Expanding Roles of PIFs in Signal Integration from Multiple Processes

PHYTOCHROME-INTERACTING FACTORs (PIFs) are members of the basic helix-loop-helix (bHLH) family of transcription factors in Arabidopsis. Since their discovery in phytochrome-mediated light signaling pathways, recent studies have unraveled new functions of PIFs in integrating multiple signaling pathways not only through their role as transcription factors directly targeting gene expression but also by interacting with diverse groups of factors to optimize plant growth and development. These include endogenous (e.g., hormonal) as well as abiotic (light, circadian, and elevated temperature) and biotic (defense responses) pathways. PIFs interact with key factors in each of these pathways and tailor the outcome of the signal integration among these pathways. This review discusses the roles of PIFs as pivotal signal integrators in regulating plant growth and development.