高温抑制水稻韧皮部蔗糖转运的作用机制

Heat stress frequently occurs at flowering stage of rice these years that seriously damages the grain yield. This stress not only signifcanly decrease the seed-setting rate but also the kernel weight and grain quality. Previous results suggested that heat stress during flowering stage impaired the photosynthesis of flag leaves and grain filling key enzyme activities were the main reasons for reducing the 1000-grain weight of rice. However, the results in our experiment indicated that heat stress occurred at flowering stage caused little adverse effect on the photosynthesis of flag leaves and Q enzyme activity of grains at the mid and late grain filling stage. There was little changes showed in the “source” and “sink” whereas the kernel weight was significantly decreased when encountered heat stress suggested that“flow”rather than “resource” and “sink” was the main cause in decreasing 1000-grain weight. Thereby, two rice genotypes with different heat resistance(Nipponbare and its mutant, HTS) were used as materials in this experiment to study the effect of heat stress on the translocation of photoassimilates in the phloem of stem, sheath and caryopsis and apoplasmic unloading in the grains. The main factor in limiting the assimilate translocation in the phloem will be clarified in ultrastructure and molecule level, and the results provide not only the new ideas to the researches of heat stress but also the physiological basis for rice breeding with large panicle and heat resistance.

近年来极端高温天气频发，对我国水稻生产造成极大损失，花期高温不仅显著降低水稻结实率，千粒重也显著下降，严重影响到稻米品质的形成。粒重降低，以往研究多认为主要在于高温对水稻“源”和“库”的影响。然而，申请者的试验表明开花期高温对水稻剑叶光合作用及籽粒灌浆关键酶活性的影响均不大。在“源”和“库”没有明显变化的情况下，粒重显著下降，表明高温影响籽粒充实的主要原因不在于“源”和“库”的限制，而在于“流”的不畅，尤其是光合同化物在茎鞘及颖果韧皮部中的运输及卸载。鉴此，本项研究以日本晴（热钝感）及其突变体（热敏感）为材料研究高温影响水稻同化物在韧皮部运输的作用机理，即高温胁迫对水稻同化物在水稻茎鞘韧皮部及颖果背部维管束韧皮部中的运输及卸载的影响。从超微结构及分子表达水平揭示高温下影响水稻同化物运输的主要限制因素，研究结果不仅为水稻耐热性研究提供新的研究思路，还可为培育大穗耐热型水稻品种提供生理学基础。

温室气体的大量排放导致极端高温天气频繁发生，对我国水稻生产造成极大影响。开花期是水稻对高温最敏感的时期，此时遭遇高温天气，会使水稻的结实率和千粒重显著降低，严重影响稻米品质。前人研究多聚焦于高温对水稻“源”和“库”的影响，而我们的研究结果发现高温对水稻“流”的影响更为显著。结果如下：1）对NIPP（热钝感）及其突变体HTS（热敏感）进行开花期高温处理，发现高温导致结实率与千粒重显著降低，但光合速率、叶绿素荧光、粒型及籽粒关键酶活性无显著变化，推测开花期高温导致籽粒充实不足主要是“流”不畅，而不是“源”及“库”的限制。高温下，穗部干物质量及非结构性碳水化合物比例显著降低，而在茎鞘及叶片中比例明显增加，进一步的试验表明高温抑制叶片和颖果中蔗糖卸载相关基因的表达。此外，高温下，水稻叶片及叶鞘胞间连丝连接处观察到胼胝质沉积，HTS籽粒背部维管束筛管部分有糊状物质堵塞，表明韧皮部同化物-蔗糖转运的质外体及共质体途径均受到高温胁迫抑制。进一步证明了高温下蔗糖作为信号分子协调“源”、“库”及“流”之间的关系。2）对NIPP与HTS在灌浆期高温处理，干物质量-同化物分配有较大的变化，叶片与茎鞘同化物比例增加，籽粒同化物比例减少，导致粒重降低。转录组分析其调控网络显示高温胁迫下籽粒下调基因的差异较为显著，差异基因功能集中在对刺激及逆境的响应、碳氮代谢、次生代谢、信号转导等，其中碳代谢、信号转导及次生代谢物合成受影响最大。外源蔗糖一定程度上减缓高温对水稻籽粒充实的抑制。3）分别在开花期和灌浆期研究蔗糖与ABA互作对水稻同化物转运的影响。中度高温胁迫下，蔗糖与ABA互作促进同化物-蔗糖向穗部转运，为柱头花粉管伸长及籽粒灌浆提供充足的能量，可显著增加水稻产量，并一定程度改善稻米品质，表明高温外源调节糖信号能改善水稻“源”、“库”及“流”之间的关系。该结果不仅可为水稻耐热研究提供思路，还可为培育耐热型大穗水稻提供生理学基础。