Abstract
The xanthophylls of the light-harvesting complexes of photosystem II (LHCII), zeaxanthin, and lutein are thought to be essential for non-photochemical quenching (NPQ). NPQ is a process of photoprotective energy dissipation in photosystem II (PSII). The major rapidly reversible component of NPQ, qE, is activated by the transmembrane proton gradient, and involves the quenching of antenna chlorophyll excited states by the xanthophylls lutein and zeaxanthin. Using diaminodurene (DAD), a mediator of cyclic electron flow around photosystem I, to enhance Delta pH we demonstrate that qE can still be formed in the absence of lutein and light-induced formation of zeaxanthin in chloroplasts derived from the normally qE-deficient lut2npq1 mutant of Arabidopsis. The qE induced by high Delta pH in lut2npq1 chloroplasts quenched the level of fluorescence when all PSII reaction centers were in the open state (F-o state), protected PSII reaction centers from photoinhibition, was sensitive to the uncoupler nigericin, and was accompanied by absorption changes in the 410-565 nm region. Titrations show the Delta pH threshold for activation of qE in lut2npq1 chloroplasts lies outside the normal physiological range and is highly cooperative. Comparison of quenching in isolated trimeric (LHCII) and monomeric (CP26) light-harvesting complexes from lut2npq1 plants revealed a similarly shifted pH dependency compared with wild-type LHCII. The implications for the roles of lutein and zeaxanthin as direct quenchers of excitation energy are discussed. Furthermore, we argue that the control over the proton-antenna association constant, pK, occurs via influence of xanthophyll structure on the interconnected phenomena of light-harvesting antenna reorganization/aggregation and hydrophobicity.