Rice OsMYB44 regulates <i>Early heading date 1</i> expression for photoperiodic flowering

Photoperiod is a critical environmental cue that determines the timing of flowering in plants. During the domestication of rice (Oryza sativa), natural allelic variation in photoperiodic flowering regulators played an important role in adaptation to a wide range of latitudes. Here, we demonstrate the involvement of the R2R3-type MYB protein OsMYB44 in photoperiodic flowering in rice. Overexpression of OsMYB44 accelerated flowering under short-day (SD) conditions and delayed flowering under long-day conditions. Meanwhile, osmyb44 knockout mutants showed delayed flowering under SD conditions. We determined that the C-terminal region of OsMYB44 interacts with Heading date 1 (Hd1), a primary photoperiodic flowering regulator in rice. OsMYB44 also interacts with Days to heading 8 (DTH8) and OsNF-YC2, forming an NF-Y transcriptional complex with Hd1. This suggests that OsMYB44 participates in transcriptional regulation along with NF-Y components. We also showed that OsMYB44 binds to MYB-core type I motifs in the promoter region of Early heading date 1 (Ehd1) and activates its expression. Analysis of natural variation in OsMYB44 identified 4 haplotypes, OsMYB44a-d, and revealed that a single amino acid substitution in OsMYB44b may contribute to delayed flowering by reducing the binding affinity of OsMYB44b to Hd1. Further examination of the geographical distribution of the 4 OsMYB44 haplotypes suggested that natural variation in the OsMYB44 haplotypes contributed to regional adaptation during rice domestication. Together, these findings underscore the pivotal role of OsMYB44 in the transcriptional regulation of photoperiodic flowering in rice.