- Trees from temperate latitudes transition between growth and dormancy to survive dehydration and freezing stress during winter months. We employed activation tagging to isolate a dominant mutation affecting release from dormancy, and identified the corresponding gene EARLY BUD-BREAK 1 (EBB1). We demonstrate through positioning of the tag, expression analysis, and retransformation experiments that EBB1 encodes a putative AP2/ERF transcription factor. Transgenic upregulation of the gene caused early bud-flush, while down-regulation delayed bud-break. Native EBB1 expression was highest in actively-growing apices, undetectable during the dormancy period, but rapidly increased prior to bud-break. The EBB1 transcript was localized in the L1/L2 layers of the shoot meristem and leaf primordia. EBB1-overexpressing transgenic plants displayed enlarged shoot meristems, open and poorly differentiated buds, and a higher rate of cell division in the apex. Transcriptome analyses of the EBB1 transgenics identified 971 differentially-expressed genes whose expression correlated with the EBB1 expression changes in the transgenic plants. Promoter analysis among the differentially expressed genes for presence of a canonical EBB1 binding site identified 65 putative target genes indicative of a broad regulatory context of EBB1 function. Our results suggest that EBB1 has a major and integrative role in reactivation of meristem activity after winter dormancy.