Concerns over climate change and energy security have heightened interest in the development of renewables for heat, power and transport fuels, and willow (Salix) has a recognised contribution to make. Optimisation of elite lines is needed to realise fully the potential of Short Rotation Coppice (SRC) willow as a bioenergy crop. In this study, knowledge ascertained from dissecting the regulation of bud outgrowth in Arabidopsis is transferred to Salix spp. with the long-term aim of understanding branching control in willow and applying this knowledge to crop improvement. Bud outgrowth and associated stem number is a trait that shows great variability in the UK National Willow Collection (NWC). Some of this diversity has been captured in twelve mapping populations at Rothamsted Research, U.K. Alleles of the More AXillary branching (MAX) genes 1 to 4 within these populations are being cloned and introduced into their respective Arabidopsis knock-out mutants. The Arabidopsis mutants are being used as a screening tool to test for relative functionality of the different Salix allele protein products. To date, MAX4 has yielded interesting data with two particular Salix alleles exhibiting ‘poor’ and ‘excellent’ ability to rescue Arabidopsis max4-1. Crosses are now being made to produce populations including homozygotes for these alleles to test whether the differing degree of Salix MAX4 allele functionality has an effect in the SRC willow crop that could be informative in the generation of improved lines.