Abstract
Increasing the grain number is the most direct route toward enhancing the grain yield in cereals. In rice, grain number can be amplified through increasing the shoot branching (tillering), panicle branching, panicle length, and seed set percentage. Phytohormones have been conclusively shown to control the above characteristics by regulating molecular factors and their cross-interactions. The dynamic equilibrium of cytokinin levels in both shoot and inflorescence meristems, maintained by the regulation of its biosynthesis, activation, and degradation, determines the tillering and panicle branching, respectively. Auxins and gibberellins are known broadly to repress the axillary meristems, while jasmonic acid is implicated in the determination of reproductive meristem formation. The balance of auxin, gibberellin, and cytokinin determines meristematic activities in the inflorescence. Strigolactones have been shown to repress the shoot branching but seem to regulate panicle branching positively. Ethylene, brassinosteroids, and gibberellins regulate spikelet abortion and grain filling. Further studies on the optimization of endogenous hormonal levels can help in the expansion of the grain yield potential of rice. This review focuses on the molecular machinery, involving several genes and quantitative trait loci (QTL), operational in the plant that governs hormonal control and, in turn, gets governed by the hormones to regulate grain number and yield in rice.
Highlights
The world’s population is predicted to crest over 9 billion by 2050, with the current growth rate
The current review provides an overview of the recent progress of the genetics of phytohormone actions and their crosstalks in the context of grain number as a complex agronomic trait in rice
Preferential expression of PANICLE MORPHOLOGY MUTANT 1 (PMM1) in developing young panicles, in branches and spikelet primordia, established the role of PMM1 in determining inflorescence architecture. Mutant screening identified another regulator of grain size, grain number, and grain yield named, SMALL GRAIN 11 (SMG11), which is a novel allele of DWARF2 and encodes a cytochrome P450 (CYP90D2) involved in BR biosynthesis (Figure 5)
Summary
The world’s population is predicted to crest over 9 billion by 2050, with the current growth rate. The mutant, regulator of Gn1a (reg1), was identified as a semidominant allele of DST called DSTreg1 that showed increased activity in the inflorescence meristem due to higher CK accumulation, resulting in more panicle branching and greater grain yield. Mutation in ONAC096, a gene encoding NAC domaincontaining transcription factor, results in a 16% increase in grain yield due to a 15% increase in the number of panicles (representation of increased tillering).
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