Effects of alfalfa nod gene-inducing flavonoids on nodABC transcription in Rhizobium meliloti strains containing different nodD genes.

Transcription of the nodulation genes nodABC in Rhizobium meliloti requires a plant flavonoid signal and nodD, a family of bacterial regulatory genes (nodD1, nodD2, and nodD3). Results from this study show that all previously identified nod gene inducers released by alfalfa seeds and roots induced nodABC-lacZ transcription in R. meliloti containing extra copies of nodD1, but only 4,4'-dihydroxy-2'-methoxychalcone gave high levels of induction with extra copies of nodD2. While mixtures of the methoxychalcone and luteolin showed a positive synergism with extra NodD1 protein, they apparently competed for binding to the NodD2 protein.