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First published online September 8, 2006; 10.1104/pp.106.085670 Plant Physiology 142:984-992 (2006) © 2006 American Society of Plant Biologists Extracellular ATP in Plants. Visualization, Localization, and Analysis of Physiological Significance in Growth and Signaling1,[W]National Center for Soybean Biotechnology and Division of Plant Sciences (S.-Y.K., G.S.) and Division of Biological Sciences and Molecular Cytology Core (M.S.), University of Missouri, Columbia, Missouri 65211
Extracellular ATP (eATP) in animals is well documented and known to play an important role in cellular signaling (e.g. at the nerve synapse). The existence of eATP has been postulated in plants; however, there is no definitive experimental evidence for its presence or an explanation as to how such a polar molecule could exit the plant cell and what physiological role it may play in plant growth and development. The presence of eATP in plants (Medicago truncatula) was detected by constructing a novel reporter; i.e. fusing a cellulose-binding domain peptide to the ATP-requiring enzyme luciferase. Application of this reporter to plant roots allowed visualization of eATP in the presence of the substrate luciferin. Luciferase activity could be detected in the interstitial spaces between plant epidermal cells and predominantly at the regions of actively growing cells. The levels of eATP were closely correlated with regions of active growth and cell expansion. Pharmacological compounds known to alter cytoplasmic calcium levels revealed that ATP release is a calcium-dependent process and may occur through vesicular fusion, an important step in the polar growth of actively growing root hairs. Reactive oxygen species (ROS) activity at the root hair tip is not only essential for root hair growth, but also dependent on the cytoplasmic calcium levels. Whereas application of exogenous ATP and a chitin mixture increased ROS activity in root hairs, no changes were observed in response to adenosine, AMP, ADP, and nonhydrolyzable ATP (
1 This work was supported by the National Research Initiative of the U.S. Department of Agriculture Cooperative State Research, Education and Extension Service (grant no. 2005–35319–16192). 2 Present address: Institute for Genomic Biology, University of Illinois, 1207 West Gregory Dr., Urbana, IL 61801. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Gary Stacey (staceyg{at}missouri.edu). [W] The online version of this article contains Web-only data. www.plantphysiol.org/cgi/doi/10.1104/pp.106.085670 * Corresponding author; e-mail staceyg{at}missouri.edu; fax 573–884–9676. Received June 22, 2006; accepted August 28, 2006; published September 8, 2006. Related articles in Plant Physiol.:
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meATP). However, application of exogenous potato (Solanum tuberosum) apyrase (ATPase) decreased ROS activity, suggesting that cytoplasmic calcium gradients and ROS activity are closely associated with eATP release. 
