Summary-Analysis of a synthetic ABA agonist uncovers a new family of ABA binding proteins that control signal transduction by directly regulating the activity of type 2C protein phosphatases.-PP2Cs are vital phosphatases that play important roles in abscisic acid (ABA) signaling. Using chemical genetics, we previously identified a synthetic growth inhibitor called pyrabactin. Here we show that pyrabactin is a selective ABA agonist that acts through PYR1, the founding member of a family of START proteins called PYR/PYLs, which are necessary for both pyrabactin and ABA signaling in vivo. We show that ABA binds to PYR1, which in turn binds to and inhibits PP2Cs. We therefore suggest that PYR/PYLs are ABA-receptors that function at the apex of a negative regulatory pathway that controls ABA signaling by inhibiting PP2Cs. Our results

The Arabidopsis NPR1 protein is essential for regulating salicylic acid-dependent gene expression during systemic acquired resistance. NPR1 interacts differentially with members of the TGA class of basic domain/Leu zipper transcription factors and regulates their DNA binding activity. Here, we report that although TGA1 does not interact with NPR1 in yeast two-hybrid assays, treatment with salicylic acid induces the interaction between these proteins in Arabidopsis leaves. This phenomenon is correlated with a reduction of TGA1 Cys residues. Furthermore, site-directed mutagenesis of TGA1 Cys-260 and Cys-266 enables the interaction with NPR1 in yeast and Arabidopsis. Together, these results indicate that TGA1 relies on the oxidation state of Cys residues to mediate the interaction with NPR1. An intramolecular disulfide bridge in TGA1 precludes interaction with NPR1, and NPR1 can only stimulate the DNA binding activity of the reduced form of TGA1. Unlike its animal and yeast counterparts, the DNA binding activity of TGA1 is not redox regulated; however, this property is conferred by interaction with the NPR1 cofactor.

Plant and animal pathogenic bacteria can suppress host immunity by injecting type III secreted effector (T3SE) proteins into host cells. However, T3SEs can also elicit host immunity if the host has evolved a means to recognize the presence or activity of specific T3SEs. The diverse YopJ/HopZ/AvrRxv T3SE superfamily, which is found in both animal and plant pathogens, provides examples of T3SEs playing this dual role. The T3SE HopZ1a is an acetyltransferase carried by the phytopathogen Pseudomonas syringae that elicits effector-triggered immunity (ETI) when recognized in Arabidopsis thaliana by the nucleotide-binding leucine-rich repeat (NB-LRR) protein ZAR1. However, recognition of HopZ1a does not require any known ETI-related genes. Using a forward genetics approach, we identify a unique ETI-associated gene that is essential for ZAR1-mediated immunity. The hopZ-ETI-deficient1 (zed1) mutant is specifically impaired in the recognition of HopZ1a, but not the recognition of other unrelated T3SEs or in pattern recognition receptor (PRR)-triggered immunity. ZED1 directly interacts with both HopZ1a and ZAR1 and is acetylated on threonines 125 and 177 by HopZ1a. ZED1 is a nonfunctional kinase that forms part of small genomic cluster of kinases in Arabidopsis. We hypothesize that ZED1 acts as a decoy to lure HopZ1a to the ZAR1-resistance complex, resulting in ETI activation.ZED1-related kinase | ZRK | hypersensitive response

Plant pathogenic Pseudomonas syringae deliver type III effector proteins into the host cell, where they function to manipulate host defense and metabolism to benefit the extracellular bacterial colony. The activity of these virulence factors can be monitored by plant disease resistance proteins deployed to ''guard'' the targeted host proteins. The Arabidopsis RIN4 protein is targeted by three different type III effectors. Specific manipulation of RIN4 by each of them leads to activation of either the RPM1 or RPS2 disease resistance proteins. The type III effector AvrRpt2 is a cysteine protease that is autoprocessed inside the host cell where it activates RPS2 by causing RIN4 disappearance. RIN4 contains two sites related to the AvrRpt2 cleavage site (RCS1 and RCS2). We demonstrate that AvrRpt2-dependent cleavage of RIN4 at RCS2 is functionally critical in vivo. This event leads to proteasome-mediated elimination of all but a membrane-embedded Ϸ6.4-kDa C-terminal fragment of RIN4. One or more of three consecutive cysteines in this C-terminal fragment are required for RIN4 localization; these are likely to be palmitoylation and͞or prenylation sites. AvrRpt2-dependent cleavage at RCS2, and release of the remainder of RIN4 from the membrane, consequently prevents RPM1 activation by AvrRpm1 or AvrB. RCS2 is contained within the smallest tested fragment of RIN4 that binds AvrB in vitro. Thus, at least two bacterial virulence factors target the same domain of RIN4, a Ϸ30-aa plant-specific signature sequence found in a small Arabidopsis protein family that may be additional targets for these bacterial virulence factors. disease resistance ͉ plant immune system P lant disease resistance (R) genes control the plant immune response upon pathogen recognition. R proteins initiate cellular events that efficiently limit pathogen reproduction (1). Most plant R proteins defined to date possess a central nucleotide binding (NB) domain together with C-terminal leucinerich repeats (LRR), and hence are termed NB-LRR proteins. NB-LRR proteins are specific in that each is activated by a particular pathogen-encoded molecule. These are polymorphic across the pathogen population. The proteins from Gramnegative plant-pathogenic bacteria that trigger NB-LRR action are type III effector proteins delivered into plant cells through the type III secretion system (2). These are called avirulence (Avr) proteins because their presence renders a given pathogen strain avirulent on a resistant plant genotype (R). Avr proteins, however, can also contribute to pathogen virulence on susceptible plant genotypes (r).An Avr ligand-R receptor model could explain the genetic specificity of disease resistance, although experimental support for this model is limited. Rather, accumulating evidence suggests that Avr proteins from pathogenic bacteria can be recognized indirectly by their action on one or more host targets (1, 3, 4). In this model, type III effector proteins manipulate host targets, thereby contributing to pathogen virulence. The plant NB-LRR pro...