Osmotic stress enhances the competence of Beta vulgaris vacuoles to respond to inositol 1,4,5‐trisphosphate

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Abstract
Summary: Isolated vacuoles from Beta vulgaris storage roots respond to the intracellular signalling molecule inositol 1,4,5‐trisphosphate (InsP3). Whole vacuole patch clamp enables measurement of an inward current (cytosol‐directed) induced by cytosolic InsP3 which is fully reversible upon removal of InsP3. The reversal potentials of the InsP3‐induced whole vacuolar currents indicate a permeability ratio (P,Ca:P,K) of 200:1. Competence of vacuoles to respond to InsP3 is dependent upon the root tissue undergoing hyperosmotic stress before vacuole isolation. The magnitude of the hyperosmotic stress and the density of InsP3‐induced current per unit membrane area are exponentially related. A standing osmotic gradient across the vacuolar membrane further enhances the InsP3‐induced current, the current being larger when there is net water flux from the cytosol to the vacuolar lumen. InsP3‐induced currents are not affected by the cytosolic free Ca2+ concentration. The conductance of InsP3‐induced single channel currents varied greatly between individual outside‐out patches, but all showed a non‐linear increase in single channel current at physiological potentials. The reversal potentials of these currents indicated a PCa:PK of between 100:1 and 800:1.The significance of these findings is discussed in relation to technical aspects of monitoring InsP3‐induced currents in plant vacuoles and in the context of the physiological roles of InsP3 and its receptor in cell water relations.

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