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 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.
Highlights
Directional organ growth allows the plant root system to strategically cover its surroundings
Despite the apparent importance of the root system depth, the molecular mechanisms regulating the direction of Lateral roots (LRs) growth are poorly understood
Anticipating that natural variation could provide valuable insights on how to sustainably engineer root systems, we focused on the primary growth direction of LR in natural Arabidopsis accessions
Summary
Directional organ growth allows the plant root system to strategically cover its surroundings. The asymmetric distribution of auxin eventually reduces cellular elongation rates at the lower root flank, which leads to differential growth within the organ and bending towards gravity[6,7,8]. Transcription factors FOUR LIPS and MYB88 define PIN3 expression in columella cells of LRs10 and its transient expression in columella cells temporally defines asymmetric auxin distribution as well as differential elongation rates in stage II LRs7,11 This developmental stage lasts about 8–9 h and is characterized by asymmetric growth towards gravity at a slower rate than in primary roots[7,12]. The derepression of PIN3 and PIN4 in columella cells of older stage III LRs does not necessarily correlate with additional bending to gravity[8] This finding illustrates that the primary GSA is developmentally maintained, determining an important root architectural trait. Physiological, computational, biochemical, and cell biological approaches, we reveal that two opposing hormonal cues at the lower and upper lateral root flank counterbalance each other and set directional LR growth
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