The role of auxin transport in the control of shoot branching

Branching is a highly plastic trait, enabling plants to adapt their growth form in response to environmental stimuli. In flowering plants, shoot branching is regulated through the activity of axillary buds, which grow into branches. Several classes of plant hormones have been shown to play pivotal roles in regulating bud outgrowth. Auxin derived from the primary shoot apex and active branches inhibits bud outgrowth, whereas cytokinin promotes it. Strigolactones also inhibit bud outgrowth, by changing properties of the auxin transport network, increasing the competition between buds. This occurs by modulating access to the polar auxin transport stream (PATS) in the main stem. The PATS provides directional, long distance transport of auxin down the stem, involving basal localisation of the auxin transporter PIN-FORMED1 (PIN1). Buds need to export their auxin across the stem towards the PATS in order to activate, but since PIN1 is mainly expressed in narrow files of cells associated with the stem vasculature, PIN1 itself it is unlikely to facilitate this connectivity. This thesis re-examines the role of auxin transport in the stem, showing that, besides the PIN1-mediated PATS, other auxin transport proteins constitute a more widespread and less polar auxin transport stream, allowing auxin exchange between the PATS and surrounding tissues. Disruption of this transport stream is shown to reduce bud-bud communication and to partially rescue the increased branching observed in strigolactone mutants. Furthermore, it is shown that distinct classes of auxin transport proteins within this stream can differentially affect bud outgrowth mediated by BRANCHED1 (BRC1). BRC1 is a transcription factor proposed to determine bud activation potential. Taken together, the data presented here provide a more comprehensive understanding of the shoot auxin transport network and its role in shoot branching regulation.