Different roles of Enhanced Disease Susceptibility1 (EDS1) bound to and dissociated from Phytoalexin Deficient4 (PAD4) in Arabidopsis immunity

Enhanced Disease Susceptibility1 (EDS1) is an important regulator of plant basal and receptor-triggered immunity. Arabidopsis EDS1 interacts with two related proteins, Phytoalexin Deficient4 (PAD4) and Senescence Associated Gene101 (SAG101), whose combined activities are essential for defense signaling. The different sizes and intracellular distributions of EDS1-PAD4 and EDS1-SAG101 complexes in Arabidopsis leaf tissues suggest that they perform nonredundant functions. The nature and biological relevance of EDS1 interactions with PAD4 and SAG101 were explored using yeast three-hybrid assays, in vitro analysis of recombinant proteins purified from Escherichia coli, and characterization of Arabidopsis transgenic plants expressing an eds1 mutant (eds1(L262P)) protein which no longer binds PAD4 but retains interaction with SAG101. EDS1 forms molecularly distinct complexes with PAD4 or SAG101 without additional plant factors. Loss of interaction with EDS1 reduces PAD4 post-transcriptional accumulation, consistent with the EDS1 physical association stabilizing PAD4. The dissociated forms of EDS1 and PAD4 are fully competent in signaling receptor-triggered localized cell death at infection foci. By contrast, an EDS1-PAD4 complex is necessary for basal resistance involving transcriptional up-regulation of PAD4 itself and mobilization of salicylic acid defenses. Different EDS1 and PAD4 molecular configurations have distinct and separable functions in the plant innate immune response.