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首页> 外文期刊>Current Science: A Fortnightly Journal of Research >Photoprotection of green leaves by zeaxanthin, a two-channel process
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Photoprotection of green leaves by zeaxanthin, a two-channel process

机译：玉米黄芩苷的绿色叶子的光保护，双通道工艺

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The green plants have developed the skill to not only harvest light with great efficiency, but also to dispose of the excess energy with safety. The excess light energy can cause damage to the photosynthetic apparatus of green leaves through production of singlet oxygen and other harmful oxidative species. The photosystem-II (PSI1) of the thylakoid membrane of the chloroplast is the most vulnerable site of damage, obviously, due to the higher oxidizing potential of the special pigment P680 located at the reaction centre (RCII) of the photosystem. Such a high potential (estimated to be 1.17 V) is required for extraction of electrons by cleavage of water that takes place at the water-oxidizing complex. The RCII is supplemented with a group of light-harvesting antenna proteins (CP47, CP43 and LHCII) containing chlorophylls (Chi) and carotenoids. The carotenoids (carotenes and xanthophylls), besides acting as accessory pigments, are associated with photoprotection. One of the important mechanisms of photoprotection in higher plants is the dissipation of excess light energy through the xanthophyll cycle, with the formation of zeaxanthin from violaxanthin. At higher irradiances, a pH gradient develops across the thylakoid membrane, which triggers the conversion of diepoxide violaxanthin (number of conjugated double bonds, n = 9) to zeaxanthin (n = I 1) through the monoepoxide antheraxanthin (n = 10; Figure 1). The de-epoxidized xanthophyll, zeaxanthin, is known to facilitate dissipation of excess light energy by thermal relaxation. The three major competitive routes for quenching of light energy in the thylakoid are photochemistry, thermal dissipation and emission of light, primarily by Chla fluorescence. Therefore, an increase in rate of thermal relaxation is reflected in a decrease in fluorescence, called non-photochemical quenching of Chla fluorescence (NPQ). In spite of the accumulation of vast literature on energy quenching (qE) by the xanthophyll cycle, its exact location and molecular mechanism remain obscure. Moreover, the modus operandi of zeaxanthin in qE is still controversial. A group of workers suggest that zeaxanthin can directly quench the singlet Chi energy due to its lower S_1 energy level. On the other hand, Ruban et al. describe zeaxanthin as a modulator of qE. Their experiments, of course with isolated LHCII, reveal that violaxanthin causes disaggregation of LHCs favouring light harvesting and its de-epoxidation to zeaxanthin triggers a quenched state by LHC aggregation. Experiments with mutants of Chlamydomonas suggest that lutein, in addition to zeaxanthin and antheraxanthin, contributes to NPQ. Although most of the studies accentuate the role of zeaxanthin in NPQ, the antioxidant effect of the xanthophyll cycle, with zeaxanthin as a better quencher of singlet oxygen, has also been demonstrated. In a recent paper, Li et al. working with Arabidopsis mutant, emphasize that a 22-kDa PsbS protein has an exclusive role in, and is essential for photoprotective energy dissipation. In the same paper, they conclude that the appearance of LHC proteins involved in photoprotection precedes those involved in light harvesting during the evolution of oxygenic photosynthesis.
机译：绿色植物已经开发了技能，不仅能够以效率的效率而收获光，而且为了安全地处理过量的能量。通过生产单线氧和其他有害的氧化物种，过量的光能可能导致绿叶的光合液损坏。叶绿体上囊体膜的光照系统-II（psi1）是最脆弱的损伤部位，显然是由于位于光系统的反应中心（Rcii）的特殊颜料P680的氧化潜力较高。通过在水 - 氧化复合物中裂解水分来提取电子需要这种高电位（估计为1.17V）。 RCII补充有一组含有叶绿素（CHI）和类胡萝卜素的光收集天线蛋白（CP47，CP43和LHCII）。除了用作配件颜料之外，类胡萝卜素（类胡萝卜素和Xanthophyls）与光保护相关。高等植物中光保护的重要机制是通过叶黄素循环耗散过量的光能，从紫外线中形成玉米黄质。在较高的辐射术中，pH梯度在囊体膜上发育，其通过单氧化氮杂素（n = 10;图1，触发Diemoxide violaxanthin（缀合的双键数，n = 9）转化为Zeaxanthin（n = i 1）（n = 10;图1 ）。已知脱氧氧化胺，Zeaxanthin，促进通过热弛豫耗散过量的光能。三个主要竞争路线用于猝灭的光能是光化学，热耗散和光的发光，主要由CHLA荧光。因此，热弛豫率的增加在荧光的降低中反映，称为非光化学猝灭的CHLA荧光（NPQ）。尽管叶蝉循环对能量猝灭（QE）积累了大量文献，但其确切的位置和分子机制仍然模糊不清。此外，QE中的玉米黄芩苷的Modus operandi仍然存在争议。一组工人表明，玉米蛋白由于其下部S_1能量水平而直接终止单线态能量。另一方面，Ruban等人。描述Zeaxanthin作为QE的调节剂。当然，他们的实验当然是含有分离的LHCII，揭示了violaxanthin导致LHC的分解，LHC有利于光收获，并且其脱氧对ZeAxanthin触发通过LHC聚集的猝灭状态。含有衣原体突变体的实验表明，除了玉米黄芩苷和氧化体之外，叶黄素有助于NPQ。虽然大多数研究突出了玉米黄素在NPQ中的作用，但番茄素循环的抗氧化效果也已经证明了Zeaxanthin作为单线氧的更好的淬火器。在最近的一篇论文中，李等人。与拟南芥突变体一起使用，强调22 kDa psbs蛋白在光保护能量耗散方面具有独特的作用。在相同的纸张中，他们得出结论，参与光保护的LHC蛋白的出现在含氧光合作用的演变期间参与光收获的那些。

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