A MAJOR ISOFORM OF THE MAIZE PLASMA-MEMBRANE H-ATPASE - CHARACTERIZATION AND INDUCTION BY AUXIN IN COLEOPTILES()

The plasma membrane (PM) HC-ATPase has been proposed to play important transport and regulatory roles in plant physiology, including its par ticipation in auxin-induced acidification in coleoptile segments. This enzyme is encoded by a family of genes differing in tissue distributi on, regulation, and expression level. A major expressed isoform of the maize PM H+-ATPase (MHA2) has been characterized. RNA gel blot analys is indicated that MHA2 is expressed in all maize organs, with highest levels being in the roots. In situ hybridization of sections from maiz e seedlings indicated enriched expression of MHA2 in stomatal guard ce lls, phloem cells, and root epidermal cells. MHA2 mRNA was induced thr eefold when nonvascular parts of the coleoptile segments were treated with auxin. This induction correlates with auxin-triggered proton extr usion by the same part of the segments. The PM H+-ATPase in the vascul ar bundles does not contribute significantly to auxin-induced acidific ation, is not regulated by auxin, and masks the auxin effect in extrac ts of whole coleoptile segments, We conclude that auxin-induced acidif ication in coleoptile segments most often occurs in the nonvascular ti ssue and is mediated, at least in part, by increased levels of MHA2.