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Ineffective Frankia endophytes were retrieved from various wet soils by using Alnus glutinosa clones as trapping plants. No pure cultures could be isolated from these ineffective nodules. Therefore, the phylogenetic position of these endophytes was determined by sequence analysis of cloned PCR products of bacterial 16S rDNA, derived from nodules. The results showed that all nodule endophytes belong to a hitherto undescribed cluster of the Frankia phylogenetic tree. The position of these uncultured ineffective Frankia nodule endophytes is different from that of the ineffective Frankia isolates derived from A. glutinosa nodules, even when originating from the same geographical location. This suggests a bias in current isolation techniques.

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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 168 rRNA probe showed that Frankia is degraded at an early stage of development of infected cells. The mRNAs of two plant genes, ag12 and 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.Resume.-Plusieurs souches de Frankia induisent des nodules non-effectifs, c'est-a-dire non fixateurs d'azote, parfois sous Ia dependance de Ia plante-h6te. De precedentes etudes ont montre que Ia resistance d'Ainus glutinosa a l'infection par des souches de Frankia non-effectives repond a un determinisme genetique. Au cours de Ia presents etude, des nodules non-effectifs, induits sur des clones susceptibles d'Ainus glutinosa par des suspensions de sols locaux, ont fait l'objet d'une analyse cytologique. Une comparaison avec des nodules effectifs montre que les nodules non-effectifs contiennent plus de polyphenols, ce qui indique qu'une reaction de defense a lieu. Des polyphenols ont egalement ete observes dans les cellules infectees du cortex. Une hybridation in situ avec une sonde antisens pour I'ARNr 168 de Frankia montre que Frankia subit Ires tot une degradation lors du developpement des cellules infectees. La localisation des ARNm d'ag12 et ag13, deux genes de Ia plante qui sont exprimes dans les cellules infectees des nodules effectifs, a ete realises. Une expression analogue semble exister dans les deux types de nodules.