Externally applied ATP (xATP) can induce increased [Ca²⁺] cyt in plant cells (Demidchik et al., 2003; Jeter et al., 2004), just as it does in animal cells. To evaluate the physiological significance of this signaling response, we have tested the effects of xATP on growth and development in Arabidopsis. Lower µM levels of xATP enhance and higher µM levels inhibit the hypocotyl growth of seedlings. Sub-µM xATP can induce in leaves an enhanced production of superoxide and downstream gene expression changes associated with wounding. Neither the growth nor superoxide responses can be induced by equivalent concentrations of phosphate or AMP. The inhibitory growth effects of xATP can be blocked by ethylene inhibitors, and the ability of xATP to induce superoxide production can be blocked by antagonists of animal purinoceptors and by inhibitors of calcium uptake into cells.
Ectoapyrases (ecto-NTPDases) are ATP-hydrolyzing enzymes that play a key role in controlling the [xATP]. Two highly similar apyrases in Arabidopsis, Atapy1 and Atapy2, have putative signal peptides and have been postulated to function as ectoapyrases. They are important for pollen germination and other aspects of plant cell growth. Pollen from plants not expressing Atapy1 and Atapy2 (= double knock-out (DKO) plants) cannot germinate. DKO pollen from plants that are complemented with a wild-type gene under the control of a steroid-inducible promoter can be induced to germinate by dexamethasone. Fertilization by this pollen yields seeds that germinate normally, but in the absence of further dexamethasone treatment, the resulting DKO plants are stunted, typically < 10 cm tall at maturity. As evaluated by promoter:GUS constructs and other methods, the highest mRNA level for both apyrases was found in tissue poised for rapid growth (mature pollen), or tissue actually undergoing rapid growth (elongation zone of the root, hypocotyls of etiolated seedlings), consistent with the postulate that apyrases play a key role in growth control. (Supported by NSF IBN0344221 to S.J.R.).

1. Demidchik V. et al. (2003) Plant Physiol. 133: 456-461
2. Jeter C.R. et al. (2004) Plant Cell 16: 2652-2664