Set‐point control of RD 21 protease activity by A t S erpin1 controls cell death in A rabidopsis

Summary Programmed cell death ( PCD ) in plants plays a key role in defense response and is promoted by the release of compartmentalized proteases to the cytoplasm. Yet the exact identity and control of these proteases is poorly understood. Serpins are an important group of proteins that uniquely curb the activity of proteases by irreversible inhibition; however, their role in plants remains obscure. Here we show that during cell death the A rabidopsis serpin protease inhibitor, A t S erpin1, exhibits a pro‐survival function by inhibiting its target pro‐death protease, RD 21. A t S erpin1 accumulates in the cytoplasm and RD 21 accumulates in the vacuole and in endoplasmic reticulum bodies. Elicitors of cell death, including the salicylic acid agonist benzothiadiazole and the fungal toxin oxalic acid, stimulated changes in vacuole permeability as measured by the changes in the distribution of marker dye. Concomitantly, a covalent A t S erpin1– RD 21 complex was detected indicative of a change in protease compartmentalization. Furthermore, mutant plants lacking RD 21 or plants with A t S erpin1 over‐expression exhibited significantly less elicitor‐stimulated PCD than plants lacking A t S erpin1. The necrotrophic fungi B otrytis cinerea and S clerotina sclerotiorum secrete oxalic acid as a toxin that stimulates cell death. Consistent with a pro‐death function for RD 21 protease, the growth of these necrotrophs was compromised in plants lacking RD 21 but accelerated in plants lacking A t S erpin1. The results indicate that AtSerpin1 controls the pro‐death function of compartmentalized protease RD 21 by determining a set‐point for its activity and limiting the damage induced during cell death.