Very low concentrations of extracellular purines (ATP, ADP and AMP) can function as neurotransmitters in animal cells (1). At the cell surface they interact with specialised receptors (“purinoceptors”) which form transmembrane cation-selective channels (P2X receptors) or activate G-proteins (P2Y-receptors). Low concentrations of extracellular ATP (eATP) have recently been demonstrated to modify auxin distribution, gene expression and gravitropic responses in Arabidopsis roots (2). Since these effects have not been related to phosphorylation, a possible explanation is that eATP can act in plants in a neurotransmitter-like mode (2). Here, we test the hypothesis that ATP is a signalling agent in plants that functions in a similar way to animal neurotransmitters (activating ion fluxes at very low concentrations). We have found that [eATP] as low as 300 nM induced transient elevations of root ([Ca⁺²] cyt) in plants constitutively expressing cytosolic aequorin (3). ADP and non-hydrolysable analogues of ATP also caused [Ca⁺²] cyt increases, showing that the effect was not induced by phosphorylation. UTP was far less potent than all tested purines. Purine-induced [Ca⁺²] cyt increases were inhibited by classic purinoceptor blockers (suramin and PPADS) and by Gd⁺³ (3). These results have now been confirmed by Jeter et al. (2004) (4). Using the MIFE ® technique we have measured the effect of extracellular purines on net Ca⁺² and K + fluxes in intact Arabidopsis root epidermis. ATP, ADP and the non-hydrolysable ATP analogue ab meATP (up to 100 m M) simultaneously caused transient Ca⁺² influx and K⁺ efflux. Purine-induced fluxes were far larger in the elongation zone than in the mature epidermis. Using patch-clamp we have identified p urine- induced whole-cell cation conductances and single cation channels in root epidermis-derived protoplasts. Activity resembled animal ionotropic purinoceptors. We propose that extracellular ATP and ADP are important signalling agents in plants acting via plasma membrane cation channels.

1. Khakh BS (2001) Nat Rev Neurosci 2: 165.
2. Tang et al. (2003) Plant Physiol 131: 147.
3. Demidchik et al. (2003) Plant Physiol 133: 456.
4. Jeter et al. (2004) Plant Cell 16: 2652.