Extracellular ATP activates an Arabidopsis plasma membrane Ca(2+)-permeable conductance

Extracellular ATP has been found to elevate cytosolic free Ca(2+) in Arabidopsis thaliana and trigger gene transcription, suggesting that it acts as a plant cell regulator. Recent findings place extracellular ATP upstream of Arabidopsis thaliana NADPH oxidase activity and plasma membrane Ca(2+)-permeable channels in the root epidermis. Here we show that increasing extracellular ATP concentration evokes a larger but more irregular Ca(2+) influx conductance in root epidermal protoplasts. This may help modulate changes in cytosolic free Ca(2+) as a second messenger and help explain the dose-dependent effects of extracellular ATP on cell function. The receptors for ATP and the downstream plasma membrane Ca(2+) channels remain unknown at the protein or gene level. No equivalents of animal ATP receptors have been identified in higher plant genomes. We propose here that annexins could perceive extracellular ATP and participate in Ca(2+) influx.