Abstract
Soybean (Glycine max) RPG1-B (for resistance to Pseudomonas syringae pv glycinea) mediates species-specific resistance to P. syringae expressing the avirulence protein AvrB, similar to the nonorthologous RPM1 in Arabidopsis (Arabidopsis thaliana). RPM1-derived signaling is presumably induced upon AvrB-derived modification of the RPM1-interacting protein, RIN4 (for RPM1-interacting 4). We show that, similar to RPM1, RPG1-B does not directly interact with AvrB but associates with RIN4-like proteins from soybean. Unlike Arabidopsis, soybean contains at least four RIN4-like proteins (GmRIN4a to GmRIN4d). GmRIN4b, but not GmRIN4a, complements the Arabidopsis rin4 mutation. Both GmRIN4a and GmRIN4b bind AvrB, but only GmRIN4b binds RPG1-B. Silencing either GmRIN4a or GmRIN4b abrogates RPG1-B-derived resistance to P. syringae expressing AvrB. Binding studies show that GmRIN4b interacts with GmRIN4a as well as with two other AvrB/RPG1-B-interacting isoforms, GmRIN4c and GmRIN4d. The lack of functional redundancy among GmRIN4a and GmRIN4b and their abilities to interact with each other suggest that the two proteins might function as a heteromeric complex in mediating RPG1-B-derived resistance. Silencing GmRIN4a or GmRIN4b in rpg1-b plants enhances basal resistance to virulent strains of P. syringae and the oomycete Phytophthora sojae. Interestingly, GmRIN4a- or GmRIN4b-silenced rpg1-b plants respond differently to AvrB-expressing bacteria. Although both GmRIN4a and GmRIN4b function to monitor AvrB in the presence of RPG1-B, GmRIN4a, but not GmRIN4b, negatively regulates AvrB virulence activity in the absence of RPG1-B.
Highlights
Soybean (Glycine max) RPG1-B mediates species-specific resistance to P. syringae expressing the avirulence protein AvrB, similar to the nonorthologous RPM1 in Arabidopsis (Arabidopsis thaliana)
All four GmRIN4 proteins contained several conserved domains present in AtRIN4, including those involved in binding to AvrB, an amino acid sequence required for AvrRpt2-mediated cleavage, and a putative palmitoylation site for plasma membrane localization (Chisholm et al, 2005; Kim et al, 2005a; Takemoto and Jones, 2005, Desveaux et al, 2007; Fig. 1A)
AvrB bacteria showed more pronounced virulence in the rpg1-b sojae. GmRIN4a (S4a) plants, accumulating to levels approximately 4-fold higher than vir bacteria in rpg1-b V plants (P, 0.001). These levels were 2-fold higher than the AvrB bacteria in rpg1-b V plants (P, 0.05). These results suggest that GmRIN4a negatively regulates the virulence of AvrB bacteria on plants lacking RPG1-B
Summary
Soybean (Glycine max) RPG1-B (for resistance to Pseudomonas syringae pv glycinea) mediates species-specific resistance to P. syringae expressing the avirulence protein AvrB, similar to the nonorthologous RPM1 in Arabidopsis (Arabidopsis thaliana). Similar to RPM1, RPG1-B does not directly interact with AvrB but associates with RIN4-like proteins from soybean. Silencing either GmRIN4a or GmRIN4b abrogates RPG1-B-derived resistance to P. syringae expressing AvrB. GmRIN4a- or GmRIN4b-silenced rpg1-b plants respond differently to AvrBexpressing bacteria. RIN4 is required for RPM1-induced resistance to AvrRpm1/AvrBexpressing P. syringae (Mackey et al, 2002). Both AvrRpm and AvrB induce the phosphorylation of RIN4, which is thought to induce RPM1-mediated resistance signaling. RIN4 associates with a second Arabidopsis R protein, RPS2 (for resistance to P. syringae), which mediates resistance against P. syringae expressing AvrRpt. The AvrRpt2-triggered loss of RIN4 compromises RPM1mediated resistance, because RIN4 is not available for phosphorylation (Ritter and Dangl, 1996; Axtell and Staskawicz, 2003; Mackey et al, 2003)
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