低砷积累小麦品种对砷的阻遏机制

Food is an important source of inorganic arsenic (As), a non-threshold carcinogen, for human intake. An effective approach to reduce As intake is to decrease As accumulation in edible parts of crops. Wheat (Triticum aestivum) is the second most important staple food in China. Arsenic concentrations in wheat grains are very low when the plants were grown in uncontaminated soils. However, where soils were contaminated, significantly higher concentrations of As were found, which resulted in a potentially health risk for populations. Selection of wheat crops with lower As accumulation is a matter of health concern for inhabitants near the contaminated area. Therefore, it's important to understand the properties and mechanisms of As uptake and accumulation by wheat, in order to reduce inorganic As accumulation in wheat grains. .The objectives of the project are: 1) to determine As uptake kinetics, as well as As transportation and accumulation in two cultivars of winter wheat with significant difference in As accumulation; 2) to analyze As influx and efflux from roots, the expression of high affinity phosphate transporter and silicon transporter Lsi1 gene in wheat roots under As exposure, and the relationship of gene expression and As accumulation in Escherichia coli, to illustrate their roles during As transport across cell membrane of wheat root; 3)to investigate As speciations and distribution in root cells; and 4) to study the characteristics of As translocation in xylem and phloem, redistribution and As speciation in wheat grains for better understanding the repression of As transfer from soil to grains by wheats with lower As accumulation. .The results of the proposed study will provide further insights into the biological process involved in As uptake, translocation and accumulation by wheat, which is helpful to decrease As transfer from soil to edible parts of crops.

小麦是我国重要的农作物。在砷污染的条件下，小麦籽粒中的砷含量显著增加，对人体健康的危害加大，研究低砷积累小麦对砷的吸收和转运的阻遏及其机制，对生产上选育低砷积累品种，减少砷对人体的潜在风险具有重要的意义。本项目拟以两种砷积累能力差异较大的小麦栽培品种为实验材料，研究两种小麦对砷的吸收动力学过程，砷在小麦体内的转运、积累和分布特征；通过比较砷胁迫下2种小麦根系对砷的吸收和泌出量和形态、根系高亲和力磷转运蛋白PT1和硅转运通道si1基因表达，及该蛋白基因在大肠杆菌中的表达与砷吸收的关系，初步阐明这两种基因在小麦根系跨膜转运砷的过程中的作用；研究两种小麦根系砷的形态和在根系细胞中的分布、木质部和韧皮部中砷的形态和转运能力与籽粒中砷的形态变化，明确低砷积累小麦阻碍砷从土壤向小麦籽粒中迁移的可能机制。本项目的研究结果，可以为生产上选育砷低积累小麦品种、减少砷在农作物可食部分的积累提供一定的理论依据。

砷在农作物可食部位的积累是导致人体摄入砷的重要途径，降低农作物中砷的积累，是减少砷经食物链进入人体的主要方式。本项目针对农田和农作物砷污染现状，从农作物栽培品种对砷的积累差异的角度，从不同小麦品种对砷的耐性、吸收和外排、吸收动力学和砷耐性机制、砷在植物体内的分布及再分配等方面揭示低砷积累小麦对砷的阻遏机制。同时研究了磷硅对小麦砷吸收和转运的影响。研究结果表明，小麦幼苗根系和地上部积累的As以三价砷（As (III)）为主。不同品种小麦幼苗对As(V)的耐性及积累量存在显著差异。小麦幼苗对As(V)的耐性与As(V)的吸收和根系As含量呈显著正相关。对4种小麦根系砷吸收动力学的研究结果表明，小麦幼苗根系和地上部砷的积累量与根系对砷的亲和性没有显著的相关性。小麦根系各亚细胞组分中砷的含量显著不同，可溶性胞液中最多，细胞壁中次之，细胞器膜碎片中最少。小麦中砷向地上部的转运系数与可溶性胞液中砷所占的比例呈正相关。这可能与液泡的区隔化作用及巯基化合物的络合作用有关。小麦根系非蛋白巯基主要有Cys (半胱氨酸)、GSH（谷胱甘肽）和PC2（植物螯合肽），其含量在砷胁迫下显著增加。BSO（GSH合成抑制剂）可以显著抑制小麦幼苗体内GSH和PCs的合成，显著降低根系中砷的含量，增加地上部砷含量。这些结果表明，巯基化合物在不同小麦品种幼苗As(V)耐性和积累差异方面起着关键的作用。小麦籽粒、秸秆和根系中As和Cd含量在12个小麦品种之间存在显著差异。小麦籽粒中As含量与秸秆和根系中As含量没有显著相关性，而与As向籽粒中的转运系数呈正相关；籽粒中Cd的含量与Cd吸收量呈正相关。4个小麦品种根系As和Cd的浓度随着生长期的延长而显著降低，而叶片中显著增加，秸秆中则先降后升。不同的生长阶段叶片中As和Cd的浓度变化趋势相似: I < II < III < IV <V。小麦各节间As的分布规律为：V > IV > III > II > I。麦穗各部位As和Cd浓度为：穗轴 > 颖壳 > 籽粒。外源添加磷酸盐能显著增加土壤溶液、小麦秸秆和籽粒中砷的含量，叶面喷施硅对土壤溶液和籽粒中砷的含量没有显著影响。本项研究详细分析了不同小麦品种对砷的吸收和积累差异，明确了小麦根系中砷积累和向地上部转运的品种差异与根系中非蛋白巯基化合物的合成有关，小麦籽粒中As和Cd积累量主要取决于其在植物体内的分配。