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.