Functional redundancy in the Arabidopsis Cathepsin B gene family contributes to basal defence, the hypersensitive response and senescence

Cysteine proteases are required for programmed cell death (PCD) in animals. Recent work in Nicotiana benthamiana has implicated cathepsin B-like cysteine proteases in the hypersensitive response (HR) in plants, a form of PCD involved in disease resistance. Here, we investigate the function and regulation of Cathepsin B (CathB) genes in plant defence, and in both pathogen-inducible and developmental forms of PCD. Single, double and triple knockout mutants were isolated for the three Arabidopsis thaliana AtCathB genes. AtCathB genes were redundantly required for full basal resistance against the virulent bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. By contrast, AtCathB genes were not required for R gene-mediated resistance to Pst DC3000 expressing AvrB or AvrRps4. Neither did they contribute to PCD triggered by AvrRps4, although they were crucial for the full development of PCD during HR triggered by AvrB. Cathepsin B has also been proposed to play a positive regulatory role in senescence. Atcathb triple mutants showed a delay in senescence and a seven-fold decrease in accumulation of senescence marker gene SAG12. Our results demonstrate a redundant function for AtCathB genes in basal defence as well as a potential regulatory role in distinct forms of plant PCD.