Mechanistic aspects of xanthophyll cycle‐dependent photoprotection in higher plant chloroplasts and leaves

Abstract

Higher plants must dissipate absorbed light energy that exceeds the photosynthetic capacity to avoid molecular damage to the pigments and proteins that comprise the photosynthetic apparatus. Described in this minireview is a current view of the biochemical, biophysical and bioenergetic aspects of the primary photoprotective mechanism responsible for dissipating excess excitation energy as heat from photosystem II (PSII). The photoprotective heat dissipation is measured as nonphotochemical quenching (NPQ) of the PSII chlorophyll a (Chl a) fluorescence. The NPQ mechanism is controlled by the trans‐thylakoid membrane pH gradient (ΔpH) 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) strongly affect their interactions with protonated chlorophyll binding proteins (CPs) of the PSII inner antenna. The CP protonation sites and steps are influenced by proton domains sequestered within the proteo‐lipid core of the thylakoid membrane. Xanthophyll cycle enrichment around the CPs may explain why changes in the peripheral PSII antenna size do not necessarily affect either the concentration of the xanthophyll cycle pigments on a per PSII unit basis or the NPQ mechanism. Recent time‐resolved PSII Chi a fluorescence studies suggest the NPQ mechanism switches PSII units to an increased rate constant of heat dissipation in a series of steps that include xanthophyll de‐epoxidation, CP‐protonation and binding of the xanthophylls to the protonated CPs; the concerted process can be described with a simple two‐step, pH‐activation model. The xanthophyll cycle‐dependent NPQ mechanism is profoundly influenced by temperatures suboptimal for photosynthesis via their effects on the trans‐thylakoid membrane energy coupling system. Further, low temperature effects can be grouped into either short term (minutes to hours) or long term (days to seasonal) series of changes in the content and composition of the PSII pigment‐proteins. This minireview concludes by briefly highlighting primary areas of future research interest regarding the NPQ mechanism.