Abstract
Shoot branching is a major determinant of plant architecture and is highly regulated by endogenous and environmental signals. Two classes of hormones, auxin and cytokinin, have long been known to regulate shoot branching. Previous studies using a series of enhanced shoot branching mutants suggested the existence of a new class of hormone that inhibits shoot branching, but its chemical identity has been unknown. We have recently found that levels of strigolactones, a group of terpenoid lactones, are significantly reduced in some of the branching mutants. Furthermore, an application of strigolactone inhibited shoot branching in these mutants. Strigolactones were previously found in root exudates as communication chemicals with root parasitic plants and arbuscular mycorrhizal fungi. Thus, we propose that strigolactones act as a new hormone class, or their biosynthetic precursors, in regulating aboveground plant architecture, and also have a function in underground communication with other neighboring organisms.
