An environmentally responsive root system is crucial for plant growth and crop yield, especially in suboptimal soil conditions. This responsiveness enables the plant to exploit regions of high nutrient density while simultaneously minimizing abiotic stress. Despite the vital importance of root systems in regulating plant growth, significant gaps of knowledge exist in the mechanisms that regulate their architecture. Auxin defines both the frequency of lateral root (LR) initiation and the rate of LR outgrowth. Here, we describe a search for proteins that regulate root system architecture (RSA) by interacting directly with a key auxin transporter, PIN1. The native separation of Arabidopsis plasma membrane protein complexes identified several PIN1 co-purifying proteins. Among them, AZG1 was subsequently confirmed as a PIN1 interactor. Here, we show that, in Arabidopsis, AZG1 is a cytokinin (CK) import protein that co-localizes with and stabilizes PIN1, linking auxin and CK transport streams. AZG1 expression in LR primordia is sensitive to NaCl, and the frequency of LRs is AZG1-dependent under salt stress. This report therefore identifies a potential point for auxin:cytokinin crosstalk, which shapes RSA in response to NaCl.

BIOSS Centre for Biological Signalling Studies University of Freiburg 79104 Freiburg Germany

Centre of Biological Systems Analysis University of Freiburg 79104 Freiburg Germany

Conservation Ecology Department Biology Philipps Universität Marburg Karl von Frisch Straße 8 35032 Marburg Germany

Faculty of Medicine Institute of Physiology 2 University of Freiburg Hermann Herder Strasse 7 79104 Freiburg Germany

Institute of Biology 2 University of Freiburg Schänzlestrasse 1 79104 Freiburg Germany

Instituto Multidisciplinario de Biología Vegetal Velez Sarsfield 249 5000 Córdoba Argentina

Laboratory of Growth Regulators Institute of Experimental Botany ASCR and Palacky Šlechtitelů 27 783 71 Olomouc Czech Republic

Labormedizinisches Zentrum Ostschweiz Lagerstrasse 30 9470 Buchs SG Switzerland

Molecular Plant Physiology Institute of Cellular and Molecular Botany University of Bonn Kirschallee 1 53115 Bonn Germany

Zentrum für Molekularbiologie der Pflanzen Universität Tübingen Auf der Morgenstelle 1 72076 Tübingen Germany

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Membrane transport of root-borne trans-zeatin riboside maintains the cytokinin homeostasis in shoots