Down-regulation of OsSAG12-1 results in enhanced senescence and pathogen-induced cell death in transgenic rice plants.

Senescence is a highly regulated process accompanied by changes in gene expression. While the mRNA levels of most genes decline, the mRNA levels of specific genes (senescence associated genes, SAGs) increase during senescence. Arabidopsis SAG12 (AtSAG12) gene codes for papain-like cysteine protease. The promoter of AtSAG12 is SA-responsive and reported to be useful to delay senescence by expressing cytokinin biosynthesis gene isopentenyltransferase specifically during senescence in several plants including Arabidopsis, lettuce and rice. The physiological role of AtSAG12 is not known; the homozygous atsag12 mutant neither fails to develop senescenceassociated vacuoles nor shows any morphological phenotype. Through BLAST search using AtSAG12 amino acid sequences as query, we identified a few putative homologues from rice genome (OsSAGs; Oryza sativa SAGs). OsSAG12-1 is the closest homologue of AtSAG12 with 64% similar amino acid composition. Expression of OsSAG12-1 is induced during senescence and pathogen-induced cell death. To evaluate the possible role of OsSAG12-1 we generated RNAi transgenic lines in Japonica rice cultivar TP309. The transgenic lines developed early senescence at varying levels and showed enhanced cell death when inoculated with bacterial pathogen Xanthomonas oryzae pv.oryzae. Our results suggest that OsSAG12-1 is a negative regulator of cell death in rice.