The Role of Flavonoids in Nodulation Host-Range Specificity: An Update

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.

Keywords: daidzein; genistein; medicarpin; methoxychalcone; phytoalexins.

Figure 1

The isoflavonoid biosynthesis pathway. PAL (Phenylalanine ammonia-lyase), C4H (Cinnamate 4-hydroxylase), 4CL (4-coumarate CoA-ligase), CHS (Chalcone synthase), CHR (Chalcone reductase), CHI (Chalcone isomerase), IFS (Isoflavone synthase), ChOMT (Chalcone O-methyltransferase). Legume specific steps are indicated in green.

Figure 2

ChOMT1 expression in pathogen-inoculated roots of Medicago truncatula. Data are taken from the Medicago Gene Expression Atlas [79]. Original data for Cotton Root Rot (Phymatotrichopsis omnivore) are from Reference [80], and data for Macrophomina phaseolina were described by the authors of [81]. Data for Ralstonia solanacearum has not been described in a publication. hpi = hours post inoculation. Bars are SD.

Figure 3

The association between high C/N ratios and flavonoids in nodulation. Areas undergoing infection by rhizobia are dominated by flavonoid-induced Nod factor (NF) signalling and accumulate carbon within amyloplasts. In the bacteroid-containing nitrogen fixation zone, carbon stores have been depleted, flavonoid-related gene expression is low and infection threads are mostly absent.

Figure 4

The production and secretion of flavonoids at different stages of growth and development in Medicago spp. (Top left) Luteolin and other flavonoids are produced in seed coat and are released in the exudate upon imbibition and may play a role in chemoattraction of rhizobia [17,44,45]. (Top right) Flavonoids are produced in the root hair elongation zone and some are secreted into the rhizosphere [31,48,106]. (Bottom left) ChOMT genes are expressed in rhizobially infected root hairs, suggesting that the nod gene-inducer methoxychalcone is produced locally [50,51]. (Bottom right) ChOMT genes are also expressed in the nodule apex/infection zone where infection threads are present, but not in the nitrogen fixation zone [49,50].