Nitrogen (N) is an essential nutrient that affects multiple plant developmental processes, including flowering. As flowering requires resources to develop sink tissues for reproduction, nutrient availability is tightly linked to this process. Low N levels accelerate floral transition; however, the molecular mechanisms underlying this response are not well understood. Here, we identify the FLOWERING BHLH 4 (FBH4) transcription factor as a key regulator of N-responsive flowering in Arabidopsis. Low N-induced early flowering is compromised in fbh quadruple mutants. We found that FBH4 is a highly phosphorylated protein and that FBH4 phosphorylation levels decrease under low N conditions. In addition, decreased phosphorylation promotes FBH4 nuclear localization and transcriptional activation of the direct target CONSTANS (CO) and downstream florigen FLOWERING LOCUS T (FT) genes. Moreover, we demonstrate that the evolutionarily conserved cellular fuel sensor SNF1-RELATED KINASE 1 (SnRK1), whose kinase activity is down-regulated under low N conditions, directly phosphorylates FBH4. SnRK1 negatively regulates CO and FT transcript levels under high N conditions. Together, these results reveal a mechanism by which N levels may fine-tune FBH4 nuclear localization by adjusting the phosphorylation state to modulate flowering time. In addition to its role in flowering regulation, we also showed that FBH4 was involved in low N-induced up-regulation of nutrient recycling and remobilization-related gene expression. Thus, our findings provide insight into N-responsive growth phase transitions and optimization of plant fitness under nutrient-limited conditions.

氮 (N) 是影响多种植物发育过程（包括开花）的必需营养素。由于开花需要资源来发育用于繁殖的水槽组织，营养供应与这一过程密切相关。低氮水平加速花的转变；然而，这种反应背后的分子机制尚不清楚。在这里，我们将 FLOWERING BHLH 4 (FBH4) 转录因子鉴定为拟南芥中 N 响应性开花的关键调节因子。低氮诱导的早花在 fbh 中受到影响四重突变体。我们发现 FBH4 是一种高度磷酸化的蛋白质，并且在低氮条件下 FBH4 磷酸化水平降低。此外，磷酸化降低促进 FBH4 核定位和直接靶标CONSTANS ( CO ) 和下游 florigen FLOWERING LOCUS T ( FT ) 基因的转录激活。此外，我们证明了进化上保守的细胞燃料传感器 SNF1 相关激酶 1 (SnRK1)，其激酶活性在低氮条件下被下调，直接磷酸化 FBH4。SnRK1 负调节CO和FT高氮条件下的转录水平。总之，这些结果揭示了 N 水平可以通过调节磷酸化状态来调节开花时间来微调 FBH4 核定位的机制。除了在开花调节中的作用外，我们还表明 FBH4 参与了低氮诱导的养分循环和再动员相关基因表达的上调。因此，我们的研究结果提供了对 N 响应性生长相变和在营养有限条件下植物适应性优化的见解。