NRL3 is essential for the growth and development of rice leaves. In this study, we found that the loss function of NRL3 also delayed heading date under natural long daylight and short daylight conditions. The yeast two-hybrid and the bimolecular fluorescence complementation proved that NRL3 interacts with OsK4, a Snf1-related kinase. OsK4 localized to the nucleus and expressed in various rice tissues. The rhythmic expression pattern of OsK4 was similar to NRL3 under long daylight and short daylight conditions. Knock-out mutants of OsK4 exhibited early heading under long daylight conditions, indicating that it acts as a negative regulator of heading date in rice. Interestingly, the OsK4 mutant under the nrl3 mutant background rescued the late heading phenotype of nrl3 under long daylight conditions, suggesting that OsK4 functions downstream of NRL3. Moreover, both NRL3 and OsK4 controlled heading date through regulating the expression of Hd3a and RFT1 genes. These findings shed light on the heading date regulation in rice and provide a sound theoretical base to improve regional adaptability of rice.
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