The plant hormone auxin and its directional transport are known to play a crucial role in defining the embryonic axis and subsequent development of the body plan. Although the role of PIN auxin efflux transporters has been clearly assigned during embryonic shoot and root specification, the role of the auxin influx carriers AUX1 and LIKE-AUX1 (LAX) proteins is not well established. Here, we used chemical and genetic tools on Brassica napus microspore-derived embryos and Arabidopsis thaliana zygotic embryos, and demonstrate that AUX1, LAX1 and LAX2 are required for both shoot and root pole formation, in concert with PIN efflux carriers. Furthermore, we uncovered a positive-feedback loop between MONOPTEROS (ARF5)-dependent auxin signalling and auxin transport. This MONOPTEROS-dependent transcriptional regulation of auxin influx (AUX1, LAX1 and LAX2) and auxin efflux (PIN1 and PIN4) carriers by MONOPTEROS helps to maintain proper auxin transport to the root tip. These results indicate that auxin-dependent cell specification during embryo development requires balanced auxin transport involving both influx and efflux mechanisms, and that this transport is maintained by a positive transcriptional feedback on auxin signalling.

Department of Plant Systems Biology Flanders Institute for Biotechnology 3400 Klosterneuburg Austria

Department of Plant Systems Biology Flanders Institute for Biotechnology and Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Gent Belgium

Department of Plant Systems Biology Flanders Institute for Biotechnology and Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Gent Belgium Mendel Centre for Genomics and Proteomics of Plants Systems CEITEC MU Central European Institute of Technology Masaryk University 625 00 Brno Czech Republic

Laboratory of Biochemistry Wageningen University 6703 HA Wageningen The Netherlands

School of Biosciences and Centre for Plant Integrative Biology University of Nottingham Nottingham LE12 5RD UK

University of Potsdam Institute of Biochemistry and Biology D 14476 Potsdam Germany Departamento Molecular de Plantas Centro Nacional de Biotecnología Consejo Superior de Investigaciones Cientificas 28049 Madrid Spain

Wageningen University and Research Centre P O Box 619 6700 AP Wageningen The Netherlands

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