine insensitive 1 (coi1) and ethylene insensitive 3 (ein3) mutants (jah3 coi1 and jah3 ein3) demonstrated that the hypersensitive phenotypes of the jah3 mutants are mediated by JA and ET signalling components COI1 and EIN3. Therefore, we propose that JAH3 is a negative regulator of both JA and ET signalling.

The plant pathogen enzyme AldC is a long-chain aliphatic aldehyde dehydrogenase.

Aldehyde dehydrogenases are versatile enzymes that serve a range of biochemical functions. Although traditionally considered metabolic housekeeping enzymes because of their ability to detoxify reactive aldehydes, like those generated from lipid peroxidation damage, the contributions of these enzymes to other biological processes are widespread. For example, the plant pathogen Pseudomonas syringae strain PtoDC3000 uses an indole-3-acetaldehyde dehydrogenase to synthesize the phytohormone indole-3-acetic acid to elude host responses. Here we investigate the biochemical function of AldC from PtoDC3000. Analysis of the substrate profile of AldC suggests that this enzyme functions as a long-chain aliphatic aldehyde dehydrogenase. The 2.5 Å resolution X-ray crystal of the AldC C291A mutant in a dead-end complex with octanal and NAD+ reveals an apolar binding site primed for aliphatic aldehyde substrate recognition. Functional characterization of site-directed mutants targeting the substrate- and NAD(H)-binding sites identifies key residues in the active site for ligand interactions, including those in the "aromatic box" that define the aldehyde-binding site. Overall, this study provides molecular insight for understanding the evolution of the prokaryotic aldehyde dehydrogenase superfamily and their diversity of function.

Dual Role of Auxin in Regulating Plant Defense and Bacterial Virulence Gene Expression During Pseudomonas syringae PtoDC3000 Pathogenesis.

Modification of host hormone biology is a common strategy used by plant pathogens to promote disease. For example, the bacterial pathogen strain Pseudomonas syringae DC3000 (PtoDC3000) produces the plant hormone auxin (indole-3-acetic acid [IAA]) to promote PtoDC3000 growth in plant tissue. Previous studies suggest that auxin may promote PtoDC3000 pathogenesis through multiple mechanisms, including both suppression of salicylic acid (SA)-mediated host defenses and via an unknown mechanism that appears to be independent of SA. To test if host auxin signaling is important during pathogenesis, we took advantage of Arabidopsis thaliana lines impaired in either auxin signaling or perception. We found that disruption of auxin signaling in plants expressing an inducible dominant axr2-1 mutation resulted in decreased bacterial growth and that this phenotype was suppressed by introducing the sid2-2 mutation, which impairs SA synthesis. Thus, host auxin signaling is required for normal susceptibility to PtoDC3000 and is involved in suppressing SA-mediated defenses. Unexpectedly, tir1 afb1 afb4 afb5 quadruple-mutant plants lacking four of the six known auxin coreceptors that exhibit decreased auxin perception, supported increased levels of bacterial growth. This mutant exhibited elevated IAA levels and reduced SA-mediated defenses, providing additional evidence that auxin promotes disease by suppressing host defense. We also investigated the hypothesis that IAA promotes PtoDC3000 virulence through a direct effect on the pathogen and found that IAA modulates expression of virulence genes, both in culture and in planta. Thus, in addition to suppressing host defenses, IAA acts as a microbial signaling molecule that regulates bacterial virulence gene expression.

Indole-3-acetaldehyde dehydrogenase-dependent auxin synthesis contributes to virulence of Pseudomonas syringae strain DC3000.

The bacterial pathogen Pseudomonas syringae modulates plant hormone signaling to promote infection and disease development. P. syringae uses several strategies to manipulate auxin physiology in Arabidopsis thaliana to promote pathogenesis, including its synthesis of indole-3-acetic acid (IAA), the predominant form of auxin in plants, and production of virulence factors that alter auxin responses in the host; however, the role of pathogen-derived auxin in P. syringae pathogenesis is not well understood. Here we demonstrate that P. syringae strain DC3000 produces IAA via a previously uncharacterized pathway and identify a novel indole-3-acetaldehyde dehydrogenase, AldA, that functions in IAA biosynthesis by catalyzing the NAD-dependent formation of IAA from indole-3-acetaldehyde (IAAld). Biochemical analysis and solving of the 1.9 Å resolution x-ray crystal structure reveal key features of AldA for IAA synthesis, including the molecular basis of substrate specificity. Disruption of aldA and a close homolog, aldB, lead to reduced IAA production in culture and reduced virulence on A. thaliana. We use these mutants to explore the mechanism by which pathogen-derived auxin contributes to virulence and show that IAA produced by DC3000 suppresses salicylic acid-mediated defenses in A. thaliana. Thus, auxin is a DC3000 virulence factor that promotes pathogenicity by suppressing host defenses.

The roles of auxin during interactions between bacterial plant pathogens and their hosts.

Plant pathogens have evolved several strategies to manipulate the biology of their hosts to facilitate colonization, growth to high levels in plant tissue, and production of disease. One of the less well known of these strategies is the synthesis of plant hormones and hormone analogs, and there is growing evidence that modulation of host hormone signaling is important during pathogenesis. Several plant pathogens produce the auxin indole-3-acetic acid (IAA) and/or virulence factors that modulate host auxin signaling. Auxin is well known for being involved in many aspects of plant growth and development, but recent findings have revealed that elevated IAA levels or enhanced auxin signaling can also promote disease development in some plant-pathogen interactions. In addition to stimulating plant cell growth during infection by gall-forming bacteria, auxin and auxin signaling can antagonize plant defense responses. Auxin can also act as a microbial signaling molecule to impact the biology of some pathogens directly. In this review, we summarize recent progress towards elucidating the roles that auxin production, modification of host auxin signaling, and direct effects of auxin on pathogens play during pathogenesis, with emphasis on the impacts of auxin on interactions with bacterial pathogens.

Arabidopsis Rab Geranylgeranyltransferases Demonstrate Redundancy and Broad Substrate Specificity in Vitro.

Posttranslational lipid modifications mediate the membrane attachment of Rab GTPases, facilitating their function in regulating intracellular vesicular trafficking. In Arabidopsis, most Rab GTPases have two C-terminal cysteines and potentially can be double-geranylgeranylated by heterodimeric Rab geranylgeranyltransferases (Rab-GGTs). Genes encoding two putative α subunits and two putative ß subunits of Rab-GGTs have been annotated in the Arabidopsis thaliana genome, but little is known about Rab-GGT activity in Arabidopsis. In this study, we demonstrate that four different heterodimers can be formed between putative Arabidopsis Rab-GGT α subunits RGTA1/RGTA2 and ß subunits RGTB1/RGTB2, but only RGTA1·RGTB1 and RGTA1·RGTB2 exhibit bona fide Rab-GGT activity, and they are biochemically redundant in vitro. We hypothesize that RGTA2 function might be disrupted by a 12-amino acid insertion in a conserved motif. We present evidence that Arabidopsis Rab-GGTs may have preference for prenylation of C-terminal cysteines in particular positions. We also demonstrate that Arabidopsis Rab-GGTs can not only prenylate a great variety of Rab GTPases in the presence of Rab escort protein but, unlike Rab-GGT in yeast and mammals, can also prenylate certain non-Rab GTPases independently of Rab escort protein. Our findings may help to explain some of the phenotypes of Arabidopsis protein prenyltransferase mutants.

Auxin promotes susceptibility to Pseudomonas syringae via a mechanism independent of suppression of salicylic acid-mediated defenses.

Auxin is a key plant growth regulator that also impacts plant-pathogen interactions. Several lines of evidence suggest that the bacterial plant pathogen Pseudomonas syringae manipulates auxin physiology in Arabidopsis thaliana to promote pathogenesis. Pseudomonas syringae strategies to alter host auxin biology include synthesis of the auxin indole-3-acetic acid (IAA) and production of virulence factors that alter auxin responses in host cells. The application of exogenous auxin enhances disease caused by P. syringae strain DC3000. This is hypothesized to result from antagonism between auxin and salicylic acid (SA), a major regulator of plant defenses, but this hypothesis has not been tested in the context of infected plants. We further investigated the role of auxin during pathogenesis by examining the interaction of auxin and SA in the context of infection in plants with elevated endogenous levels of auxin. We demonstrated that elevated IAA biosynthesis in transgenic plants overexpressing the YUCCA 1 (YUC1) auxin biosynthesis gene led to enhanced susceptibility to DC3000. Elevated IAA levels did not interfere significantly with host defenses, as effector-triggered immunity was active in YUC1-overexpressing plants, and we observed only minor effects on SA levels and SA-mediated responses. Furthermore, a plant line carrying both the YUC1-overexpression transgene and the salicylic acid induction deficient 2 (sid2) mutation, which impairs SA synthesis, exhibited additive effects of enhanced susceptibility from both elevated auxin levels and impaired SA-mediated defenses. Thus, in IAA overproducing plants, the promotion of pathogen growth occurs independently of suppression of SA-mediated defenses.

The Pseudomonas syringae type III effector AvrRpt2 promotes pathogen virulence via stimulating Arabidopsis auxin/indole acetic acid protein turnover.

To accomplish successful infection, pathogens deploy complex strategies to interfere with host defense systems and subvert host physiology to favor pathogen survival and multiplication. Modulation of plant auxin physiology and signaling is emerging as a common virulence strategy for phytobacteria to cause diseases. However, the underlying mechanisms remain largely elusive. We have previously shown that the Pseudomonas syringae type III effector AvrRpt2 alters Arabidopsis (Arabidopsis thaliana) auxin physiology. Here, we report that AvrRpt2 promotes auxin response by stimulating the turnover of auxin/indole acetic acid (Aux/IAA) proteins, the key negative regulators in auxin signaling. AvrRpt2 acts additively with auxin to stimulate Aux/IAA turnover, suggesting distinct, yet proteasome-dependent, mechanisms operated by AvrRpt2 and auxin to control Aux/IAA stability. Cysteine protease activity is required for AvrRpt2-stimulated auxin signaling and Aux/IAA degradation. Importantly, transgenic plants expressing the dominant axr2-1 mutation recalcitrant to AvrRpt2-mediated degradation ameliorated the virulence functions of AvrRpt2 but did not alter the avirulent function mediated by the corresponding RPS2 resistance protein. Thus, promoting auxin response via modulating the stability of the key transcription repressors Aux/IAA is a mechanism used by the bacterial type III effector AvrRpt2 to promote pathogenicity.

Auxin Plays Multiple Roles during Plant-Pathogen Interactions.

The plant hormone auxin governs many aspects of normal plant growth and development. Auxin also plays an important role in plant-microbe interactions, including interactions between plant hosts and pathogenic microorganisms that cause disease. It is now well established that indole-3-acetic acid (IAA), the most well-studied form of auxin, promotes disease in many plant-pathogen interactions. Recent studies have shown that IAA can act both as a plant hormone that modulates host signaling and physiology to increase host susceptibility and as a microbial signal that directly impacts the pathogen to promote virulence, but large gaps in our understanding remain. In this article, we review recent studies on the roles that auxin plays during plant-pathogen interactions and discuss the virulence mechanisms that many plant pathogens have evolved to manipulate host auxin signaling and promote pathogenesis.

Pseudomonas syringae type III effector AvrRpt2 alters Arabidopsis thaliana auxin physiology.

The Pseudomonas syringae type III effector AvrRpt2 promotes bacterial virulence on Arabidopsis thaliana plants lacking a functional RPS2 gene (rps2 mutant plants). To investigate the mechanisms underlying the virulence activity of AvrRpt2, we examined the phenotypes of transgenic A. thaliana rps2 seedlings constitutively expressing AvrRpt2. These seedlings exhibited phenotypes reminiscent of A. thaliana mutants with altered auxin physiology, including longer primary roots, increased number of lateral roots, and increased sensitivity to exogenous auxin. They also had increased levels of free indole acetic acid (IAA). The presence of AvrRpt2 also was correlated with a further increase in free IAA levels during infection with P. syringae pv. tomato strain DC3000 (PstDC3000). These results indicate that AvrRpt2 alters A. thaliana auxin physiology. Application of the auxin analog 1-naphthaleneacetic acid promoted disease symptom development in PstDC3000-infected plants, suggesting that elevated auxin levels within host tissue promote PstDC3000 virulence. Thus, AvrRpt2 may be among the virulence factors of P. syringae that modulate host auxin physiology to promote disease.

Investigating the reaction and substrate preference of indole-3-acetaldehyde dehydrogenase from the plant pathogen Pseudomonas syringae PtoDC3000.

Aldehyde dehydrogenases (ALDHs) catalyze the conversion of various aliphatic and aromatic aldehydes into corresponding carboxylic acids. Traditionally considered as housekeeping enzymes, new biochemical roles are being identified for members of ALDH family. Recent work showed that AldA from the plant pathogen Pseudomonas syringae strain PtoDC3000 (PtoDC3000) functions as an indole-3-acetaldehyde dehydrogenase for the synthesis of indole-3-acetic acid (IAA). IAA produced by AldA allows the pathogen to suppress salicylic acid-mediated defenses in the model plant Arabidopsis thaliana. Here we present a biochemical and structural analysis of the AldA indole-3-acetaldehyde dehydrogenase from PtoDC3000. Site-directed mutants targeting the catalytic residues Cys302 and Glu267 resulted in a loss of enzymatic activity. The X-ray crystal structure of the catalytically inactive AldA C302A mutant in complex with IAA and NAD+ showed the cofactor adopting a conformation that differs from the previously reported structure of AldA. These structures suggest that NAD+ undergoes a conformational change during the AldA reaction mechanism similar to that reported for human ALDH. Site-directed mutagenesis of the IAA binding site indicates that changes in the active site surface reduces AldA activity; however, substitution of Phe169 with a tryptophan altered the substrate selectivity of the mutant to prefer octanal. The present study highlights the inherent biochemical versatility of members of the ALDH enzyme superfamily in P. syringae.

Mqo, a tricarboxylic acid cycle enzyme, is required for virulence of Pseudomonas syringae pv. tomato strain DC3000 on Arabidopsis thaliana.

Plant pathogenic bacteria, such as Pseudomonas syringae pv. tomato strain DC3000, the causative agent of tomato bacterial speck disease, grow to high levels in the apoplastic space between plant cells. Colonization of plant tissue requires expression of virulence factors that modify the apoplast to make it more suitable for pathogen growth or facilitate adaptation of the bacteria to the apoplastic environment. To identify new virulence factors involved in these processes, DC3000 Tn5 transposon insertion mutants with reduced virulence on Arabidopsis thaliana were identified. In one of these mutants, the Tn5 insertion disrupted the malate:quinone oxidoreductase gene (mqo), which encodes an enzyme of the tricarboxylic acid cycle. mqo mutants do not grow to wild-type levels in plant tissue at early time points during infection. Further, plants infected with mqo mutants develop significantly reduced disease symptoms, even when the growth of the mqo mutant reaches wild-type levels at late stages of infection. Mutants lacking mqo function grow more slowly in culture than wild-type bacteria when dicarboxylates are the only available carbon source. To explore whether dicarboxylates are important for growth of DC3000 in the apoplast, we disrupted the dctA1 dicarboxylate transporter gene. DC3000 mutants lacking dctA1 do not grow to wild-type levels in planta, indicating that transport and utilization of dicarboxylates are important for virulence of DC3000. Thus, mqo may be required by DC3000 to meet nutritional requirements in the apoplast and may provide insight into the mechanisms underlying the important, but poorly understood process of adaptation to the host environment.

The phytotoxin coronatine contributes to pathogen fitness and is required for suppression of salicylic acid accumulation in tomato inoculated with Pseudomonas syringae pv. tomato DC3000.

The roles of the phytotoxin coronatine (COR) and salicylic acid (SA)-mediated defenses in the interaction of Pseudomonas syringae pv. tomato DC3000 and tomato (Solanum lycopersicum) were investigated. Unlike findings reported for Arabidopsis thaliana, DC3000 mutants impaired for production of COR or one of its components, coronafacic acid (CFA) or coronamic acid (CMA), induced distinctly different disease lesion phenotypes in tomato. Tomato plants inoculated with the CFA- CMA- mutant DB29 showed elevated transcript levels of SlICS, which encodes isochorismate synthase, an enzyme involved in SA biosynthesis in S. lycopersicum. Furthermore, expression of genes encoding SA-mediated defense proteins were elevated in DB29-inoculated plants compared with plants inoculated with DC3000, suggesting that COR suppresses SlICS-mediated SA responses. Sequence analysis of SlICS revealed that it encodes a protein that is 55 and 59.6% identical to the A. thaliana ICS-encoded proteins AtICS1 and AtICS2, respectively. Tomato plants silenced for SlICS were hypersusceptible to DC3000 and accumulated lower levels of SA after infection with DC3000 compared with inoculated wild-type tomato plants. Unlike what has been shown for A. thaliana, the COR- mutant DB29 was impaired for persistence in SlICS-silenced tomato plants; thus, COR has additional roles in virulence that are SA independent and important in the latter stages of disease development. In summary, the infection assays, metabolic profiling, and gene expression results described in this study indicate that the intact COR molecule is required for both suppression of SA-mediated defense responses and full disease symptom development in tomato.

CorR regulates multiple components of virulence in Pseudomonas syringae pv. tomato DC3000.

The phytotoxin coronatine (COR) is produced by various pathovars of Pseudomonas syringae, including P. syringae pv. tomato DC3000, which is pathogenic on crucifers and tomato, and P. syringae pv. glycinea PG4180, a soybean pathogen. The COR molecule contains two distinct components, coronafacic acid (CFA) and coronamic acid (CMA), which are intermediates in the COR biosynthetic pathway. In P. syringae pv. tomato DC3000, it is not clear whether corR, which encodes a response regulator, positively regulates CFA and CMA synthesis as it does in P. syringae pv. glycinea PG4180. In this study, a corR mutant of P. syringae pv. tomato DC3000 was constructed and was shown to be defective in the production of COR, CFA, and CMA. Furthermore, disease severity was greatly reduced in tomato plants inoculated with the corR mutant compared with wild-type P. syringae pv. tomato DC3000. We also showed that a mutation in hrpL, which encodes an alternate RNA polymerase sigma factor (sigmaL) required for the expression of genes encoding components of the type III secretion system, abrogated production of COR in P. syringae pv. tomato DC3000. The presence of a potential hrp box, the recognition site for sigmaL, upstream of corR suggested that corR might be regulated by hrpL. This was confirmed in reverse-transcription polymerase chain reaction experiments showing that the upstream effector gene holPtoAA, which was associated with the hrp box, was cotranscribed with corR. Furthermore, studies also were conducted to investigate whether mutations in corR had effects on the expression of hrpL. The corR mutant of P. syringae pv. tomato DC3000 showed both a reduction and delay in the expression of hrpL and was impaired in its ability to elicit a hypersensitive response on Nicotiana benthamiana. A putative CorR-binding site was identified upstream of hrpL, and gel shift studies confirmed the binding of CorR to this region. These results indicate that corR directly impacts the expression of the hrp regulon in P. syringae.

The Arabidopsis thaliana JASMONATE INSENSITIVE 1 gene is required for suppression of salicylic acid-dependent defenses during infection by Pseudomonas syringae.

Many plant pathogens suppress antimicrobial defenses using virulence factors that modulate endogenous host defenses. The Pseudomonas syringae phytotoxin coronatine (COR) is believed to promote virulence by acting as a jasmonate analog, because COR-insensitive 1 (coil) Arabidopsis thaliana and tomato mutants are impaired in jasmonate signaling and exhibit reduced susceptibility to P. syringae. To further investigate the role of jasmonate signaling in disease development, we analyzed several jasmonate-insensitive A. thaliana mutants for susceptibility to P. syringae pv. tomato strain DC3000 and sensitivity to COR. Jasmonate-insensitive 1 (jin1) mutants exhibit both reduced susceptibility to P. syringae pv. tomato DC3000 and reduced sensitivity to COR, whereas jasmonate-resistant 1 (jar1) plants exhibit wild-type responses to both COR and P. syringae pv. tomato DC3000. A jin1 jar1 double mutant does not exhibit enhanced jasmonate insensitivity, suggesting that JIN1 functions downstream of jasmonic acid-amino acid conjugates synthesized by JAR1. Reduced disease susceptibility in jin1 mutants is correlated with elevated expression of pathogenesis-related 1 (PR-1) and is dependent on accumulation of salicylic acid (SA). We also show that JIN1 is required for normal P. syringae pv. tomato DC3000 symptom development through an SA-independent mechanism. Thus, P. syringae pv. tomato DC3000 appears to utilize COR to manipulate JIN1-dependent jasmonate signaling both to suppress SA-mediated defenses and to promote symptom development.

Cross talk between signaling pathways in pathogen defense.

Plant defense in response to microbial attack is regulated through a complex network of signaling pathways that involve three signaling molecules: salicylic acid (SA), jasmonic acid (JA) and ethylene. The SA and JA signaling pathways are mutually antagonistic. This regulatory cross talk may have evolved to allow plants to fine-tune the induction of their defenses in response to different plant pathogens.

The Arabidopsis Auxin Receptor F-Box Proteins AFB4 and AFB5 Are Required for Response to the Synthetic Auxin Picloram.

The plant hormone auxin is perceived by a family of F-box proteins called the TIR1/AFBs. Phylogenetic studies reveal that these proteins fall into four clades in flowering plants called TIR1, AFB2, AFB4, and AFB6. Genetic studies indicate that members of the TIR1 and AFB2 groups act as positive regulators of auxin signaling by promoting the degradation of the Aux/IAA transcriptional repressors. In this report, we demonstrate that both AFB4 and AFB5 also function as auxin receptors based on in vitro assays. We also provide genetic evidence that AFB4 and AFB5 are targets of the picloram family of auxinic herbicides in addition to indole-3-acetic acid. In contrast to previous studies we find that null afb4 alleles do not exhibit obvious defects in seedling morphology or auxin hypersensitivity. We conclude that AFB4 and AFB5 act in a similar fashion to other members of the family but exhibit a distinct auxin specificity.

The Pseudomonas syringae avrRpt2 gene contributes to virulence on tomato.

In order to cause disease on plants, gram-negative phytopathogenic bacteria introduce numerous virulence factors into the host cell in order to render host tissue more hospitable for pathogen proliferation. The mode of action of such bacterial virulence factors and their interaction with host defense pathways remain poorly understood. avrRpt2, a gene from Pseudomonas syringae pv. tomato JL1065, has been shown to promote the virulence of heterologous P. syringae strains on Arabidopsis thaliana. However, the contribution of avrRpt2 to the virulence of JL1065 has not been examined previously. We show that a mutant derivative of JL1065 that carries a disruption in avrRpt2 is impaired in its ability to cause disease on tomato (Lycopersicon esculentum), indicating that avrRpt2 also acts as a virulence gene in its native strain on a natural host. The virulence activity of avrRpt2 was detectable on tomato lines that are defective in either ethylene perception or the accumulation of salicylic acid, but could not be detected on a tomato mutant insensitive to jasmonic acid. The enhanced virulence conferred by the expression of avrRpt2 in JL1065 was not associated with the suppression of several defense-related genes induced during the infection of tomato.