Hormonal and Genetic Regulation of Axillary Bud Outgrowth in Chrysanthemum morifolium during Floral Initiation

Shoot branching or the outgrowth of axillary buds is a crucial aspect in the production and breeding of Chrysanthemum sp. The outgrowth of axillary buds is inhibited by the shoot apex through apical dominance. Central to this regulation is the interaction of the plant hormones auxin, cytokinin and strigolactone. In this research we aim to study the hormonal and genetic regulation of apical dominance and axillary bud outgrowth in a cut flower cultivar of Chrysanthemum. For this we combined measurements of axillary bud outgrowth with UPLC-MS/MS quantification of auxin and cytokinin levels and RT-qPCR analysis of two genes involved in the regulation of branching. The cultivar used displayed apical dominance during vegetative growth. A release from apical dominance occurs during floral initiation with outgrowth and elongation of shoots under the apex. The outgrowth of axillary buds after release from apical dominance was preceded by decreased auxin levels in the shoot apex, axillary buds and stem and increased cytokinin levels in the axillary buds. RT-qPCR showed a decreased expression of the local bud outgrowth regulator BRC1 preceding the release from apical dominance at the time of floral initiation.