Expression of harpin(xoo) in transgenic tobacco induces pathogen defense in the absence of hypersensitive cell death

HarpinXoo, encoded by the hpaGXoo gene of Xanthomonas oryzae pv. oryzae, is a member of the harpin group of proteins that induce pathogen resistance and hypersensitive cell death (HCD) in plants. We elaborated whether both processes are correlated in hpaGXoo-expressing tobacco (HARTOB) plants, which produced harpinXoo intracellularly. Resistance to fungal, bacterial, and viral pathogens increased in HARTOB, in correlation with the expression of hpaGXoo, the gene NPR1 that regulates several resistance pathways, and defense genes GST1, Chia5, PR-1a, and PR-1b that are mediated by different signals. However, reactive oxygen intermediate burst, the expression of HCD marker genes hsr203 and hin1, and cell death did not occur spontaneously in HARTOB, though they did in untransformed and HARTOB plants treated exogenously with harpinXoo. Thus, the transgenic expression of harpinXoo confers nonspecific pathogen defense in the absence of HCD.