Actin filaments modulate hypoosmotic-responsive K⁺ efflux channels in specialised cells of developing bean seed coats

In developing bean (Phaseolus vulgaris L.) seeds, nutrients move in the symplasm from sieve elements to ground-parenchyma cells where they are transported across the plasma membrane into the seed apoplasm. Release of nutrients to the seed apoplasm is related to the osmotic conditions of the apoplasm. A hypoosmotic solution, resulting from enhanced uptake of nutrients by cotyledons, stimulates nutrient release from seed coat to the apoplasm. We investigated hypoosmotic nutrient release by examining the ionic membrane currents that respond to hypoosmotic treatment in protoplasts derived from three important cell types that occur at the seed coat-cotyledonary boundary. A non-selective but predominantly K⁺ efflux current that displayed a distinct time-dependent inactivation was elicited by membrane depolarisation under hypoosmotic conditions only in ground-parenchyma protoplasts. Hypoosmotic treatment had little effect on whole-cell ionic currents in protoplasts derived from coat chlorenchyma cells and cotyledon dermal cells. The inactivating K⁺ efflux current was elicited under isosmotic conditions by treatment with cytochalasin D, which disrupts actin filaments. Hypoosmotic treatment and cytochalasin D failed to induce the K⁺ current in ground-parenchyma protoplasts in the presence of the actin stabiliser, phalloidin. The net efflux of K⁺ from intact seed coats was enhanced by hypoosmotic treatment and cytochalasin D, and the stimulation of K⁺ efflux induced by the hypoosmotic treatment and cytochalasin D was abolished by phalloidin. A bursting Cl^- channel previously described showed a similar pattern of responses. These results suggest that hypoosmotic-dependent KCl efflux from seed coats is mediated by the inactivating K⁺ outward current and bursting Cl^- channel, and that actin filaments act as components of the transduction process that is a function of cell volume.