Double-stranded RNA-binding protein 4 is required for resistance signaling against viral and bacterial pathogens.

Plant viruses often encode suppressors of host RNA silencing machinery, which occasionally function as avirulence factors that are recognized by host resistance (R) proteins. For example, the Arabidopsis R protein, hypersensitive response to TCV (HRT), recognizes the turnip crinkle virus (TCV) coat protein (CP). HRT-mediated resistance requires the RNA-silencing component double-stranded RNA-binding protein 4 (DRB4) even though it neither is associated with the accumulation of TCV-specific small RNA nor requires the RNA silencing suppressor function of CP. HRT interacts with the cytosolic fraction of DRB4. Interestingly, TCV infection both increases the cytosolic DRB4 pool and inhibits the HRT-DRB4 interaction. The virulent R8A CP derivative, which induces a subset of HRT-derived responses, also disrupts this interaction. The differential localization of DRB4 in the presence of wild-type and R8A CP implies the importance of subcellular compartmentalization of DRB4. The requirement of DRB4 in resistance to bacterial infection suggests a universal role in R-mediated defense signaling.

Oleic acid-dependent modulation of NITRIC OXIDE ASSOCIATED1 protein levels regulates nitric oxide-mediated defense signaling in Arabidopsis.

The conserved cellular metabolites nitric oxide (NO) and oleic acid (18:1) are well-known regulators of disease physiologies in diverse organism. We show that NO production in plants is regulated via 18:1. Reduction in 18:1 levels, via a genetic mutation in the 18:1-synthesizing gene SUPPRESSOR OF SA INSENSITIVITY OF npr1-5 (SSI2) or exogenous application of glycerol, induced NO accumulation. Furthermore, both NO application and reduction in 18:1 induced the expression of similar sets of nuclear genes. The altered defense signaling in the ssi2 mutant was partially restored by a mutation in NITRIC OXIDE ASSOCIATED1 (NOA1) and completely restored by double mutations in NOA1 and either of the nitrate reductases. Biochemical studies showed that 18:1 physically bound NOA1, in turn leading to its degradation in a protease-dependent manner. In concurrence, overexpression of NOA1 did not promote NO-derived defense signaling in wild-type plants unless 18:1 levels were lowered. Subcellular localization showed that NOA1 and the 18:1 synthesizing SSI2 proteins were present in close proximity within the nucleoids of chloroplasts. Indeed, pathogen-induced or low-18:1-induced accumulation of NO was primarily detected in the chloroplasts and their nucleoids. Together, these data suggest that 18:1 levels regulate NO synthesis, and, thereby, NO-mediated signaling, by regulating NOA1 levels.

Cryptochrome 2 and phototropin 2 regulate resistance protein-mediated viral defense by negatively regulating an E3 ubiquitin ligase.

Light harvested by plants is essential for the survival of most life forms. This light perception ability requires the activities of proteins termed photoreceptors. We report a function for photoreceptors in mediating resistance (R) protein-derived plant defense. The blue-light photoreceptors, cryptochrome (CRY) 2 and phototropin (PHOT) 2, are required for the stability of the R protein HRT, and thereby resistance to Turnip Crinkle virus (TCV). Exposure to darkness or blue-light induces degradation of CRY2, and in turn HRT, resulting in susceptibility. Overexpression of HRT can compensate for the absence of PHOT2 but not CRY2. HRT does not directly associate with either CRY2 or PHOT2 but does bind the CRY2-/PHOT2-interacting E3 ubiquitin ligase, COP1. Application of the proteasome inhibitor, MG132, prevents blue-light-dependent degradation of HRT, consequently these plants show resistance to TCV under blue-light. We propose that CRY2/PHOT2 negatively regulate the proteasome-mediated degradation of HRT, likely via COP1, and blue-light relieves this repression resulting in HRT degradation.

Enhanced disease susceptibility 1 and salicylic acid act redundantly to regulate resistance gene-mediated signaling.

Resistance (R) protein-associated pathways are well known to participate in defense against a variety of microbial pathogens. Salicylic acid (SA) and its associated proteinaceous signaling components, including enhanced disease susceptibility 1 (EDS1), non-race-specific disease resistance 1 (NDR1), phytoalexin deficient 4 (PAD4), senescence associated gene 101 (SAG101), and EDS5, have been identified as components of resistance derived from many R proteins. Here, we show that EDS1 and SA fulfill redundant functions in defense signaling mediated by R proteins, which were thought to function independent of EDS1 and/or SA. Simultaneous mutations in EDS1 and the SA-synthesizing enzyme SID2 compromised hypersensitive response and/or resistance mediated by R proteins that contain coiled coil domains at their N-terminal ends. Furthermore, the expression of R genes and the associated defense signaling induced in response to a reduction in the level of oleic acid were also suppressed by compromising SA biosynthesis in the eds1 mutant background. The functional redundancy with SA was specific to EDS1. Results presented here redefine our understanding of the roles of EDS1 and SA in plant defense.

HRT-mediated hypersensitive response and resistance to Turnip crinkle virus in Arabidopsis does not require the function of TIP, the presumed guardee protein.

The Arabidopsis resistance protein HRT recognizes the Turnip crinkle virus (TCV) coat protein (CP) to induce a hypersensitive response (HR) in the resistant ecotype Di-17. The CP also interacts with a nuclear-targeted NAC family of host transcription factors, designated TIP (TCV-interacting protein). Because binding of CP to TIP prevents nuclear localization of TIP, it has been proposed that TIP serves as a guardee for HRT. Here, we have tested the requirement for TIP in HRT-mediated HR and resistance by analyzing plants carrying knockout mutation in the TIP gene. Our results show that loss of TIP does not alter HR or resistance to TCV. Furthermore, the mutation in TIP neither impaired the salicylic acid-mediated induction of HRT expression nor the enhanced resistance conferred by overexpression of HRT. Strikingly, the mutation in TIP resulted in increased replication of TCV and Cucumber mosaic virus, suggesting that TIP may play a role in basal resistance but is not required for HRT-mediated signaling.

Plastidial fatty acid levels regulate resistance gene-dependent defense signaling in Arabidopsis.

In Arabidopsis, resistance to Turnip Crinkle Virus (TCV) depends on the resistance (R) gene, HRT, and the recessive locus rrt. Resistance also depends on salicylic acid (SA), EDS1, and PAD4. Exogenous application of SA confers resistance in RRT-containing plants by increasing HRT transcript levels in a PAD4-dependent manner. Here we report that reduction of oleic acid (18:1) can also induce HRT gene expression and confer resistance to TCV. However, the 18:1-regulated pathway is independent of SA, rrt, EDS1, and PAD4. Reducing the levels of 18:1, via a mutation in the SSI2-encoded stearoyl-acyl carrier protein-desaturase, or by exogenous application of glycerol, increased transcript levels of HRT as well as several other R genes. Second-site mutations in the ACT1-encoded glycerol-3-phosphate acyltransferase or GLY1-encoded glycerol-3-phosphate dehydrogenase restored 18:1 levels in HRT ssi2 plants and reestablished a dependence on rrt. Resistance to TCV and HRT gene expression in HRT act1 plants was inducible by SA but not by glycerol, whereas that in HRT pad4 plants was inducible by glycerol but not by SA. The low 18:1-mediated induction of R gene expression was also dependent on ACT1 but independent of EDS1, PAD4, and RAR1. Intriguingly, TCV inoculation did not activate this 18:1-regulated pathway in HRT plants, but instead resulted in the induction of several genes that encode 18:1-synthesizing isozymes. These results suggest that the 18:1-regulated pathway may be specifically targeted during pathogen infection and that altering 18:1 levels may serve as a unique strategy for promoting disease resistance.

Plant signal transduction and defense against viral pathogens.

Viral infection of plants is a complex process whereby the virus parasitizes the host and utilizes its cellular machinery to multiply and spread. In turn, plants have evolved signaling mechanisms that ultimately limit the ingress and spread of viral pathogens, resulting in resistance. By dissecting the interaction between host and virus, knowledge of signaling pathways that are deployed for resistance against these pathogens has been gained. Advances in this area have shown that resistance signaling against viruses does not follow a prototypic pathway but rather different host factors may play a role in resistance to different viral pathogens. Some components of viral resistance signaling pathways also appear to be conserved with those functioning in signaling pathways operational against other nonviral pathogens, however, these pathways may or may not overlap. This review aims to document the advances that have improved our understanding of plant resistance to viruses.

Light-dependent hypersensitive response and resistance signaling against Turnip Crinkle Virus in Arabidopsis.

Resistance to Turnip Crinkle Virus (TCV) in Arabidopsis ecotype Dijon (Di)-17 is conferred by the resistance gene HRT and a recessive locus rrt. In Di-17, TCV elicits a hypersensitive response (HR), which is accompanied by increased expression of pathogenesis-related (PR) genes and high levels of salicylic acid (SA). We have previously shown that HRT-mediated resistance to TCV is dependent on SA-mediated signal transduction and that increased levels of SA confer enhanced resistance to TCV via upregulation of the HRT gene. Here we show that HRT-mediated HR and resistance are dependent on light. A dark treatment immediately following TCV inoculation suppressed HR, resistance and activation of the majority of the TCV-induced genes. However, the absence of light did not affect either TCV-induced elevated levels of free SA or the expression of HRT. Interestingly, in the dark, transgenic plants overexpressing HRT showed susceptibility, but overexpression of HRT coupled with high levels of endogenous SA resulted in pronounced resistance. Consistent with these results is the finding that exogenous application of SA prior to TCV inoculation partially overcame the requirement for light. Light was also required for N gene-mediated HR and resistance to Tobacco Mosaic Virus, suggesting that it is an important factor which may be generally required during defense signaling.

Signaling requirements and role of salicylic acid in HRT- and rrt-mediated resistance to turnip crinkle virus in Arabidopsis.

Inoculation of turnip crinkle virus (TCV) on the resistant Arabidopsis ecotype Di-17 elicits a hypersensitive response (HR), which is accompanied by increased expression of pathogenesis-related (PR) genes. Previous genetic analyses revealed that the HR to TCV is conferred by HRT, which encodes a coiled-coil (CC), nucleotide-binding site (NBS) and leucine-rich repeat (LRR) class resistance (R) protein. In contrast to the HR, resistance to TCV requires both HRT and a recessive allele at a second locus designated rrt. Here, we demonstrate that unlike most CC-NBS-LRR R genes, HRT/rrt-mediated resistance is dependent on EDS1 and independent of NDR1. Resistance is also independent of RAR1 and SGT1. HRT/rrt-mediated resistance is compromised in plants with reduced salicylic acid (SA) content as a consequence of mutations eds5, pad4, or sid2. By contrast, HR is not affected by mutations in eds1, eds5, pad4, sid2, ndr1, rar1, or sgt1b. Resistance to TCV is restored in both SA-deficient Di-17 plants expressing the nahG transgene and mutants containing the eds1, eds5, or sid2 mutations by exogenous application of SA or the SA analog benzo(1,2,3)thiadiazole-7-carbothioic acid (BTH). In contrast, SA/BTH treatment failed to enhance resistance in HRT pad4, Col-0, or hrt homozygous progeny of a cross between Di-17 and Col-0. Thus, HRT and PAD4 are required for SA-induced resistance. Exogenously supplied SA or high endogenous levels of SA, due to the ssi2 mutation, overcame the suppressive effects of RRT and enhanced resistance to TCV, provided the HRT allele was present. High levels of SA upregulate HRT expression via a PAD4-dependent pathway. As Col-0 transgenic lines expressing high levels of HRT were resistant to TCV, but lines expressing moderate to low levels of HRT were not, we conclude that SA enhances resistance in the RRT background by upregulating HRT expression. These data suggest that the HRT-TCV interaction is unable to generate sufficient amounts of SA required for a stable resistance phenotype, and the presence of rrt possibly corrects this deficiency.