Alfalfa (Medicago sativa L) root exudates contain of isoflavonoids in the presence of rhizobium meliloti.
Dakora, Felix D. Joseph, Cecillia M
Dakora, Felix D.
Joseph, Cecillia M
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Abstract
Root exudates of alfalfa (Medicago sativa 1.) 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-7-0-(6”-0-malonyl glycoside), a conjugated form of the medicarpin precursor formononetin. lhe medicarpin molecules did not induce nod gene transcription in R. meliloti, but the formononetin-7-0-(6”-O-malonylglycoside) induced nod genes regulated by both NodDl and NodD2 proteins in R. meliloti. Hydrolysis of either the malonyl or the glycosyl linkage from the formononetin conjugate eliminated nod gene-inducing activity. lhe 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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Date
1993-01-01
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Oxford University Press and American Society of Plant Biologists
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Keywords
Root Exudates, Alfafa, Rhizobium, lsof lavonoids