Institute of Crop Science, NARO, 2-1-2 Kannondai, Tsukuba, Ibaraki, Japan.
Central Region Agricultural Research Center, NARO, 1-2-1, Inada, Joetsu, Niigata, Japan.
J Exp Bot. 2021 Mar 29;72(7):2570-2583. doi: 10.1093/jxb/erab016.
Understanding the limiting factors of grain filling is essential for the further improvement of grain yields in rice (Oryza sativa). The relatively slow grain growth of the high-yielding cultivar 'Momiroman' is not improved by increasing carbon supply, and hence low sink activity (i.e. the metabolic activity of assimilate consumption/storage in sink organs) may be a limiting factor for grain filling. However, there is no metabolic evidence to corroborate this hypothesis, partly because there is no consensus on how to define and quantify sink activity. In this study, we investigated the carbon flow at a metabolite level from photosynthesis in leaves to starch synthesis in grains of three high-yielding cultivars using the stable isotope 13C. We found that a large amount of newly fixed carbon assimilates in Momiroman was stored as hexose instead of being converted to starch. In addition, the activity of ADP-glucose pyrophosphorylase and the expression of AGPS2b, which encodes a subunit of the ADP-glucose pyrophosphorylase enzyme, were both lower in Momiroman than in the other two cultivars in grains in superior positions on panicle branches. Hence, slower starch synthesis from hexose, which is partly explained by the low expression level of AGPS2b, may be the primary metabolic reason for the lower sink activity observed in Momiroman.
了解灌浆限制因素对于进一步提高水稻（Oryza sativa）的产量至关重要。高产品种“Momiroman”的籽粒灌浆相对较慢，增加碳供应并不能改善这种情况，因此低库活性（即库器官中同化产物消耗/储存的代谢活性）可能是灌浆的限制因素。然而，目前还没有代谢证据支持这一假说，部分原因是对于如何定义和量化库活性尚无共识。在这项研究中，我们使用稳定同位素 13C 研究了三个高产品种叶片光合作用到籽粒淀粉合成过程中代谢物水平的碳流动。我们发现，Momiroman 中大量新固定的碳同化为己糖而不是转化为淀粉。此外，ADP-葡萄糖焦磷酸化酶的活性以及编码 ADP-葡萄糖焦磷酸化酶亚基的 AGPS2b 的表达，在 Momiroman 中都低于另外两个品种，在穗分支上具有优势位置的籽粒中。因此，己糖合成淀粉的速度较慢，部分原因是 AGPS2b 的表达水平较低，这可能是 Momiroman 中观察到的低库活性的主要代谢原因。
