Strigolactone-Based Node-to-Bud Signaling May Restrain Shoot Branching in Hybrid Aspen

Area of Science:

• Plant Biology
• Molecular Biology
• Forestry

Background:

• Strigolactone (SL) research has focused on herbaceous plants, with limited understanding in trees.
• Axillary bud (AXB) dormancy in *Populus* species offers a model for studying bud activation.

Purpose of the Study:

• To investigate the presence, expression, and role of SL pathway genes in *Populus tremula* × *Populus tremuloides*.
• To determine the source of SLs and their precursors for AXB dormancy and activation.

Main Methods:

• Identified *Populus* homologs of key SL pathway genes (D27, LBO, D53-like).
• Analyzed spatial and temporal gene expression in root tips, shoot tissues, nodal bark, and AXBs.
• Investigated the effect of decapitation, node isolation, and GR24 (synthetic SL) feeding on gene expression and AXB outgrowth.

Main Results:

• Poplar AXBs express SL perception/signaling genes (MAX2, D14, D53) but lack key biosynthesis genes (MAX3, MAX4).
• Nodal bark shows high expression of SL biosynthesis genes, suggesting nodes supply SLs/precursors to AXBs.
• AXB activation involves rapid downregulation then upregulation of SL pathway genes; exogenous SLs are ineffective post-activation.

Conclusions:

• Poplar AXBs rely on nodal supply of strigolactones and their precursors.
• Strigolactones likely play a role in maintaining embryonic shoot elongation arrest during AXB formation and para-dormancy.
• Nodes, rather than roots, are the primary source of SLs for regulating bud dormancy in poplar.