Biological roles of cyclic-nucleotide-gated ion channels in plants: What we know and don't know about this 20 member ion channel family

Cyclic-nucleotide-gated ion channels (CNGCs) are nonselective cation channels that were first identified in vertebrate retinal photoreceptors and olfactory sensory neurons. The role of CNGCs in animals is well documented, and they are believed to localize to the plasma membrane as heterotetramers. So far, only six CNGC genes (CNGA1-4, CNGB1, and CNGB3) have been identified in mammalian genomes. In contrast, the Arabidopsis thaliana (L.) Heynh. genome encodes 20 putative CNGC subunits that are thought to assemble in the same tetrameric form as animal subunits do. Although the study of Arabidopsis CNGCs is still in its infancy, current research revealed their ability to transport cations that play a role in mediating various biotic and abiotic stresses and developmental processes. This review discusses the current knowledge of plant CNGCs and focuses on functional aspects, with references to heterologous expression studies and reverse genetics analyses. In addition, structural aspects of these channels are discussed.