Rhizobial Adaptation to Hosts, a New Facet in the Legume Root-Nodule Symbiosis

Abstract

Rhizobia are able to infect legume roots, elicit root nodules, and live therein as endosymbiotic, nitrogen-fixing bacteroids. Host recognition and specificity are the results of early programming events in bacteria and plants, in which important signal molecules play key roles. Here, we introduce a new aspect of this symbiosis: the adaptive response to hosts. This refers to late events in bacteroids in which specific genes are transcribed and translated that help the endosymbionts to meet the disparate environmental requirements imposed by the hosts in which they live. The host-adaptation concept was elaborated with Bradyrhizobium japonicum and three different legumes (soybean, cowpea, and siratro). Transcriptomes and proteomes in root-nodule bacteroids were analyzed and compared, and genes and proteins were identified which are specifically induced in only one of the three hosts. We focused on those determinants that were congruent in the two data sets of host-specific transcripts and proteins: seven for soybean, five for siratro, and two for cowpea. One gene cluster for a predicted ABC-type transporter, differentially expressed in siratro, was deleted in B. japonicum. The respective mutant had a symbiotic defect on siratro rather than on soybean or cowpea. This result demonstrates the value of the applied approach and corroborates the host-specific adaptation concept.

A Link between Arabinose Utilization and Oxalotrophy in Bradyrhizobium japonicum

“…This led us to speculate that glycolaldehyde is oxidized to glyoxylate, which can be fed into at least two pathways: (i) reduction to glycerate through the activity of glyoxylate carboligase (blr3166) (42) and tartronate semialdehyde reductase (blr3168), or (ii) oxidation to oxalate followed by the stepwise, complete oxidation to formate and CO 2 through the activities of Oxc and Frc (43,44) and formate dehydrogenase. We previously showed that only the enzymes Oxc and Frc involved in the second pathway were detected in bacteroids during symbiosis with all different host plants (12,21) (Table 1). Interestingly, using qPCR analysis, an elevated expression of bll3157 and bll3156 was not observed in B. japonicum cultures grown in minimal medium with succinate as the carbon source.…”

“…To further analyze the hypothesis that oxalate degradation is linked to arabinose utilization, a ⌬frc-oxc mutant strain was con- (12,21). The proteomics data summarize expression from three independent biological replicates.…”

“…Oxalate content in roots and root nodules induced by B. japonicum. Since both Frc and Oxc had been detected in soybean, cowpea, and siratro bacteroids using a global proteomics approach (12,21) (Table 1), we became interested in exploring whether oxalate is present in nodules as a potential C source for the bacteroids. Notably, it has been hypothesized previously that oxalate is a potential energy-yielding substrate sufficient to sustain nitrogen fixation in Vicia faba (46).…”

“…cv. Williams), mungbean (Vigna radiata), cowpea (Vigna unguiculata), and siratro (Macroptilium atropurpureum) seeds was done as described earlier (21). Plants were inoculated with cultures of B. japonicum that had been grown for 5 days in full medium and diluted to approximately 100 bacteria per plant.…”

“…1). Results from previous transcriptome and proteome studies in B. japonicum nodules (12,21) suggested that glyoxylate is preferentially degraded via oxalate oxidation during symbiosis. In fact, in contrast to the glyoxylate carboligase and tartronate semialdehyde reductase, the Frc and Oxc enzymes were detected in nodules where oxalate presence also could be measured.…”