Abstract
Flavonoids are crucial signaling molecules in the symbiosis between legumes and their nitrogen-fixing symbionts, the rhizobia. The primary function of flavonoids in the interaction is to induce transcription of the genes for biosynthesis of the rhizobial signaling molecules called Nod factors, which are perceived by the plant to allow symbiotic infection of the root. Many legumes produce specific flavonoids that only induce Nod factor production in homologous rhizobia, and therefore act as important determinants of host range. Despite a wealth of evidence on legume flavonoids, relatively few have proven roles in rhizobial infection. Recent studies suggest that production of key “infection” flavonoids is highly localized at infection sites. Furthermore, some of the flavonoids being produced at infection sites are phytoalexins and may have a role in the selection of compatible symbionts during infection. The molecular details of how flavonoid production in plants is regulated during nodulation have not yet been clarified, but nitrogen availability has been shown to play a role.
Highlights
Nodulation in legumes evolved as a highly specific interaction between the legumes and gram-negative soil bacteria called rhizobia
The infection thread acts as a conduit to provide access for the rhizobia to the inner root tissues where they are endocytosed into nodule cells and begin to fix nitrogen [3]
“strong” inducers of nod genes in homologous rhizobia secreted by roots increased biosynthesis in response to rhizobia or Nod factors required for rhizobial infection
Summary
Nodulation in legumes evolved as a highly specific interaction between the legumes and gram-negative soil bacteria called rhizobia. As actinorhizal nodulation predates the appearance of legumes this suggests either ancient origins for, or convergent evolution of, the role of flavonoids in nodulation While it is clear the main role for flavonoids in legume nodulation is to induce Nod factor production by rhizobia, they regulate other rhizobial responses that are important for symbiosis Even though flavonoids have been studied extensively in legumes, relatively little information is available on which flavonoids play a role in determination of host range Work in this area focused on the identification of the key flavonoids being produced and their effects on the rhizobia. A relationship between flavonoid production and the carbon-nitrogen status is evident, and this may be reflected in flavonoid production in nodules
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