Résumé
Among the Leguminosae, two genera were recently described as forming efficient nodulation with the bacterial genus Methylobacterium: Crotalaria and Lotononis. Methylobacterium are able to utilize methanol and other C 1 compounds as a sole source of carbon and energy. Originally isolated from Crotalaria podocarpa, C. perrotetti and C. glaucoides, M. nodulans was proved to possess the gene mxaF coding for methanol dehydrogenase large subunit. Plants inoculated by mutants altered in methylotrophy were found to be drastically affected in nodule number and growth. This study aims at describing some histological and cytological characteristics of C. podocarpa nodules induced by Methylobacterium ORS 20601 strain. Mature C. podocarpa nodules are multilobed anchored to the root at one single pod. Within one mature lobe tissues are organized as described in classical indeterminate nodules like Medicago. Within invasion zone II, bacteria are present as classical rods both in intercellular spaces and also intracellularly. Infection threads are not observed nor within root hairs, neither delivering bacteria to young plant cells. Bacteroids within fixation zone III are globally spherical with an average diameter of 4 µm. In the apical zone of mature, fully elongated nodules, the invasion zone rapidly appears to take an unusual shape with a progressive loss of plant cell wall integrity, clearly revealed by cell wall stains. The whole zone is disorganized with no more subsistence of any plant cell wall. Mutants affected in methylotrophy do not induce such lysis. Interestingly, beyond their methylotrophy impairment, mxaF mutants also loose their ability to use methylated pectins in pure culture. We hypothesize that methanol could be produced from cell-wall pectin demethylation in this apical zone, where mxaF gene expression has been localized. (Texte intégral)