Plant growth flexibly adapts to environmental conditions, implying cross-talk between environmental signalling and developmental regulation. Here we show that the PIN auxin efflux carrier family possesses three highly conserved putative mitogen-activated protein kinase (MAPK) sites, adjacent to the phosphorylation sites of the well-characterised AGC kinase, PINOID, which regulates the polar localisation of PINs and directional auxin transport, thereby underpinning organ growth. The conserved sites of PIN1 are phosphorylated in vitro by two environmentally-activated MAPKs, MPK4 and MPK6. In contrast to AGC kinases, MAPK-mediated phosphorylation of PIN1 at adjacent sites leads to a partial loss of the plasma-membrane localisation of PIN1. MAPK-mediated modulation of PIN trafficking may participate in environmental adjustment of plant growth.