Strigolactones: discovery of the elusive shoot branching hormone

The control of axillary bud outgrowth involves a network of hormonal signals and feedback regulation. A repressor of bud outgrowth that is central to the story has been missing since it was first postulated more than 70 years ago. This hormone moves upward in plant stems and can act as a long-distance messenger for auxin. Strigolactones, previously known as carotenoid-derived signals exuded from roots, fit the role of this elusive hormone. The discovery of branching inhibition by strigolactones will help solve many confusing aspects of branch control, including interactions with other signals, and is a great step forward toward uncovering the links between environment, genetics and plant form.