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Protection of plants against light stress: role of structural rearrangements in the light harvesting antenna system of photosystem II


A basic feature of the photosynthetic system of higher plants is its ability to be regulated by short-term variations in the external environmental conditions. This is achieved by multilevel regulatory processes, in which structural rearrangements in the hyracoid membranes evidently play an important role. When the illumination experienced by a plant exceeds the level that can be used in photosynthesis, protective mechanisms are induced, which prevent this excess excitation energy from causing sustained damage to the plant. In hyaloids exposed thigh light different components of non-photochemical quenching (NPQ) of the singled excited states of Chi-a can down-regulate the photosynthetic energy conversion. NPQ consists mainly of two types of quenching: a rapidly relaxing we form dependent upon the Depth and the xanthophylls cycle and a more sustained all form occurring at higher irradiance about which very little is known.LHCII is capable of undergoing light-induced reversible structural changes and fluorescence quenching in away resembling NPQ in granola hyaloids.
Recently, in a joint work with the host laboratory, we have shown that excess light induces a significant degree of monomerization, which is in contrast with the preferentially trimericorganization of the isolated complexes in the dark; disruption of trimmers to monomers also occurs in isolatedthylakoid membranes and whole plants. These structural reorganizations are explained in terms of thermo-optic mechanism, effect of fast thermal transients due to dissipated photon energies that lead to elementary structural transitions; they are likely to provide plants with a novel regulatory mechanism in excess light. In this project, with a combination of biochemical and spectroscopy techniques, and investigating the role of different antenna components, in mutants, a multidisciplinary approach will be applied to elucidate the nature, mechanism and physiological significance.

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