中国科学院植物研究所、中国农科院作物科学研究所与澳大利亚联邦科学和工业组织合作，通过半粒种子筛选方法获得了一个糊粉层增厚的水稻品系ta2，使水稻的维生素、微量元素和膳食纤维等营养品质因子得到普遍提升。这是国际上首次发现的一种可用于培育高营养水稻的新型育种材料。该成果于10月2日在《美国科学院院刊》上在线发表。

人类70%的粮食来自禾本科作物的胚乳，胚乳由糊粉层和淀粉胚乳两部分组成。白米来自淀粉胚乳，主要成分为淀粉类碳水化合物，而外部糊粉层则富含蛋白质、维生素、膳食纤维和微量元素。尽管二者发育起源相同，但分化命运和营养物质积累迥异，这方面的研究既可以回答植物发育学领域的前沿问题，又可为禾本科作物营养改良和品质提高提供理论和技术支持。

通讯作者、中国农科院作物科学研究所研究员刘春明研究组创建了一种半粒种子筛选体系，并利用这一体系筛检了近3万粒种子获得了这样特殊的水稻品系。基因克隆发现这一表型是由于一个DNA去甲基化酶基因OsROS1显性负突变体产生，其糊粉层从一般水稻的1层细胞增加到4~10层。小麦、玉米、大麦等禾本科作物中均存在与OsROS1蛋白序列相似性很高的去甲基化酶，并在胚乳发育过程中表达。

“这一发现不仅为了解禾本科作物胚乳发育和营养物质积累的表观遗传调控提供了理论基础，我们还利用自主开发的水稻高通量TILLING平台获得了一批OsROS1基因等位突变新种质，为水稻营养品质改良育种提供了重要的遗传材料。”刘春明表示。

目前，该研究组正在尝试利用基因编辑技术调控小麦、玉米等作物中去甲基化酶ROS1基因的表达活性，有望开拓禾本科作物营养品质育种的新路径。（来源：科学网 王方）

Mutations in the DNA demethylase OsROS1 result in a thickened aleurone and improved nutritional value in rice grains

Abstract  The rice endosperm, consisting of an outer single-cell layer aleurone and an inner starchy endosperm, is an important staple food for humans. While starchy endosperm stores mainly starch, the aleurone is rich in an array of proteins, vitamins, and minerals. To improve the nutritional value of rice, we screened for mutants with thickened aleurones using a half-seed assay and identified thick aleurone 2–1 (ta2-1), in which the aleurone has 4.8 ± 2.2 cell layers on average. Except for starch, the contents of all measured nutritional factors, including lipids, proteins, vitamins, minerals, and dietary fibers, were increased in ta2-1 grains. Map-based cloning showed that TA2 encodes the DNA demethylase OsROS1. A point mutation in the 14th intron of OsROS1 led to alternative splicing that generated an extra transcript, mOsROS1, with a 21-nt insertion from the intron. Genetic analyses showed that the ta2-1 phenotype is inherited with an unusual gametophytic maternal effect, which is caused not by imprinted gene expression but rather by the presence of the mOsROS1 transcript. Five additional ta2 alleles with the increased aleurone cell layer and different inheritance patterns were identified by TILLING. Genome-wide bisulfite sequencing revealed general increases in CG and CHG methylations in ta2-1 endosperms, along with hypermethylation and reduced expression in two putative aleurone differentiation-related transcription factors. This study thus suggests that OsROS1-mediated DNA demethylation restricts the number of aleurone cell layers in rice and provides a way to improve the nutrition of rice.

原文链接：http://www.pnas.org/content/pnas/early/2018/09/26/1806304115.full.pdf