土壤-水稻植株系统中镉迁移的稳定同位素分馏示踪研究

The excessive content of cadmium (Cd) in rice grain has seriously threatened human health. How to effectively reduce the accumulation of Cd in rice grain has become an urgent environmental problem and should be resolved as soon as possible. The object of the present project is the migration rule of Cd in the system of soil-root-grain. This project focus on the migration of Cd in the system of soil-root. A combination of stable isotope fractionation and tracer, noninvasive microelectrode, electron probe and quantitative PCR technology was used in order to investigate the characteristics of isotope Cd fractionation in the system of soil-root. The simulation and pot experiment will be conducted and then the characteristics of isotope Cd fractionation soil, root, stem, leave and grain will be analyzed at key growth stages of rice. The content of Cd in grain, the expression of cadmium-related gene in root and Cd isotope composition in the system of root (apoplast, symplast and cell wall) will be analyzed for these relationship. And the main migration pathway of Cd from soil to root will be clarified. Meanwhile, the characteristic of Cd isotope fractionation in soil-rice system was investigated in the different soil pH/Eh and temperature (different soil, water management and different season). The contribute content of Cd was quantitatively analyzed in the systems of soil-root and root-grain with the influence of these factors. The results of this project could provide a new theoretical basis to effectively reduce the accumulation of Cd in rice grain.

稻米镉超标已严重威胁人类健康，如何有效控制稻米镉积累已成为亟需解决的环境问题。本申请以土壤-根系-籽粒系统中镉的迁移规律为核心科学问题，以土壤-水稻根系镉的迁移为核心过程，采用水培与土培盆栽试验体系，综合运用稳定性镉同位素分馏与示踪、非损伤微电极、电子探针与水稻镉迁移功能基因定量PCR等技术，重点研究水稻关键生育期土壤-根系中整体土壤、根际土壤、根系、茎、叶和籽粒中镉的同位素分馏特征，探究土壤-水稻植株系统中籽粒镉含量与根系(质外体、共质体和细胞壁)镉同位素组成的关系，以及根系镉相关基因表达量与镉同位素分馏的关系，明确从土壤至水稻根系镉迁移的主要途径；同时系统研究不同母质发育土壤、水分管理条件、季节所代表的土壤pH与Eh、气温等重要因素影响下，土壤-水稻系统的镉同位素分馏特征，定量评价土壤-根系与根系-籽粒系统镉迁移的相对贡献。为探索有效控制稻米镉积累的技术途径提供理论依据。

(1) 水稻倾向于吸收轻镉同位素，且铁膜作为根土界面的屏障偏爱吸附更轻的镉离子；根部优先隔离轻镉同位素和较重镉同位素转运进入地上部。水稻灌浆期，铁膜不能减缓水稻对镉的吸收作用，主要通过隔离更多的轻镉同位素于根部。茎同样保持对轻镉同位素的累积；尤其是节点Ⅰ，镉含量提高两倍且镉同位素更轻，而叶片镉含量与质量均略有降低且变重，这表明镉转运进入稻米时更多轻镉同位素被扣押累积于节点和轻镉同位素从叶片回迁至稻米。.(2) 落干条件下， OsNRAMP1 和 OsNRAMP5 的表达水平上调，证实这些转运蛋白优先吸收轻镉同位素。镉同位素在落干条件下从根到地上部呈负分馏，但在淹水条件下相 反，这与根中 CAL上调有关，促进了富集轻镉同位素的Cd-CAL 络合物装载至木质部。.镉通过与金属硫蛋白OsMT1e 的结合固定也可能有利于较轻同位素在老叶中富集。落干条件下，茎和老叶的 Cd-PCs 易被转运至顶叶中。因此，落干条件下水稻优先吸收轻镉同位素.以及从根向茎叶转运轻镉同位素，促进了更多镉累积于地上部。.(3) 水分管理都表现出相似的同位素分馏特征：相对于土壤，土壤孔隙水具有更重同位素，铁膜富集轻同位素。落干条件下，水稻叶/穗轴镉同位素比节点轻，而在淹水条件下呈分馏相反。铁膜优先吸附轻镉同位素，且淹水和落干条件改变了水稻系统中节点控制地再分布过程中镉的主要物种。.(4) 淹水条件下，水稻植株倾向于吸收重铁同位素且相比于落干条件更重，这证实根部吸收途径主要通过根际泌氧致使铁被氧化、分泌的 DMA 络合物络合(OsNAAT1 与 OsTOM2 在根部表达上调)和相继地通过根部 OsYSL15 蛋白吸收进入根部，而 Fe2+通过类似机理 I 的吸收策略则贡献较小。然而从根部至地上部，节点至叶片/穗轴的迁移转运过程，以及从叶片至稻米的过程均可产生大尺度的不同方向上铁同位素分馏。