Long Hypocotyl 1 mediates nitrogen regulation of flowering time and nitrogen use efficiency

Flowering time is of great significance for crop yield, reproduction and regional adaptability, which is regulated by complex gene network and environmental signals, such as nitrogen (N) nutrition. Long hypocotyl 1 (HY1) encodes a heme oxygenase, which is necessary for the biosynthesis of phytochrome chromophore biosynthesis, and promotes photomorphogenesis in plants. It has been reported that HY1 regulates rice flowering time, though the molecular mechanism remains elusive. Meanwhile, whether HY1 involves in N-regulated flowering is also unclear. Here, we found that HY1 knock-out mutant showed early flowering and decreased grain yield. HY1 is highly expressed in young rice leaf tissues, and its expression is induced by both light and N supply. We also revealed that HY1 follows a circadian rhythm, and alters the expression of circadian-clock genes to regulate following time. In rice, high N supply delayed flowering time, while the flowering time of hy1 mutant is insensitive to N supply in comparison to WT. Furthermore, HY1 knock-out decreases N uptake and use efficiency at both low N and high N levels under long-day conditions. These results suggested a critical role of HY1 in N-dependent signaling to regulate the function of clock and thus flowering under long-day conditions.