水稻灌浆期叶片和茎鞘中蔗糖韧皮部装载机理及调控途径研究

Grain filling and yield formation of rice depend on sucrose that produced in, and translocated from photosynthetically active leaves and stems with non-structural carbohydrates stored prior to heading. Phloem loading of sucrose is the first crucial step for transporting of sucrose to grains. Aim at the scientific problem that enhancing phloem loading to improve the efficient translocation of sucrose from leaves and stems to grains to increase rice yield, the experiments will be conducted under different nitrogen application rates, using techniques and methods combining morphological structure, physiology, and molecular, focusing on: (1) sucrose phloem loading in leaf and stem based on the investigations of ultrastructural characteristics of phloem, characteristics of sucrose concentration and distribution, activities of key enzymes involved in sucrose metabolism, gene and protein expressions of sucrose transporter and SWEET, to elucidate the mechanisms of sucrose phloem loading and its relationships with sucrose translocation (transport rate and amount) and yield formation (grain filling percentage, grain weight and harvest index); (2) the effects of different nitrogen application rates on sucrose phloem loading in rice leaf and stem and its relationships with sucrose translocation and yield formation, to clarify the effect of nitrogen on sucrose phloem loading and its physiological mechanism. These studies may provide rationales for promoting translocation and distribution of assimilates in leaf and stem, excavating yield potential of rice and breeding of high-yield varieties, and adopting reasonable regulation technology to increase grain yield of rice.

水稻灌浆期叶片光合产物和花前茎鞘储存的非结构性碳水化合物以蔗糖的形式向穗转运，是籽粒灌浆的物质来源。韧皮部装载是叶片和茎鞘蔗糖向籽粒运输的第一个关键步骤。针对增强韧皮部装载促进叶片和茎鞘蔗糖向籽粒高效转运提高水稻产量的科学问题，本项目在不同氮水平下开展试验，结合形态结构、生理和分子等水平的技术方法，重点研究：（1）水稻叶片和茎鞘维管束韧皮部超微结构、蔗糖浓度变化及分配、蔗糖代谢关键酶活性变化、SWEET蛋白和蔗糖转运蛋白的基因及蛋白差异表达，阐明灌浆期叶片和茎鞘蔗糖韧皮部装载机理及其与蔗糖转运（转运速率和转运量）和产量形成（结实率、粒重和收获指数）的关系；（2）不同氮水平对叶片和茎鞘蔗糖韧皮部装载的影响及其与蔗糖转运和产量形成的关系，阐明氮对蔗糖韧皮部装载的影响及生理机制。研究结果将为促进叶片和茎鞘同化物运转与分配，进一步挖掘水稻产量潜力、选育高产品种及采取合理的高产调控技术提供理论支撑。

水稻籽粒灌浆的物质来源于灌浆期叶片光合产物和花前茎鞘储存的光合同化物，蔗糖是同化物向籽粒运输的主要形式，增强蔗糖韧皮部装载和维管束运输对产量形成具有重要作用。本项目选用不同类型水稻品种结合不同氮水平处理进行盆栽试验，研究了不同品种灌浆期茎鞘同化物转运特征、茎秆基部节间维管束特征、茎叶韧皮部装载机理及其与产量形成的关系，并探究了氮对蔗糖韧皮部装载的影响及其生理机理。主要结果如下：（1）不同类型品种茎鞘非结构性碳水化合物（NSC）转运具有差异，籼粳杂交稻最高，其次是杂交籼稻和常规籼稻，常规粳稻最低；抽穗期茎鞘NSC含量和灌浆期茎鞘NSC转运与产量、结实率和收获指数显著正相关，成熟期茎鞘NSC含量与产量和收获指数显著负相关。（2）不同类型品种茎秆基部节间维管束特征具有差异，籼粳杂交稻的维管束数量、横截面积和韧皮部面积等性状最高，杂交籼稻和常规籼稻其次，常规粳稻最低；上述维管束性状与茎鞘NSC转运、结实率和产量正相关。（3）通过透射电镜观察结合韧皮部特异运输荧光染料处理分析表明水稻剑叶和茎秆维管束韧皮部筛管与伴胞和周围薄壁细胞间缺乏共质体连接，进一步采用qRT-PCR、免疫组化等技术分析蔗糖转运蛋白基因和蛋白表达并结合蛋白抑制剂处理分析表明基因OsSUT1、OsSUT2、OsSUT4、OsSUT5、OsSWEETT11和OsSWEETT14在剑叶和茎秆中大量表达，SUT和SWEET蛋白在剑叶和茎秆维管束韧皮部大量表达；抑制蔗糖转运蛋白活性，叶片和茎秆光合同化物向穗部和茎鞘的分配显著降低，表明水稻叶片和茎秆蔗糖采用质外体装载途径。（4）氮通过调节碳代谢酶活性、SUT和SWEET的基因表达和蛋白水平影响水稻剑叶和茎秆蔗糖韧皮部装载。低氮处理增加了水稻茎鞘α-淀粉酶、β-淀粉酶、蔗糖磷酸合成酶（SPS）和叶片SPS活性，增加了SUT和SWEET的基因表达和蛋白水平，增强了蔗糖韧皮部装载，促进了叶片和茎鞘NSC转运，有利于水稻籽粒灌浆和产量形成。研究结果对调控水稻源库碳分配，进一步挖掘产量潜力和选育高产品种具有重要参考价值。