Auxin and strigolactones in shoot branching: intimately connected?

Axillary meristems form in the axils of leaves. After an initial phase of meristematic activity during which a small axillary bud is produced, they often enter a state of suspended growth from which they may be released to form a shoot branch. This post-embryonic growth plasticity is typical of plants and allows them to adapt to changing environmental conditions. The shoot architecture of genotypically identical plants may display completely contrasting phenotypes when grown in distinct environmental niches, with one having only a primary inflorescence and many arrested axillary meristems and the other displaying higher orders of branches. In order to cease and resume growth as required, the plant must co-ordinate its intrinsic developmental programme with the responses to environmental cues. It is thought that information from the environment is integrated throughout the plant using plant hormones as long-distance signals. In the present review, we focus primarily on how two of these hormones, auxin and strigolactones, may be acting to regulate shoot branching.