The Initiation of Inferior Grain Filling is Affected by Sugar Translocation Efficiency in Large Panicle Rice

Background Large panicle rice has a large sink capacity, but inferior spikelet filling is poor in this variety of rice due to asynchronous grain filling. The understanding of the factors that cause asynchronous grain filling will help to propose a model for how to regulate the rice inferior spikelets grain filling. Results In this study, two large panicle rice varieties, W1844 and CJ03, with the same sink capacity but with differences in asynchronous grain filling were used. The difference in the grain filling rate between superior and inferior spikelets in W1844 was much smaller than that in CJ03. We found that superior spikelet filling was initiated earlier in W1844 than in CJ03. The source-to-sink translocation rate of sucrose during the grain filling stage was more efficient in W1844 than in CJ03, and the gene expression levels of sucrose transporters (OsSUTs) were higher in W1844 functional leaves than in those of CJ03. In addition, carbon output, the transport ratio, and the contribution rate from the stem and sheath to the panicle were much higher at the early filling stage than at later filling stages in W1844. Conclusion Efficient sugar translocation can satisfy high sink strength, and also the strong sink activity can facilitate the sugar unloading in spikelets. All the above results indicate that an efficient sugar translocation rate at the early grain filling stage can improve sink strength and inferior grain filling initiation. Strategies to limit asynchronous grain filling in rice were also discussed based on our findings.

背景 大穗型水稻具有较大的库容量，但由于籽粒灌浆不同步，该品种水稻的弱势粒灌浆不良。了解导致籽粒灌浆不同步的因素将有助于提出如何调控水稻弱势粒灌浆的模型。 结果 在本研究中，选用了两个库容量相同但籽粒灌浆不同步情况存在差异的大穗型水稻品种W1844和CJ03。W1844中强势粒和弱势粒之间的灌浆速率差异远小于CJ03。我们发现W1844中强势粒开始灌浆的时间比CJ03早。在灌浆期，W1844中蔗糖从源到库的转运速率比CJ03更高效，并且W1844功能叶中蔗糖转运蛋白（OsSUTs）的基因表达水平高于CJ03。此外，在W1844中，灌浆前期茎鞘的碳输出量、运输比例以及对穗的贡献率远高于灌浆后期。 结论 高效的糖分转运能够满足高库强，并且强大的库活性能够促进小穗中的糖分卸载。上述所有结果表明，灌浆前期高效的糖分转运速率能够提高库强以及弱势粒灌浆的起始。基于我们的研究结果，还讨论了限制水稻籽粒灌浆不同步的策略。