Cytokinin functions as an asymmetric and anti-gravitropic signal in lateral roots

Abstract

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 rootflank. 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 naturalArabidopsis thaliana accessions. Cytokinin signaling interferes with growth at the upper lateral rootflank and thereby prevents downward bending. Our interdisciplinary approach proposes that two phytohormonal cues at opposite organflanks counterbalance each other’s negative impact on growth, suppressing organ growth towards gravity and allow for radial expansion of the root system.