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 senescence-associated 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.

Arabidopsis MYC2 (AtMYC2) is a bHLH class transcription factor that mediates light-dependent seedling development,disease defence, JA and ABA signalling. AtMYC2 gene modulates hypocotyl elongation and expression of chlorophyll A/Bbinding protein 1 (CAB1) and rubisco small subunit protein1 (RBCS1) under blue light. The atmyc2 mutants are resistantagainst virulent bacterial pathogens. MYC2 orthologues from several crop plants have been characterized. The rice geneOs10g42430 has been referred earlier as OsMYC2 and has been shown to promote expression of JA-inducible genes.However, the role of OsMYC2 in seedling development under ABA, dark or light of specific wavelengths was not known.It was also not known whether OsMYC2 complements AtMYC2 function in Arabidopsis. We show here that expression ofOsMYC2 in the atmyc2 mutant of Arabidopsis complements the blue-light-mediated defects in hypocotyl elongation andexpression of CAB1 and RBCS1. We generated multiple transgenic rice lines for over-expression and RNAi-mediatedsuppression of OsMYC2. In agreement with AtMYC2 function, OsMYC2 over-expression and RNAi lines showedenhanced and suppressed seedling growth compared to WT plants respectively under blue light, and showed little effectunder white light or dark. In agreement with the negative regulatory role of AtMYC2 in disease defence, the RNAi linesshowed enhanced resistance against bacterial pathogen Xanthomonas oryzae pv oryzae. However, in contrast to AtMYC2function, OsMYC2 influences seedling development under red light and show no effect in ABA-mediated seed germination.Thus, the results suggest evolutionarily conserved as well as the distinct role of OsMYC2 in comparison with AtMYC2.