The role of flavonoids in the establishment of plant roots endosymbioses with arbuscular mycorrhiza fungi, rhizobia and Frankia bacteria

Abstract

Flavonoids are a group of secondary metabolites derived from the phenylpropanoid pathway. They are ubiquitous in the plant kingdom and have many diverse functions including key roles at different levels of root endosymbioses. While there is a lot of information on the role of particular flavonoids in the Rhizobium-legume symbiosis, yet their exact role during the establishment of arbuscular mycorrhiza and actinorhizal symbioses still remains unclear. Within the context of the latest data suggesting a common symbiotic signaling pathway for both plant-fungal and plant bacterial endosymbioses between legumes and actinorhiza-forming fagales, this mini-review highlights some of the recent studies on the three major types of root endosymbioses. Implication of the molecular knowledge of endosymbioses signaling and genetic manipulation of flavonoid biosynthetic pathway on the development of strategies for the transfer and optimization of nodulation are also discussed.

Figure 1. Simplified scheme of the flavonoid biosynthetic pathway adapted from.^, Some critical enzymes are indicated and abbreviated as follows: CHS, chalcone synthase; CHR, chalcone reductase; CHI, chalcone isomerase; IFS, isoflavone synthase; IFR, isoflavone reductase; F3H, flavanone 3 hydroxylase; F3′H, flavanoid 3′ hydroxylase; F3′,5′H, flavanoid 3′,5′ hydroxylase; DFR, dihydroflavonol reductase; FNS, flavone synthase; FLS, flavonol synthase; LDOX, leucoanthocyanidin dioxygenase; LAR, leucanthocyanidin reductase; ANR, anthocyanidin reductase. Major classes of end products are emphasized in bold italic.

Figure 2. Schematic overview of flavonoid functions in the establishment of plant root endosymbioses. Flavonoids play an essential role in rhizobium-legume symbiosis as chemoattractant and nod gene inducers. They are suggested to act on mycorrhization by stimulating germination of spores and hyphal growth. The recent identification of strigolactones as host-recognition signals for AM fungi raises the questions about flavonoids as signal molecules in AM endosymbioses and of a possible interaction with strigolactones. Recent data suggest also a role for flavonoids in actinorhizal symbioses.