Ion channel activity and tip growth: tip-localized stretch-activated channels generate an essential Ca2+ gradient in the oomycete Saprolegnia ferax.

The plasma membrane of tip-growing hyphae of the oomycete Saprolegnia ferax contains stretch-activated (SA) Ca(2+)-permeable and Ca(2+)-activated K+ ion channels. Patch clamp measurements on protoplasts derived from specific regions of hyphae demonstrated that SA channels were most abundant in the tip. Gadolinium (Gd3+) inhibited SA channel activity and stopped tip growth. The Ca(2+)-sensitive fluorochrome INDO 1 revealed a tip-high gradient of free cytoplasmic Ca2+ in growing hyphae. This gradient could be dissipated with the addition of Gd3+. The calcium gradient returned and growth resumed when Gd3+ was washed out. This implies a fundamental requirement for growth for Ca2+ influx through the SA channels. Ca(2+)-activated K+ channels were distributed evenly along the hyphae. These channels were inhibited by tetraethylammonium concentrations which caused a rapid but transient decrease in growth. We suggest that the SA channels at the apex act as feedback sensors, responding to membrane stretching at the tip. They are an obligate requirement for tip growth. The Ca(2+)-activated K+ channels may act to maintain turgor pressure, but are not obligatory for growth.