Retrograde signalling pathways that are triggered by changes in cellular redox homeostasis remain poorly understood.

Transformed rice plants that are deficient in peroxisomal ascorbate peroxidase APX4 (OsAPX4-RNAi) are

known to exhibit more effective protection of photosynthesis against oxidative stress than controls when catalase

(CAT) is inhibited, but the mechanisms involved have not been characterized. An in-depth physiological and proteomics

analysis was therefore performed on OsAPX4-RNAi CAT-inhibited rice plants. Loss of APX4 function led to an

increased abundance of several proteins that are involved in essential metabolic pathways, possibly as a result of

increased tissue H2O2 levels. Higher photosynthetic activities observed in the OsAPX4-RNAi plants under CAT inhibition

were accompanied by higher levels of Rubisco, higher maximum rates of Rubisco carboxylation, and increased

photochemical efficiencies, together with large increases in photosynthesis-related proteins. Large increases were

also observed in the levels of proteins involved in the ascorbate/glutathione cycle and in other antioxidant-related

pathways, and these changes may be important in the protection of photosynthesis in the OsAPX4-RNAi plants. Large

increases in the abundance of proteins localized in the nuclei and mitochondria were also observed, together with

increased levels of proteins involved in important cellular pathways, particularly protein translation. Taken together,

the results show that OsAPX4-RNAi plants exhibit significant metabolic reprogramming, which incorporates a more

effective antioxidant response to protect photosynthesis under conditions of impaired CAT activity.