Suppression of zeaxanthin epoxidation by chloroplast phosphatase inhibitors in rice leaves

Abstract

Leaves of Oryza sativa L. plants were fed with fluoride or molybdate, two classical inhibitors of chloroplast phosphatases, through their epicotyls for 3 h in darkness, in each case followed by a 6-h exposure to a photosynthetic photon flux density of 150 μmol m −2 s −1 at 4°C. During chilling treatment, the PSII photochemical efficiency ( F v/ F m) decreased in parallel with an increase in the level of zeaxanthin. Pretreatment with the phosphatase inhibitors led to a greater reduction in F v/ F m and an increased rate of zeaxanthin accumulation. During dark-incubation at 22°C after chilling, the zeaxanthin epoxidation was strikingly suppressed by the phosphatase inhibitors. When leaves were preincubated with either fluoride or salicylaldoxime, an inhibitor of epoxidase, even an illumination of 60 μmol m −2 s −1 at room temperature for 1.5 h could cause an accumulation of zeaxanthin up to the level comparable to that observed in 6-h light-chilled leaves. The zeaxanthin epoxidation during subsequent dark-incubation was significantly suppressed by these two chemicals. In vitro experiments with uncoupled thylakoids isolated from chilled leaves revealed that the retarding effect of phosphatase inhibitors on zeaxanthin epoxidation could not be ascribed to the maintenance of a trans-thylakoid proton gradient nor to the limitation of cosubstrates for epoxidase. The possible mechanistic basis for inhibitory effect of phosphatase inhibitors on zeaxanthin epoxidation is discussed.