Redox balance in chloroplasts as a modulator of environmental stress responses: The role of ascorbate peroxidase and nudix hydrolase in Arabidopsis

The chloroplast is a well-established organelle that generates reactive oxygen species (ROS) due to photosynthesis. It is now widely accepted that the redox state in chloroplasts is a crucial factor in various physiological activities such as stress responsiveness and immunity in plants. The redox state is characterized by the balance between oxidants including ROS and reductants such as antioxidants, NAD(P)H, and FAD. The water-water cycle involving ascorbate peroxidase (APX) is a significant regulator of ROS levels in chloroplasts. Chloroplastic APX isoforms, particularly the thylakoid membrane-bound form (tAPX), are regarded as a bottleneck in the water-water cycle because of their high susceptibility to H2O2. A recent study on conditional tAPX suppression system indicated that the susceptibility of tAPX enables the flexible use of H2O2 as a signaling molecule in the regulation of various nuclear genes, named Responsive to tAPX Silencing (RTS) genes, which ultimately promotes various metabolic pathways related to abiotic stress acclimation and plant immunity. In terms of redox balance related to NAD(P) H, the identification and molecular physiological analysis of novel Nudix (nucleoside diphosphate linked to some moiety, X) hydrolase (NUDX) family have shown that the degradation processes of NAD(P)H in chloroplasts extend into many aspects of the regulation of metabolism and stress responses in plants. These findings have provided novel insights into the role of chloroplastic APX and NUDX as signaling modulators through the spatiotemporal regulation of redox balance.