Modulation of herbicide-binding by the redox state of Q 400, an endogenous component of Photosystem II

Abstract

Preincubation of isolated chloroplasts with ferricyanide, prior to addition of DCMU, unmasks a high-potential electron acceptor (Q 400) in Photosystem II that acts as an additional quencher and prolongs the fluorescence induction curve in the presence of DCMU (Ikegami, I. and Katoh, S. (1973) Plant Cell Physiol. 14, 829–836). This study confirms that Q 400 is endogenous to Photosystem II and is not a bound ferricyanide, and several new characteristics of this high potential acceptor are established. (a) It is accessible to ferricyanide even in the presence of DCMU. The rate of oxidation, however, is very slow, consistent with access only via Q A. Accessibility may be enhanced by magnesium, reminiscent of the oxidation of Q − A by ferricyanide. (b) Oxidation of Q 400 drastically suppresses the binding of DCMU at neutral and alkaline pH. Below pH 6, however, DCMU binding is essentially normal. The pH dependence of DCMU binding is consistent with the known pH dependence of the redox midpoint potential of Q 400. (c) Binding of many other inhibitors of Q A-to-Q B electron transfer is much less affected or even completely unaffected. These results have implications for current notions of herbicide binding and may also bear on the origin of slow phases of fluorescence induction in the presence of DCMU.