Background: Caspases are a family of aspartate-specific cysteine proteases that play an essential role in initiating and executing programmed cell death (PCD) in metazoans. Caspase-like activities have been shown to be required for the initiation of PCD in plants, but the genes encoding those activities have not been identified. VPEγ, a cysteine protease, is induced during senescence, a form of PCD in plants, and is localized in precursor protease vesicles and vacuoles, compartments associated with PCD processes in plants. Results: We show that VPEγ binds in vivo to a general caspase inhibitor and to caspase-1-specific inhibitors, which block the activity of VPEγ. A cysteine protease inhibitor, cystatin, accumulates to 20-fold higher levels in vpeγ mutants. Homologs of cystatin are known to suppress hypersensitive cell death in plant and animal systems. We also report that infection with an avirulent strain of Pseudomonas syringae results in an increase of caspase-1 activity, and this increase is partially suppressed in vpeγ mutants. Plants overexpressing VPEγ exhibit a greater amount of ion leakage during infection with P. syringae, suggesting that VPEγ may regulate cell death progression during plant-pathogen interaction. VPEγ expression is induced after infection with P. syringae, Botrytis cinerea, and turnip mosaic virus, and knockout of VPEγ results in increased susceptibility to these pathogens. Conclusions: We conclude that VPEγ is a caspase-like enzyme that has been recruited in plants to regulate vacuole-mediated cell dismantling during cell death, a process that has significant influence in the outcome of a diverse set of plant-pathogen interactions.
Bibliographical noteFunding Information:
The authors would like to thank Dr. Shauna Somerville for many useful suggestions and Ms. Jocelyn Brimo for helping with the artwork and formatting the manuscript. This work was supported by the Spanish Ministerio de Educacion y Ciencia (#BMC2003-08039 to E.R.), the National Science Foundation (#MCB-0296080 to N.V.R.), the National Institutes of Health (National Research Service Award Postdoctoral Fellowship #GM069087 to C.C.), National Institutes of Health (grant GM48707 to F.M.A.), and the National Science Foundation Genome Fund (#DBI-0218166 to B.B.).