水稻砷酸还原酶OsHAC4调控As(Ⅴ)抗性的分子机制

Arsenic (As) contamination is widespread in paddy soils in south and southeast Asia due to mining and irrigation of As-contaminated groundwater. Build-up of As in paddy soils can not only lead to phytotoxicity and substantial yield losses in rice, but also increase As accumulation in rice grain, thus posing a risk to food safety and human health. It is therefore important to understand the mechanisms of As tolerance and accumulation in rice in order to develop strategies to minimise the risk of As contamination in soil.It is clear that As(V) reduction is a key step of its detoxification in plants. Following As(V) uptake, As(V) is readily reduced to As(III), suggesting a high capacity of As(V) reduction.Until recently, two recent studies have independently identified a new As(V) reductase in Arabidopsis thaliana, named HAC1 (High As Content 1) or ATQ1 (Arsenic Tolerance QTL 1). However, detail molecular mechamism of As(V) tolerance regulated by arsenate reductase in planta Is still not clear. In the present study, we isolated an As(V) hypersensitive mutant of rice and identified OsHAC4 as the causal gene for the mutant phenotype. Our results show that OsHAC4 encodes an As(V) reductase that could be critical for As(V) tolerance. It is possible to play an important role in controlling As accumulation in rice plants.

由于栽培模式的原因，在东南亚地区水稻田的砷污染是广泛的。富集在水稻地里的砷不但导致植物的毒害，水稻产量的下降，而且增加了谷粒里的砷含量，进而威胁到人类的健康。因此了揭示植物对砷抗性和砷积累的分子机制，有利于我们制定合理的措施来减少土壤砷带来的风险。在植物体内，As(V)还原是植物去毒性关键的一步。随着As(V) 的吸收，As(V)很快就在植物体内还原成 As(III)，这表明植物体内存在着很强As(V)还原能力。直到最近，两个独立的研究首次鉴定出植物中新的砷酸还原酶HAC1或ATQ1。但是，砷酸还原酶调控砷抗性的分子机制研究还是很缺乏的。在我们目前的研究中，我们从水稻分离到一个 As(V) 超敏突变体，最后鉴定出OsHAC4 是导致As(V)超敏的目的基因。我们的结果显示OsHAC4编码一个砷酸还原酶，可能参与了调控As(V)抗性，并在控制水稻砷积累上发挥关键的作用。

通过水稻EMS突变体库我们筛选到两个个砷酸盐敏感突变体，通过构建遗传群体，利用重测序和回复试验分别鉴定到两个目标基因HAC4 和LPD1。通过相关的分子和生理生化试验，揭示了HAC4是水稻一个关键的砷酸还原酶，通过高丰度表达在根系的表皮与外皮层细胞中，快速还原砷酸根离子，并将一部分还原的亚砷酸通过水通道蛋白外排到体外，从而提高水稻的抗砷性。通过对同源的HACs家族蛋白的研究发现，砷酸还原酶活性的一个关键位点以及HAC7蛋白可能参与水稻二甲基砷的抗性。另外，对LPD1的研究结果表明，质体定位的LPD1的突变显著的影响根细胞的NAD+/NADH的平衡，从而降低根细胞的还原力，导致ROS的增加。此外，研究结果揭示超表达OsHAC4基因的水稻材料可以降低水稻地上部砷含量，为保障粮食安全、发展绿色水稻提供了一个关键的基因资源。