Résumé
Actinorhizal nodules or actinorhizae represent the most typical example of modified non pathogenic roots (nodule lobes), induced by signals synthetized by the microsymbiont. The formation of a nodule robe occurs in four steps : infection of the root hair by Frankia, formation of the prenodule, initiation and nodule robe infection. The nodule lobe comprises four distinct zones which are the result of the Frankia acropetal growth and of the differentiation of tissues originating from the apical meristem. In situ hybridization studies of the expression of nif genes of Frankia show that the zone of infected mature cells (zone III) is the zone where the nitrogen fixation is the greatest. Similitudes and differences with organogenesis of the nodules of legumes are displayed and the interest of this primitive symbiosis is emphasized.