Exogenous and Endogenous Signals: Critical Factors for Regulation of Flowering Time and Grain Yield in Rice

The pattern and timing of flowering within the rice panicle are the key agronomic characteristics for determination of grain yield and maintenance of genetic continuity across generations. A variety of exogenous and endogenous signals such as abiotic environmental extremes and phytohormones largely influence the sequence and timing of flowering. Timing of flowering is also crucial for assessing regional diversity and adaptation to local climatic conditions. Generally flowering of spikelets occurs asynchronously, while synchronous flowering boosts yield; additionally, early flowering helps mitigate heat stress. Like panicle, tillers in a rice hill also exhibit substantial variations in flowering times, causing yield reductions. Multiple physiological, and molecular characteristics influence rice flowering time which include absorption and loss of water for swelling and shrinkage of lodicule cells respectively which ultimately regulates opening and closing of rice spikelets. Moreover, methyl esterification, presence of sugars, and temporal and spatial distribution of Ca2+ between cytoplasm and vacuoles in the epidermal cells of lodicules, and expression of genes encoding flowering hormones RFT1 and Hd3a, along with proteins involved in flowering signal transduction pathway, also influence rice flowering. Basically, rice demonstrates the Hd1-Hd3a flowering pathway in short-day photoperiods whereas Ghd7-Ehd1-Hd3a/RFT1 route is prominent under long-day conditions. A thorough analysis of the morpho-physiological and molecular genetic network controlling rice flowering is lacking, despite the fact that unprecedented strides have been made in the recent years. This review cites all recent research in an effort to show a clear and thorough regulatory network that mediates rice flowering, encompassing all biological events.