We studied the effect of constitutive expression of pathogenesis-related proteins (PRs) in tobacco plants on vesicular-arbuscular mycorrhiza. Tobacco lines genetically transformed to express various PRs constitutively under the control of the cauliflower mosaic virus 35S promoter of tobacco were examined. Immunoblot analysis and activity measurements demonstrated high levels of expression of the PRs in the root systems of the plants. Constitutive expression of the following acidic isoforms of tobacco PRs did not affect the time course or the final level of colonization by the vesicular-arbuscular mycorrhizal fungus Glomus mosseae: PR-1a, PR-3 (=PR-Q), PR-Q(prm1), PR-4, and PR-5. Similarly, constitutive expression of an acidic cucumber chitinase, of a basic tobacco chitinase with and without its vacuolar targeting peptide, of a basic (beta)-1,3-glucanase, and of combinations of PR-Q and PR-Q(prm1) or basic chitinase and basic (beta)-1,3-glucanase did not affect colonization by the mycorrhizal fungus. A delay of colonization by G. mosseae was observed in tobacco plants constitutively expressing the acidic isoform of tobacco PR-2, a protein with (beta)-1,3-glucanase activity.