Senescence is the final stage of plant development. Although expression of most of the genes is suppressed duringsenescence, a set of genes referred as senescence-associated genes (SAGs) is induced. Arabidopsis thaliana SAG12(AtSAG12) is one such gene that has been mostly studied for its strict association with senescence. AtSAG12 encodes apapain-like cysteine protease, expressed predominantly in senescence-associated vacuoles. Rice genome containsmultiple AtSAG12 homologues (OsSAGs). OsSAG12-1, the closest structural homologue of AtSAG12, is a negativeregulator of developmental and stress-induced cell death. Proteolytic activity has not been established for any SAG12homologues in vitro. Here, we report that OsSAG12-2, the second structural homologue of AtSAG12 from rice, codesfor a functional proteolytic enzyme. The recombinant OsSAG12-2 protein produced in Escherichia coli undergoesautolysis to generate a functional protease. The matured OsSAG12-2 protein shows 27% trypsin-equivalent proteolyticactivity on azocasein substrate. Dark-induced senescence activates OsSAG12-2 expression. Down-regulation of
OsSAG12-2 in the transgenic artificial miRNA lines results in enhanced salt- and UV-induced cell death, even thoughit does not affect cell viability in the stress-free condition. Our results show that OsSAG12-2 codes for a functionalprotease that negatively regulates stress-induced cell death in rice.
Volume 44 | Issue 3
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