Same same, but different: growth responses of primary and lateral roots.

The root system architecture describes the shape and spatial arrangement of roots within the soil. Its spatial distribution depends on growth and branching rates as well as directional organ growth. The embryonic primary root gives rise to lateral (secondary) roots, and the ratio of both root types changes over the life span of a plant. Most studies have focused on the growth of primary roots and the development of lateral root primordia. Comparably less is known about the growth regulation of secondary root organs. Here, we review similarities and differences between primary and lateral root organ growth, and emphasize particularly how external stimuli and internal signals differentially integrate root system growth.

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

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.

Growth Rate Normalization Method to Assess Gravitropic Root Growth.

Time-lapse imaging of roots is highly suitable for depicting gravitropic growth behaviors. However, roots may show faster or slower bending kinetics when compared to control as a result of differences in overall root growth. Accordingly, conditions that cause differential organ growth require growth rate normalization to compare gravitropic curvature. Here, we describe a simple normalization method for gravitropic root growth evaluation. We exemplify this method by exposing seedlings to distinct environmental conditions or disturbing the cellular auxin contents. This data shows that the method is suitable to discriminate between gravitropic and overall organ growth defects.