Abstract
Genomes of unicellular and multicellular green algae, mosses, grasses and dicots harbor genes encoding cation-chloride cotransporters (CCC). CCC proteins from the plant kingdom have been comparatively less well investigated than their animal counterparts, but proteins from both plants and animals have been shown to mediate ion fluxes, and are involved in regulation of osmotic processes. In this review, we show that CCC proteins from plants form two distinct phylogenetic clades (CCC1 and CCC2). Some lycophytes and bryophytes possess members from each clade, most land plants only have members of the CCC1 clade, and green algae possess only the CCC2 clade. It is currently unknown whether CCC1 and CCC2 proteins have similar or distinct functions, however they are both more closely related to animal KCC proteins compared to NKCCs. Existing heterologous expression systems that have been used to functionally characterize plant CCC proteins, namely yeast and Xenopus laevis oocytes, have limitations that are discussed. Studies from plants exposed to chemical inhibitors of animal CCC protein function are reviewed for their potential to discern CCC function in planta. Thus far, mutations in plant CCC genes have been evaluated only in two species of angiosperms, and such mutations cause a diverse array of phenotypes—seemingly more than could simply be explained by localized disruption of ion transport alone. We evaluate the putative roles of plant CCC proteins and suggest areas for future investigation.
Highlights
Cation chloride (Cl−) cotransporters (CCC) are membrane-integral solute carriers that mediate electroneutral translocation of Cl−, coupled to potassium (K+) and/or sodium (Na+)
Can the phenotypes of plant ccc mutants be explained by disrupted ion transport? Are there other in planta functions of plant CCC proteins that are yet to be revealed? This review describes what is currently known about CCC proteins in plants, investigates their evolutionary origins and phylogenetic relationships, evaluates the current methods used for characterizing plant CCC proteins, and describes avenues for future investigations to broaden our understanding of this unique class of plant transporter
Plant CCC proteins reach the plasma membrane when expressed in X. laevis oocytes [7]
Summary
Cation chloride (Cl−) cotransporters (CCC) are membrane-integral solute carriers that mediate electroneutral translocation of Cl−, coupled to potassium (K+) and/or sodium (Na+). Belonging to the Solute Carrier 12 (SLC12) family of proteins, CCC members are present in plants, animals, fungi and prokaryotes [1]. Phylogenetic analyses have revealed that all plant CCC proteins belong to the KCC subfamily of CCCs [1,2,3]. Plant CCC transporters have been cloned and functionally characterized from Arabidopsis (Arabidopsis thaliana) [3], rice (Oryza sativa) [6] and grapevine (Vitis vinifera) [7]. Disruption of genes that encode CCC proteins in plants leads to severe growth and developmental phenotypes [3,6]. Can the phenotypes of plant ccc mutants be explained by disrupted ion transport? Are there other in planta functions of plant CCC proteins that are yet to be revealed? Can the phenotypes of plant ccc mutants be explained by disrupted ion transport? Are there other in planta functions of plant CCC proteins that are yet to be revealed? This review describes what is currently known about CCC proteins in plants, investigates their evolutionary origins and phylogenetic relationships, evaluates the current methods used for characterizing plant CCC proteins, and describes avenues for future investigations to broaden our understanding of this unique class of plant transporter
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