水稻弱势粒灌浆机理与调控途径

Mechanism and Regulation in the Filling of Inferior Spikelets of Rice

水稻籽粒充实优劣和粒重高低与颖花在穗上着生的部位有密切关系。通常,着生在稻穗中上部早开花的强势粒,灌浆快、充实好、粒重高;着生在稻穗下部迟开花的弱势粒,灌浆慢、充实差、粒重低。这种强、弱势粒灌浆的差异在大穗型超级稻品种上表现更为突出。弱势粒充实差和粒重低不仅阻碍了水稻产量潜力的发挥,而且还会降低稻米品质,尤其是加工品质和外观品质。关于弱势粒灌浆差的机理有许多假设,包括同化物供应限制、库容限制、激素间不平衡、蔗糖-淀粉代谢途径关键酶活性或基因表达量低、"流"不畅等。最近研究表明,灌浆始期籽粒库生理活性低和活跃灌浆期蔗糖转化为淀粉的生化效率低是弱势粒灌浆差的重要原因;增加抽穗期糖花比(抽穗期茎与鞘中非结构性碳水化合物与颖花数之比)及灌浆期脱落酸与乙烯比值可以显著提高籽粒库生理活性和籽粒灌浆速率。从环境(含栽培)、植株整体水平以及籽粒内在因素等不同层次上深入研究水稻弱势粒灌浆差的机理及其调控途径,对于破解弱势粒灌浆差的科学难题、挖掘水稻生产潜力具有十分重要的意义。

Grain filling is the final growth stage in cereals when fertilized ovaries develop into caryopses. The degree and rate of grain filling in rice spikelets differ largely with their positions on a panicle. In general, earlier-flowering superior spikelets, usually located on apical primary branches, fill fast and produce larger and heavier grains. While later-flowering inferior spikelets, usually located on proximal secondary branches, are either sterile or fill slowly and produce smaller grains. The poor grain-filling of inferior spikelets is more aggravated in the new bred ＂super＂ rice cultivars that have numerous spikelets on a panicle. Poor filling of inferior spikelets not only limits the realization of great yield potential, but also degrades rice quality, especially milling and apparent qualities. There are many explanations to the poor filling of inferior spikelets, including carbon limitation, sink capacity limitation, unbalance in hormone levels, low activities and/or expressions of enzymes involved in sucrose-to-starch conversion, and ＂flow＂ impediment. Recent studies have shown that low physiological activities of sink （grains） at the initial grain filling and low conversion efficiency from sucrose to starch during the active grain filling period contribute to the poor filling of inferior spikelets. It is observed that the ratio of sugar to spikelets at the heading stage （amount of non-structural carbohydrate in the stems and sheaths over the number of spikelets at heading） significantly correlats with the physiological activities of sink, and the ratio of abscisic acid （ABA） to 1-aminocylopropane-l-carboxylic acid （ACC, a precursor of ethylene） significantly correlats with the grain filling rate, indicating that increases in the ratios of sugar to spikelets and of ABA to ethylene would be two important regulatory approaches to improve the filling of inferior spikelets. Further studies are essential by investigating how environmental factors （including cultivation techniques）, factors of the whole plant, and factors within the spikelets regulate the filling of inferior spikelets. A deep understanding of the regulation mechanism that limits the filling of inferior spikelets would lead to efforts that could greatly enhance grain filling and, consequently, increase the yield performance of rice.