Molecular Mechanisms for Activation of Non-Photochemical Fluorescence Quenching: From Unicellular Algae to Mosses and Higher Plants

Summary

All oxygenic photosynthetic organisms have mechanisms for regulation of light-harvesting efficiency in response to variable light intensity. Non-photochemical quenching of chlorophyll fluorescence (NPQ) is a marker for the rapid dissipation of excess excitation energy as heat, activated in order to prevent formation of reactive oxygen species. Although widespread among oxygenic photosynthetic organisms, NPQ is activated through distinct molecular effectors depending on taxa. In unicellular green algae as well as other algal groups (such as diatoms), NPQ activity depends on a light-harvesting complex (LHC)-like protein, called light-harvesting-complex stress-related (LHCSR). In land plants, such as Arabidopsis thaliana, NPQ instead relies on a different LHC-like protein: PsbS. This protein responds to low lumenal pH via protonation of two glutamate residues essential for activity. PsbS induces a reorganization of thylakoid membrane complexes upon lumen acidification, which is indispensable for the generation of the dissipative state. Upon reorganization, two distinct quenching components are generated: one is tightly connected to the photosystem II (PS II) reaction center and the other is located in a domain of the PS II antenna system, disconnected from the reaction center. Low lumenal pH also induces activation of the violaxanthin (V) de-epoxidase enzyme, leading to the synthesis of zeaxanthin (Z), which, in turn, up-regulates heat dissipation/NPQ upon binding to antenna proteins. PsbS-dependent NPQ is present in land plants and in the moss Physcomitrella patens. Although the PsbS gene sequence is conserved in all green algae whose genome has been sequenced thus far, the corresponding protein was not found to be accumulated in algal cells under any of many growth conditions explored, suggesting it may be a pseudogene. PsbS and LHCSR play a similar role in activating protective heat dissipation of excess light by responding to low lumenal pH. In contrast to PsbS, LHCSR exhibits a clear capacity for binding xanthophylls and chlorophylls. Thus LHCSR appears to combine the pH-responsive activity typical of PsbS with the capacity for binding pigments, chlorophylls and carotenoids, as a basis for engagement of energy dissipation reflected in NPQ. LHCSR appears to directly participate in energy dissipation possibly avoiding the need for reorganization of thylakoid complexes.