Transport of Plant Growth Regulators Special Issue doi:10.1093/jxb/erm134 Advance Access publication 28 August, 2007 SPECIAL ISSUE REVIEW PAPER

Shoot branching is the process by which axillary buds, located on the axil of a leaf, develop and form new flowers or branches. The process by which a dormant bud activates and becomes an actively growing branch is complex and very finely tuned. Bud out-growth is regulated by the interaction of environmental signals and endogenous ones, such as plant hor-mones. Thus these interacting factors have a major effect on shoot system architecture. Hormones known to have a major influence are auxin, cytokinin, and a novel, as yet chemically undefined, hormone. Auxin is actively transported basipetally in the shoot and inhibits bud outgrowth. By contrast, cytokinins travel acropetally and promote bud outgrowth. The novel hormone also moves acropetally but it inhibits bud outgrowth. The aim of this review is to integrate what is known about the hormonal control of shoot branch-ing in Arabidopsis, focusing on these three hormones and their interactions