Luteolin absorption in Rhizobium meliloti wild-type and mutant strains

Summary: Luteolin is a flavonoid produced by plants which is required for induction of nod genes in Rhizobium meliloti. R. meliloti absorbed luteolin at higher rate than all other bacteria tested, including R. leguminosarum. The flavonoids naringenin and quercetin, which do not induce the expression of nodulation genes of R. meliloti, were absorbed at a lower rate by this species, suggesting a certain degree of species specificity of flavonoid absorption. Luteolin accumulated preferentially in the outer membrane, but a small amount was always found in the inner membrane. Luteolin strongly inhibited NADH oxidase, an enzyme of the respiratory chain, raising the possibility that the site of luteolin absorption in the outer membrane allows the protection of the respiratory chain located in the inner membrane from an excess of flavonoids. The incorporation of luteolin was found to be affected in some exo or nod mutants of R. meliloti. The exoB mutant, which does not produce exopolysaccharides, accumulated lower amounts of luteolin in the outer membrane than the exo + parent. Among the nod mutants affected in nodulation genes, those not expressing any of the three nodD genes accumulated luteolin at a significantly lower level in both the outer and the inner membrane. A strain overexpressing the nod genes, particularly the nodD genes, absorbed luteolin at a higher level in both membranes. These results indicate that absorption of luteolin by R. meliloti involves several gene products, including the NodD protein.