The significance of different isoforms of the plant plasma membrane H(+)-ATPase is essentially unknown. At least some of the isoforms exhibit a tissue-specific and developmental pattern of expression. The three major isoforms of Arabidopsis thaliana plasma membrane H(+)-ATPase have been expressed in a functional form in yeast internal membranes. The isoforms differ significantly from each other in several kinetic parameters. Thus, the isoform diversity of plasma membrane H(+)-ATPase might provide the individual cell with enzymes specifically adapted to that cell.