Figure 4. A simplified model of photomorphogenic pathway in Arabidopsis. Different spectrum of light are absorbed by the corresponding photoreceptors (phytochromes and cryptochromes) and the light signals are then transduced to suppress three main types of negative factors, including COP1, PIFs and PKL/EPP1. COP1 represses HY5 through mediating its protein turnover, while EPP1 inhibits the transcriptional activity of HY5. PIFs (including PIF1, PIF3, PIF4 and PIF5) and HY5 are transcription factors that promote or suppress the transcription of cell elongation genes, respectively. Solid lines indicate a direct regulation, while dotted lines denote an indirect effect. Modified from Lau and Deng.⁹