Shoot branching in plants occurs after bud activation in axillary meristems, which are stem cell clusters along the shoot. This process is repressed by strigolactones by a poorly understood mechanism. One model suggests that strigolactones do not act on the bud directly, but inhibit shoot branching by preventing auxin transport out of the bud. Ottoline Leyser and co-workers now show, on , direct evidence in support of this model. The researchers treated Arabidopsis stem segments with a synthetic strigolactone, GR24, and found that this reduced auxin transport. Furthermore, they ruled out a direct effect of strigolactones on bud growth by showing that treatment of a solitary bud with GR24 alone had no effect on bud outgrowth, but that GR24 increased the inhibitory effect of an additional auxin source. In addition, they demonstrated that GR24 enhanced competition between two buds on the same stem. Collectively, these data support the model that strigolactone inhibits shoot branching by regulating the efflux of auxin from axillary buds.
