Vacuolar processing enzyme is up-regulated in the lytic vacuoles of vegetative tissues during senescence and under various stressed conditions

Vacuolar processing enzyme (VPE) has been shown to be responsible for maturation of various seed proteins in protein-storage vacuoles. Arabidopsis has three VPE homologues; beta VPE is specific to seeds and alpha VPE and gamma VPE are specific to vegetative organs. To investigate the activity of the vegetative VPE, we expressed the gamma VPE in a pep4 strain of the yeast Saccharomyces cerevisiae and found that gamma VPE has the ability to cleave the peptide bond at the carbonyl side of asparagine residues. An immunocytochemical analysis revealed the specific localization of the gamma VPE in the lytic vacuoles of Arabidopsis leaves that had been treated with wounding. These findings indicate that gamma VPE functions in the lytic vacuoles as the beta VPE does in the protein-storage vacuoles. The beta VPE promoter was found to direct the expression of the beta-glucuronidase reporter gene in seeds and the root tip of transgenic Arabidopsis plants. On the other hand, both the alpha VPE and gamma VPE promoters directed the expression in senescent tissues, but not in young intact tissues. The mRNA levels of both alpha VPE and gamma VPE were increased in the primary leaves during senescence in parallel with the increase of the mRNA level of a senescence-associated gene (SAG2). Treatment with wounding, ethylene and salicylic acid up-regulated the expression of alpha VPE and gamma VPE, while jasmonate slightly up regulated the expression of gamma VPE. These gene expression patterns of the VPEs were associated with the accumulation of vacuolar proteins that are known to respond to these treatments. Taken together, the results suggest that vegetative VPE might regulate the activation of some functional proteins in the lytic vacuoles.