Phytohormone-Mediated Molecular Mechanisms Involving Multiple Genes and QTL Govern Grain Number in Rice

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.

Keywords: grain number; panicle branching; phytohormones; rice; tiller; yield.

FIGURE 1

Cytokinin (CK)-dependent meristem activity is central to panicle architecture. Enzymes like OsCKX2, OsCKX9, LOG1, and OsIPTs regulate the concentration of CK. These genes are further regulated by transcription factors, epigenetic regulators, and signaling factors. CK robustly interacts with other hormones, and a complex network determines the inflorescence meristem activity and hence panicle architecture. See the text for details. Solid lines represent direct regulation; dashed lines represent indirect regulation; arrows represent positive regulation; bar-headed lines represent negative regulation; block arrows represent the effect of a response. P represents phosphorylation.

FIGURE 2

Regulators of auxin signaling regulating plant architecture and affecting grain number. Factors affecting auxin biosynthesis, homeostasis, signaling, and transport identified in rice with functions known in affecting grain number are shown. Interconnections of auxin signaling with CK and SL also contribute to the plant architecture that affects grain number. Arrows represent positive regulation; bar-headed lines represent negative regulation.

FIGURE 3

Model of strigolactone (SL) regulated pathways specifically involved in the regulation of panicle and tiller development. SL inhibits tillering but promotes panicle size and branching. Crosstalk of SL with BR and CK regulates tillering and panicle branching, respectively. Core components of SL biosynthesis and signaling are shown as red hexagons. Other interacting factors are shown as yellow hexagons. Arrows represent positive regulation; bar-headed lines represent negative regulation.

FIGURE 4

Factors involved in gibberellin (GA)-mediated regulation of panicle architecture in rice. The model depicts the factors (white boxes) that are involved in GA biosynthesis, catabolism, or homeostasis and influence panicle development and morphology. Blue boxes represent the regulated panicle phenotypes. Arrows represent positive regulation; bar-headed lines represent negative regulation. Cytokinin (CK) and GA have antagonistic roles in regulating reproductive meristem activity. GA modulating factors also interact with other hormones (BR, SL, ethylene) mediated factors to make a complex meshwork.

FIGURE 5

Model of brassinosteroid (BR) regulation controlling panicle development, panicle architecture, and tillering. Various factors (blue hexagons) regulate BR biosynthesis and signaling. Their coordinated actions regulate the downstream targets (green hexagons) of BR signaling that control various aspects of spikelet differentiation, panicle development, and tillering. Nitrogen and moderate soil drying conditions serve as additional factors affecting panicle development by increasing BR levels and signaling. Arrows represent positive regulation; bar-headed lines represent negative regulation; dotted arrows represent the formation of protein complex.

FIGURE 6

Mode of action of ethylene during panicle development and grain filling. The model summarizes the role of ethylene-related factors (pink hexagons) in the regulation of panicle development and grain filling. Inferior spikelets accumulate ethylene by promoting its biosynthesis. Polyamines compete and reroute ethylene metabolism, thus antagonizing the ethylene response. Meristem transition determination by FZP greatly affects the branching potential of panicles. FZP interacts with BR and auxin pathway components. OsEATB coordinately with GA affects the tillering and panicle branching. PA, polyamines; D-SAM, decarboxylated SAM; SAMDC, SAM decarboxylase. Arrows represent positive regulation; bar-headed lines represent negative regulation.