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 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.