Kobayashi Koichi, Fujii Sho, Sato Mayuko, Toyooka Kiminori, Wada Hajime
Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo, 153-8902, Japan.
Plant Cell Rep. 2015 Apr;34(4):631-42. doi: 10.1007/s00299-014-1719-z. Epub 2014 Dec 5.
With phosphate deficiency, the role of phosphatidylglycerol is compensated by increased glycolipid content in thylakoid membrane biogenesis but not photosynthetic electron transport in Arabidopsis chloroplasts. In plants and cyanobacteria, anionic phosphatidylglycerol (PG) is the only major phospholipid in thylakoid membranes, where neutral galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are predominant. In addition to provide a lipid bilayer matrix, PG plays a specific role in photosynthetic electron transport. Non-phosphorous sulfoquinovosyldiacylglycerol (SQDG) is another anionic lipid in thylakoids; it substitutes for PG under phosphate (Pi) deficiency to maintain proper balance of anionic charge in thylakoid membranes. Although the crucial role of PG in photosynthesis has been deeply analyzed in cyanobacteria, its physiological function in seed plants other than photosynthesis remains unclear. To reveal specific roles of PG and functional overlaps with other thylakoid lipids, we characterized a PG-deficient Arabidopsis mutant (pgp1-2) under Pi-controlled conditions. Under Pi-sufficient conditions, the proportion of PG and other thylakoid lipids was decreased in pgp1-2, which led to severe disruption of thylakoid membrane biogenesis. Under Pi-deficient conditions, the proportion of all glycolipids in the mutant was greatly increased, with that of PG further decreased. In Pi-deficient pgp1-2, thylakoid membranes remarkably developed, which was accompanied by a change in nucleoid morphology and restored expression of nuclear- and plastid-encoded photosynthesis genes. Increase in glycolipid content with Pi deficiency may compensate for the loss of PG in terms of thylakoid membrane biogenesis. Although Pi deficiency increased chlorophyll and photosynthesis protein content in pgp1-2, it critically decreased photochemical activity in PSII. Further deprivation of PG in photosynthesis complexes may abolish the PSII activity in Pi-deficient pgp1-2, which suggests that glycolipids cannot replace PG in photosynthesis.
在拟南芥叶绿体中，缺磷时，类囊体膜生物合成中糖脂含量的增加可补偿磷脂酰甘油的作用，但光合电子传递过程不能得到补偿。在植物和蓝细菌中，阴离子型磷脂酰甘油（PG）是类囊体膜中唯一主要的磷脂，而中性半乳糖脂单半乳糖基二酰甘油（MGDG）和二半乳糖基二酰甘油（DGDG）占主导地位。除了提供脂质双分子层基质外，PG在光合电子传递中还发挥着特定作用。非磷磺基喹喔啉二酰甘油（SQDG）是类囊体中的另一种阴离子脂质；在缺磷（Pi）条件下，它可替代PG以维持类囊体膜中阴离子电荷的适当平衡。尽管PG在光合作用中的关键作用已在蓝细菌中得到深入分析，但其在种子植物中除光合作用外的生理功能仍不清楚。为了揭示PG的特定作用及其与其他类囊体脂质的功能重叠，我们在Pi控制条件下对一个PG缺陷型拟南芥突变体（pgp1-2）进行了表征。在Pi充足的条件下，pgp1-2中PG和其他类囊体脂质的比例降低，导致类囊体膜生物合成严重受损。在缺磷条件下，突变体中所有糖脂的比例大幅增加，而PG的比例进一步降低。在缺磷的pgp1-2中，类囊体膜显著发育，同时伴随着类核形态的变化以及核编码和质体编码的光合作用基因表达的恢复。缺磷时糖脂含量的增加可能在类囊体膜生物合成方面补偿了PG的损失。尽管缺磷增加了pgp1-2中的叶绿素和光合作用蛋白含量，但它严重降低了PSII中的光化学活性。光合作用复合物中PG的进一步缺失可能会消除缺磷的pgp1-2中的PSII活性，这表明糖脂在光合作用中不能替代PG。
