In arbuscular mycorrhizal symbioses, solutes such as phosphate are transferred to the plant in return for photoassimilates. The uptake mechanism is probably facilitated by a proton gradient generated by proton H(+)-ATPases. We investigated expression of Lycopersicon esculentum Mill. H(+)-ATPases in mycorrhizal and non-mycorrhizal plants to determine if any are specifically regulated in response to colonization. Tissue expression and cellular localization of H(+)-ATPases were determined by RNA gel blot analysis and in situ hybridization of mycorrhizal and non-mycorrhizal roots. LHA1, LHA2, and LHA4 had high levels of expression in roots and were expressed predominantly in epidermal cells. LHA1 and LHA4 were also expressed in cortical cells containing arbuscules. The presence of arbuscules in root sections was correlated with lower levels of expression of these two isoforms in the epidermis. These results suggest that LHA1 and LHA4 expression is decreased in epidermal cells located in regions of the root that contain arbuscules. This provides evidence of differential regulation between molecular mechanisms involved in proton-coupled nutrient transfer either from the soil or fungus to the plant.