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

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.

Activation of the isoflavonoid pathway in actinorhizal symbioses

We investigated the involvement of flavonoids in the actinorhizal nodulation process resulting from the interaction between the tropical tree Casuarina glauca Sieb. ex Spreng. and the actinomycete Frankia. Eight C. glauca genes involved in flavonoid biosynthesis: chalcone synthase (CHS), chalcone isomerase (CHI), isoflavone reductase (IFR), flavonoid-3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H), flavonoid 3',5' hydroxylase (F3'5'H), dihydroflavonol 4-reductase (DFR) and flavonol synthase (FLS), were identified from a unigene database and gene expression patterns were monitored by quantitative real-time PCR (qRT-PCR) during the nodulation time course. Results showed that FLS and F3'5'H transcripts accumulated in mature nodules whereas CHI and IFR transcripts accumulated preferentially early after inoculation with Frankia. Comparison of IFR and CHI expression in inoculated plants and in control plants cultivated with or without nitrogen confirmed that early expression of IFR is specifically linked to symbiosis. Taken together, these data suggest for the first time that isoflavonoids are implicated in actinorhizal nodulation.

Strain specificity in the Myricaceae-Frankia symbiosis is correlated to plant root phenolics

Plant secondary metabolites play an important role in the interaction between plants and their environment. For example, mutualistic nitrogen-fixing symbioses typically involve phenolic-based recognition between host plants and bacteria. Although these mechanisms are well studied in the rhizobia-legume symbiosis, little is known about the role of plant phenolics in the symbiosis between actinorhizal plants and the actinobacterium Frankia. In this study, the responsiveness of two Myricaceae plant species, Myrica gale L. and Morella cerifera L., to Frankia inoculation was correlated with the plant-bacteria compatibility status. Two Frankia strains were inoculated: ACN14a, compatible with both M. gale and M. cerifera and Ea112, compatible only with M. cerifera. The effect of inoculation on root phenolic metabolism was evaluated by metabolic profiling based on high-performance liquid chromatography (HPLC) and principal component analysis (PCA). Our results revealed that: (i) both Frankia strains induced major modifications in root phenolic content of the two Myricaceae species and (ii) strain-dependant modifications of the phenolic contents were detected. The main plant compounds differentially affected by Frankia inoculation are phenols, flavonoids and hydroxycinnamic acids. This work provides evidence that during the initial phases of symbiotic interactions, Myricaceae plants adapt their secondary metabolism in accordance with the compatibility status of Frankia bacterial strains.

Perception and modification of plant flavonoid signals by rhizosphere microorganisms

Flavonoids are a diverse class of polyphenolic compounds that are produced as a result of plant secondary metabolism. They are known to play a multifunctional role in rhizospheric plant-microbe and plant-plant communication. Most familiar is their function as a signal in initiation of the legume-rhizobia symbiosis, but, flavonoids may also be signals in the establishment of arbuscular mycorrhizal symbiosis and are known agents in plant defence and in allelopathic interactions. Flavonoid perception by, and impact on, their microbial targets (e.g. rhizobia, plant pathogens) is relatively well characterized. However, potential impacts on 'non-target' rhizosphere inhabitants ('non-target' is used to distinguish those microorganisms not conventionally known as targets) have not been thoroughly investigated. Thus, this review first summarizes the conventional roles of flavonoids as nod gene inducers, phytoalexins and allelochemicals before exploring questions concerning 'non-target' impacts. We hypothesize that flavonoids act to shape rhizosphere microbial community structure because they represent a potential source of carbon and toxicity and that they impact on rhizosphere function, for example, by accelerating the biodegradation of xenobiotics. We also examine the reverse question, 'how do rhizosphere microbial communities impact on flavonoid signals?' The presence of microorganisms undoubtedly influences the quality and quantity of flavonoids present in the rhizosphere, both through modification of root exudation patterns and microbial catabolism of exudates. Microbial alteration and attenuation of flavonoid signals may have ecological consequences for below-ground plant-microbe and plant-plant interaction. We have a lack of knowledge concerning the composition, concentration and bioavailability of flavonoids actually experienced by microbes in an intact rhizosphere, but this may be addressed through advances in microspectroscopic and biosensor techniques. Through the use of plant mutants defective in flavonoid biosynthesis, we may also start to address the question of the significance of flavonoids in shaping rhizosphere community structure and function.