Thiol protease that plays a central role in plant programmed cell death (PCD). In addition to its role in protein degradation, may cleave and/or degrade a number of target proteins, activating signaling towards PCD. Contributes to the increase of caspase-3-like activity after UV-C-induced PCD and is required for abiotic stress-induced PCD (PubMed:27058316). Functions redundantly with CATHB2 and CATHB3 in basal defense and distinct forms of plant programmed cell death (PCD). Participates in the establishment of basal resistance against the bacterial pathogen Pseudomonase syringae pv. tomato DC3000. Required for full levels of PCD during resistance (R) gene-mediated hypersensitive response (HR). Involved in the regulation of senescence, a developmental form of PCD in plants (PubMed:19453434). By dark-induced senescence. Down-regulated by infection with an avirulent strain of the bacterial pathogen Pseudomonase syringae pv. tomato DC3000. Belongs to the peptidase C1 family.