| Summary: | There is considerable interest in investigating conserved roles for protease in the Hypersensitive Response (HR), a plant defence response which shares some morphological characteristics with apoptosis. a cysteine protease, with homology to mammalian Cathepsin B proteases, was isolated in a screen for genes up-regulated in the HR. The focus of this current research is to examine the roles of Cathepsin B genes in the model plant <i>Arabidopsis.</i> There are three Cathepsin B homologues in <i>Arabidopsis </i>for which knock-out lines were isolated and genetically crossed using a combination of T-DNA insert lines and RNAi to generate double and triple mutants. These genes were found to act redundantly with triple mutants showing increased susceptibility to both virulent and avirulent strains of <i>Pseudomonas syringae </i>DC3000. Moreover, these genes are also involved in non-host resistance to fungal pathogen <i>Blumeria graminis </i>f.sp. <i>tritici,</i> where they positively regulate the HR but negatively regulate <i>Pathogenesis-Related 1 </i>(<i>PR1</i>) expression downstream of <i>Enhanced Disease Susceptible 1</i>(<i>EDS1</i>). In addition, this work also implicates <i>Cathepsin B </i>genes in senescence, a developmental form of PCD, via regulation of the senescence marker gene <i>Senescence Associated Gene 12 </i>(<i>SAG12</i>). Furthermore, it was shown that NbCathB is localised to the plant apoplast where it is activated upon secretion. Partially purified Cathepsin B protein was inhibited by a variety of peptide inhibitors but evidence of inhibition by several pathogen-derived inhibitors that are secreted during infection was inconclusive.
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