Directional organ growth allows the plant root system to strategically cover its surroundings. Intercellular auxin transport is aligned with the gravity vector in the primary root tips, facilitating downward organ bending at the lower root flank. Here we show that cytokinin signaling functions as a lateral root specific anti-gravitropic component, promoting the radial distribution of the root system. We performed a genome-wide association study and reveal that signal peptide processing of Cytokinin Oxidase 2 (CKX2) affects its enzymatic activity and, thereby, determines the degradation of cytokinins in natural Arabidopsis thaliana accessions. Cytokinin signaling interferes with growth at the upper lateral root flank and thereby prevents downward bending. Our interdisciplinary approach proposes that two phytohormonal cues at opposite organ flanks counterbalance each other's negative impact on growth, suppressing organ growth towards gravity and allow for radial expansion of the root system.

Centro de Biotecnología y Genómica de Plantas Autopista M 40 Km 38 Pozuelo de Alarcón 28223 Madrid Spain

Department of Applied Genetics and Cell Biology University of Natural Resources and Life Sciences Vienna Muthgasse 18 1190 Vienna Austria

Department of Biology Stanford University 260 Panama Street Stanford CA 94305 USA

Department of Plant Biology Carnegie Institution for Science 260 Panama Street Stanford CA 94305 USA

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science of Palacký University and Institute of Experimental Botany of the Czech Academy of Sciences Šlechtitelů 27 78371 Olomouc Czech Republic

Plant Ecophysiology Institute of Environmental Biology Utrecht University Padualaan 8 Utrecht 3584 CH The Netherlands

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Trends Plant Sci. 2020 Feb;25(2):121-123. doi: 10.1016/j.tplants.2019.12.001. PubMed

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