Photosynthetic and photorespiratory responses to high H2O2 accumulation triggered by peroxisomal ascorbate peroxidase knockdown and catalase inhibition in rice plants: physiological and proteomic approaches

Abstract

Synthesis of reactive oxygen species (ROS) is a natural process in plant cells. The excessive ROS accumulation implies in damage for important cellular processes, such as photosynthesis and protein synthesis. However, several evidences have shown the role of ROS as signaling molecules. Among them, hydrogen peroxide (H2O2) is considered the main signaling molecule, since it has a relative high half-life and can migrate into different cellular compartments. Several works have reported H2O2 signaling inducing tolerance to oxidative stress. Peroxisome is the main site of H2O2 production, by photorespiration. Many antioxidants act in the H2O2 detoxification in peroxisomes and catalase (CAT) is the most important of them. The absence of CAT results in several damages for plant metabolism by excessive H2O2 accumulation, such as strong impairment in growth and photosynthesis. The H2O2 detoxification by ascorbate/glutathione cycle, through the enzyme ascorbate peroxidase (APX), is also an important H2O2 scavenger. This cycle is present in many cellular compartments, including peroxisomes. However, the role of peroxisomal ascorbate peroxidases (pAPX) is still unclear. It has shown that APX4 knockdown, a pAPX isoform, triggers less sensibility to CAT inhibition in rice plants. Nevertheless, it is not completely understood how pAPX knockout induces this favorable outcome. In the present study, proteomic and physiological approaches were utilized to evaluate which antioxidant systems are triggered by pAPX-knocked-down plants upon CAT inhibition and how this impacts the photosynthetic performance. High photosynthesis resilience and accumulation of protein from antioxidant metabolism, mainly the ascorbate/glutathione cycle, were observed in APX4-knocked-down plants (APX4) under inhibition of CAT. Additionally, the participation of photorespiration in photosynthesis resilience of APX4 plants was evaluated. The better photosynthesis performance of APX4 plants upon induced photorespiration was associated to enhanced photorespiratory flux and accumulation of chloroplastic antioxidant proteins. Therefore, the present thesis allows the conclusion that the deficiency of APX4 induces a previous signaling that triggers an efficient antioxidant response to peroxisomal H2O2 accumulation by different protective mechanisms. This response protects the photosynthetic apparatus against oxidative damage caused by high H2O2. The results obtained in this work expand the understanding of the role of pAPX and H2O2 signaling in peroxisomes.