Suppressive Effects of Nitric Oxide (NO) on Photosynthetic Electron Transport System

Abstract

The air pollution studies have shown that nitric oxide (NO), a gaseous free radical, could inhibit photosynthesis. However, the mechanism for the inhibitory effects of NO on molecular basis remains to be clarified. By using S-nitroso-N-acetyl-DL-penicillamine (SNAP) as an NO donor, we demonstrate here that NO suppresses in vitro photosynthetic electron transport activity of thylakoid membranes. The activity of the linear electron transport from water to methylviologen was inhibited by NO. The extent of the steady-state D pH formation was also decreased by the presence of NO, which resulted in a severe inhibition of the ATP synthesis. We investigated changes in chlorophyll a florescence from photosystem II (PSII) to examine effects of NO on the partial electron transport activity. The electron transport rate of PSII (ETR) was inhibited by NO whereas the maximum efficiency of PSII (Fv/Fm) was not affected. Under these conditions, NO was consumed by thylakoid membranes upon illumination. The extent of NO consumption was light dependent and was not decreased by the inhibitor DUMU that binds to the QB site of PSII. We conclude that NO suppresses the electron transport activity of PSII. The mechanism of the inhibition will be discussed in terms of NO as an alternative electron acceptor.