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 with 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 cytosolic 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+] between 0.01 and 10 mM with transient changes in growth intensity and cytosolic [Ca2+]: increased external [Ca2+] elevates cytosolic [Ca2+] followed by a growth burst. Whereas external [Ca2+] lower than 1 μM is inhibitory to steady state tip growth, concentrations up to 30 mM are not. Vibrating probe analysis reveals inwardly directed net Ca2+-currents in the tip only. The calcium channel antagonists nifedipine and La3+ decrease cytosolic free [Ca2+], inhibit the inwardly directed Ca2+-current and tip growth. Dibromo-BAPTA, injected into the cells, also decreases cytosolic [Ca2+] and inhibits growth, but only marginally depolarizes the cells. Abrupt changes in external pH between 5 and 9 affect cytosolic pH and transiently inhibit tip growth, regardless of the direction of the pH-shift. Acetic acid and NH4Cl both inhibit tip growth only, when the cytosolic pH is shifted from its steady state value. Tip growth is inhibited in the presence of the ATPase inhibitors DCCD, vanadate and erythrosin B. We argue that several Ca2+- and pH-related processes are pivotal for tip growth in root hairs: with respect to Ca2+, these are an inwardly directed Ca2+-current, localized elevated cytosolic [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.