Stretch-activated Ca2+ and Ca2+-activated K+ channels in the hyphal tip plasma membrane of the oomycete Saprolegnia ferax

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Abstract
We have used the patch clamp technique to study ion channels involved in hyphal tip growth in the oomycete Saprolegnia ferax. The development of a technique in which protoplasts are produced in distinct linear arrays has permitted the study of transport proteins in protoplasts derived from different regions of the hypha. Using the cell-attached mode we find two K+ channels of different amplitudes, both of which are activated by Ca2+, as shown by the addition of the Ca2+ ionophore A23187. In nearly all the recordings (95%) there was a characteristic but variable oscillatory nature to the activity of these channels. K+ fluxes through these channels are inward and are of a sufficient magnitude to have a significant impact on cell turgor. Hyphal tip, but rarely distal protoplasts, also contain two stretch-activated channels, one displaying larger amplitudes and permeable to both Ca2+ and K+ and a smaller channel permeable to Mg2+. Stretch-activated ion channels permeable to calcium are of particular interest in relation to tip growth and their concentration at the hyphal tip supports a direct role in the tip growth process. This represents a significant advance toward understanding the mechanisms of regulation of this mode of cellular growth.