A genetic framework for regulation and seasonal adaptation of shoot architecture in hybrid aspen

Shoot architecture is critical for optimizing plant adaptation and productivity. In contrast with annuals, branching in perennials native to temperate and boreal regions must be coordinated with seasonal growth cycles. How branching is coordinated with seasonal growth is poorly understood. We identified key components of the genetic network that controls branching and its regulation by seasonal cues in the model tree hybrid aspen. Our results demonstrate that branching and its control by seasonal cues is mediated by mutually antagonistic action of aspen orthologs of the flowering regulators TERMINAL FLOWER 1 (TFL1) and APETALA1 (LIKE APETALA 1/LAP1). LAP1 promotes branching through local action in axillary buds. LAP1 acts in a cytokinin-dependent manner, stimulating expression of the cell-cycle regulator AIL1 and suppressing BRANCHED1 expression to promote branching. Short photoperiod and low temperature, the major seasonal cues heralding winter, suppress branching by simultaneous activation of TFL1 and repression of the LAP1 pathway. Our results thus reveal the genetic network mediating control of branching and its regulation by environmental cues facilitating integration of branching with seasonal growth control in perennial trees.