**1. Introduction**

The transition from vegetative to reproductive stage is a critical developmental event in plants for ensuring offspring survival under favorable environments [1]. Understanding the genetic basis of flowering time control is especially important in crop species for breeding cultivars adapted well in the target environment. In rice, flowering (also referred to as heading) time is regulated by the complex genetic mechanisms involving hundreds of quantitative trait loci (QTLs) and at least 14 cloned genes [2,3].

Various environmental cues are integrated to modulate the expression of genes encoding florigen, which is synthesized in leaves and transferred to the apical meristem to initiate flower development [4]. In rice, florigen is encoded by *Heading Date 3a* (*Hd3a*) and *RICE FLOWERING LOCUS T 1* (*RFT1*), the orthologs of Arabidopsis *FT* [5,6]. *Hd3a* expression is mainly regulated by *Hd1*, the ortholog of Arabidopsis *CONSTANS* (*CO*), which upregulates *Hd3a* under short day and downregulates *Hd3a* under long day [7]. *Hd1* expression is upregulated under both short and long days by *OsGI*, the ortholog of Arabidopsis *GIGANTEA* (*GI*) [8]. Unlike bifunctional *Hd1*, *EARLY HEADING DATE 1* (*Ehd1*) can accelerate heading under both short and long days by upregulating *Hd3a* and *RFT1* under short and long days, respectively [9]. *Ehd1* expression is negatively regulated by *Grain number*, *plant height*, *and heading date 7* (*Ghd7*) under long day [10]. While the *OsGI*-*Hd1*-*Hd3a* pathway in rice is

orthologous to the *GI*-*CO*-*FT* pathway in Arabidopsis, the *Ghd7*-*Ehd1*-*Hd3a*/*RFT1* pathway is unique in rice without clear orthologs in Arabidopsis [1]. Recent studies revealed more complex rice-specific gene networks regulating the *Ghd7*-*Ehd1*-*Hd3a*/*RFT1* pathway (e.g., *Hd16* and *Hd17* upregulating *Ghd7* under long day) reviewed in [2,3].

To fine-tune days to heading (DH) in breeding programs and maximize yield and grain quality under the target environment, it is essential to characterize the e ffects of major genes controlling DH, their allelic variation, epistasis, and interaction with environmental factors including daylength and temperature. In Korea, developing early heading rice cultivars is especially important for boosting farmers' income by enabling diverse double cropping patterns in the rice paddies, e.g., late-planting of rice after harvesting winter crops such as cabbage, barley, and wheat, or early-planting of rice followed by cash crops such as garlic and onion [11,12]. The utilization of early heading rice cultivars can be also useful for reducing cropping duration in order to minimize damages from erratic weather events. Although over 80 rice cultivars classified as the early heading group have been released in Korea, they occupy less than 10% of the rice cultivation area in Korea as many rice growers generally prefer mid-late heading cultivars because of their higher yield and grain quality compared to the early heading cultivars [13].

Baegilmi is an extremely early heading rice cultivar recently released in Korea exhibiting high yield in the mid-north plain (milled rice yield 5.01 MT/ha) and north-east coastal (5.27 MT/ha) regions in Korea [14]. However, the genes conferring early heading in Baegilmi have remained unknown. In this study, we used a recombinant inbred line (RIL) population derived from the cross between Koshihikari and Baegilmi to identify the chromosomal regions harboring genes controlling DH. Candidate genes underlying the major DH QTLs were sequenced in Koshihikari and Baegilmi and their allelic compositions were screened among commercial rice cultivars. Molecular breeding strategy using the allelic variations in major DH genes was discussed to facilitate the fine-tuning of DH in breeding programs.
