Role of peroxidases in the compensation of cytosolic ascorbate peroxidase knockdown in rice plants under abiotic stress

Abstract

Current studies, particularly in Arabidopsis, have demonstrated that mutants deficient in cytosolic ascorbate peroxidases (APXs) are susceptible to the oxidative damage induced by abiotic stress. In contrast, we demonstrate here that rice mutants double silenced for cytosolic APXs (APx1/2s) up-regulated other peroxidases, making the mutants able to cope with abiotic stress, such as salt, heat, high light and methyl viologen, similar to non-transformed (NT) plants. The APx1/2s mutants exhibited an altered redox homeostasis, as indicated by increased levels of H2O2 and ascorbate and glutathione redox states. Both mutant and NT plants exhibited similar photosynthesis (CO2 assimilation and photochemical efficiency) under both normal and stress conditions. Overall, the antioxidative compensatory mechanism displayed by the mutants was associated with increased expression of OsGpx genes, which resulted in higher glutathione peroxidase (GPX) activity in the cytosolic and chloroplastic fractions. The transcript levels of OsCatA and OsCatB and the activities of catalase (CAT) and guaiacol peroxidase (GPOD; type III peroxidases) were also up-regulated. None of the six studied isoforms of OsApx were up-regulated under normal growth conditions. Therefore, the deficiency in cytosolic APXs was effectively compensated for by up-regulation of other peroxidases. We propose that signalling mechanisms triggered in rice mutants could be distinct from those proposed for Arabidopsis. We demonstrate in this study that rice mutants double silenced for cytosolic APXs, differently of Arabidopsis, up-regulate other peroxidases making the mutants able to cope with abiotic stress. The compensatory mechanism for the deficiency of cytosolic Apx in rice plants was associated with the overexpression of two OsCat and four OsGpx isoforms and the upregulation of GPOD, GPX and CAT activities. For the first time, this work demonstrates a comprehensive study employing a multidisciplinary approach to assess the role of increased endogenous levels of H2O2 in ROS signaling networks involved in a new redox homeostasis in rice plants deficient in cytosolic APX. © 2011 Blackwell Publishing Ltd.