LOCALIZATION OF PLASMA-MEMBRANE H-ATPASE IN NODULES OF PHASEOLUS-VULGARIS L()

Legume nodules have specialized transport functions for the exchange o f carbon and nitrogen compounds between bacteroids and root cells. Pla sma membrane-type (vanadate-sensitive) H+-ATPase energizes secondary a ctive transporters in plant cells and it could drive exchanges across peribacteroidal and plasmatic membranes. A nodule cDNA corresponding t o a major isoform of Phaseolus vulgaris H+-ATPase (designated BHA1) ha s been cloned. BHA1 is a functional proton pump because after removal of its inhibitory domain and can complement a yeast mutant unable to s ynthesize a H+-ATPase. BHA1 is not nodule-specific, since it is also e xpressed in roots of uninfected plants. It belongs to the subfamily of plasma membrane H+-ATPases defined by the Arabidopsis AHA1, AHA2 and AHA3 genes and the tobacco PMA4 and corn MHA2 genes. In situ hybridiza tion in nodule sections indicates high expression of BHA1 limited to u ninfected cells. These results were confirmed by immunocytochemistry. The relatively low expression of plasma membrane-type H+-ATPase in Rhi zobium-infected cells put a note of caution on the origin of the vanad ate-sensitive ATPase described in preparations of peribacteroidal memb ranes. Also, our results indicate that active transport in symbiotic n odules is most intense at the plasma membrane of uninfected cells and support a specialized role of uninfected tissue for nitrogen transport .