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 relative effectiveness of other nucleotides was ATP=ADP= GTP>AMP >TTP (= adenosine)> CTP. Phosphate had no effect, indicating that the membrane potential changes were not a consequence of nucleotide hydrolysis and phosphate uptake. The ATP and ADP effects were characterized in more detail: half-maximal depolarization occurred at 0.4 mM for ATP, and at 10 mu M for ADP; membrane conductance was unchanged after treatment with either nucleotide. After wash-out. the potential usually did not completely recover, and conductance declined. Additions of ADP at a concentration resulting in depolarization did not affect cytosolic Ca2+ levels as monitored directly with dextran-conjugated calcium green or indirectly by cytoplasmic streaming (which was unaffected). Growth increased slightly (22-38%) after ADP perfusion. Since purines were more potent, the cause of the depolarization could be a plant homolog of an animal purinergic receptor. The surprisingly high specificity of 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 activity near the root surface. (C) 2000 Published by Elsevier Science Ireland Ltd. All rights reserved.