Study of factors controlling the photoprotection capacity of the thylakoid membrane
Plants require light for the process of photosynthesis, but excess of light absorption can cause photooxidative damage. To avoid this damage, plants have evolved a photoprotective mechanism to dissipate excess light energy as heat in a process called nonphotochemical quenching (NPQ). This regulatory mechanism of light harvesting involves both pigment and protein constituents of antenna complexes. Two xanthophylls, lutein and zeaxanthin, have been implicated to contribute to the rapidly relaxing qE component of NPQ, acting as quenchers of the chlorophyll excitation energy. To determine the molecular mechanism of NPQ and role of these xanthophylls in it, the kinetics of qE and qE-related conformational changes were measured in Arabidopsis thaliana mutant plants with altered xanthophyll contents. The effect of xanthophyll composition on the chlorophyll excited state lifetime was also compared - in leaves and native isolated antenna complexes. The data reveal that the replacement of lutein by either zeaxanthin or violaxanthin in the internal binding sites of the antenna complexes affects the qE kinetics and amplitude as well as the absolute chlorophyll fluorescence lifetime. This demonstrates the role of lutein in maintaining the efficient photoprotective state. The PsbS protein of photosystem II has also been demonstrated to play a significant role in controlling the qE component of NPQ. Thereby, enhancement of PsbS and resultant increase in qE and qE-related conformational changes was achieved in Arabidopsis by physiological and genetic means in the absence of zeaxanthin. This helps to dissect the relationship between zeaxanthin and PsbS in NPQ, suggesting both as independent entities. The results support allosteric role of zeaxanthin and not as the direct quencher alone or in combination with the PsbS in the process, whilst the role of PsbS is suggested as kinetic modulator of conformational change which results in NPQ.
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