Shoot branching is an important factor that determines the plant architecture. Two classes of hormones, auxin and cytokinin, have long been known to play key roles in controlling shoot branching. Previous studies using a series of mutants with enhanced shoot branching suggested the existence of a third class of hormone(s), but it has not been chemically identified. Because some of these branching mutants are defective in a carotenoid cleavage dioxygenase (CCD), this new hormone is thought to be synthesized from carotenoids. Strigolactones are a class of terpenoids that were previously found...