A Basic Helix-Loop-Helix Gene from Poplar is Regulated by a Basic Leucine-Zipper Protein and is Involved in the ABA-Dependent Signaling Pathway

Basic helix-loop-helix (bHLH) transcription factors (TF) comprise a large group of proteins that are involved in many developmental and physiological processes in plants. In this study, a bHLH gene (PkbHLH2), along with its promoter, was cloned from Populus koreana Rehd. A PkbHLH2 promoter::GUS gene fusion construct was generated to investigate the expression of PkbHLH2. The results demonstrated that PkbHLH2 was expressed mainly in leaf stalks, leaf veins and roots. Yeast one-hybrid assays showed that a bZIP gene product (PkbZIP2) can bind specifically to the ABA-responsive elements (ABRE) that exist in the promoter region of PkbHLH2, regulating the expression of PkbHLH2. In addition, the "GC" of the ABRE core motif "ACGTG" was very important for PkbZIP2 recognition, because its mutation to "TT" completely prevented the interaction between PkbZIP2 and ABRE. Furthermore, both PkbHLH2 and PkbZIP2 can be up-regulated by abscisic acid (ABA) and osmotic stress, and share similar expression patterns when exposed to ABA and osmotic stress. These results suggest that PkbZIP2 is an upstream regulator of PkbHLH2, which can control the expression of PkbHLH2 through an ABA-dependent signaling pathway.