Ion transport and metal sensitivity of vacuolar channels from the roots of the aquatic plant Eichhornia crassipes

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 mill-seconds. Similarly to SV channel previously identified in the tonoplast of terrestrial plants, SV currents in E. crassipes were activated by micromolar concentrations of Ca2+ and current slightly increased (25%) on addition (10 mM) 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+ > NH4+ ≈ 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 Ni2+ and Zn2+ decreased the vacuolar current in a reversible manner. However, although Zn2+ 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 Ni2+ 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.