我国科学家在水稻光合效率提高分子机理研究中取得重要进展

7月28日，国际杂志《PLoS Genetics》在线发表了题为“A Rice Plastidial Nucleotide Sugar Epimerase Is Involved in Galactolipid Biosynthesis and Improves Photosynthetic Efficiency”的论文（DOI:10.1371/journal.pgen.1002196）。文章揭示，由中国科学院遗传与发育生物学研究所基因组生物学研究中心储成才领导的研究组，通过大规模筛选鉴定水稻光合能力和碳同化突变体（photoassimilate defective1, phd1），克隆和鉴定了编码一个新型UDP-葡萄糖差向异构酶（UDP-glucose epimerase, UGE）基因PHD1。以往人们一直认为UGE仅存在于细胞质中，研究组通过遗传学、分子生物学和免疫细胞学等手段，证明PHD1存在于叶绿体基质中；进一步生化实验也表明，这个酶催化生成糖脂合成底物——UDP-半乳糖的过程是在叶绿体内进行。这是首次发现UGE也存在于叶绿体中。进一步功能鉴定表明，PHD1参与了糖脂的生物合成以及光合膜的生物发生过程。因此，这项研究不仅发现了一种新型质体定位的UDP-葡萄糖差向异构酶，而且阐明了高等植物光合膜上糖脂糖基组分的来源和发生机制，并暗示了该机制在绿色植物中是保守的，为进一步研究光合膜糖脂在植物光合作用以及生长发育过程中的重要作用奠定了基础。光合作用是绿色植物及光合细菌在光下利用光合色素将二氧化碳和水转化为碳水化合物并释放氧气的过程，是整个生物界赖以生存的基础。提高光合作用效率是农作物增产的一个根本途径。

光合作用在绿色植物所特有的细胞器——叶绿体中进行，存在于叶绿体上的光合膜含有丰富的糖脂（半乳糖甘油酯），而UDP-半乳糖是合成这些糖脂的主要供体。目前有关光合膜上糖脂组装的遗传和生化分析已有研究，但对糖脂合成过程中UDP-半乳糖底物供应来源和产生机制一直不甚清楚。

phd1突变体中糖脂的不足严重影响了光合色素的含量并降低了有效光化学效率。有意思的是，在过表达PHD1的转基因株系中决定水稻产量的两个主要因子：有效穗数和籽粒千粒重都显著增加，并最终提高了水稻产量。PHD1不仅对水稻分蘖、千粒重及株高等产生适度(fine-tuning)促进效应，尤为重要的是，过表达植株的光合能力，尤其是在低光强下光合能力得到显著提高。因此，PHD1为新一代水稻以及能源作物高产育种，特别对西南如云贵川等光强不足地区的作物高产育种改良提供了一种全新的思路，具有显著的潜在应用价值和经济意义。

论文摘要：

Photosynthesis is the final determinator for crop yield. To gain insight into genes controlling photosynthetic capacity, we selected from our large T-DNA mutant population a rice stunted growth mutant with decreased carbon assimilate and yield production named photoassimilate defective1 (phd1). Molecular and biochemical analyses revealed that PHD1 encodes a novel chloroplast-localized UDP-glucose epimerase (UGE), which is conserved in the plant kingdom. The chloroplast localization of PHD1 was confirmed by immunoblots, immunocytochemistry, and UGE activity in isolated chloroplasts, which was approximately 50% lower in the phd1-1 mutant than in the wild type. In addition, the amounts of UDP-glucose and UDP-galactose substrates in chloroplasts were significantly higher and lower, respectively, indicating that PHD1 was responsible for a major part of UGE activity in plastids. The relative amount of monogalactosyldiacylglycerol (MGDG), a major chloroplast membrane galactolipid, was decreased in the mutant, while the digalactosyldiacylglycerol (DGDG) amount was not significantly altered, suggesting that PHD1 participates mainly in UDP-galactose supply for MGDG biosynthesis in chloroplasts. The phd1 mutant showed decreased chlorophyll content, photosynthetic activity, and altered chloroplast ultrastructure, suggesting that a correct amount of galactoglycerolipids and the ratio of glycolipids versus phospholipids are necessary for proper chloroplast function. Downregulated expression of starch biosynthesis genes and upregulated expression of sucrose cleavage genes might be a result of reduced photosynthetic activity and account for the decreased starch and sucrose levels seen in phd1 leaves. PHD1 overexpression increased photosynthetic efficiency, biomass, and grain production, suggesting that PHD1 plays an important role in supplying sufficient galactolipids to thylakoid membranes for proper chloroplast biogenesis and photosynthetic activity. These findings will be useful for improving crop yields and for bioenergy crop engineering. 详细信息：

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