Induction of systemic resistance to Xanthomonas oryzae pv. oryzae by salicylic acid in Oryza sativa (L.)

Abstract

The effect of salicylic acid (SA) is hypothesized to be a natural signal that triggers the systemic induction of phenolics, pathogenesis-related proteins and disease resistance in rice against the bacterial leaf blight pathogen Xanthomonas oryzae pv. oryzae (Xoo). Rice plants pretreated with 1000 μmol/l Sa showed resistance to challenge inoculation with Xanthomonas oryzae pv. oryzae and the effectiveness persisted in the susceptible cv. ‘Ir 50’ for at least 3 days prior to inoculation with Xoo. To investigate the role of SA in rice disease resistance, we examined the endogenous levels of SA in the Sa-pretreated rice plants with Xoo inoculation. A three-fold increase in the endogenous Sa levels was observed in the rice tissues pretreated with 1000 μmol/l Sa and the resistance persisted for at least 3 days after Sa treatment prior to inoculation with Xoo. Increasing the endogenous level of Sa in rice leaves to those naturally observed during systemic acquired resistance resulted in increased resistance to Xanthomonas oryzae pv. oryzae, expressed as a reduction in leaf blight lesion length. Immunoblot analysis revealed an induction of a 25 kDa protein cross-reacting with rice thaumatin-like protein (TLP) antiserum in response to Sa-pretreated and Sa non-pretreated rice plants followed by pathogen inoculation. A significant increase in the induction of Tlps 3 days after Xoo inoculation in the tissues pretreated with Sa was observed when compared with the 2 days and 1 day after Xoo inoculation in SA-treated plants. Increased phenolics content and enhanced activities of some pathogenesis-related (PR) proteins, viz., TLP, chitinase and β-1,3-glucanase were observed in rice plants treated with SA. Based on these experiments, it was investigated that the defense responses are induced locally at the infection site only after pathogen attack and are augumented when the rice tissue has been pretreated with Sa. These data further support the hypothesis that the defense responses in rice can be rapidly triggered and induced in a genetically susceptible cultivar after treatment with SA.