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Mechanisms of solute efflux from seed coats: whole-cell K+ currents in transfer cell protoplasts derived from coats of developing seeds of Vicia faba L.

In developing seed of Vicia faba L., solutes imported through the phloem of the coats move symplastically from the sieve elements to a specialized set of cells (the thin-walled parenchyma transfer cells) for release to the seed apoplast. Potassium (K + ) is the predominant cation released from the seed coats. To elucidate the mechanisms of K + efflux from seed coat to seed apoplast, whole-cell currents across the plasma membranes of protoplasts of thin-walled parenchyma transfer cells were measured using the whole-cell patch-clamp technique. Membrane depolarization elicited a time-dependent and an instantaneous outward current. The reversal potential (E R ) of the time-dependent outward current was close to the potassium equilibrium potential (E K ) and it shifted in the same direction as E K upon changing the external K + concentration, indicating that this current was largely carried by an efflux of K + . The activation of the time-dependent outward K + current could be well fitted by two exponential components plus a constant. The instantaneous outward current could also be carried by K + efflux as suggested by ion substitution experiments. These K + outward rectifier currents elicited by membrane depolarization are probably too small to represent the mechanism for the normal K + efflux from seed coat cells. Membrane hyperpolarization more negative than -80 mV activated a time-dependent inward current. K + influx was responsible for the inward current as the current reversed at membrane voltage close to E K and shifted in the same direction as E K when external [K + ] was varied. Activation of this K + inward rectifier current was well fitted with two exponential components plus a constant. A regulating function for this current is suggested.