Gene expression in ineffective actinorhizal nodules ofAlnus glutinosa

Summary Several Frankia strains have been shown to induce ineffective, i.e. non-nitrogen fixing nodules, sometimes in a host-plant dependent manner. Previous studies have demonstrated that the resistance to nodulation of Alnus glutinosa by ineffective Frankia strains is genetically determined. In this study, ineffective nodules induced on susceptible Alnus glutinosa clones by soil suspensions from a local swamp were analysed cytologically. Comparisons with effective nodules showed that ineffective nodules contain higher amounts of polyphenols than effective nodules, indicating a plant defense reaction. Polyphenols were found even in the infected cortical cells. In situ hybridization with a Frankia antisense 16S rRNA probe showed that Frankia is degraded at an early stage of development of infected cells. The mRNAs of two plant genes, ag12ar\6 ag13, which had been found to be expressed in the infected cells of effective nodules, were localized in ineffective nodules. Their expression patterns seemed to be analogous in both types of nodules.