Immunolocalization of the Plasma Membrane H+ -ATPase in Minor Veins of Vicia faba in Relation to Phloem Loading.

The immunolocalization of the plasma membrane H+ -ATPase, which generates a proton motive force energizing the uptake of inorganic and organic solutes, was studied by electron microscopy. The cells studied were in minor veins of Vicia faba L. exporting leaves, where photosynthates are supposed to be absorbed from the apoplast by phloem transfer cells. Immunologically detectable H+ -ATPase varied among the different cell types and was considerably denser in the transfer cells than in the other cell types, particularly in the sieve tube. Moreover, the distribution of the H+ -ATPase was not homogeneous in transfer cells, that pump being more concentrated in the region adjacent to the bundle sheath, phloem parenchyma, and xylem vessels than along the smooth part of the wall bordering the sieve tube. These results show that the plasma membrane infoldings of transfer cells possess the proton-pumping machinery required to energize an efficient uptake of photosynthates from the phloem apoplast and an efficient retrieval of nitrogenous compounds from the vascular sap.