Arabidopsis PAD4 promotes the maturation and nuclear accumulation of immune-related cysteine protease RD19.

Arabidopsis PHYTOALEXIN DEFICIENT 4 (PAD4) has an essential role in pathogen resistance as a heterodimer with ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1). Here we investigated an additional PAD4 role in which it associates with and promotes the maturation of the immune-related cysteine protease RESPONSIVE TO DEHYDRATION 19 (RD19). We find that RD19 and its paralog RD19c promote EDS1- and PAD4-mediated effector-triggered immunity (ETI) to an avirulent Pseudomonas syringe strain DC3000 expressing the effector AvrRps4 and basal immunity against a fungal pathogen Golovinomyces cichoracearum. Overexpression of RD19, but not RD19 protease-inactive catalytic mutants, in Arabidopsis transgenic lines causes EDS1- and PAD4-dependent autoimmunity and enhanced pathogen resistance. In these lines, RD19 maturation to a pro-form requires its catalytic residues, suggesting that RD19 undergoes auto-processing. In transient assays, PAD4 interacts preferentially with the RD19 pro-protease and promotes its nuclear accumulation in leaf cells. The data lead us to propose a model for PAD4-stimulated defense potentiation. PAD4 promotes maturation and nuclear accumulation of processed RD19, and RD19 then stimulates EDS1-PAD4 dimer activity to confer pathogen resistance. This study highlights potentially important additional PAD4 functions which eventually converge onto canonical EDS1-PAD4 dimer signaling in plant immunity.