Phenolic acids act as signaling molecules in plant-microbe symbioses

Abstract

Phenolic acids are the main polyphenols made by plants. These compounds have diverse functions and are immensely important in plant-microbe interactions/symbiosis. Phenolic compounds act as signaling molecules in the initiation of legumerhizobia symbioses, establishment of arbuscular mycorrhizal symbioses and can act as agents in plant defense. Flavonoids are a diverse class of polyphenolic compounds that have received considerable attention as signaling molecules involved in plant-microbe interactions compared to the more widely distributed, simple phenolic acids; hydroxybenzoic and hydroxycinnamic acids, which are both derived from the general phenylpropanoid pathway. This review describes the well-known roles attributed to phenolic compounds as nod gene inducers of legume-rhizobia symbioses, their roles in induction of the GmGin1 gene in fungus for establishment of arbuscular mycorrhizal symbiosis, their roles in inducing vir gene expression in Agrobacterium, and their roles as defense molecules operating against soil borne pathogens that could have great implications for rhizospheric microbial ecology. Amongst plant phenolics we have a lack of knowledge concerning the roles of phenolic acids as signaling molecules beyond the relatively well-defined roles of flavonoids. This may be addressed through the use of plant mutants defective in phenolic acids biosynthesis or knock down target genes in future investigations.

Figure 1

Summarized phenolic acids and flavonoids branch of the phenylpropanoid biosynthetic pathway.

Figure 2

Diagrammatic representation of phenolic compounds signal exchange between the legumes and rhizobia during nodulation. Plant signal activates the nodD product that controls the transcription of other nod genes—in the same way as is it occurs for phenolic acids? The nod boxes are highlighted with black boxes in the organization of nod gene cluster of Rhizobia and the arrows with different nod gene indicate the direction of transcription.