Panigrahi Rashmi, Kariali Ekamber, Panda Binay Bhusan, Lafarge Tanguy, Mohapatra Pravat Kumar
Funct Plant Biol. 2019 Jun;46(6):595. doi: 10.1071/FP18153_CO.
The advent of dwarf statured rice varieties enabled a major breakthrough in yield and production, but raising the ceiling of genetically determined yield potential even further has been the breeding priority. Grain filling is asynchronous in the rice panicle; the inferior spikelets particularly on secondary branches of the basal part do not produce grains of a quality suitable for human consumption. Of the various strategies being considered, the control of ethylene production at anthesis has been a valuable route to potentially enhance genetic yield level of rice. The physiology underlying spikelet development has revealed spikelet position-specific ethylene levels determine the extent of grain filling, with higher levels resulting in ill-developed spikelet embodying poor endosperm starch content. To break the yield barrier, breeders have increased spikelet number per panicle in new large-panicle rice plants. However, the advantage of panicles with numerous spikelets has not resulted in enhanced yield because of poor filling of inferior spikelets. High spikelet number stimulates ethylene production and downgrading of starch synthesis, suggesting a trade-off between spikelet number and grain filling. High ethylene production in inferior spikelets suppresses expression of genes encoding endosperm starch synthesising enzymes. Hence, ethylene could be a retrograde signal that dictates the transcriptome dynamics for the cross talk between spikelet number and grain filling in the rice panicle, so attenuation of its activity may provide a solution to the problem of poor grain filling in large-panicle rice. This physiological linkage that reduces starch biosynthesis of inferior kernels is not genetically constitutive and amenable for modification through chemical, biotechnological, surgical and allelic manipulations. Studies on plant genotypes with different panicle architecture have opened up possibilities of selectively improving starch biosynthesis of inferior spikelets and thereby increasing grain yield through a physiological route.
矮秆水稻品种的出现使产量和生产取得了重大突破，但进一步提高由基因决定的产量潜力上限一直是育种工作的重点。水稻穗部的籽粒灌浆是不同步的；尤其是基部二次枝上的弱势小穗不能产生适合人类食用品质的籽粒。在正在考虑的各种策略中，花期控制乙烯生成是潜在提高水稻遗传产量水平的一条有价值的途径。小穗发育的生理学研究表明，小穗位置特异性的乙烯水平决定了籽粒灌浆的程度，乙烯水平较高会导致小穗发育不良，胚乳淀粉含量低。为了突破产量瓶颈，育种者在新的大穗型水稻植株中增加了每穗小穗数。然而，由于弱势小穗灌浆不良，具有大量小穗的穗部优势并未带来产量的提高。高小穗数会刺激乙烯生成并降低淀粉合成，这表明小穗数与籽粒灌浆之间存在权衡。弱势小穗中高乙烯生成抑制了编码胚乳淀粉合成酶基因的表达。因此，乙烯可能是一种逆向信号，它决定了水稻穗部小穗数与籽粒灌浆之间相互作用的转录组动态，因此减弱其活性可能为解决大穗型水稻籽粒灌浆不良的问题提供一种解决方案。这种降低弱势籽粒淀粉生物合成的生理联系不是遗传固定的，可以通过化学、生物技术、手术和等位基因操作进行修饰。对具有不同穗型结构的植物基因型的研究为选择性改善弱势小穗的淀粉生物合成从而通过生理途径提高籽粒产量开辟了可能性。
