BpAHL23 promotes shoot branching in white birch by regulating cell cycle

Lateral branch development is critical for woody plants, as it determines crown architecture, photosynthetic efficiency, and wood quality. Although auxin and cytokinin are known to regulate lateral branch development in herbaceous plants, underlying transcription factor networks, and the downstream genes that mediate responses to these phytohormones in forest trees remain largely unclear. In this study, we characterized BpAHL23 (AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED 23), a transcription factor that regulates lateral branch development in white birch (Betula platyphylla). Loss-of-function bpahl23 mutants exhibited delayed axillary bud elongation, whereas BpAHL23-overexpressing transgenic plants displayed enhanced lateral branching. Transcriptomic and molecular analyses suggest that BpAHL23 responds to cytokinin and auxin and regulates the expression of the cell cycle – related gene BpCYCD3;1. The CYCD3;1 protein can interact with CDKA;1 to control cell cycle progression, thereby promoting cell proliferation in axillary bud and contributing to axillary bud elongation. Overall, these results indicate that BpAHL23 is induced by cytokinin and auxin, activates BpCYCD3;1, and regulates the cell cycle through its interaction with CDKA;1. These findings provide insights into the molecular mechanisms underlying the regulation of tree architecture and suggest potential strategies for the molecular breeding of forest trees.