Abbreviations: AM, arbuscular mycorrhizal; CCD, CAROTENOID CLEAVAGE DIOXYGENASE; SL, strigolactone.

The discovery of strigolactone (SL) or its derivatives as a new hormone that inhibits shoot branching was a recent signifi cant breakthrough in plant biology ( Gomez-Roldan et al. 2008, Umehara et al. 2008). It was discovered independently by two groups, who submitted their papers for publication only a few weeks apart; it was clearly the moment for SL to be revealed. Shoot branching is controlled by two distinct mechanisms, namely formation and subsequent outgrowth of axillary buds produced in the axils of leaves ( Bennett and Leyser 2006). The latter aspect, the control of bud outgrowth, is generally more important in the determination of plant architecture. After initiation, axillary buds make the decision to continue to grow or to stay dormant, depending on a number of internal and environmental cues. The presence or absence of an apical bud is one such cue. Auxin supplied by the apical bud inhibit