Posidonia oceanica (L) Delile, a seagrass endemic of the Mediterranean sea, provides food and shelter to marine organisms. As environment contamination and variation in physico-chemical parameters may compromise the survival of the few Posidonia genotypes living in the Mediterranean, comprehending the molecular mechanisms controlling Posidonia growth and development is increasingly important. In the present study the properties of ion channels in P. oceanica plasma membranes studied by the patch-clamp technique in protoplasts obtained from the young non-photosynthetic leaves were investigated. In protoplasts that were presumably originated from sheath cells surrounding the vascular bundles of the leaves, an outwardrectifying time-dependent channel with a single channel conductance of 58 ± ± ± ± 2 pS which did not inactivate, was selective for potassium and impermeable to monovalent cations such as Na + , Li + and Cs + was identified. In the same protoplasts, an inward-rectifying channel that has a timedependent component with single channel conductance of the order of 10 pS, a marked selectivity for potassium and no permeation to sodium was also identified, as was a third type of channel that did not display any ionic selectivity and was reversibly inhibited by tetraethylammonium and lanthanum. A comparison of Posidonia channel characteristics with channels identified in terrestrial plants and other halophytic plants is included.
Using the patch-clamp technique, we investigated the transport properties of vacuolar ion channels from the roots of water hyacinth, Eichhornia crassipes (Mart. Solms, Pontederiacae). Eichhornia crassipes vacuoles displayed large voltage-dependent rectifying slow-vacuolar (SV) currents, which activated in a few seconds at positive potentials and deactivated at negative voltages in a few hundreds of millseconds. Similarly to SV channel previously identified in the tonoplast of terrestrial plants, SV currents in E. crassipes were activated by micromolar concentrations of Ca 2+ and current slightly increased (25%) on addition (10 m M ) of the reducing agent dithiothreitol (DTT). Eichhornia crassipes SV channels were equally permeable to K + and Na + . The permeability sequence derived from current values is: K + ≈ ≈ ≈ ≈ Na + > Rb + > NH 4 + ≈ ≈ ≈ ≈ Cs + >> TEA + . Excised membrane patches displayed single channel transitions typical of SV-type single channel openings with a conductance of (83·0 ± ± ± ± 5·6) pS; a smaller channel with a conductance of (31·0 ± ± ± ± 2·7) pS was also identified. Metals such as Ni 2+ and Zn 2+ decreased the vacuolar current in a reversible manner. However, although Zn 2+ inhibition is comparable to that induced by the same metal in vacuoles from the main root of sugar beet ( Beta vulgaris L.), the inhibition of the SV currents by Ni 2+ is not as substantial in E. crassipes as in sugar beet. To our knowledge, this is the first electrophysiological characterization of ionic transport in E. crassipes , a pervasive troublesome aquatic weed, which has exceptional absorption properties of several water contaminants such as heavy metals, pesticides and phenols.
Kdc1 is a novel K+-channel gene cloned from carrot roots, and which is also present in cultured carrot cells. We investigated the characteristics of the ionic current elicited in Xenopus oocytes coinjected with KDC1 (K+-Daucus carota 1) and KAT1 (from Arabidopsis thaliana) RNA. Expressed heteromeric channels displayed inward-rectifying potassium currents whose kinetics, voltage characteristics, and inhibition by metal ions depended on KDC1:KAT1 ratios. At low KDC1:KAT1 ratios, Zn2+ inhibition of heteromeric K+ current was less pronounced compared to homomeric KAT1 channels, while at higher KDC1:KAT1 ratios, the addition of Zn2+ even produced an increase in current. Under the same conditions, the Ni2+ inhibition of the current was also reduced, but no current increase was observed. These effects might be explained by the unusual amino acid composition of the KDC1 protein in terms of histidine residues that are absent in the pore region, but abundant (four per subunit) in the proximity of the pore entrance. Channels like KDC1 could be at least partially responsible for the higher resistance of carrot cells in the presence of metals.
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