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WIREs Dev Biol
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Root branching: mechanisms, robustness, and plasticity

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Abstract Plants are sessile organisms that must efficiently exploit their habitat for water and nutrients. The degree of root branching impacts the efficiency of water uptake, acquisition of nutrients, and anchorage. The root system of plants is a dynamic structure whose architecture is determined by modulation of primary root growth and root branching. This plasticity relies on the continuous integration of environmental inputs and endogenous developmental programs controlling root branching. This review focuses on the cellular and molecular mechanisms involved in the regulation of lateral root distribution, initiation, and organogenesis with the main focus on the root system of Arabidopsis thaliana. We also examine the mechanisms linking environmental changes to the developmental pathways controlling root branching. Recent progress that emphasizes the parallels to the formation of root branches in other species is discussed. WIREs Dev Biol 2012, 1:329–343. doi: 10.1002/wdev.17 This article is categorized under: Plant Development > Root Development

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Spatiotemporal organization of Arabidopsis lateral root formation. Pre‐initiation occurs in the basal meristem. As the primary root grows, the primed pericycle divides to form a unicellular primordium (initiation) that will further divide, creating a dome‐shaped primordium (growth) which will emerge from the parental root (emergence).

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Nitrogen‐induced lateral root plasticity (a) Effect of uniform low nitrogen abundance on Arabidopsis root system architecture. (b) The miRNA mediated and nitrogen controlled gene networks modulating lateral root growth. (c) Modification of auxin transport during lateral root development in response to nitrate levels. Green arrows indicate acropetal auxin flow while blue arrows indicate NRT1‐mediated basipetal auxin reflux. Under high nitrate abundance, NO inhibits NRT1 auxin transport; as a consequence, the flow of auxin in the lateral root primordium increases, leading to increased lateral root growth. The color coding used is identical to Figure 1.

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Coupling between lateral root formation and gravistimulation. (a) Oscillating gene expression in the basal meristem determines competence for periodic Arabidopsis root branching. (b) Upon gravistimulation, concentrations of the plant hormone auxin increase along the outside of the bend as consequence of cell stretching. This stretching initiates changes in auxin transport. AUX1 up‐regulation enhances the auxin maxima that specify the lateral root founder cells at the bend, while PIN down‐regulation modulates the lateral spacing of the roots along the main root axis. Auxin flow and maximum are indicated by green arrows and cell, respectively. The color coding used is identical to Figure 1.

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Auxin signaling modules controlling lateral root development. For each phase of lateral root development, from pre‐initiation to growth stage (a) and emergence (b), the gene networks downstream of auxin are indicated. The flow of auxin is indicated by green arrows. The color coding used is identical to Figure 1.

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Cellular events of Arabidopsis lateral root development. (a) Each stage of lateral root development, from pre‐initiation (stage 0) to emerging primordium (stage 8) is represented. Stages are numbered according to Ref 9. (b) Details of the early cellular events during lateral root initiation. The nuclei of two primed pericycle cells become round and migrate toward their common cell wall (gray arrows). This step is followed by two asymmetric divisions (red arrowheads) creating two smaller cells in the center and two larger at the periphery. The ACR4 receptor controls these divisions, favoring them in the primed pericycle cells while repressing them in the neighboring cells. (c) Flow of auxin in the lateral root primordium. Auxin coming from the shoot flows toward the primary root tip and the lateral root primordium via PIN1‐mediated transport (green arrows). An auxin maximum is formed at the tip of the lateral root primordium (green cells). From there, part of the auxin is retrieved by a PIN2‐dependent auxin route through the outer layers (blue arrows). Color coding is identical to Figure 1.

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