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WIREs Dev Biol
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Molecular mechanisms of root gravity sensing and signal transduction

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Abstract Plants use gravity as a guide to direct their roots down into the soil to anchor themselves and to find resources needed for growth and development. In higher plants, the columella cells of the root tip form the primary site of gravity sensing, and in these cells the sedimentation of dense, starch‐filled plastids (amyloplasts) triggers gravity signal transduction. This generates an auxin gradient across the root cap that is transmitted to the elongation zone where it promotes differential cell elongation, allowing the root to direct itself downward. It is still not well understood how amyloplast sedimentation leads to auxin redistribution. Models have been proposed to explain how mechanosensitive ion channels or ligand–receptor interactions could connect these events. Although their roles are still unclear, possible second messengers in this process include protons, Ca2+, and inositol 1,4,5‐triphosphate. Upon gravistimulation, the auxin efflux facilitators PIN3 and PIN7 relocalize to the lower side of the columella cells and mediate auxin redistribution. However, evidence for an auxin‐independent secondary mechanism of gravity sensing and signal transduction suggests that this physiological process is quite complex. Furthermore, plants must integrate a variety of environmental cues, resulting in multifaceted relationships between gravitropism and other directional growth responses such as hydro‐, photo‐, and thigmotropism. WIREs Dev Biol 2012, 1:276–285. doi: 10.1002/wdev.14 This article is categorized under: Plant Development > Root Development

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(a) When a plant is turned on its side, the columella cells in the root tip sense this change and promote root curvature. (b) In a vertically growing root, starch‐filled plastids called amyloplasts are sedimented at the bottoms of the columella cells. (c) On reorientation, the amyloplasts sediment to the new lower sides of the columella cells. This promotes an auxin gradient, which is ultimately responsible for tip curvature at the elongation zones. Large green dots represent higher auxin levels and small green dots represent lower auxin levels. (d) In a vertically growing plant, the PIN3 and PIN7 auxin efflux facilitators (shown in blue) are localized symmetrically on all sides of the plasma membrane of the columella cells. (e) On reorientation, PIN3 and PIN7 relocalize to the lower sides of the columella cells where they contribute to the formation of the auxin gradient.

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