Metabolism of Flavonoids by Rhizobia

Abstract

In common with many other soil microorganisms rhizobia are capable of catabolizing a variety of aromatic compounds. Monocylic aromatics and hydroaromatics such as benzoate and shikimate can be degraded to 3-oxoadipate via catechol or protocatechuate prior to entry into the tricarboxylic acid cycle (Chen et al., 1984; Parke, Ornston, 1984) and protocatechuate itself is a universal growth substrate for rhizobia. The possibility that rhizobia could catabolize polycyclic flavonoids responsible for nod gene induction has been raised as a consequence of experiments which showed that such compounds also induced expression of Rhizobium genes with unknown functions and showing no homologies with nodulation gene promoters (Sadowsky et al., 1988; Perret et al., 1994). The first demonstration of flavonoid catabolism by Rhizobium involved utilization of the flavan-3-ol catechin by an isolate from Leucaena leucocephala with attendant formation of phloroglucinol carboxylic acid and protocatechuate (Gajendiran, Mahadevan, 1988). Rao et al. (1991) subsequently reported that R. loti could cleave the C-ring of the pentahydroxy flavone quercetin by means of a novel mechanism which yielded phloroglucinol and protocatechuate among the degradation products. Further studies with other Rhizobium species/biovars and their respective nod gene inducers confirmed that flavonoid degradation is a common metabolic feature in members of this genus (Rao, Cooper, 1994). In this paper we report on recent developments from our studies of flavonoid metabolism by free-living rhizobia:

• 1) evidence for C-ring modification and fission during interactions between rhizobia and their flavonoid or isoflavonoid nod gene inducers;

• 2) the establishment of a contribution from carbon atoms of nod gene-inducing naringenin to the structure of R. leguminosarum bv viciae Nod factor Rlv-IV.