Salicylic acid (SA) is a plant defense hormone required for immunity. Arabidopsis NPR1 and NPR3/NPR4 were previously shown to bind SA and all three proteins were proposed as SA receptors. NPR1 functions as a transcriptional co-activator, whereas NPR3/NPR4 were suggested to function as E3 ligases that promote NPR1 degradation. Here we report that NPR3/NPR4 function as transcriptional co-repressors and SA inhibits their activities to promote the expression of downstream immune regulators. npr4-4D, a gain-of-function npr4 allele that renders NPR4 unable to bind SA, constitutively represses SA-induced immune responses. In contrast, the equivalent mutation in NPR1 abolishes its ability to bind SA and promote SA-induced defense gene expression. Further analysis revealed that NPR3/NPR4 and NPR1 function independently to regulate SA-induced immune responses. Our study indicates that both NPR1 and NPR3/NPR4 are bona fide SA receptors, but play opposite roles in transcriptional regulation of SA-induced defense gene expression.

Salicylic acid (SA) is a defense hormone required for both local and systemic acquired resistance (SAR) in plants. Pathogen infections induce SA synthesis through up-regulating the expression of Isochorismate Synthase 1 (ICS1), which encodes a key enzyme in SA production. Here we report that both SAR Deficient 1 (SARD1) and CBP60g are key regulators for ICS1 induction and SA synthesis. Whereas knocking out SARD1 compromises basal resistance and SAR, overexpression of SARD1 constitutively activates defense responses. In the sard1-1 cbp60g-1 double mutant, pathogen-induced ICS1 upregulation and SA synthesis are blocked in both local and systemic leaves, resulting in compromised basal resistance and loss of SAR. Electrophoretic mobility shift assays showed that SARD1 and CBP60g represent a plant-specific family of DNA-binding proteins. Both proteins are recruited to the promoter of ICS1 in response to pathogen infections, suggesting that they control SA synthesis by regulating ICS1 at the transcriptional level.

In both plants and animals, nucleotide-binding (NB) domain and leucine-rich repeat (LRR)-containing proteins (NLR) function as sensors of pathogen-derived molecules and trigger immune responses. Although NLR resistance (R) proteins were first reported as plant immune receptors more than 15 years ago, how these proteins activate downstream defense responses is still unclear. Here we report that the Toll-like/interleukin-1 receptor (TIR)-NB-LRR R protein, suppressor of npr1-1, constitutive 1 (SNC1) functions through its associated protein, Topless-related 1 (TPR1). Knocking out TPR1 and its close homologs compromises immunity mediated by SNC1 and several other TIR-NB-LRR-type R proteins, whereas overexpression of TPR1 constitutively activates SNC1-mediated immune responses. TPR1 functions as a transcriptional corepressor and associates with histone deacetylase 19 in vivo. Among the target genes of TPR1 are Defense no Death 1 (DND1) and Defense no Death 2 (DND2), two known negative regulators of immunity that are repressed during pathogen infection, suggesting that TPR1 activates R protein-mediated immune responses through repression of negative regulators.histone deacetylase 19 | plant immunity | Topless | Topless-related 1 P lant resistance (R) proteins play important roles in defense against pathogens. The majority of R proteins contain either a Toll-like/interleukin 1 receptor (TIR) or a coiled coil (CC) domain at their N terminus domain, a central nucleotide-binding (NB) domain, and C-terminal leucine-rich repeats (LRRs). Downstream components for TIR-and CC-NB-LRR R proteins appear to be different. Mutations in enhanced disease susceptibility 1 (EDS1), phytoalexin deficient 4 (PAD4), and senescence-associated gene101 (SAG101) affect the resistance specified by TIR-NB-LRR but not by CC-NB-LRR R proteins (1-3). On the other hand, mutations in non-race-specific disease resistance 1 (NDR1) compromise resistance mediated by CC-NB-LRR but not by TIR-NB-LRR R proteins (1, 4). EDS1, PAD4, and SAG101 encode three related proteins with homology to acyl hydrolases (3,5,6). How these proteins regulate R protein signaling is not clear.Increasing evidence suggests that certain R proteins accumulate in the nucleus and that the nuclear pools of these R proteins are important for the activation of defense responses (7-10). Multiple TIR-NB-LRR R proteins, including nicotiana glutinosa virus resistance protein (N) in tobacco and resistance to Pseudomonas syringae 4 (RPS4) and suppressor of npr1-1, constitutive 1 (SNC1) in Arabidopsis, have been shown to localize to the nucleus, and reduction of the nuclear R protein pool attenuates the activation of downstream defense responses (7-10). These findings are consistent with that the nucleocytoplasmic trafficking machinery is required for R protein-mediated immunity (9,11,12). However, the function of these R proteins in the nucleus and whether they participate directly or indirectly in transcriptional regulation of defense genes is unclear.Despite tremendous progress has been made ...

Recognition of pathogens by host plants leads to rapid transcriptional reprogramming and activation of defence responses. The expression of many defence regulators is induced in this process, but the mechanisms of how they are controlled transcriptionally are largely unknown. Here we use chromatin immunoprecipitation sequencing to show that the transcription factors SARD1 and CBP60g bind to the promoter regions of a large number of genes encoding key regulators of plant immunity. Among them are positive regulators of systemic immunity and signalling components for effector-triggered immunity and PAMP-triggered immunity, which is consistent with the critical roles of SARD1 and CBP60g in these processes. In addition, SARD1 and CBP60g target a number of genes encoding negative regulators of plant immunity, suggesting that they are also involved in negative feedback regulation of defence responses. Based on these findings we propose that SARD1 and CBP60g function as master regulators of plant immune responses.