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. 1993 Mar;101(3):819–824. doi: 10.1104/pp.101.3.819

Alfalfa (Medicago sativa L.) Root Exudates Contain Isoflavonoids in the Presence of Rhizobium meliloti.

F D Dakora 1, C M Joseph 1, D A Phillips 1
PMCID: PMC158695  PMID: 12231731

Abstract

Root exudates of alfalfa (Medicago sativa L.) inoculated with symbiotic Rhizobium meliloti bacteria contained three isoflavonoids that were not found in exudates of uninoculated plants. Data from proton nuclear magnetic resonance, mass spectrometry, and ultraviolet-visible absorbance analyses indicated that root exudates of inoculated plants contained aglycone and glycoside forms of the phytoalexin medicarpin and a formononetin-7-O-(6"-O-malonylglycoside), a conjugated form of the medicarpin precursor formononetin. The medicarpin molecules did not induce nod gene transcription in R. meliloti, but the formononetin-7-O-(6"-O-malonylglycoside) induced nod genes regulated by both NodD1 and NodD2 proteins in R. meliloti. Hydrolysis of either the malonyl or the glycosyl linkage from the formononetin conjugate eliminated nod gene-inducing activity. The nod gene-inducing activity of crude root exudates was increased 200 and 65% upon inoculation with R. meliloti or R. leguminosarum bv phaseoli, respectively. When root exudate from uninoculated alfalfa was incubated with R. meliloti, high performance liquid chromatography analyses showed no evidence that bacterial metabolism produced medicarpin. These results indicate that alfalfa responds to symbiotic R. meliloti by exuding a phytoalexin normally elicited by pathogens and that the microsymbiont can use a precursor of the phytoalexin as a signal for inducing symbiotic nod genes.

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Selected References

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  1. Beringer J. E. R factor transfer in Rhizobium leguminosarum. J Gen Microbiol. 1974 Sep;84(1):188–198. doi: 10.1099/00221287-84-1-188. [DOI] [PubMed] [Google Scholar]
  2. Fisher R. F., Egelhoff T. T., Mulligan J. T., Long S. R. Specific binding of proteins from Rhizobium meliloti cell-free extracts containing NodD to DNA sequences upstream of inducible nodulation genes. Genes Dev. 1988 Mar;2(3):282–293. doi: 10.1101/gad.2.3.282. [DOI] [PubMed] [Google Scholar]
  3. Hartwig U. A., Maxwell C. A., Joseph C. M., Phillips D. A. Interactions among Flavonoid nod Gene Inducers Released from Alfalfa Seeds and Roots. Plant Physiol. 1989 Nov;91(3):1138–1142. doi: 10.1104/pp.91.3.1138. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Jacobs T. W., Egelhoff T. T., Long S. R. Physical and genetic map of a Rhizobium meliloti nodulation gene region and nucleotide sequence of nodC. J Bacteriol. 1985 May;162(2):469–476. doi: 10.1128/jb.162.2.469-476.1985. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Kape R., Parniske M., Brandt S., Werner D. Isoliquiritigenin, a strong nod gene- and glyceollin resistance-inducing flavonoid from soybean root exudate. Appl Environ Microbiol. 1992 May;58(5):1705–1710. doi: 10.1128/aem.58.5.1705-1710.1992. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Kessmann H., Edwards R., Geno P. W., Dixon R. A. Stress Responses in Alfalfa (Medicago sativa L.) : V. Constitutive and Elicitor-Induced Accumulation of Isoflavonoid Conjugates in Cell Suspension Cultures. Plant Physiol. 1990 Sep;94(1):227–232. doi: 10.1104/pp.94.1.227. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Köster J., Strack D., Barz W. High Performance Liquid Chromatographic Separation of Isoflavones and Structural Elucidation of Isoflavone 7-O-glucoside 6''-malonates from Cicer arietinum. Planta Med. 1983 Jul;48(7):131–135. doi: 10.1055/s-2007-969907. [DOI] [PubMed] [Google Scholar]
  8. León-Barrios M., Dakora F. D., Joseph C. M., Phillips D. A. Isolation of Rhizobium meliloti nod Gene Inducers from Alfalfa Rhizosphere Soil. Appl Environ Microbiol. 1993 Feb;59(2):636–639. doi: 10.1128/aem.59.2.636-639.1993. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Maxwell C. A., Hartwig U. A., Joseph C. M., Phillips D. A. A Chalcone and Two Related Flavonoids Released from Alfalfa Roots Induce nod Genes of Rhizobium meliloti. Plant Physiol. 1989 Nov;91(3):842–847. doi: 10.1104/pp.91.3.842. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Pankhurst C. E., Biggs D. R. Sensitivity of Rhizobium to selected isoflavonoids. Can J Microbiol. 1980 Apr;26(4):542–545. doi: 10.1139/m80-092. [DOI] [PubMed] [Google Scholar]
  11. Parniske M., Ahlborn B., Werner D. Isoflavonoid-inducible resistance to the phytoalexin glyceollin in soybean rhizobia. J Bacteriol. 1991 Jun;173(11):3432–3439. doi: 10.1128/jb.173.11.3432-3439.1991. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Phillips D. A., Joseph C. M., Maxwell C. A. Trigonelline and Stachydrine Released from Alfalfa Seeds Activate NodD2 Protein in Rhizobium meliloti. Plant Physiol. 1992 Aug;99(4):1526–1531. doi: 10.1104/pp.99.4.1526. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Recourt K., van Tunen A. J., Mur L. A., van Brussel A. A., Lugtenberg B. J., Kijne J. W. Activation of flavonoid biosynthesis in roots of Vicia sativa subsp. nigra plants by inoculation with Rhizobium leguminosarum biovar viciae. Plant Mol Biol. 1992 Jun;19(3):411–420. doi: 10.1007/BF00023389. [DOI] [PubMed] [Google Scholar]
  14. van Brussel A. A., Recourt K., Pees E., Spaink H. P., Tak T., Wijffelman C. A., Kijne J. W., Lugtenberg B. J. A biovar-specific signal of Rhizobium leguminosarum bv. viciae induces increased nodulation gene-inducing activity in root exudate of Vicia sativa subsp. nigra. J Bacteriol. 1990 Sep;172(9):5394–5401. doi: 10.1128/jb.172.9.5394-5401.1990. [DOI] [PMC free article] [PubMed] [Google Scholar]

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