Dynamical and allosteric regulation of photoprotection in light harvesting complex II
Hao LiYingjie WangManping YeShanshan LiDeyong LiHaisheng RenMohan WangLuchao DuHeng LiGianluigi VegliaJiali GaoYuxiang Weng
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Keywords:
Non-photochemical quenching
Photoprotection
Light-harvesting complex
Trimer
Violaxanthin
Plant Physiology
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Photoprotection
Non-photochemical quenching
Photoinhibition
Light-harvesting complex
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Under natural conditions, photosynthesis has to be adjusted to fluctuating light intensities. Leaves exposed to high light dissipate excess light energy in form of heat at photosystem II (PSII) by a process called non-photochemical quenching (NPQ). Upon fast transition from light to shade, plants lose light energy by a relatively slow relaxation from photoprotection. Combined overexpression of violaxanthin de-epoxidase (VDE), PSII subunit S (PsbS) and zeaxanthin epoxidase (ZEP) in tobacco accelerates relaxation from photoprotection, and increases photosynthetic productivity. In Arabidopsis, expression of the same three genes (VPZ) resulted in a more rapid photoprotection but growth of the transgenic plants was impaired. Here we report on VPZ expressing potato plants grown under various light regimes. Similar to tobacco and Arabidopsis, induction and relaxation of NPQ was accelerated under all growth conditions tested, but did not cause an overall increased photosynthetic rate or growth of transgenic plants. Tuber yield of VPZ expressing plants was unaltered as compared to control plants under constant light conditions and even decreased under fluctuating light conditions. Under control conditions, levels of the phytohormone abscisic acid (ABA) were found to be elevated, indicating an increased violaxanthin availability in VPZ plants. However, the increased basal ABA levels did not improve drought tolerance of VPZ transgenic potato plants under greenhouse conditions. The failure to benefit from improved photoprotection is most likely caused by a reduced radiation use efficiency under high light conditions resulting from a too strong NPQ induction. Mitigating this negative effect in the future might help to improve photosynthetic performance in VPZ expressing potato plants.
Photoprotection
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Non-photochemical quenching
Antheraxanthin
Photoinhibition
Light intensity
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Photoinhibition is the light-induced reduction in photosynthetic efficiency and is usually associated with damage to the D1 photosystem II (PSII) reaction centre protein. This damage must either be repaired, through the PSII repair cycle, or prevented in the first place by nonphotochemical quenching (NPQ). Both NPQ and D1 repair contribute to light tolerance because they ensure the long-term maintenance of the highest quantum yield of PSII. However, the relative contribution of each of these processes is yet to be elucidated. The application of a pulse amplitude modulation fluorescence methodology, called protective NPQ, enabled us to evaluate of the protective effectiveness of the processes. Within this study, the contribution of NPQ and D1 repair to the photoprotective capacity of Arabidopsis thaliana was elucidated by using inhibitors and mutants known to affect each process. We conclude that NPQ contributes a greater amount to the maintenance of a high PSII yield than D1 repair under short periods of illumination. This research further supports the role of protective components of NPQ during light fluctuations and the value of protective NPQ and qPd as unambiguous fluorescence parameters, as opposed to qI and Fv /Fm , for quantifying photoinactivation of reaction centre II and light tolerance of photosynthetic organisms.
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Non-photochemical quenching
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Abstract To avoid photodamage plants regulate the amount of excitation energy in the membrane at the level of the light-harvesting complexes (LHCs). It has been proposed that the energy absorbed in excess is dissipated via protein conformational changes of individual LHCs. However, the exact quenching mechanism remains unclear. Here we study the mechanism of quenching in LHCs that bind a single carotenoid species and are constitutively in a dissipative conformation. Via femtosecond spectroscopy we resolve a number of carotenoid dark states, demonstrating that the carotenoid is bound to the complex in different conformations. Some of those states act as excitation energy donors for the chlorophylls, whereas others act as quenchers. Via in silico analysis we show that structural changes of carotenoids are expected in the LHC protein domains exposed to the chloroplast lumen, where acidification triggers photoprotection in vivo. We propose that structural changes of LHCs control the conformation of the carotenoids, thus permitting access to different dark states responsible for either light harvesting or photoprotection.
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Solar energy is used by
photosynthetic organisms to drive energy required cellular processes. Is
absorbed by two groups of pigments, located in the LHCs. These proteins are
essential for the performance of photosynthesis, because they are involved in
harvesting the light and because they
protect the photosynthetic system from excess of light that cause photodamage.
I performed in vitro studies mimicking the two functions of LHCII by inserting
the protein in nanodiscs and in liposomes. I demonstrate that Chl excitation
quenching is dependent on protein-protein interactions.
I investigated the specific interactions of LHCII with PsbS. The fluorescence
study of our minimal membrane models strongly suggests that the pH-dependent
role of PsbS lies in creating membrane rearrangements and supercomplex
remodeling that could facilitate LHCII aggregation quenching.
I successfully produced 13C lutein-rLhcb1 protein in detergent, mimicking the
unquenched state, and protein aggregates, mimicking the quenched state, were
biochemically and spectroscopically characterized and further analysed with
solid state NMR.Ring current shifts of the lutein head signals indicate that
the heads are in close proximity to specific Chls (Chl a610 and Chl a602),
providing for the first-time structural information about lutein-Chl
interactions in LHCII in its unquenched state.
Photoprotection
Non-photochemical quenching
Light-harvesting complex
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Photoprotection
Phycobilisome
Non-photochemical quenching
Light-harvesting complex
Photoinhibition
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