Flavones induce expression of nodulation genes in Rhizobium

Successful establishment of nitrogen-fixing root nodules¹ in legume plants requires the presence of several plasmid-borne genes in the Rhizobium symbionts^2-12. Those genes so far defined (designated nodABCDEF) are arranged in three separate operons (nodABC, nodD and nodFE), based on DNA sequence analysis and expression data using nod genes fused to Escherichia coli lac genes^4-10. Only one gene, nodD, is expressed by Rhizobium in culture, and its gene product, together with factors secreted by the plant host, induces expression of genes in nodABC and nodFE^7-9. In addition, lac fusions to presently undefined nod genes, located in regions II and IV of the 14-kilobase (kb) nodulation fragment of Rhizobium trifolii, also respond to the presence of plant exudate⁷. We report here that flavones found in washings of undamaged clover roots induce nod gene expression. The flavones are not the same as other plant factors recently identified as inducers of the expression of virulence genes in the plant-pathogenic bacterium Agrobacterium tumefaciens^13,14, which suggests that there may be a unique set of signals for each type of plant-bacterium interaction. This result has profound implications for the understanding of these interactions and provides strategies for investigating the control of nitrogen fixation and plant disease.