Arabidopsis cryptochrome 1 functions in nitrogen regulation of flowering

The phenomenon of delayed flowering after the application of nitrogen (N) fertilizer has long been known in agriculture, but the detailed molecular basis for this phenomenon is largely unclear. Here we used a modified method of suppression-subtractive hybridization to identify two key factors involved in N-regulated flowering time control in Arabidopsis thaliana, namely ferredoxin-NADP(+)-oxidoreductase and the blue-light receptor cryptochrome 1 (CRY1). The expression of both genes is induced by low N levels, and their loss-of-function mutants are insensitive to altered N concentration. Low-N conditions increase both NADPH/NADP(+) and ATP/AMP ratios, which in turn affect adenosine monophosphate-activated protein kinase (AMPK) activity. Moreover, our results show that the AMPK activity and nuclear localization are rhythmic and inversely correlated with nuclear CRY1 protein abundance. Low-N conditions increase but high-N conditions decrease the expression of several key components of the central oscillator (e.g., CCA1, LHY, and TOC1) and the flowering output genes (e.g., GI and CO). Taken together, our results suggest that N signaling functions as a modulator of nuclear CRY1 protein abundance, as well as the input signal for the central circadian clock to interfere with the normal flowering process.