Initial Bud Outgrowth Occurs Independent of Auxin Flow from Out of Buds

Apical dominance is the process whereby the shoot tip inhibits the growth of axillary buds along the stem. It has been proposed that the shoot tip, which is the predominant source of the plant hormone auxin, prevents bud outgrowth by suppressing auxin canalization and export from axillary buds into the main stem. In this theory, auxin flow out of axillary buds is a prerequisite for bud outgrowth, and buds are triggered to grow by an enhanced proportional flow of auxin from the buds. A major challenge of directly testing this model is in being able to create a bud- or stem-specific change in auxin transport. Here we evaluate the relationship between specific changes in auxin efflux from axillary buds and bud outgrowth after shoot tip removal (decapitation) in the pea (Pisum sativum). The auxin transport inhibitor 1-N-naphthylphthalamic acid (NPA) and to a lesser extent, the auxin perception inhibitor p-chlorophenoxyisobutyric acid (PCIB), effectively blocked auxin efflux from axillary buds of intact and decapitated plants without affecting auxin flow in the main stem. Gene expression analyses indicate that NPA and PCIB regulate auxin-inducible, and biosynthesis and transport genes, in axillary buds within 3 h after application. These inhibitors had no effect on initial bud outgrowth after decapitation or cytokinin (benzyladenine; BA) treatment. Inhibitory effects of PCIB and NPA on axillary bud outgrowth only became apparent from 48 h after treatment. These findings demonstrate that the initiation of decapitation- and cytokinin-induced axillary bud outgrowth is independent of auxin canalization and export from the bud.