Role of Phenolic Compounds in Disease Resistance to Plants

Plants produce a diverse range of compounds that have no direct role in their normal growth and development known as secondary metabolites. These metabolites essentially play an important role in plant defense. Many secondary metabolite groups are known, such as terpenes, phenolic compounds, and alkaloids. All these organic compounds are classified on the basis of their biosynthetic origin. In natural systems, plants face plethora potential enemies. All the types of ecosystems contain a wide variety of organisms such as bacteria, viruses, fungi, nematodes, mites, insects, mammals, and other herbivorous animals that can cause damage to plants. However, plants have evolved myriad defense mechanisms for their protection. Secondary metabolites are one of the incredibly known such mechanisms by which plants are able to cope with various kinds of biotic and abiotic stress. Phenolic compounds are one such group of plant secondary metabolites which play an essential key role in defense against environmental stresses and other stresses from pathogen infections, herbivores, and nutrient deficiency. Phenolic compounds act as the primary molecules of defense in plants against pathogens. Phenolic compounds act as the natural defense in plants against potential fungal attack. Tannins, lignins, and some simple phenolic compound serve as a defense against pathogens. During any kind of stress, phenolic compound concentration in plants increases dramatically. Activity of phenylammonium lyase (PAL) increases during pathogen attack. Many stress conditions such as wounding, viral, fungal, and insect attack increase PAL activity in majority of the plants. Production of lignin and lignin polymers increases the cell wall rigidity and mechanical resistance and also increases the cell wall hydrophobicity against number of diseases caused to carrot, tobacco, Japanese radish root, muskmelon, etc. Phenolic compounds also produce an “allelopathic” effect to decrease the growth of other plants around. Role of phenols was found to be highly effective in apple against Penicillium expansum, Monilinia fructigena, Gloeosporium spp., and Venturia inequalis, pear against Venturia nashicola, etc. This chapter has focused to cover the effect of the phenols in plant disease resistance, especially fungal infections.