Dual Role of Gibberellin in Perennial Shoot Branching: Inhibition and Activation

Shoot branching from axillary buds (AXBs) is regulated by a network of inhibitory and promotive forces, which includes hormones. In perennials, the dwarfed stature of the embryonic shoot inside AXBs is indicative of gibberellin (GA) deficiency, suggesting that AXB activation and outgrowth require GA. Nonetheless, the role of GA in branching has remained obscure. We here carried out comprehensive GA transcript and metabolite analyses in hybrid aspen, a perennial branching model. The results indicate that GA has an inhibitory as well as promotive role in branching. The latter is executed in two phases. While the expression level of GA2ox is high in quiescent AXBs, decapitation rapidly downregulated it, implying increased GA signaling. In the second phase, GA3ox2-mediated de novo GA-biosynthesis is initiated between 12 and 24 h, prior to AXB elongation. Metabolite analyzes showed that GA(1/4) levels were typically high in proliferating apices and low in the developmentally inactive, quiescent AXBs, whereas the reverse was true for GA(3/6). To investigate if AXBs are differently affected by GA(3), GA(4), and GR24, an analog of the branch-inhibitor hormone strigolactone, they were fed into AXBs of single-node cuttings. GA(3) and GA(4) had similar effects on GA and SL pathway genes, but crucially GA(3) induced AXB abscission whereas GA(4) promoted outgrowth. Both GA(3) and GA(4) strongly upregulated GA2ox genes, which deactivate GA(1/4) but not GA(3/6). Thus, the observed production of GA(3/6) in quiescent AXBs targets GA(1/4) for GA2ox-mediated deactivation. AXB quiescence can therefore be maintained by GA(3/6), in combination with strigolactone. Our discovery of the distinct tasks of GA(3) and GA(4) in AXB activation might explain why the role of GA in branching has been difficult to decipher. Together, the results support a novel paradigm in which GA(3/6) maintains high levels of GA2ox expression and low levels of GA(4) in quiescent AXBs, whereas activation and outgrowth require increased GA(1/4) signaling through the rapid reduction of GA deactivation and subsequent GA biosynthesis.