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IONIC SIGNALLING IN PLANT GRAVITROPISM.  S. Gilroy. Biology Department, The Pennsylvania State University

   Although gravitropism directs the downward growth of the root, other stimuli such as gradients in nutrients, water, and physical obstacles such as rocks all act on the root sensory systems to modify this gravitropic response and so optimize exploration of the soil. We have therefore investigated both the signaling events in the root cap that might regulate root tropic growth in response to the gravity signal and the interaction of this gravisensory system with responses to other stimuli such as touch. Laser ablation studies have mapped the most gravisensitive cells to the core of the columella region of the root cap. These cells produce a rapid cytoplasmic pH transient upon gravistimulation and blocking this transient using caged protons suggests the pH signal is required for gravisensing/signal transduction. Concomitantly, the cell wall of the root cap becomes acidified around the sensory cells implying regulation of the H⁺/ATPase at the plasma membrane is involved in generating these gravity-related pH signals. Consistent with a role for plasma membrane transport processes in gravity signaling, mutations in the AKT1 plasma membrane K⁺ channel reduce gravity sensitivity, perhaps through disruption of the systems that are needed to regulate membrane potential during gravity-induced H⁺ pump activation. Touch stimulation of the root cap inhibits both the cytoplasmic and wall pH-dependent gravity signaling events and does appear to reduce root gravity sensitivity and subsequent gravitropic response. These results suggest touch and gravity may interact at the level of signal transduction events in the columella cells. Touch stimulation of root cap peripheral cells elicits an increase in cytosolic that propagates from cell to cell throughout the cap. Touch of these peripheral cap cells also alters the dynamics of statolith movement in the columella cells implying a signal is passing from the surface cells to the gravity sensing cells at the core of the root cap. These observations suggest a model whereby touch and gravity coordinate root growth through the interaction of pH and Ca²⁺ signaling events in the columella cells. (Supported by NASA: NAG 1366 and NSF: MCB 98-74445).

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