Phenolic Compounds in Plant Defense and Pathogen Counter-defense Mechanisms

Phenolics have been in the center of many discoveries on plant defenses to pathogens, including bacteria, fungi, and viruses. More recently, this fascinating group of metabolites was also pointed out as a target of invading microbes to defeat such defenses. Here, we summarize findings in several host-pathogen interactions involving the oomycete Phytophthora infestans and soilborne pathogens such as Verticillium dahliae, depicting the role of phenolics in plant defenses against them, and how they target such responses to settle their infection. The defenses described here include the upregulation of genes in the phenylpropanoid pathway, and the accumulation of phenolics such as scopoletin (a coumarin), p-coumaric acid methyl ester (a hydroxycinnamic acid derivative), or rutin (a flavonoid) in response to several pathogens. Counter-defense mechanisms developed by these pathogens, such as downregulation of defense genes and degradation of phenolics, are described as well.