Methylotrophy: A great asset in symbiosis process?

The bacterial genus Methylobacterium belongs to the [alpha]-proteobacteria. The main feature of members of this genus is their ability to oxidize methanol, a methylotrophic property based on the presence of methanol dehydrogenase (MDH). Recently, strains isolated from three Crotalaria spp. were described as being a single novel methylotrophic Methylobacterium spp. (Sy et al., 2001). This new species was named M. nodulans for its ability to both nodulate and fix nitrogen specifically during symbiosis with Crotalaria spp. (Jourand et al., 2004; Sy et al., 2001). The aim of this work was to study the relationship between the methylotrophic function of M. nodulans during symbiotic processes with the host legume Crotalaria podocarpa. The expression of the methylotroph genes was investigated during the symbiosis between M. nodulans strain ORS2060T and its host legume, C. podocarpa, by using a lacZ fusion to mxaF that encodes the largest [alpha]-polypeptide subunit of MDH. The role of methylotrophy on the plant growth was determined by studying the effect of methylotroph mutants. The mxaF gene of M. nodulans was expressed in apex of root nodules during symbiosis. This suggests that methanol is present inside the nodule and subsequently used by the bacteria (Jourand et al., 2005). In agreement, a substantial amount of methanol was detected in the nodule tissue. During symbiosis between M nodulans and Crotalaria spp., we hypothesize that methanol could be produced from pectin cell-wall demethylation. The growth of C. podocarpa plants inoculated with M. nodulans ORS2060 T methylotrophic mutants was reduced substantially compared to plants inoculated with the wild-type strain. The decrease in plant growth, which represented up to 46% biomass loss, was correlated with a drastic drop in the total root nodule number (up to 60%) and a significant reduction of the nitrogen fixation ability (up to 42%) (Jourand et al., 2005). This suggests that methylotrophy might be implicated at different levels of symbiosis perhaps in the bacterial ability to colonize or invade plant roots. During symbiosis, the methylotrophic function could contribute as supplementary energy source for the bacteria nitrogenase activity by recycling methanol produced through nodule tissue degradation. (Texte intégral