Nodulation gene-inducing factors have been isolated and identified. A number of substituted flavones, some of which are components of exudates and extracts of leguminous plants, have been found to induce expression of certain nodulation genes in Rhizobium strains. Three components of clover extract 7,4ʹ-dihydroxyflavone, 7,4ʹ-dihydroxy-3ʹ-methoxyflavone, and 4ʹ-hydroxy-7-methoxyflavone have been identified as nodulation inducers of certain Rhizobiumtrifoliinod genes. A number of other flavanoids including flavones, flavanones, flavanols and dihydroflavanols, not necessarily found in legume exudates, were also found to induce expression of certain nodulation genes.
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