Role of the xanthophyll cycle in photoprotection in Arabidopsis thaliana

The xanthophyll cycle has a major role in protecting plants from photo-oxidative stress, although the mechanism of its action is unclear. Here, Arabidopsis plants overexpressing a gene encoding {3-carotene hydroxylase, containing nearly three-times the amount of xanthophyll cycle carotenoids present in the wild-type have been investigated. Although the capacity of non-photochemical quenching is not significantly different in transformed plants compared to wild-type, altered kinetics were observed. In transformed plants the kinetics of non-photochemical quenching were found to be less responsive to changes in light environment. The kinetics were found to correlate with de-epoxidation state of the xanthophyll cycle pool rather than absolute zeaxanthin concentration. This result strongly suggests zeaxanthin and violaxanthin act as allosteric regulators of non-photochemi~al quenching. In high light at low temperature wild-type plants exhibited symptoms of severe oxidative stress - lipid peroxidation, chlorophyll bleaching and photoinhibition. In transformed plants, which accumulate over twice as much zeaxanthin as the wild-type, these symptoms were significantly ameliorated. As no enhancement in nonphotochemical quenching was detected an enhancement of this mechanism cannot be the cause of the stress tolerant phenotype. Rather, It is concluded that it results from the antioxidant effect of zeaxanthin. 80-90% of violaxanthin and zeaxanthin in wild-type and transformed plants was localized to an oligomeric fraction of the light harvesting complex of photosystem II (LHCII) prepared from thylakoid membranes. The binding of these pigments in intact membranes was confirmed by resonance Raman spectroscopy. Based on the structural model of LHCII, it is suggested that the protein/lipid interface is the active site for the anti-oxidant activity of zeaxanthin, which mediates stress tolerance by the protection of bound lipids. Therefore both roles of the xanthophyll cycle in photoprotection involve the same conserved binding sites in the thylakoid membrane.EThOS - Electronic Theses Online ServiceGBUnited Kingdo