Abstract: Phytochromes are red (R) and far-red (FR) light photoreceptors in plants, and phytochrome A (phyA) is the only plant photoreceptor that perceives FR light and then mediates various responses to this signal. The phyA is synthesized in the cytosol in the inactive Pr form; upon light illumination, phyA is converted to the biologically active Pfr form, and interact directly with the shuttle proteins FHY1 and FHL. The phyA is then imported into the nucleus by FHY1/FHL, while the phyA-FHY1/FHL complexes dissociate in the nucleus and FHY1/FHL return to the cytosol for the next phyA transportation cycle. A recently developed mathematical model suggested that the photoconversion of phyA between Pr and Pfr forms and FHY1/FHL-dependent nuclear trafficking cycles determine phyA’s response profile to far-red light. In the nucleus, phyA interacts with COP1 and SPA proteins directly, leading to the disruption and inactivation of the COP1/SPA complexes and the accumulation of HY5, a bZIP family transcription factor acting as a key positive regulator of photomorphogenesis. The Pfr form of phyA also interacts directly with the PIF proteins, a group of bHLH family transcription factors acting as repressors of photomorphogenesis, and induces rapid phosphorylation and degradation of PIFs. FHY3 and FAR1 are transposase-derived transcription factors which directly activate the transcription of FHY1 and FHL under farred light; however, HY5 negatively regulates FHY3/FAR1-activated FHY1/FHL expression, thus playing a role in fine-tuning phyA signaling homeostasis. The Pfr form of phyA could be phosphorylated in the nucleus, and the phosphorylated phyA form may serve as the preferred substrate for the COP1/SPA complex-mediated degradation. It was recently shown that the phosphorylated phyA form may represent a moreactive form of phyA, thus playing an essential role in inducing the FR light response.