Summary: The basic helix-loop-helix (bHLH) transcription factor PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) is a central regulator that promotes stem growth by activating growth-related gene expression during shade-avoidance responses. Studying the co-factors of PIF7 can facilitate understanding of the mechanism of PIFs and light signal transduction. Here, we describe the identification of two bHLH transcription factors, bHLH48 and bHLH60 (bHLH48/bHLH60), as essential partners for PIF7-dependent modulation of hypocotyl elongation and function downstream of phytochrome B. These two bHLH factors display DNA binding activity and interact with PIF7. Genetic analysis indicated that bHLH48/bHLH60 and PIF7 are interdependent in promoting hypocotyl elongation. Chromatin immunoprecipitation sequencing (ChIP-seq) analysis identified the substantially overlapping downstream targets of bHLH60 and PIF7. Biochemical analysis revealed that bHLH48/bHLH60 enhance the DNA binding ability of PIF7. These results provide evidence that bHLH48/bHLH60 act as positive partners of PIF7 for mutual benefit in the regulation of hypocotyl elongation.
