Flavonoid activation of nodulation genes in Rhizobium reversed by other compounds present in plants

Bacteria of the genus Rhizobium specifically recognize and infect legume plants, resulting in a complex morphological and bio-chemical differentiation into nitrogen-fixing root nodules. In R. leguminosarum , whose host plant is the pea, and several other Rhizobium species, groups of linked plasmid-borne genes are involved in nitrogen fixation ( nif genes) and nodulation ( nod genes) 1,2 . The nod genes are organized in several transcription units on the symbiotic plasmid pRL1JI of R. leguminosarum ; these comprise nodABCIJ , nodD and nodFE 3–6 with nodD being a positively acting regulatory gene whose product activates transcription of the other two operons which contain genes essential for the nodulation process 5,7 . Although the precise biochemical functions of most of the pRL1JI nod genes are unknown, it has recently been discovered that their expression requires not only the product of the regulatory nodD gene, but also a factor (or factors) present in pea root exudate 5,7 , and similar results have been reported for Rhizobium meliloti 8 and Rhizobium trifolii 9 . The finding of plant-produced products that ‘signal’ the Rhizobium offers the possibility of identifying one of the key biochemical reactions required for the infection process. Here we describe the characteristics of several plant-specified compounds that activate the transcription of the pRL1JI nodABCIJ and nodFE genes and present a novel phenomenon, namely that other, naturally occurring compounds can antagonize this induction.