Directional transport of the plant hormone auxin plays an essential role in plant development. To date, most studies of auxin transport have focussed on the PIN family of auxin efflux transporters but now Jiri Friml and colleagues show that the AUX1 and LIKE-AUX1 (LAX) auxin influx carriers are required during plant embryogenesis (p. ). The researchers first demonstrate that the pharmacological inhibition of auxin influx in both microspore-derived Brassica napus embryos and Arabidopsis thaliana embryos results in defects in early embryogenesis. They further reveal that AUX1, LAX1 and LAX2, but not LAX3, are expressed during Arabidopsis embryo development. These differentially expressed influx carriers, they report, are required for correct embryo development; patterning defects and defective cotyledon and root formation are observed in aux1 lax1 lax2 triple mutants. Further genetic interaction studies reveal that aux/lax and pin mutations have additive effects on cotyledon development, suggesting that AUX/LAX carriers act in concert with PIN transporters. Finally, the researchers uncover a positive-feedback loop between MONOPTEROS-dependent auxin signalling and auxin transport, highlighting a role for balanced and regulated auxin influx and efflux during plant development.
