NIP aquaporins: new tools to reduce rice arsenic accumulation

大米是世界上大约一半人口的主食。在主要粮食作物中，水稻在砷的积累方面尤其有效，而砷是有毒和致癌的。这种积累构成了潜在的严重健康风险，因为对于世界上任何地方以大米为食的人来说，食用大米是无机砷摄入量的很大一部分。由于过去使用含砷除草剂和杀虫剂、采矿和用砷污染的地下水灌溉，许多水稻产区的问题更加严重。迫切需要制定战略，以减少这种对食物链的广泛污染。这需要更好地了解水稻对砷的吸收、运输和分配机制。在好氧土壤中，砷酸盐是砷的主要化学种类，而在淹水水稻土的还原性环境中，砷酸盐形式占主导地位。我们最近发现亚砷酸盐通过硅吸收途径被水稻根系吸收。水稻积累了大量的硅，这保护植物免受生物和非生物胁迫。一种叫做NIP2的水通道蛋白；1向根细胞输送硅，不经意间也输送亚砷酸盐。在水稻中有10个NIP蛋白家族，其中一些主要在叶片和籽粒组织中表达。我们假设这些NIP通道蛋白中的一些参与了砷向稻谷的运输。我们将利用一系列分子和生理方法评估NIP蛋白在砷向叶片和稻谷分布中的作用。我们将研究籽粒发育过程中不同NIP基因在叶片和籽粒组织中的表达模式以及NIP蛋白对砷化合物的转运功能。我们将确定不同NIP蛋白对砷化合物的特异性，并操纵蛋白质关键区域的氨基酸组成来改变它们对砷的转运选择性。本项目的研究结果将有助于深入了解砷在植物体内的转运机制，并有助于通过分子育种或转基因方法制定减少水稻籽粒中砷积累的对策。

Rice is the staple food for about half of the world's population. Among major food crops, rice is especially efficient at the accumulation of arsenic which is toxic and carcinogenic. This accumulation presents a potentially serious health risk, because consumption of rice contributes a large proportion of inorganic As intake for people living on a rice-diet anywhere in the world. The problem is exacerbated in many rice-producing regions by the past use of arsenic-based herbicides and insecticides, mining, and irrigation with arsenic-contaminated groundwater. There is an urgent need to develop strategies to reduce this widespread contamination of the food chain. This requires a better understanding of the mechanisms responsible for uptake, transport and distribution of arsenic into rice grain. Unlike aerobic soils where arsenate is the predominant chemical species of arsenic, the arsenite form dominates in the reducing environment of flooded paddy soils. We have recently discovered that arsenite is taken up by rice roots through the silicon uptake pathway. Rice accumulates a large amount of silicon, which protects the plant against biotic and abiotic stresses. An aquaporin channel protein called NIP2;1 transports silicon, and also inadvertently arsenite, into the root cells. There is a family of 10 NIP proteins in rice, some of which are expressed mainly in leaf and grain tissues. We hypothesise that some of these NIP channel proteins are involved in arsenic transport to the rice grain. We will evaluate the role of NIP proteins in arsenic distribution to the leaf and rice grain using a range of molecular and physiological methods. We will investigate the pattern of expression of different NIP genes in leaf and grain tissues during grain development and the transport function of NIP proteins for arsenic compounds. We will determine the specificity of different NIP proteins for arsenic compounds and manipulate the amino acid composition in a key region of the proteins to alter their transport selectivity for arsenic. Results obtained from this project will provide insight into the mechanisms of arsenic transport in plants and help the development of counter measures to reduce arsenic accumulation in rice grain through molecular breeding or transgenic approaches.