The EARLY BUD-BREAK regulatory module in poplar

Bud-break after winter dormancy is economically and environmentally important process in trees and shrubs from boreal and temperate latitudes but its molecular mechanisms are poorly understood. Previously we have reported the isolation and characterization of the EARLY BUD BREAK 1 (EBB1) gene, encoding an AP2/ERF transcription factor. We show here that EBB1 directly represses the poplar SVL (SHORT VEGETATIVE PHASE-LIKE), a MADS-box protein which was recently found to negatively regulate bud-break in Populus. We also report the identification and characterization of the EARLY BUD-BREAK 3 (EBB3) gene, which encodes another AP2/ERF transcription factor. We demonstrate that EBB3 is a positive regulator of bud-break and acts downstream of EBB1 and SVL. The effect of EBB3 on bud-break is mediated via control of cell cycle by directly and positively regulating CYCLIND3.1 gene. Our results show the molecular architecture of a novel regulatory module that controls bud-break in poplar by controlling progression of cell cycle.