Extracellular ATP Signaling in Animals and Plants: Comparison and Contrast

Abstract

Although the key role of extracellular nucleotides as signaling agents in animals and plants is not often discussed in text books, it is a major topic in the primary literature, with typically over 400 papers published on this topic every year for the past two decades. For research in animal cells, this literature became quite extensive following the discovery, over three decades ago, of multiple purinergic receptors for extracellular nucleotides such as extracellular ATP (eATP) in mammals and other vertebrates. On the other hand, research on eATP signaling in plant cells is relatively more recent and limited, but it has begun to expand significantly after the discovery of an eATP receptor in Arabidopsis in 2014. Although the structural characteristics of the purinergic receptors in animals and plants differ significantly, the signaling steps that follow the activation of these receptors are similar in plants and animals, both having an increase in [Ca2+]cyt within seconds as one of the earliest steps, and both leading to increased levels of reactive oxygen species within minutes as a critical intermediate in the signaling pathway. New downstream molecular and physiological responses to receptor activation by extracellular nucleotides are being discovered every year, and this chapter will discuss underlying similarities and distinct differences in these responses in plants and animals. In both animals and plants, the main enzyme limiting the [eATP] is a nucleoside triphosphate-diphosphohydrolase (NTPDase), more often referred to in the plant literature as apyrase. These enzymes have features that have been conserved throughout evolution, from primitive algae through to humans. This fact, plus the observation that physiologically significant levels of ATP can be found in the open ocean, suggest that eATP signaling is an ancient method of regulating cellular responses.