Many short-day plants have a critical day length that fixes the schedule for flowering time, limiting the range of natural growth habitats (or growth and cultivation areas). Thus, fine-Tuning of the critical day-length setting in photoperiodic flowering determines ecological niches within latitudinal clines
however, little is known about the molecular mechanisms controlling the fine-Tuning of the critical day-length setting in plants. Previously, we determined that florigen genes are regulated by day length, and identified several key genes involved in setting the critical day length in rice. Using a set of chromosomal segment substitution lines with the genetic background of an elite temperate japonica cultivar, we performed a series of expression analyses of flowering-Time genes to identify those responsible for setting the critical day-length in rice. Here, we identified two casein kinase genes, Hd16 and Hd6, which modulate the expression of florigen genes within certain restricted ranges of photoperiod, thereby fine-Tuning the critical day length. In addition, we determined that Hd16 functions as an enhancer of the bifunctional action of Hd1 (the Arabidopsis CONSTANS ortholog) in rice. Utilization of the natural variation in Hd16 and Hd6 was only found among temperate japonica cultivars adapted to northern areas. Therefore, this fine-Tuning of the setting of the critical day length may contribute to the potential northward expansion of rice cultivation areas.