THE ACTINORHIZAL NODULE

Actinorhizal nodules or actinorhizae represent the most typical exampl e of modified non pathogenic roots (nodule lobes), induced by signals synthetized by the microsymbiont. The formation oi a nodule robe occur s in four steps : infection of the root hair by Frankia, formation of the prenodule, initiation and nodule robe infection. The nodule lobe c omprises four distinct zones which are the result of the Frankia acrop etal growth and of the differentiation of tissues originating from the apical meristem. In situ hybridization studies of the expression of n if genes of Frankia show that the zone of infected mature cells (zone III) is the zone where the nitrogen fixation is the greatest. Similitu des and differences with organogenesis of the nodules of legumes are d isplayed and the interest of this primitive symbiosis is emphasized.