Monogalactosyldiacylglycerol (MGDG) is the main lipid constituent of thylakoids and a structural component of photosystems and photosynthesis-related proteo-lipid complexes in green tissues. Previously reported changes in MGDG abundance upon stress treatments are hypothesized to reflect mobilization of MGDG-based polyunsaturated lipid intermediates to maintain extraplastidial membrane integrity. While exchange of lipid intermediates between compartmental membranes is well documented, physiological consequences of mobilizing an essential thylakoid lipid, such as MGDG, for an alternative purpose are not well understood. Arabidopsis seedlings exposed to mild (50 mM) salt treatment displayed significantly increased abundance of both MGDG and the extraplastidial lipid, phosphatidylcholine (PC). Interestingly, similar increases in MGDG and PC were observed in Arabidopsis fad3 mutant seedlings defective in endoplasmic reticulum (ER)–localized linolenic acid formation, in which compensatory plastid-to-ER-directed mobilization of linolenic acid–containing intermediates takes place. The postulated (salt) or evident (fad3) plastid–ER exchange of intermediates concurred with altered thylakoid function according to parameters of photosynthetic performance. While salt treatment of wild-type seedlings inhibited photosynthetic parameters in a dose-dependent manner, interestingly, untreated fad3 mutants did not show overall reduced photosynthetic quantum yield. By contrast, we observed a reduction specifically of non-photochemical quenching (NPQ) under high light, representing only part of observed salt effects. The decreased NPQ in the fad3 mutant was accompanied by reduced activity of the xanthophyll cycle, leading to a reduced concentration of the NPQ-effective pigment zeaxanthin. The findings suggest that altered ER-located fatty acid unsaturation and ensuing inter-organellar compensation impacts on the function of specific thylakoid enzymes, rather than globally affecting thylakoid function.

中文翻译：

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拟南芥 fad3 突变体中内质网定位缺陷的亚麻酸形成对类囊体脂质组成、叶绿素荧光的非光化学猝灭和叶黄素循环活性的细胞间影响

单半乳糖二酰基甘油（MGDG）是类囊体的主要脂质成分，也是绿色组织中光系统和光合作用相关蛋白脂质复合物的结构成分。先前报道的应激处理后 MGDG 丰度的变化被假设反映了基于 MGDG 的多不饱和脂质中间体的动员，以维持质体外膜的完整性。虽然区室膜之间脂质中间体的交换已有充分记录，但出于替代目的而动员必需的类囊体脂质（例如 MGDG）的生理后果尚不清楚。暴露于温和 (50 mM) 盐处理的拟南芥幼苗显示出 MGDG 和质体外脂质磷脂酰胆碱 (PC) 的丰度显着增加。有趣的是，在内质网（ER）局部亚麻酸形成缺陷的拟南芥fad3突变体幼苗中观察到MGDG和PC的类似增加，其中发生了质体到ER定向的含亚麻酸中间体的补偿性动员。根据光合性能参数，假定的（盐）或明显的（fad3）质体-ER 中间体交换与类囊体功能的改变相一致。虽然野生型幼苗的盐处理以剂量依赖性方式抑制光合参数，但有趣的是，未经处理的 fad3 突变体并未表现出整体光合量子产率降低。相比之下，我们观察到高光下非光化学猝灭（NPQ）的减少，仅代表观察到的盐效应的一部分。fad3 突变体中 NPQ 的降低伴随着叶黄素循环活性的降低，导致 NPQ 有效色素玉米黄质的浓度降低。研究结果表明，内质网脂肪酸不饱和度的改变和随后的细胞器间补偿会影响特定类囊体酶的功能，而不是整体影响类囊体功能。