Global vegetation and agricultural productivity are severely affected by harsh environmental conditions. To ensure their survival and sustain reproduction, plant cells have developed an elegant system for the uptake of ions that are present at extremely low levels in the soil plus a unique mechanism of maintaining cellular ion homeostasis as an effective adaptive measure. Plant membrane transport systems differ from those in animals in several ways. The unique properties of plant channels and transporters must be part of a fundamental understanding of solute transport across membranes and the formation of membrane energetic motive force in prokaryotes and eukaryotes.
The genome of terrestrial plants contains neither sodium channels nor sodium/potassium exchanger genes. This is consistent with the fact that plants do not require sodium as an essential nutrient, unlike animal cells; on the contrary, sodium can cause severe salinity stress for plants. The proton motive force is mainly formed in plant cells as an electrochemical membrane potential, unlike the sodium motive force in animal cells. Plant cells contain potassium as the major cation, together with effective anion membrane transport systems and a calcium signaling mechanism, which are also commonly found in animal cells. The distribution of these ions is similar in both eukaryotic systems; anions and potassium at high concentrations in the cells form an electrochemical potential across the membrane, and calcium at a low concentration in the cells acts as a versatile cytosolic signal. The similarity of the basic ion transport equipment of plants and animals reflects their common evolutionary ancestry.
In the last two decades, ion channels and transporters in plasma and vacuolar membranes of plant cells have been identified and our knowledge of the molecular aspects of the mechanism of ion transport across membranes has increased. This minireview series covers recent developments with respect to three areas of plant membrane transport systems for the flux of calcium, potassium and anions in plant cells.
In the first minireview, the Kwak group focuses on recent findings regarding the central role of Ca channels in cellular responses. It is well known that calcium signaling plays a pivotal role in physiological cytosolic events in plants in response to endogenous and environmental cues. Higher plant cells contain genes that have been suggested to act as Ca channels, including cyclic nucleotide gated ion channels and homologs of animal glutamate receptors. It appears that several classes of Ca-permeable channels are needed for the wide range of physiological responses in plant cells to environmental changes. In the second minireview, Dreyer and Uozumi summarize the unique transport systems for potassium accumulation and release. Extensive research on plant K channels and transporters reveals the central role of the signal transduction processes controlling guard cell volume and nutrient acquisition from soil as well as intracellular ion homeostasis. The third minireview, contributed by the groups of Kollist and Thomine, highlights the most recent progress on anion channels which regulate guard cell volume and loading/unloading of anions and organic acids into/from the xylem in the root. Newly identified anion channels contribute to our understanding of their regulation by abscisic acid, carbon oxide and calcium. Root anion channels mediate organic acid excretion in the rhizosphere for aluminum tolerance and are also involved in the regulation of nitrate assimilation.
These reviews cover the most up-to-date knowledge of plant ion channels. They should provide a stimulating starting point for the further study of plant membrane ion transport systems.
[ Nobuyuki Uozumi is a Professor in the Department of Biomolecular Engineering in the Graduate School of Engineering at Tohoku University, Japan. He moved there in 2007 from a faculty professor position at Nagoya University. His research field of interest is the study of membrane transporters involved in the adaptation to environmental stress in plant cells and bacteria. During the preparation of this minireview series, his laboratory was affected by the earthquake that hit the main island of Japan on 11 March 2011. He is now restarting his research with the intention of continuing to contribute to the progress of science. ]