Root hairs secrete ATP as they grow, and extracellular ATP and ADP can trigger signaling pathways that regulate plant cell growth. In several plant tissues the level of extracellular nucleotides is limited in part by ectoapyrases (ecto-NTPDases), and the growth of these tissues is strongly influenced by their level of ectoapyrase expression. Both chemical inhibition of ectoapyrase activity and suppression of the expression of two ectoapyrase enzymes by RNAi in Arabidopsis resulted in inhibition of root hair growth. As assayed by a dose-response curve, different concentrations of the poorly hydrolysable nucleotides, ATPγS and ADPβS, could either stimulate (at 7.5-25 μM) or inhibit (at ≥ 150 μM) the growth rate of root hairs in less than an hour. Equal amounts of AMPS, used as a control, had no effect on root hair growth. Root hairs of nia1nia2 mutants, which are suppressed in nitric oxide (NO) production, and of atrbohD/F mutants, which are suppressed in the production of H(2)O(2), did not show growth responses to applied nucleotides, indicating that the growth changes induced by these nucleotides in wild-type plants were likely transduced via NO and H(2)O(2) signals. Consistent with this interpretation, treatment of root hairs with different concentrations of ATPγS induced different accumulations of NO and H(2)O(2) in root hair tips. Two mammalian purinoceptor antagonists also blocked the growth responses induced by extracellular nucleotides, suggesting that they were initiated by a receptor-based mechanism.