Flavone Limitations to Root Nodulation and Symbiotic Nitrogen Fixation in Alfalfa

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Abstract
Transcription of the nodABC genes in Rhizobium meliloti is required for root nodule formation in alfalfa (Medicago sativa L.) and occurs when specific compounds, such as the flavone luteolin, are supplied by the host plant. Results reported here indicate how luteolin in the root and rhizosphere can affect subsequent N2 fixation and plant growth. Previous experiments with `Hairy Peruvian 32' (HP32), an alfalfa population produced from `Hairy Peruvian' (HP) by two generations of selection for increased N2 fixation and growth, found that HP32 had more root nodules and fixed more N2 than the parental HP population. In the present study, flavonoid extracts of HP32 seedling roots are shown to contain a 60% higher concentration of compounds that induce transcription of a nodABC-lacZ fusion in R. meliloti than comparable extracts of HP roots. Chromatographic data indicated that HP32 roots had a 77% higher concentration of luteolin than HP roots. Adding 10 micromolar luteolin to the rhizosphere of HP seedlings increased nodulation, N2 fixation, total N, and total dry weight but had no effect on nitrate assimilation. These data show that normal levels of flavone nodulation signals in the rhizosphere of HP alfalfa can limit root nodulation, symbiotic N2 fixation, and seedling growth and suggest that one mechanism for increasing N2 fixation can be the genetic enhancement of specific biochemical signals which induce nodulation genes in Rhizobium.