A Chalcone and Two Related Flavonoids Released from Alfalfa Roots Induce nod Genes of Rhizobium meliloti.

Flavonoid signals from alfalfa (Medicago sativa L.) induce transcription of nodulation (nod) genes in Rhizobium meliloti. Previous investigations identified the flavone luteolin as an active inducer in alfalfa seed extracts, but the nature of nod inducers released from roots has not been reported. Root exudate from 3-day-old alfalfa seedlings was purified and then assayed for biological activity with a nodABC-lacZ fusion in R. meliloti. Indentities of major nod inducers were established by spectroscopic analyses (ultraviolet/visible, proton nuclear magnetic resonance, and mass spectroscopy) and comparison with authentic standards. Major nod inducers, which were identified as 4',7-dihydroxyflavone, 4'-7-dihydroxyflavanone, and 4,4'-dihydroxy-2'-methoxychalcone, were released from seedling roots at 54, 22, and 20 picomole.plant(-1).day(-1), respectively. Luteolin was not found in these root exudates. The 4,4'-dihydroxy-2'-methoxychalcone induced nod genes at a concentration one order of magnitude lower than luteolin and is the first naturally released chalcone reported to have this function. Moderate and weak nod-inducing activity was associated, respectively, with 4',7-dihydroxyflavone and 4',7-dihydroxyflavanone.

[1]  G. Caetano-Anollés,et al.  Chemotaxis of Rhizobium meliloti to the plant flavone luteolin requires functional nodulation genes , 1988, Journal of bacteriology.

[2]  Jeffrey H. Miller.  Experiments in molecular genetics , 1972 .

[3]  J. W. Frost,et al.  A plant flavone, luteolin, induces expression of Rhizobium meliloti nodulation genes. , 1986, Science.

[4]  J. Ingham.  Isoflavonoid phytoalexins of the genus Medicago , 1979 .

[5]  Y. Kapulnik,et al.  Flavone limitations to root nodulation and symbiotic nitrogen fixation in alfalfa. , 1987, Plant physiology.

[6]  D. Dolphin,et al.  Pisum sativum stress metabolites: Two cinnamylphenols and a 2′-methoxychalcone , 1982 .

[7]  Shinji Yata,et al.  A β-Hydroxychalcone and Flavonoids from Alfalfa Callus Stimulated by a Fungal Naphthoquinone, PO-1 , 1988 .

[8]  S. Long,et al.  Induction of Rhizobium meliloti nodC expression by plant exudate requires nodD. , 1985, Proceedings of the National Academy of Sciences of the United States of America.

[9]  D. Phillips,et al.  Combined Nitrogen and Vegetative Regrowth of Symbiotically‐grown Alfalfa1 , 1981 .

[10]  R. Kosslak,et al.  Induction of Bradyrhizobium japonicum common nod genes by isoflavones isolated from Glycine max. , 1987, Proceedings of the National Academy of Sciences of the United States of America.

[11]  D. Verma,et al.  Two host-inducible genes of Rhizobium fredii and characterization of the inducing compound , 1988, Journal of bacteriology.

[12]  N. Peters,et al.  Alfalfa Root Exudates and Compounds which Promote or Inhibit Induction of Rhizobium meliloti Nodulation Genes. , 1988, Plant physiology.

[13]  G. Towers,et al.  Specificity of signal compounds detected by Agrobacterium tumefaciens , 1988 .

[14]  S. Long,et al.  Expression of Rhizobium meliloti nod genes in Rhizobium and Agrobacterium backgrounds , 1987, Journal of bacteriology.

[15]  B. Rolfe,et al.  Clovers secrete specific phenolic compounds which either stimulate or repress nod gene expression in Rhizobium trifolii , 1987, The EMBO journal.

[16]  B. Rolfe,et al.  Flavones induce expression of nodulation genes in Rhizobium , 1986, Nature.

[17]  S. C. Witt,et al.  Identification of 7,4′-dihydroxyflavone in forages , 1965 .

[18]  T. Dejong,et al.  Nitrogen Stress and Apparent Photosynthesis in Symbiotically Grown Pisum sativum L. , 1981, Plant physiology.

[19]  J. Vincent.  A manual for the practical study of root-nodule bacteria , 1971 .

[20]  T. Mabry,et al.  The Systematic Identification of Flavonoids , 1970, Springer Berlin Heidelberg.

[21]  H. Spaink,et al.  Induction of the nodA promoter of Rhizobium leguminosarum Sym plasmid pRL1JI by plant flavanones and flavones , 1987, Journal of bacteriology.

[22]  B. Rolfe,et al.  Identification of a nodD-dependent locus in the Rhizobium strain NGR234 activated by phenolic factors secreted by soybeans and other legumes. , 1988, Molecular plant-microbe interactions : MPMI.

[23]  T. Mabry,et al.  Mass Spectrometry of Flavonoids , 1975 .

[24]  L. Rossen,et al.  Flavonoid activation of nodulation genes in Rhizobium reversed by other compounds present in plants , 1986, Nature.

[25]  T. Mabry,et al.  Ultraviolet-Visible and Proton Magnetic Resonance Spectroscopy of Flavonoids , 1975 .