Role of Phenolics in Plant–Microbe Interaction: A Review

Plants, being immobile, face many types of stresses in the environment. To combat these stresses, they have phenolics as secondary metabolites. There are many classes of phenolics such as simple hydroxybenzoic acid, free and hydroxycinnamic acids, coumarins, flavonoids and stilbenes, etc. These are classified on the basis of number of carbon atoms and basic arrangement of carbon skeleton in their structures. The types of phenol that are implicated in defense differ greatly and depend on plant species. Plants form phenolics mainly through shikimic acid pathway. The first step in the synthesis of phenolic compounds from phenylalanine in plants is deamination of phenylalanine by the enzyme phenylalanine ammonia lyase (PAL). These compounds have diverse functions and are immensely important in plant–microbe interaction/symbiosis. Phenolic compounds act as signaling molecules in the initiation of legume rhizobia symbiosis and establishment of arbuscular mycorrhizal symbiosis and can act as agents in plant defense. In plants, phenolics play many important roles in plant–microbe symbiosis, rhizobium–legume symbiosis, and arbuscular mycorrhiza symbiosis and provide defense system to the plant. They also help in quorum sensing as well as act as signaling molecules.