MECHANISTIC ASPECTS OF XANTHOPHYLL CYCLE-DEPENDENT PHOTOPROTECTION INHIGHER-PLANT CHLOROPLASTS AND LEAVES

Higher plants must dissipate absorbed light energy that exceeds the ph otosynthetic capacity to avoid molecular damage to the pigments and pr oteins that comprise the photosynthetic apparatus. Described in this m inireview is a current view of the biochemical, biophysical and bioene rgetic aspects of the primary photoprotective mechanism responsible fo r dissipating excess excitation energy as heat from photosystem II (PS II). The photoprotective heat dissipation is measured as nonphotochemi cal quenching (NPQ) of the PSII chlorophyll a (Chi a) fluorescence. Th e NPQ mechanism is controlled by the trans thylakoid membrane pH gradi ent (Delta pH) and the special xanthophyll cycle pigments. In the NPQ mechanism, the de-epoxidized endgroup moieties and the trans-thylakoid membrane orientations of antheraxanthin (A) and zeaxanthin (Z) strong ly affect their interactions with protonated chlorophyll binding prote ins (CPs) of the PSII inner antenna. The CP protonation sites and step s are influenced by proton domains sequestered within the proteo-lipid core of the thylakoid membrane. Xanthophyll cycle enrichment around t he CPs may explain why changes in the peripheral PSII antenna size do not necessarily affect either the concentration of the xanthophyll cyc le pigments on a per PSII unit basis or the NPQ mechanism. Recent time -resolved PSII Chi a fluorescence studies suggest the NPQ mechanism sw itches PSII units to an increased rate constant of heat dissipation in a series of steps that include xanthophyll de-epoxidation, CP-protona tion and binding of the xanthophylls to the protonated CPs; the concer ted process can be described with a simple two-step, pH-activation mod el. The xanthophyll cycle-dependent NPQ mechanism is profoundly influe nced by temperatures suboptimal for photosynthesis via their effects o n the trans-thylakoid membrane energy coupling system. Further, low te mperature effects can be grouped into either short term (minutes to ho urs) or long term (days to seasonal) series of changes in the content and composition of the PSII pigment-proteins. This minireview conclude s by briefly highlighting primary areas of future research interest re garding the NPQ mechanism.