Flowering time (heading date) of the rice plant is considered an important agronomic trait for environmental adaptation and grain yield. It is controlled by multiple genes and is regulated by different environmental factors, such as day length, temperature, soil moisture, etc. So far, approximately 125 genes regulating flowering process and floral organ identity or development directly or indirectly have been reported in rice. Among these genes, Heading date 3a (Hd3a), RICE FLOWERING LOCUS T1 (RFT1), Heading date 5 (Hd5), MORF-RELATED GENE702 (MRG702), Casein kinases, CKI and CK2α, Pseudo-Response Regulator 37 (PRR37), Hd gene family have been reported as the key genes regulating flowering time in rice; however, their functions are mostly inter-related. Hd3a and RFT1 that encode florigens, are known as the floral transition genes in rice. In rice, florigen immediately induces downstream genes in the shoot apical meristem (SAM) to start the transition from vegetative to reproductive phase. RFT1 gene regulates flowering time with pivotal action while Hd3a, regulates under long day conditions. The Hd5 gene regulates flowering time in variation of early heading for adaptation depending on environmental signals. MRG702, a reader protein, promotes flowering. Casein kinases, CKI and CK2α directly influence the function of the early heading regulator PRR37. Hd16/CKI hinders flowering time in the Ehd1-concerned pathway through phosphorylation of Ghd7 and PRR37. Natural variants of Hd1, PRR37, Ghd7, DTH8, Hd6, and Hd16 were found in the rice varieties that are cultivated presently in Asia and Europe, and their variants play significant roles in the down streaming of Ehd1 expression to delay flowering time in natural long day (LD) conditions.

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