Genetic analysis of a region of the Rhizobium meliloti pSym plasmid specifying catabolism of trigonelline, a secondary metabolite present in legumes

Abstract

Genes controlling the catabolism of trigonelline, a secondary metabolite that is often present in legumes, are located on the pSym megaplasmid of Rhizobium meliloti. To investigate the role of bacterial trigonelline catabolism in the Rhizobium-legume symbiosis, we identified and characterized the R. meliloti RCR2011 genetic loci (trc) controlling trigoneline catabolism. Tn5-B20 mutagenesis showed that the trc region is a continuous DNA segment of 9 kb located 4 kb downstream of the nifAB and fdxN genes. Trc mutants fell into two classes according to their phenotype and location: (i) mutants carrying Tn5-B20 insertions in the right-hand part of the trc region were incapable of growing on trigonelline as the sole carbon and/or nitrogen source, and (ii) insertions in the left-hand part of the trc region resulted in delayed growth on trigonelline as the sole carbon and/or nitrogen source. No significant defect in nodule formation or nitrogen fixation was detected for mutants of either class. Screening of a set of R. meliloti strains from various geographical origins showed that all of these strains are able to catabolize trigonelline and show sequence homology between their megaplasmids and a trc probe.Among the variety of secondary metabolites synthesized by plants, some are involved in the establishment of interactions with microorganisms, as is the case for the bacteria of the family Rhizobiaceae. Agrobacterium species are pathogens which induce gall formation or root proliferations on plants (40), while the symbiotic bacteria of the genera Rhizobium, Bradyrhizobium, and Azorhizobium induce on their leguminous hosts the formation of nodules in which they fix nitrogen (25). For these two types of bacterium-plant interactions, plant secondary metabolites have been shown to play a signalling role. Monocyclic phenolic compounds present in wounded plant tissues induce the Agrobacterium genes controlling virulence (vir genes) (28). Similarly, in Rhizobium, Bradyrhizobium, and Azorhizobium species, the expression of a set of genes controlling infection and the nodulation process (nod genes) is controlled by various complex phenolic compounds of the flavonoid family (for a review, see reference 32). Moreover, other plant secondary metabolites have been shown to play a trophic role, being used as nutrient sources by members of the family Rhizobiaceae: agrobacteria utilize opines, specific plant secondary metabolites the synthesis of which is directed by genes transferred from the bacterium to the plant during infection, for growth (40). This concept has been extended to particular Rhizobium strains able to utilize rhizopines, compounds present only in the nodules of plants infected by these strains (29,34). It would seem logical that rhizobia might have evolved catabolic functions to benefit also from secondary metabolites naturally present in their hosts. This hypothesis has been supported by recent reports of the presence in * Corresponding author.

Interrelations between Glycine Betaine Catabolism and Methionine Biosynthesis in Sinorhizobium meliloti Strain 102F34

“…The noncatabolizable betaines DMSP and HB did not support growth of the metH strain in LAS medium (data not shown). Similarly, another betaine, trigonelline, was not used as a methionine precursor by the metH strain; this observation is consistent with the fact that trigonelline is catabolized in S. meliloti by a BHMT-independent pathway (4,5). Taken together, these results are consistent with the hypothesis that BHMT is the only S. meliloti enzyme that allows methionine production from betaines.…”

“…Glycine betaine is transiently accumulated (43), and dimethylsulfonioacetate (DMSA) is catabolized only in medium of low osmolarity (32). Some betaines, like trigonelline, have been shown to be involved in the nodulation process and are catabolized through a specific pathway (4).…”

Molecular characterization and regulation of the rhizosphere-expressed genes rhiABCR that can influence nodulation by Rhizobium leguminosarum biovar viciae

“…For example, genes involved in the synthesis of melanin (3,9) and bacteriocins (28) are found on symbiotic plasmids. In Rhizobium meliloti, Boivin et al (4) identified genes involved in the catabolism of trigonelline, a secondary metabolite secreted by some legumes. Similarly, Murphy et al (36) have identified R. meliloti genes that synthesize opinelike compounds in nodules; these compounds are thought to be secreted and stimulate rhizosphere growth of strains of R. meliloti that have the capacity to utilize the opinelike metabolite.…”

Characterization of genes for synthesis and catabolism of a new rhizopine induced in nodules by Rhizobium meliloti Rm220-3: extension of the rhizopine concept

“…Trigonelline (8,10) and L-homoserine (21) catabolic genes are located on the Symbiotic (Sym) plasmid, which carries genes involved in nitrogen fixation (nif) and nodulation (nod), whereas calystegin catabolic genes in Rhizobium meliloti 41 are encoded on the cryptic plasmid pRme4la (59). The Sym plasmid location of catabolic genes points towards a symbiotic role for these compounds.…”