Inhibits metacaspase-9 (MC9) cysteine protease. Functions through cleavage of its reactive center loop and covalent binding to MC9. Involved in the control of elicitor-stimulated programmed cell death (PCD). During infection by the necrotrophic fungal pathogen Botrytis cinerea, functions to protect cells by limiting the PCD-promoting protease RD21A activity that is released from the ER body or vacuole to the cytoplasm (PubMed:23398119). Involved in the control of water stress-induced cell death by limiting the pro-death protease RD21A activity that is released from the vacuole to the cytoplasm (PubMed:26884487). Interacts with RD21A. Expressed in root tips. Expressed in siliques (at protein level). The reactive center loop (RCL) extends out from the body of the protein and directs binding to the target protease. The protease cleaves the serpin at the reactive site within the RCL, establishing a covalent linkage between the carboxyl group of the serpin reactive site and the serine hydroxyl of the protease. The resulting inactive serpin-protease complex is highly stable (By similarity). Belongs to the serpin family.