Nodulation of Crotalaria podocarpa DC. by Methylobacterium nodulans displays very unusual features
Résumé
Crotalaria are plants of the Fabaceae family whose nodulation characteristics have been little explored despite the
recent discovery of their unexpected ability to be efficiently nodulated in symbiosis with bacteria of the genus
Methylobacterium. It has been shown that methylotrophy plays a key role in this unusual symbiotic system, as it is
expressed within the nodule and as non-methylotroph mutants had a depleting effect on plant growth response.
Within the nodule, Methylobacterium is thus able to obtain carbon both from host plant photosynthesis and from
methylotrophy. In this context, the aim of the present study was to show the histological and cytological impacts of
both symbiotic and methylotrophic metabolism within Crotalaria podocarpa nodules. It was established that if
Crotalaria nodules are multilobed, each lobe has the morphology of indeterminate nodules but with a different
anatomy; that is, without root hair infection or infection threads. In the fixation zone, bacteroids display a spherical
shape and there is no uninfected cell. Crotalaria nodulation by Methylobacterium displayed some very unusual
characteristics such as starch storage within bacteroid-filled cells of the fixation zone and also the complete lysis of
apical nodular tissues (where bacteria have a free-living shape and express methylotrophy). This lysis could possibly
reflect the bacterial degradation of plant wall pectins through bacterial pectin methyl esterases, thus producing
methanol as a substrate, allowing bacterial multiplication before release from the nodule.