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 ES 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 labelling 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.