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EnvReg213-page3 - Root tip placed horizontally Migration of...

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Plant Environmental Regulators - 3 Normal cytokinin distribution Asymmetrical distribution Plants may perceive gravity by the sedimentation of amyloplasts. Gravity sensitive dense amyloplasts are called statoliths. Statoliths are found in root tip cells and in shoot vascular bundle sheath cells. Statoliths rest against the walls closer to gravity (typically the transverse walls), which promotes elongation of the opposite walls. If a root is not pointing downward, this unequal elongation will cause the root to bend. In stems, the wall where the statoliths are resting elongates, which results in a curvature away from gravity. Humans lack a satisfactory explanation for how the statoliths activate the differential growth, however.
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Unformatted text preview: Root tip placed horizontally Migration of statoliths as root curves downward Statoliths at cell base Model of statolith movement when cells are rotated • One explanation for statolith movement is that hydrostatic pressure differences exerted by the protoplast (which includes the amyloplasts) trigger cell wall expansion. • Another explanation is that Ca + + channels in the plasma membrane that connect to the cytoskeleton and to the cell wall open in response to hydrostatic pressure changes in the membrane. Opening the channels would permit movement of Ca ++ that could trigger the signal transduction pathways needed to activate auxin for the promotion of the differential cell expansion....
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