水稻磷-镉互作的生理与分子调控机制

Cadmium (Cd) contamination of the farmland and rice fields in China is a serious problem, and the rice safety production issues needs to be urgently solved. Phosphorus as a macro-element could strongly influence the accumulation of cadmium in rice. Phosphate fertilizer carrying the cadmium and phosphorus induces the rhizosphere environmental changes and other pathway, which has been widespread concerned. Our previous study found that phosphorus had significant impact on cadmium uptake and accumulation in rice plants. Significant increase of cadmium content of rice plants was induced with the increase of phosphorus under phosphorus deficiency and significant decrease was caused with the further increase of phosphorus under phosphorus sufficiency. Cadmium could be absorbed through the divalent metal ions (such as Fe2+, Mn2+) transporters by the roots and re-transport among plant organs, and these transporters are regulated by the states of these metals in environment and plant. In plant, phosphorus could form phosphate with iron and interfere strongly the state of these elements, especially iron distribution among plant organs. We speculate that phosphorus could affect the abundance of iron and other elements in plants and their distribution among plant organs, and which might be one of the important ways affecting cadmium accumulation in rice plants. The project would use two rice varieties, differing in cadmium tolerance and accumulation in rice plants, as materials, to analyze and reveal the divalent-metal-ion-mediated P-Cd interactive mechanism at physiological and molecular level under different combined treatments with phosphorus and cadmium. The project would has both important scientific significance and guidance role on rice production.

我国农田和稻米镉污染严重，水稻安全生产问题亟待解决。磷作为大量元素，可强烈影响水稻镉积累；磷肥携带镉和磷诱导根际环境变化等途径受到了广泛关注。我们前期研究发现磷本身也会对水稻根系的镉吸收和植株镉积累产生显著的影响，主要表现为磷不足时，磷增加导致水稻植株镉含量显著增加；当磷充足时，磷可降低植株镉含量。镉主要通过二价金属离子(比如二价铁离子)转运蛋白被根系所吸收，以及在植株不同器官中的转运，这些转运蛋白又受环境和植株的微量金属元素所调控。在植株体内，磷易于铁等形成磷酸盐，强烈干扰植株金属元素状态，特别是铁及其在器官中的分配。我们推测磷影响植株铁等元素的丰缺或器官分布是其影响水稻镉积累的重要途径之一。本研究拟利用镉积累差异水稻，设置不同磷处理，利用生理和分子手段解析和揭示二价金属离子介导磷-镉互作的生理与分子调控机制，不仅具有重要的科学意义，对水稻安全生产也有重大的指导作用。

我国稻田镉污染严重，如何控制稻米Cd积累已是热门课题。磷是作物养分的三大要素之一，对水稻镉积累具有明显的调控作用。本项目利用镉积累差异水稻和相关转基因材料，通过水培和土培盆栽等方式开展以下研究：①磷对水稻镉等元素积累及器官间分布的调控及其生理机制；②磷对水稻镉等吸收和转运系统及二价金属离子转运蛋白基因的表达调控；③不同磷处理水稻植株吸收和转运Cd等二价金属离子吸收的动力学研究；④磷对水稻根系镉吸收与转运的长距离调控及其分子机理。结果如下：①水培条件下，镉低积累二九南1和高积累辐品36的镉含量均受根部磷的强烈调控。随着P浓度的增加，Cd含量呈先降低后升高的趋势；同时植株P含量显著增加，而Fe含量则大幅减少。推测P处理导致地上部Fe含量大幅降低，进而调控植株的Fe吸收系统，提升对Cd吸收的能力。通过基因表达分析发现P处理增加诱导根系中Cd和Fe吸收/转运相关基因OsNRAMP1，OsIRT1，OsIRO2，OsNAS2，OsYSL2和OsYSL15等大幅提高，而其他镉积累基因则无显著变化。②动力学试验发现磷处理导致植株的Vmax和Km明显增加，表明该调控作用的动力学特征符合酶学特征，即P-Cd之间存在互作，且是由磷调控根系镉相关基因的表达来实现。③土培条件下，水稻镉积累明显受土壤P水平调控。随着P处理水平的增加，植株中的Cd含量呈先增加、后降低、再增加，后又降低的趋势。而Fe含量的变化趋势正好相反，P含量则随着P处理浓度的增加而显著增加。进一步证实P调控Cd吸收是通过介导Fe吸收和代谢机制来实现的。④利用OsNRAMP1和OsIRT1过表达和敲除的转基因材料，通过水培和土培试验，进一步验证了前述获得的试验结果。最终证实磷在调控水稻在镉积累过程中存在的P-Cd互作是由植株Fe吸收系统介导的，以OsNRAMP1基因表达量显著增加为特征的根系Cd吸收能力增强来实现的。