Early interactions between Alnus glutinosa and Frankia strain ArI3. Production and specificity of root hair deformation factor(s)

Actinomycetes from the genus Frankia are able to form symbiotic associations with more than 200 different species of woody angiosperms, so called actinorhizal plants. Many actinorhizal plants are infected via deformed root hairs. Factor(s) eliciting root hair deformation in actinorhizal symbioses have been found to be released into the culture medium, but the factor(s) has (have) not yet been characterized. In the present work, we describe the constitutive production of factor(s) by Frankia strain ArI3 causing root hair deformation on Alnus glutinosa . Deformation was detected after 4–5 h of incubation with both Frankia cultures and their cell-free culture filtrates. When culture filtrate was used, deformation was concentration dependent. A contact time of 2 min between culture filtrate and host roots was sufficient to induce subsequent root hair deformation. No root hair deformation on A. glutinosa could be detected with purified Nod factors from Rhizobium meliloti or R. leguminosarum biovar viciae . No correlation was found between Frankia strains belonging to different host specificity groups and their ability to deform root hairs on A. glutinosa. However, strains not able to deform root hairs on A. glutinosa were also unable to nodulate.