Dipartimento di Biotecnologie, Università di Verona, Strada Le Grazie 15, 37134, Verona, Italy.
Mol Plant. 2010 May;3(3):576-93. doi: 10.1093/mp/ssp117. Epub 2010 Jan 25.
When light absorbed by plants exceeds the capacity of photosynthesis, the xanthophyll violaxanthin is reversibly de-epoxidized to zeaxanthin in the so-called xanthophyll cycle. Zeaxanthin plays a key role in the protection of photosynthetic organisms against excess light, by promoting rapidly reversible (qE) and long-term (qI) quenching of excited chlorophylls, and preventing lipid oxidation. The photoprotective role of zeaxanthin, either free or bound to light-harvesting complexes (Lhcs), has been investigated by using mutants lacking Chl b (ch1) and/or specific xanthophyll species (npq, lut2). The ch1 mutation causes (1) the absence of Lhcb proteins; (2) strong reduction of the feedback de-excitation (qE); and (3) accumulation of xanthophylls as free pigments into thylakoids. Ch1 mutants showed extreme sensitivity to photo-oxidative stress in high light, due to higher singlet oxygen (¹O₂) release. The double mutant ch1npq1 was more sensitive to photo-oxidation than ch1, showing that zeaxanthin does protect lipids even when free in the membrane. Nevertheless, lack of zeaxanthin had a much stronger impact on the level of lipid peroxidation in Lhcs-containing plants (WT vs npq1) with respect to Lhc-less plants (ch1 vs ch1npq1), implying that its protective effect is enhanced by interaction with antenna proteins. It is proposed that the antioxidant capacity of zeaxanthin is empowered in the presence of PSII-LHCs-Zea complexes, while its effect on enhancement of qE only provides a minor contribution. Comparison of the sensitivity of WT vs npq1 plants to exogenous ¹O₂ suggests that besides the scavenging of ¹O₂, at least one additional mechanism is involved in chloroplast photoprotection.
当植物吸收的光超过光合作用的能力时，叶黄素玉米黄质在所谓的叶黄素循环中可逆地去环氧化为玉米黄质。玉米黄质在保护光合生物免受过量光照方面起着关键作用，通过促进激发态叶绿素的快速可逆（qE）和长期（qI）猝灭，并防止脂质氧化。通过使用缺乏 Chl b（ch1）和/或特定叶黄素物种（npq、lut2）的突变体，研究了游离或与光捕获复合物（Lhcs）结合的玉米黄质的光保护作用。ch1 突变导致：（1）缺乏 Lhcb 蛋白；（2）反馈去激发（qE）的强烈减少；（3）叶黄素作为游离色素积累到类囊体中。Ch1 突变体在高光下对光氧化应激非常敏感，因为单线态氧（¹O₂）的释放更高。双突变体 ch1npq1 比 ch1 对光氧化更敏感，表明即使在膜中游离时，玉米黄质也能保护脂质。然而，与不含 Lhc 的植物（ch1 对 ch1npq1）相比，缺乏玉米黄质对含 Lhc 的植物（WT 对 npq1）中脂质过氧化水平的影响要大得多，这意味着它的保护作用通过与天线蛋白的相互作用得到增强。有人提出，在 PSII-LHCs-Zea 复合物存在的情况下，玉米黄质的抗氧化能力得到增强，而其对增强 qE 的作用仅提供较小的贡献。WT 对 npq1 植物对外源 ¹O₂敏感性的比较表明，除了¹O₂的清除外，至少还有一种额外的机制参与叶绿体光保护。
