ION CHANNEL ACTIVITY AND TIP GROWTH - TIP-LOCALIZED STRETCH-ACTIVATEDCHANNELS GENERATE AN ESSENTIAL CA-2-FERAX( GRADIENT IN THE OOMYCETE SAPROLEGNIA)

The plasma membrane of tip-growing hyphae of the oomycete Saprolegnia ferax contains stretch-activated (SA) Ca2+-permeable and Ca2+-activate d K+ ion channels. Patch clamp measurements on protoplasts derived fro m specific regions of hyphae demonstrated that SA channels were most a bundant in the tip. Gadolinium (Gd3+) inhibited SA channel activity an d stopped tip growth. The Ca2+-sensitive fluorochrome INDO 1 revealed a tip-high gradient of free cytoplasmic Ca2+ in growing hyphae. This g radient could be dissipated with the addition of Gd2+. The calcium gra dient returned and growth resumed when Gd2+ was washed out. This impli es a fundamental requirement for growth for Ca2+ influx through the SA channels. Ca2+-activated K+ channels were distributed evenly along th e hyphae. These channels were inhibited by tetraethylammonium concentr ations which caused a rapid but transient decrease in growth. We sugge st that the SA channels at the apex act as feedback sensors, respondin g to membrane stretching at the tip. They are an obligate requirement for tip growth. The Ca2+-activated K+ channels may act to maintain tur gor pressure, but are not obligatory for growth.