Articles written in Journal of Biosciences

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

      Subaran Singh Mrunmay Kumar Giri Praveen Kumar Singh Adnan Siddiqui Ashis Kumar Nandi

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      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.

    • The rice OsSAG12-2 gene codes for a functional protease that negatively regulates stress-induced cell death


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      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.

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