The proper distribution of the plant hormone auxin is of crucial importance to plant development. To this end, a complex and strictly regulated system of active auxin transport is in place, but the role of cell-signalling molecules in this system remains largely unexplored. Now, on p. 627, Claus Schwechheimer and colleagues report that a family of plant protein kinases(PKs) regulate polar auxin transport in Arabidopsis, probably by phosphorylating PIN auxin efflux carriers. By examining mutations in a subfamily of plant AGC kinases called D6PKs, the authors establish a redundant role for these kinases in auxin transport. They find that, despite lacking any obvious localisation motifs, the D6PKs are localised at the basal membrane of various root cell types, where they colocalise with PIN proteins. Further experiments reveal that PIN proteins are in vitro and in vivo phosphorylation targets of D6PKs, leading the authors to suggest that this might be the functional interaction through which D6PKs regulate directional auxin flow.