Abstract
Plants are particularly prone to photo-oxidative damage caused by excess light. Photoprotection is essential for photosynthesis to proceed in oxygenic environments either by scavenging harmful reactive intermediates or preventing their accumulation to avoid photoinhibition. Carotenoids play a key role in protecting photosynthesis from the toxic effect of over-excitation; under excess light conditions, plants accumulate a specific carotenoid, zeaxanthin, that was shown to increase photoprotection. In this work we genetically dissected different components of zeaxanthin-dependent photoprotection. By using time-resolved differential spectroscopy in vivo, we identified a zeaxanthin-dependent optical signal characterized by a red shift in the carotenoid peak of the triplet-minus-singlet spectrum of leaves and pigment-binding proteins. By fractionating thylakoids into their component pigment binding complexes, the signal was found to originate from the monomeric Lhcb4-6 antenna components of Photosystem II and the Lhca1-4 subunits of Photosystem I. By analyzing mutants based on their sensitivity to excess light, the red-shifted triplet-minus-singlet signal was tightly correlated with photoprotection in the chloroplasts, suggesting the signal implies an increased efficiency of zeaxanthin in controlling chlorophyll triplet formation. Fluorescence-detected magnetic resonance analysis showed a decrease in the amplitude of signals assigned to chlorophyll triplets belonging to the monomeric antenna complexes of Photosystem II upon zeaxanthin binding; however, the amplitude of carotenoid triplet signal does not increase correspondingly. Results show that the high light-induced binding of zeaxanthin to specific proteins plays a major role in enhancing photoprotection by modulating the yield of potentially dangerous chlorophyll-excited states in vivo and preventing the production of singlet oxygen.
Highlights
The plant carotenoid zeaxanthin is accumulated under excess light
Effect of Zea-light-harvesting complexes (Lhc) Interaction in Limiting 1O2 Release from Arabidopsis Leaves under excess light (EL)—When photosynthetic organisms are exposed to EL, photo-oxidative stress occurs with the production of 1O2
(48), a de-stabilization of Lhc proteins is obtained; the ch1 mutation leads to a strong reduction of PS antenna size [49], whereas xanthophylls bound to Lhc in the wild type (WT) are released into the membrane [29]
Summary
The plant carotenoid zeaxanthin is accumulated under excess light. Results: Zeaxanthin induces a red shift in the carotenoid triplet excited state spectrum and reveals a higher efficiency in controlling chlorophyll triplet formation. Carotenoids play a key role in protecting photosynthesis from the toxic effect of over-excitation; under excess light conditions, plants accumulate a specific carotenoid, zeaxanthin, that was shown to increase photoprotection. Results show that the high light-induced binding of zeaxanthin to specific proteins plays a major role in enhancing photoprotection by modulating the yield of potentially dangerous chlorophyll-excited states in vivo and preventing the production of singlet oxygen. Environmental conditions unbalance the ratio between energy capture and utilization; e.g. at high photon flux densities, accumulation of excitons in the light-harvesting complexes (Lhc) of both photosystems (PS) increases the amount of 1Chl* This raises the probability of intersystem crossing to the Chl triplet state (3Chl*), a species that reacts with molecular oxygen (O2) to yield singlet oxygen (1O2) molecules [1]. Because of the high reactivity and low diffusion radius of 1O2, this reactive oxygen species (ROS) induces damage in its local environment by [2], destroying lipids and nucleic acids and proteins [3,4,5], leading to a loop of ever-
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