Dynamic transformation of vacuolar function is responsible for flexibility of differentiation and cell death events in higher plant cells. The aim of this study is to elucidate the molecular mechanism of vacuolar processing enzyme (VPE), which is known to be responsible for maturation and activation of vacuolar proteins, in the process of programmed cell death and cell differentiation.
(1) We generated the Arabidopsis knockout mutants that had a defect of each gene for the two vegetative type VPEs, αVPE and γVPE, and one seed type VPE, βVPE. Then they were crossed to obtain double and triple mutants. From the analysis of the triple mutant, we found that normally processed storage proteins were no more detected, indicating that VPEs are a key enzyme in processing of seed storage proteins.
(2) We found that a characteristic structure derived from ER, ER body. ER bodies develop in the epidermal cells of young seeding and contain the precursor proteins of stress-inducible protease such a VPE
. In contrast, rosette leaves had no ER bodies. We found that wound stress induced the formation of many ER bodies in rosette leaves. It is possible that the biological function of ER bodies is related to defense systems in higher plants.
(3) The process of controlled cell death in animals is well organized by caspases that are cysteine proteinases. In plants, many studies have shown that caspase-like activities are induced in dying cells. To identify a functional homolog of caspase in plants, we investigated hypersensitive cell death in tobacco mosaic virus (TMV)-infected tobacco plants. A VPE-specific inhibitor abolished the TMV-induced hypersensitive cell death, as did a caspase-1-specific inhibitor.
Because VPE had caspase activity toward a caspase-1-specific substrate, the proteinase responsible for the caspase-1-like activity was identified as VPE. From the appearance of VPE activity at the early stage-of the TMV-induced hypersensitive response in tobacco leaves, it was suggested that VPE regulates the early process of the TMV-induced hypersensitive cell death. Less