Rhizobia catabolize nod gene-inducing flavonoids via C-ring fission mechanisms

Abstract

Gas chromatographic and mass spectrometric analyses of derivatized culture medium extracts were used to identify the products of flavonoid metabolism by rhizobia. A number ofRhizobium species and biovars degraded their nod gene-inducing flavonoids by mechanisms which originated in a cleavage of the C-ring of the molecule and which yielded conserved A-and B-ring products among the metabolites. In contrast, Pseudomonas putida degraded quercetin via an initial fission in its A-ring, and Agrobacterium tumefaciens displayed a nonspecific mode of flavonoid degradation which yielded no conserved A-or B-ring products. When incubated with rhizobia, flavonoids with OH substitutions at the 5 and 7 positions yielded phloroglucinol as the conserved A-ring product, and those with a single OH substitution at the 7 position yielded resorcinol. A wider range of structures was found among the B-ring derivatives, including p-coumaric, p-hydroxybenzoic, protocatechuic, phenylacetic, and caffeic acids. The isoflavonoids genistein and daidzein were also degraded via C-ring fission by Rhizobium fredii and Rhizobium sp. strain NGR234, respectively. Partially characterized aromatic metabolites with potential nod gene-inducing activity were detected among the products of naringenin degradation by Rhizobium leguminosarum bv. viciae. The initial structural modification of nod gene-inducing flavonoids by rhizobia can generate chalcones, whose open C-ring system may have implications for the binding of inducers to the nodD gene product.Flavonoids are polyphenolic secondary metabolites which are synthesized by plants via the expression of two multigeneencoded enzymes: phenylalanine ammonia lyase and chalcone synthase. Subgroups of compounds such as chalcones, flavanones, flavones, flavonols, and isoflavonoids occur in legume tissues, and they can be released (15) from roots into the rhizosphere, where some of them act as molecular signals to trigger the establishment of symbioses with bacteria in the family Rhizobiaceae (18,19). Their principal function is to interact with the nodD gene products of rhizobia and the subsequent transcriptional activation of other nod genes (17). Other effects of flavonoids on rhizobia include promotion of chemotactic responses (1) and stimulation of growth rate by unspecified mechanisms (8).Although rhizobia are known to utilize various aromatic compounds as carbon and/or energy sources by degrading them to catechol and protocatechuate and channelling these products after further enzymatic cleavage into the tricarboxylic acid cycle via the 0-ketoadipate pathway (3, 16), their capacity to degrade flavonoids has received little attention. Only two examples have been reported: the utilization of catechin by a Rhizobium sp. isolated from Leucaena leucocephala, with attendant formation of phloroglucinol carboxylic acid and protocatechuate (6), and a novel form of C-ring cleavage in a pentahydroxy flavone, quercetin, by Rhizobium loti (20). One report (4) has described an alteration in the types and amounts of formon...

A dual-genome Symbiosis Chip for coordinate study of signal exchange and development in a prokaryote–host interaction

“…However, nodL and adjacent genes were increased up to 50-fold in bacteroids, a surprising result that implies an additional role for NodL besides O-acetylation of Nod factors. Genes for phenylacetic acid catabolism (paaABCD) increased in nodule bacteria, in agreement with the finding that S. meliloti degrades plant flavonoids to phenylacetic acid (46). Consistent with our earlier proposal regarding replicon specialization (11), we found a striking correlation of gene expression change with replicon.…”

Detection of and Response to Signals Involved in Host-Microbe Interactions by Plant-Associated Bacteria

“…Rhizobia are also able to catabolize nod gene inducers. Flavonoid catabolism is initiated by cleavage of the C-ring of the molecule, which yields intermediates that are themselves potential nod gene inducers (399). Chalcones are one type of such intermediates and have been reported to be especially potent nod gene inducers (399).…”

Excess Production ofBacillus subtilisQuercetin 2,3-Dioxygenase Affects Cell Viability in the Presence of Quercetin

“…Based on previous studies on the degradation of flavonols via C-ring cleavage by rhizobia 4) and the reaction mechanism of QdoI, 5) it was assumed that flavonols, such as quercetin, fisetin Lane 1, molecular mass marker; lane 2, partially purified QdoI. Its purity was estimated to be about 39% by measuring the band intensity on a gel stained with Coomassie Brilliant Blue with NIH image (http://rsb.info.nih.gov/nih-image/).…”

“…3) To avoid harmful effects, certain bacteria are equipped with a degradation system for flavonoids for detoxification. 4) A Gram-positive soil bacterium, Bacillus subtilis, possesses a quercetin 2,3-dioxygenase that converts quercetin to 2-protocatechuoyl-phloroglucinol carboxylic acid and carbon monoxide ( Fig. 1).…”