Effect of potassium deficiency on the plasma membrane H⁺-ATPASE of the wood ray parenchyma in poplar

The effect of K⁺ deficiency on the plasma membrane (PM) H ⁺-ATPase was studied in young stems of poplar plants (Populus tremula x tremuloides) grown with low or full-strength K⁺ supply. Immunological assays using different antibodies were applied to test if K ⁺ deficiency affects the amount of immunodetectable PM H ⁺-ATPases in the stem tissue. The monoclonal antibody clone 46 E5 B11 revealed an increased abundance of PM H⁺-ATPases under conditions of low K⁺ supply, and immunolabelling experiments showed that this increase was restricted to vessel-associated cells (VACs) of the wood ray parenchyma. Replacement of the monoclonal antibody by a polyclonal antibody against PM H⁺-ATPase gave a specific immunoreactivity on blots as well as tissue sections too, but the labeling intensity showed no difference between plants with low or full-strength K⁺ supply. Measurements of extracellular H⁺ concentrations using non-invasive, H ⁺-selective microelectrodes revealed a lowering of the pH at the surface of VACs and an enhancement of net efflux of H⁺ in plants grown with low K⁺ supply. The present results indicate an up-regulation of specific isoforms of the PM H⁺-ATPase in VACs under K⁺-deficient conditions and suggest a key role for these PM H ⁺-ATPases in unloading K⁺ from the xylem stream.