Extracellular nucleotide effects on the electrical properties of growing Arabidopsis thaliana root hairs

Extracellular nucleotides depolarize the membrane potential of growing root hairs. ATP and ADP (at 1 mM) caused depolarizations of about 100 mV. The r elative effectiveness of other nucleotides was ATP=ADP= GTP>AMP >TTP (= ade nosine)> CTP. Phosphate had no effect, indicating that the membrane potenti al changes were not a consequence of nucleotide hydrolysis and phosphate up take. The ATP and ADP effects were characterized in more detail: half-maxim al depolarization occurred at 0.4 mM for ATP, and at 10 mu M for ADP; membr ane conductance was unchanged after treatment with either nucleotide. After wash-out. the potential usually did not completely recover, and conductanc e declined. Additions of ADP at a concentration resulting in depolarization did not affect cytosolic Ca2+ levels as monitored directly with dextran-co njugated calcium green or indirectly by cytoplasmic streaming (which was un affected). Growth increased slightly (22-38%) after ADP perfusion. Since pu rines were more potent, the cause of the depolarization could be a plant ho molog of an animal purinergic receptor. The surprisingly high specificity o f the ADP effect on the membrane potential suggests ADP may function as an extracellular message, but its potential function is unknown. It may serve as a signal during cellular wounding, or as a sensor of bacterial/fungal ac tivity near the root surface. (C) 2000 Published by Elsevier Science Irelan d Ltd. All rights reserved.