A Tri-Antagonistic bHLH System Regulates Cell Elongation in Arabidopsis

In plants, regulation of cell elongation is pivotal for the control of proper development and pattern formation. Recently, two basic helix-loop-helix (bHLH) proteins, PACLOBUTRAZOL RESISTANCE 1 (PRE1) and ILI1 binding bHLH 1 (IBH1), were shown as important regulators for cell elongation. PRE1 and Arabidopsis IBH1 antagonistically regulate cell elongation in response to brassinosteroid and gibberellin signaling, but the detailed molecular mechanisms by which these factors regulate cell elongation remain unclear. Here, we identify the bHLH transcriptional Activators for Cell Elongation (ACEs), and demonstrate that PRE1, AtIBH1 and the ACEs constitute a tri-antagonistic bHLH system that competitively regulates cell elongation. In this system, the ACE bHLH transcription factors directly activate the expression of enzyme genes for cell elongation by interacting with their promoter regions. AtIBH1 negatively regulates cell elongation by interacting with the ACEs and thus interfering with their DNA binding. PRE1 interacts with AtIBH1 and counteracts the ability of AtIBH1 to affect ACEs. Therefore, PRE1 restores the transcriptional activity of ACEs, resulting in induction of cell elongation. The balance of tri-antagonistic bHLH proteins, ACEs, AtIBH1 and PRE1, might be important for determination of the size of plant cells. The expression of AtIBH1 and PRE1 are regulated by brassinosteroid, gibberellins and developmental phase dependent factor(s), indicating that two phytohormones and phase dependent signals are integrated by this tri-antagonistic bHLH system.