Levels of extracellular ATP in growth zones of Arabidopsis primary roots are changed by altered expression of apyrase enzymes

In both animal and plant cells extracellular nucleotides act as hormone-like signals regulating many important physiological and developmental responses. In plants, many of these responses have been studied in roots. Here we used an enzyme-based microsensor to measure the concentrations of extracellular ATP (eATP) within 2 µm of epidermal cell surfaces in growth zones of primary roots of wild-type and transgenic Arabidopsis seedlings. The concentration of eATP outside of growing wild-type roots was in the nanomolar range and was highest in in the elongation zone. The concentrations of eATP in wild-type roots were altered in two kinds of mutants, those that were overexpressing <i>AtAPY1</i> or <i>AtAPY2</i>, which encode apyrases (NTPDases) that regulate root and root hair growth, and those that were suppressed in the expression of these two transcripts. Our results indicate that the [eATP] measured varies inversely with the level of expression of these apyrases. Structural modeling of these two apyrases predicts active site configurations capable of binding ATP. Taken together these results favor the hypothesis that AtAPY1 and AtAPY2 regulate eATP levels in primary roots.