Mycorrhiza-induced changes in disease severity and PR protein expression in tobacco leaves

The development of leaf disease symptoms and the accumulation of pathogenes is-related (PR) proteins were monitored in leaves of tobacco (Nicotiana tab acum cv. Xanthinc) plants colonized by the arbuscular mycorrhizal fungus Gl omus intraradices. Leaves of mycorrhizal plants infected with the leaf path ogens Botrytis cinerea or tobacco mosaic virus showed a higher incidence an d severity of necrotic lesions than those of nonmycorrhizal controls, Simil ar plant responses were obtained at both low (0.1 mM) and high (1.0 mM) nut ritional P levels and with mutant plants (NahG) that are unable to accumula te salicylic acid. Application of PR-protein activators induced PR-1 and PR -3 expression in leaves of both nonmycorrhizal and mycorrhizal plants; howe ver, accumulation and mRNA steady-state levels of these proteins were lower , and their appearance delayed, in leaves of the mycorrhizal plants. Applic ation of 0.3 mM phosphate to the plants did not mimic the delay in PR expre ssion observed in the mycorrhizal tobacco. Together, these data strongly su pport the existence of regulatory processes, initiated in the roofs of myco rrhizal plants, that modify disease-symptom development and gene expression in their leaves.