Soybean Nodulating Rhizobia Modify nod Gene Inducers Daidzein and Genistein to Yield Aromatic Products that Can Influence Gene-Inducing Activity. J. Raghavendra Rao . Department of Applied Plant Science, The Queen's University of Belfast, Newforge Lane, Belfast BT9 5PX, Northern Ireland, U.K. James E. Cooper. Department of Applied Plant Science, The Queen's University of Belfast, Newforge Lane, Belfast BT9 5PX, Northern Ireland, U.K. MPMI 8:855-862. Accepted 10 July 1995. Copyright 1995 The American Phytopathological Society.

Degradation of nod gene-inducing isoflavones daidzein or genistein by soybean-nodulating Bradyrhizobium japonicum USDA 110spc4, Rhizobium fredii HH103, and Rhizobium sp. NGR234 was compared using gas chroma-tographic and mass spectrometric analyses of derivatized culture medium supplied with the gene inducers as substrates. The principal features of these biotransformations are an initiation of a C-ring fission mechanism and the formation of a number of structurally different subspecies of de novo flavonoids. The supernatants from the early phase of bacterial cultures contained C-ring modification products coumestrol (coumestan), liquiritigenin (flavanone), and umbelliferone (coumarin), together with a key intermediate chalcone, isoliquiritigenin. B. japoni-cum cell extracts contained coumestrol, liquiritigenin, and umbelliferone together with residual amounts of unme-tabolized daidzein or genistein. Degradation products phloroglucinol, phloroglucinol carboxylic acid, or resorci-nol, derived due to conservation of the A-ring, and the principal B-ring derived metabolites p-coumaric acid (4-hyroxy cinnamic acid) or 4-hydroxy benzoic acid and phenylacetic acid were also detected in the culture filtrates. Changes in the amounts of root exudate isofla-vonoids during incubation with individual rhizobial strains and the detection of coumestrol and isoliquiritigenin when B. japomcum was separately incubated with the isoflavone glycosides daidzin and genistin in a nitrogen-free plant growth medium confirmed the degradative activity and provided additional evidence that microbial biotranformations of preexisting flavonoid pools can contribute towards the flavonoid composition of the rhi-zosphere. Rhizobial cells accumulated some of the in-ducer-derived metabolites whose planarity or hydroxyla-tion patterns can influence gene induction. Two products in particular, umbelliferone and phenylacetic acid isolated via HPLC, affected nod gene expression in B. japonicum USDA 110spc4. The inducer-derived microbial metabolites reported here may also be involved in the development of phytoalexin resistance by rhizobia, chemotactic responses and in distinguishing a symbiotic from a pathogenic invasion.