Study of factors controlling the photoprotection capacity of the thylakoid membrane
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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.
Authors
Zia, AhmadCollections
- Theses [3822]