Comparing Plant and Animal Glutamate Receptors: Common Traits but Different Fates?

Animal ionotropic glutamate receptors (iGluRs) are ligand-gated channels whose evolution is intimately linked to that of the nervous system, where the agonist glutamate and co-agonists glycine/D-serine act as neurotransmitters or neuromodulators. While iGluRs are specialized in neuronal communication, plant glutamate receptor-like (GLR) homologs have evolved many plant-specific physiological functions, such as sperm signaling in moss, pollen tube growth, root meristem proliferation, innate immune, and wound responses. GLRs have been associated with Ca2+ signaling by directly channeling its extracellular influx into the cytosol. Nevertheless, very limited information on functional properties of GLRs is available, and we mostly rely on structure/function data obtained for animal iGluRs to interpret experimental results obtained for plant GLRs. Yet, a deeper characterization and better understanding of plant GLRs is progressively unveiling original and different functions when compared with their mammalian counterparts. Here, we review the function of plant GLRs comparing their predicted structure and physiological roles with those of the well-documented roles of iGluRs. We conclude that interpreting GLR function based on comparison with their animal counterparts calls for caution, especially when presuming physiological roles and the mode of action for plant GLRs, and when comparing iGluRs in neuronal tissues with those in peripheral, non-neuronal tissues.