BpCYCD3;1 regulates organ growth by activating the cell cycle module in white birch.

Cyclin D proteins are key regulators of cell cycle progression and plant growth. Here, we identified BpCYCD3;1, a CYCD3 family member in birch, and investigated its function. Phylogenetic and sequence analyses showed that BpCYCD3;1 is evolutionarily conserved and contains canonical cell cycle-related domains. Promoter and expression analyses suggested responsiveness to environmental and hormonal signals, with preferential expression in leaves. BpCYCD3;1 overexpression promoted leaf expansion and biomass accumulation, whereas CRISPR/Cas9 knockout reduced vegetative growth. Cytological analyses revealed increased cell density and epidermal cell number in overexpression lines, indicating enhanced cell proliferation. At the molecular level, BpCYCD3;1 upregulated G1/S transition regulators and DNA replication-related genes. Yeast two-hybrid and docking analyses suggested interaction between BpCYCD3;1 and CDKA;1, as well as the interaction between RBR and E2F transcription factors, and binding of E2FA/E2FB to downstream promoters. Together, these results indicate that BpCYCD3;1 promotes vegetative growth by activating the CDKA-RBR-E2F regulatory module, providing a potential target for improving tree growth and biomass production.