Auxin influx carriers control vascular patterning and xylem differentiation in Arabidopsis thaliana

Auxin is an essential hormone for plant growth and development. Auxin influx carriers AUX1/LAX transport auxin into the cell, while auxin efflux carriers PIN pump it out of the cell. It is well established that efflux carriers play an important role in the shoot vascular patterning, yet the contribution of influx carriers to the shoot vasculature remains unknown. Here, we combined theoretical and experimental approaches to decipher the role of auxin influx carriers in the patterning and differentiation of vascular tissues in the Arabidopsis inflorescence stem. Our theoretical analysis predicts that influx carriers facilitate periodic patterning and modulate the periodicity of auxin maxima. In agreement, we observed fewer and more spaced vascular bundles in quadruple mutants plants of the auxin influx carriers aux1lax1lax2lax3. Furthermore, we show AUX1/LAX carriers promote xylem differentiation in both the shoot and the root tissues. Influx carriers increase cytoplasmic auxin signaling, and thereby differentiation. In addition to this cytoplasmic role of auxin, our computational simulations propose a role for extracellular auxin as an inhibitor of xylem differentiation. Altogether, our study shows that auxin influx carriers AUX1/LAX regulate vascular patterning and differentiation in plants.NF is funded by an FI PhD fellowship from the Generalitat de Catalunya. PFJ acknowledges the FPU grant (FPU-AP2008-03325) funded by the Spanish Ministry of Education (2009–2011) and the Spanish Ministry of Education, Culture and Sports (2011–2013). AC is funded by a post-doctoral fellowship from Fundação para a Ciência e Tecnologia (SFRH/BPD/47280/2008). AICD is a recipient of a Marie-Curie Initial Training Network “BRAVISSIMO” (Grant PITN-GA- 2008- 215118). This work is supported by the Spanish Ministry of Science and Innovation through grants FIS2012-37655-C02-02 (PFJ and MI), FIS2009-13360-C03-01 (PFJ and MI), BIO2010-16673 (NF, AC and AICD) and BIO2013-43873 Grant Excellence to AICD, the Generalitat de Catalunya through grants 2009 SGR 0014 and 2014 SGR 878 (PFJ and MI), the Academy of Finland and the University of Helsinki (RSi and APM), Integrative Life Science Doctoral Program (RSi) and National Science Foundation Grant DBI0820755 (JMA). RSw. and MJB acknowledge the BBSRC and EPSRC for funding.Peer reviewe