Figure 1.

Schematic diagram summarizing positive and negative light effects on genes in the nitrate assimilation pathway of Arabidopsis seedlings. Continuous light (red, far-red, white) induced NIA2 and other genes involved in nitrate assimilation, whereas NRT1.1 was weakly inhibited by light. Light effects on NIA2 and NRT1.1 were enhanced by the presence of HY5/HYH. PIF4 was identified as an inhibitor of NIA2 expression. Our data are compatible with PIF4 acting through the HY5/HYH pathway because NIA2 and NRT1.1 were oppositely influenced in the pif4 mutant, and other genes of nitrate assimilation were not influenced. However, the available data do not determine if PIF4 rather act through a separate signaling cascade (not including HY5/HYH) to inhibit NIA2.