Branching gene expression during chrysanthemum axillary bud outgrowth regulated by strigolactone and auxin transport

Shoot branching is essential in ornamental chrysanthemum production and determines final plant shape and quality. Auxin
is associated with apical dominance to indirectly inhibit bud outgrowth. Two non-mutually exclusive models exist for indirect
auxin inhibition. Basipetal auxin transport inhibits axillary bud outgrowth by limiting auxin export from buds to stem
(canalization model) or by increasing strigolactone levels (second messenger model). Here we analyzed bud outgrowth in
treatments with auxin (IAA), strigolactone (GR24) and auxin transport inhibitor (NPA) using a split-plate bioassay with
isolated chrysanthemum stem segments. Besides measuring bud length, dividing cell percentage was measured with flow
cytometry and RT-qPCR was used to monitor expression levels of genes involved in auxin transport (CmPIN1) and signaling
(CmAXR2), bud dormancy (CmBRC1, CmDRM1) and strigolactone biosynthesis (CmMAX1, CmMAX3). Treatments over a
5-day period showed bud outgrowth in the control and inhibition with IAA and IAA + GR24. Bud outgrowth in the control
coincided with high dividing cell percentage, decreased expression of CmBRC1 and CmDRM1 and increased CmPIN1 expression.
Inhibition by IAA and IAA + GR24 coincided with low dividing cell percentage and unchanged or increased expressions
of CmBRC1, CmDRM1 and CmPIN1. Treatment with GR24 showed restricted bud outgrowth that was counteracted by
NPA. This restricted bud outgrowth was still concomitant with a high dividing cell percentage and coincided with decreased
expression of dormancy genes. These results indicate incomplete inhibition of bud outgrowth by GR24 treatment and suggest
involvement of auxin transport in the mechanism of bud inhibition by strigolactones, supporting the canalization model.