Photomorphogenesis in plants: The central role of phytochrome interacting factors (PIFs)

Light is the primary source of energy that powers photosynthesis and growth in plants. A wide range of signals and information for morphogenesis and many other physiological processes are triggered by light. A small group of basic helix–loop–helix domain-containing transcription factors known as phytochrome-interacting factors (PIFs) repress seed germination, regulate seedling skotomorphogenesis, and promote several developmental changes by regulating over one thousand genes. In the dark, Arabidopsis seedlings undergo skotomorphogenesis, which is a developmental strategy that seedlings use after germination to accelerate their growth in soil and optimize the plant shape before reaching the light. Light-activated phytochromes (PHYA and PHYB) bind directly to these PIF proteins which are then phosphorylated, followed by their rapid degradation and the reversal of their activities. This precise control of photomorphogenesis has been the subject of intensive investigation. In the past decade, several studies have highlighted the complex mechanism of how PIFs are regulated to fine-tune photomorphogenesis. Here, we review recent advances in this signaling pathway and present emerging evidence showing that other molecular pathways also converge to regulate PIF activity, including abiotic stress and hormonal signaling pathways. We also summarize past studies conducted on the role of PIFs in crop improvement. The roles of light-induced signaling networks and the associated molecular mechanisms have been significantly neglected in crop plants. These advances will almost certainly fuel future research on light signaling transcription factors in other crop plants for stress tolerance studies. Such information also enables breeders and plant biologists to improve crop yield and develop climate-tolerant crops that can adapt to constantly changing environmental conditions.