The mutualisitic association of arbuscular mycorrhiza (AM) is characterized by an exchange of nutrients: The fungus assists the plant with the uptake of mineral nutrients (mainly phosphate) and water from the soil, whereas the plant supplies the obligath biotrophic fungus with carbon - most likely in form of apoplastic hexoses. The cleavage of sucrose, the main form in which photosynthetically fixed carbon is translocated to the sink tissues, can be performed by plant sucrose synthases or invertases. Upon colonization with the AM-fungus Glomus intraradices promoter activation and transcript accumulation of the apoplastic invertase LIN6 near fungal structures and in the phloem cells of tomato (Lycopersicon esculentum) was shown. To analyze the role of apoplastic invertases during AM, transgenic tobacco plants (Nicotiana tabacum) expressing the chimeric gene for apoplast located yeast-derived invertase under control of an alcohol-inducible (alc) promoter system were investigated. Using low concentrated acetaldehyde the alc promoter could be locally activated. However, the formation of mycorrhizal structures as well as the accumulation of fungal specific fatty acid (C16:1Δ11) or AM-induced plant secondary metabolites (cyclohexenone- and mycorradicin-derivatives) was not affected. Additionally, no effect on the phosphate content or on the transcript accumulation of salt stress-inducible genes of mycorrhizal plants due to the increased invertase activity was found. In contrast, reduced apoplastic invertase activity, accomplished by root-specific expression of an inhibitor of acid invertases, resulted in depressed mycorrhization. A regulatory function of apoplastic invertases on AM on whole plant level was revealed by constitutive expression of the apoplast-located yeast invertase. Strongly increased invertase activity in the leaf apoplast led to hexose and PR gene transcript accumulation and reduced mycorrhization. Moderately increased invertase activity, however, resulted in a stimulation of mycorrhization. The roots of those plants were characterized by decreased levels of phenolic compounds, amines and some amino acids and a higher content of abscisic acid.