水稻灌浆期茎和颖果中同化物装载-卸载及其与维管束结构和产量形成的关系

Stem non-structural carbohydrates (NSC) are of significance for grain yield formation in rice, especially for alleviating the reduction in grain yield under an unfavorable environment. Therefore, pot and field experiments are conducted (1) to investigate the re-distribution of NSC during grain filling, and study the phloem-loading process and underlying mechanism in terms of expressions of sucrose transporters and enzymes assocated with conversion and transport of carbohydrates, structural characteristics of stem large and small vascular bundles, and apoplamic and symplasmic pathway for phloem-loading, to further elucidate the relationships among phloem-loading, NSC transport and grain yield formation(grain weight, grain filling percentage, and harvest index); (2) to study the the phloem-unloading process in caryopsis of superior and inferior spikelets by expressions of sucrose transporters and assays of key enzymes for sucrose - monosaccharide conversion, structural observation of large and small vascular bundles in rachis and caryopsis, investigation of apoplamic and symplasmic pathways for phloem-unloading, and to further elucidate the relationships among phloem-unloading，assimilate transport and grain yield formation; (3) to investigate the plasticity in processes of phloem loading and unloading and apoplamic and symplasmic pathways under different nitrogen supplies and water management, and further elucidate the relationships of the plasticity with NSC transport and grain yield formation. Those studies may provide rationale for enhancing yield potential, utilization of carbohydrates accumulated before flowering, and reliefing the loss in grain yield under adverse conditions in rice.

水稻茎鞘非结构性碳水化合物（NSC）对提高产量潜力，特别是减缓不利环境时产量下降有着重要意义。因此, 本项目利用盆栽和大田试验，(1)研究水稻灌浆期茎鞘非结构性碳水化合物再分配特征，从茎鞘糖转化相关酶和蔗糖转运蛋白表达、大小维管束功能特征、共质体和质外体途径角度研究再分配时蔗糖装载机理，阐明其与NSC转运量、产量形成(粒重、结实率和收获指数)的关系；(2)研究强势粒和弱势粒颖果同化物卸载特征，从蔗糖转化相关酶和蔗糖转运蛋白表达、颖果大小维管束功能特征、共质体和质外体途径等角度研究蔗糖卸载机理，阐明其与NSC转运量、产量形成的关系；(3)研究不同栽培条件(氮肥运筹、水分管理)下茎鞘NSC再分配与同化物装载-卸载、共质体-质外体运输等过程的可塑性，阐明其与NSC转运和产量形成的关系。本研究为合理利用花前物质积累，调控茎鞘非结构性碳水化合物再分配、减缓逆境对产量形成影响提供理论支撑。

水稻茎鞘非结构性碳水化合物(NSC)的再分配对产量形成具有重要作用。本项目主要开展了四个方面的研究：(1)分别选用不同年代育成的21个品种和两个杂交稻(汕优63和两优培九)研究了NSC积累与转运特征。与早期品种(1940s-1970s)相比，现代品种(1980s-2010s) NSC积累量和浓度显著增加。这主要是与现代品种花前叶合成蔗糖和茎鞘合成淀粉的酶活性增加有关。现代品种花后茎鞘NSC转运量和对产量贡献增加, 主要原因是花后茎鞘合成蔗糖和籽粒蔗糖水解和淀粉合成有关的酶活性增加，维管束数量和库容量增加。低氮通过调节叶、茎鞘、籽粒中蔗糖-淀粉相互转化的酶活性来调节茎鞘NSC转运和籽粒灌浆。(2)低氮和适度干旱促进了汕优63和两优培九茎鞘NSC转运和弱势粒灌浆。弱势粒具有长的有效灌浆期和低灌浆速率，所着生的枝梗维管结构发育不完整，横截面积、维管束和韧皮部面积小；弱势粒韧皮部筛管与其周围细胞间的胞间连丝数量、细胞壁转化酶活性及OsCIN、OsSUT表达量低。低氮和适度干旱增加了花后茎鞘蔗糖合成和弱势粒淀粉合成相关酶活性及韧皮部流速，增加了茎鞘NSC转运量和弱势粒同化物卸载，进而增加了弱势粒灌浆和粒重。(3)颖果同化物卸载调控茎NSC转运。R201茎鞘NSC转运、结实率和产量均低。然而R201叶面积指数、比叶重、抽穗期茎鞘NSC浓度、生物量、茎鞘同化物活化能力、库容量和活性、小维管束数量和横截面积与其它供试材料相似，具有高的SPAD和大维管束数量和横截面积。因此，R201同化物供应与运输应不是其NSC转运量低的原因。R201颖果的筛管和伴胞及其周围薄壁细胞间的胞间连丝密度低，SUT和CWI基因和蛋白表达低，蔗糖-淀粉代谢相关基因表达低。因此，颖果同化物卸载障碍应是其NSC转运量低的可能原因。这些特征导致了R201茎鞘NSC转运少，进而影响到籽粒产量形成。 (4)水稻茎鞘大、小维管束在同化物转运过程中所起作用可能不同。小维管束伴胞-薄壁细胞、薄壁细胞-薄壁细胞间胞间连丝数量和密度高于大维管束，表明小维管束细胞间共质体运输途径更为畅通。相关分析和通径分析表明小维管束数量、横截面和韧皮部面积、胞间连丝密度与NSC转运关系比大维管束更紧密。需要更直接证据来支持这一观点。