Down-Regulation of Zeaxanthin Epoxidation in Vascular Plant Leaves Under Normal and Photooxidative Stress Conditions

Abstract

The down-regulation of zeaxanthin (Zx) epoxidation is important for the regulation of Zx accumulation in xanthophyll cycle and for the development of non-photochemical quenching (NPQ). The NPQ development and Zx accumulation kinetics in rice, barley, and spinach leaves under light of different intensities were highly similar among the three plants. When the leaves were pre-treated with an inhibitor of Zx epoxidase (ZE), salicylaldoxime (SA), the two kinetics patterns in the leaves under low and moderately high light intensities became similar to those of high light intensity-treated leaves. Therefore, we propose that reversible down-regulation of Zx epoxidation plays an important role in plants, and this reversible down-regulation mechanism is a general mechanism in plants which occurs at room temperature under various light conditions as well as under different stress conditions in the presence of light. This reversible down-regulation is different from the irreversible down-regulation mechanism of ZE which involves ZE protein degradation together with D1 protein degradation under photooxidative conditions. There will be discussion on the mechanisms for the actual regulation of ZE activities involving phosphorylation/dephosphorylation of still unknown regulator(s) and/or by the redox regulation involving NADPH thioredoxin reductase C and thioredoxin m.