Shoot branching plasticity relies on integration of diverse signals to regulate the activity of buds, which is partially mediated through auxin transport by PINs and the regulation thereof. Whilst PIN behaviour has been well characterised, the mechanisms by which PINs sense and respond to auxin, strigolactone and cytokinin remain unknown. In this thesis I investigate the regulation of PIN polarity in response to these hormones, presenting evidence of an age-dependent aspect to PIN1 behaviour. I then demonstrate that NPA-sensitive auxin flux or auxin concentration are insufficient to explain PIN1 behaviour and attempt to identify a mechanism by which PIN1 senses and responds to auxin at a sub-cellular level. Following this, I demonstrate the requirement of the central region of the PIN1 hydrophilic loop to confer strigolactone sensitivity and characterise the effect of the loss of this response on plant phenotypes and bud growth dynamics. Finally, I demonstrate cross-species differences in PIN hormone responses. As a whole, this work advances our understanding of hormonal control of auxin transport in the shoot and the way in which this affects shoot architecture.