Two bHLH Transcription Factors, bHLH48 and bHLH60, Associate with PHYTOCHROME INTERACTING FACTOR 7 to Regulate Hypocotyl Elongation in <i>Arabidopsis Thaliana</i>

Abstract

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 under shade avoidance responses. However, previous report shown that PIF7 functions as a transcriptional repressor in vitro, suggesting that PIF7 may require other co-factors for its full function. Here, bHLH48/60, identified from yeast one hybrid screen, can regulate hypocotyl elongation and function downstream of phytochrome B. These two bHLH factors display DNA binding activity and interact with PIF7. Genetic analysis indicates that bHLH48/60 and PIF7 are interdependent in promoting hypocotyl elongation. ChIP-sequencing analysis identified the substantial overlapped downstream targets of bHLH60 and PIF7. Biochemical analysis explored that bHLH48/60 enhance DNA binding ability of PIF7. These results provide evidence that bHLH48/60 act as positive partners of PIF7 for mutual benefit on regulation of hypocotyl elongation.