Figure 5.

Regulation of active growth, growth cessation and dormancy in apical buds of hybrid aspen. During long days, FT and GA are produced in leaves and transported to the apex where they activate the LAP1–AIL1 pathway. This regulates active growth by controlling genes involved in the cell cycle. Short days lead to reductions in FT and GA levels in leaves. This blocks the active growth pathway (LAP1–AIL1 pathway), resulting in growth cessation and bud set. Additionally, reduction of LAP1 leads to upregulation of BRC1, which in turn interact with FT protein and antagonize its action, providing an additional control for seasonal growth. A subsequent increase in ABA level suppresses PKL and induces SVL, which establishes dormancy. During dormancy establishment, the plasmodesmata are closed by callose deposition by CALS1 and GA catabolism genes are activated to degrade any active GA remaining in the apex. The pathways shown are based on studies of hybrid aspen but most of the components are known to regulate similar responses in other species of tree.