TIP GROWTH IN ROOT HAIR-CELLS OF SINAPIS-ALBA L - SIGNIFICANCE OF INTERNAL AND EXTERNAL CA2+ AND PH

In Sinapis root hair cells, tip growth has been measured and studied w ith different electrophysiological techniques. Applying ion-selective microelectrodes, we measured 452-776 nm free [Ca2+] in the tip, which is about three times the concentration found in the base. The cytosoli c pH of 7.1-7.3 in the tip is statistically not different from values measured in the base. The cells react to changes in external [Ca2+] be tween 0.01 and 10 mM with transient changes in growth intensity and cy tosolic [Ca2+]: increased external [Ca2+] elevates cytosolic [Ca2+] fo llowed by a growth burst. Whereas external [Ca2+] lower than 1 muM is inhibitory to steady state tip growth, concentrations up to 30 mM are not. Vibrating probe analysis reveals inwardly directed net Ca2+-curre nts in the tip only. The calcium channel antagonists nifedipine and La 3+ decrease cytosolic free [Ca2+], inhibit the inwardly directed Ca2+- current and tip growth. Dibromo-BAPTA, injected into the cells, also d ecreases cytosolic [Ca2+] and inhibits growth, but only marginally dep olarizes the cells. Abrupt changes in external pH between 5 and 9 affe ct cytosolic pH and transiently inhibit tip growth, regardless of the direction of the pH-shift. Acetic acid and NH4Cl both inhibit tip grow th only, when the cytosolic pH is shifted from its steady state value. Tip growth is inhibited in the presence of the ATPase inhibitors DCCD , vandate and erythrosin B. We argue that several Ca2+- and pH-related processes are pivotal for tip growth in root hairs: with respect to C a2+, these are an inwardly directed Ca2+-current, localized elevated c ytosolic [Ca2+] in the tip, and constant Ca2+-circulation. For pH, an active H+-pump and a tightly regulated cytosolic pH at the tip appear important, however not an internal pH-gradient.