Extracellular nucleotides: Ancient signaling molecules

The ability of extracellular nucleotides to initiate diverse signaling responses in animal cells is well established, but is only just now being recognized in plants. Despite the newness of this field in the plant literature, there is reason to believe plant cells may have been using extracellular nucleotides to transduce environmental signals even before animal cells appeared on the biological scene. Recent evidence indicates that at least some green algae have purinoceptors that are subtly similar in structure to mammalian receptors, and that some algae employ extracellular ATP (eATP) as wound signals. Extracellular nucleotides can also induce superoxide and nitric oxide production in algae, two signaling intermediates that are also commonly used by other primitive plants. Another key molecular component of extracellular nucleotide signaling is the ectoapyrase enzyme, responsible for limiting the accumulation of nucleotides that are released during cell growth, wounding and pathogen attacks, and that can help terminate eATP signaling. The green alga Ostreococcus lucimarinus has several different enzymes that structurally qualify as apyrases, and one of these has a potential signal anchor, although none have been confirmed as ectoapyrases. As yet no plasma membrane-localized receptor for extracellular nucleotides has been identified in algae or in any other plant, and so it remains unclear whether the mechanisms by which algae and other plants respond to eATP and other extracellular nucleotides is similar to or has diverged from those used by animals.