Assimilation of nod gene inducer 14C-naringenin and the incorporation of labelled carbon atoms into the acyl side chain of a host-specific Nod factor produced by Rhizobium leguminosarum bv. viciae

Abstract

The fate of 14C-naringenin during its specific activation of nod genes in Rhizobium leguminosarum bv. viciae was examined. After incubation with either strain RBL5560 or its pSym-cured derivative in a medium supplemented with 14 C-naringenin at nod gene-inducing concentrations of 2 nM (ca. 12.5 kBq) plus cold acetate (0.5 μM), a radiocarbon inventory for the cells and supernatant extracts was obtained. The level of 14 C-label incorporation was also determined in the fractionated cellular components. Using 14 C-acetate at 0.5 μM (1036 kBq) and cold naringenin (2 nM) in incubations with strain RBL5560 as a separate treatment, the Nod metabolites were detected by thin layer and high performance liquid chromatographic methods and the data provided the basis for identification of the Nod factors from the supernatant obtained from 14 C-naringenin treatments. Subsequent radio-biochemical and chemical analyses revealed that RBL5560 cells assimilated 14 C-naringenin during the activation of nod genes. Our analysis also showed that labelled carbon atoms from the 14 C-naringenin were incorporated into the acyl moiety of a lipo-oligosaccharide Nod factor, NodRlv IV, present in the culture supernatants of RBL5560. The pSym-cured derivative failed to synthesize any Nod metabolites in a 14 C-naringenin supplemented medium. The tracing of flavonoid-derived carbon atoms to the acyl chain of a host-specific Nod factor, a moiety that defines host specificity for this Rhizobium, adds a new dimension to the signalling function of flavonoids in legume-Rhizobium interactions.