我国水稻细胞壁合成底物运送的分子机理研究获重要进展

细胞壁是由纤维素、半纤维素和果胶构成的复杂多糖网络结构，也是植物膨压驱动细胞生长的物质基础。水稻细胞壁研究对于抗倒伏和水稻植株形态等农艺性状的改良具有重要意义。植物细胞壁多糖除纤维素在质膜上合成外，其他多糖主要在高尔基体内合成。而所需底物、各种核苷糖分子（nucleotide sugar）却主要在细胞质中合成，需要通过核苷糖转运子跨膜运输到高尔基体内，参与多糖合成。研究证明，底物合成的缺陷可以影响细胞壁的合成，因此人们推测核苷糖转运子对细胞壁合成有重要影响。然而，这一假设由于缺乏遗传学证据，一直存在很大的争议。

在最新研究中，中科院遗传与发育生物学研究所周奕华领导的课题组和中国水稻研究所钱前领导的课题组通过鉴定一个新的水稻脆秆突变体bc14发现，其野生型基因编码高尔基体定位的尿苷二磷酸-葡萄糖（UDP-Glucose）转运子参与细胞壁多糖合成。对bc14突变体的详细表型分析发现，基因突变引起次生壁结构异常和纤维素含量下降，导致机械强度显著下降和生长发育缺陷。基因克隆和互补实验发现，突变表型是由核苷酸糖转运子(Oryza sativa Nucleotide Sugar Transporter1)基因的错义突变引起。水稻原生质体表达BC14/OsNST1融合荧光蛋白载体发现该蛋白定位于高尔基体中。体外酶活实验证明，该转运子具有尿苷二磷酸-葡萄糖的转运活性。细胞壁成分的详细分析证实，突变体基质多糖中葡萄糖含量明显下降。

本研究证实了高尔基体定位的核苷糖转运子在细胞壁多糖合成中的重要作用，为解析细胞壁生物合成的生化和生物学机制提供了关键的证据，解决了该领域中一个长期悬而未决的重要问题。

这项研究成果已于2011年3月7日在美国《国家科学院院刊》（PNAS）上在线发表（DOI:/10.1073/pnas.1016144108）。

论文摘要：

Golgi-localized nucleotide sugar transporters (NSTs) are considered essential for the biosynthesis of wall polysaccharides and glycoproteins based on their characteristic transport of a large number of nucleotide sugars to the Golgi lumen. The lack of NST mutants in plants has prevented evaluation of this hypothesis in plants. A previously undescribed Golgi NST mutant, brittle culm14 (bc14), displays reduced mechanical strength caused by decreased cellulose content and altered wall structure, and exhibits abnormalities in plant development. Map-based cloning revealed that all of the observed mutant phenotypes result from a missense mutation in a putative NST gene, Oryza sativa Nucleotide Sugar Transporter1 (OsNST1). OsNST1 was identified as a Golgi-localized transporter by analysis of a fluorescence-tagged OsNST1 expressed in rice protoplast cells and demonstration of UDP-glucose transport activity via uptake assays in yeast. Compositional sugar analyses in total and fractionated wall residues of wild-type andculms showed a deficiency in the synthesis of glucoconjugated polysaccharides inbc14, indicating that OsNST1 supplies the glucosyl substrate for the formation of matrix polysaccharides, and thereby modulates cellulose biosynthesis.OsNST1is ubiquitously expressed, with high expression in mechanical tissues. The inferior mechanical strength and abnormal development of bc14 plants suggest that OsNST1 has pleiotropic effects on cell wall biosynthesis and plant growth. Identification of OsNST1 has improved our understanding of how cell wall polysaccharide synthesis is regulated by Golgi NSTs in plants.