Branching Hormone is Busy Both Underground and Overground

Abstract

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 inhibits axillary bud outgrowth, while auxin depletion caused by decapitation triggers bud outgrowth. Physiological studies of this phenomenon, called apical dominance, have predicted the involvement of a second messenger of auxin; however, the messenger has remained unknown for decades. In the 1990s, mutants with an excess branching phenotype were collected in garden pea, Arabidopsis and petunia. They were named ramosus ( rms ), more axillary growth ( max ) and decreased apical dominance ( dad ), respectively. Grafting experiments using these mutants indicated that a long-distance, graft-transmissible signal plays a critical role in the inhibition of bud outgrowth ( Foo et al. 2001 ). It was also suggested that, among the branching mutants, rms1 and rms5 of pea and max1 , max3 and max4 of Arabidopsis are involved in the biosynthesis of the graft-transmissible signal, while rms5 and max2 probably have defects in perception or signalling. Molecular cloning of the RMS and MAX genes stimulated the progress of studies on shoot branching. First, the fi nding that RMS5/MAX3 and RMS1/MAX4 encode CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7) and CCD8 led researchers to anticipate that the hormone might be derived from carotenoids ( Sorefan et al., 2003 , Booker et al. 2004 ). In addition, the Branching Hormone is Busy Both Underground and Overground