Plant phenolics: neglected secondary metabolites in plant stress tolerance

Plants endure various biotic and abiotic stressors throughout their lives due to their sessile nature and therefore different stresses such as salinity, heavy metals, temperature (high or low), soil alkalinity or acidity, or pathogenic attack that impact plant growth and development. To combat such menace, plants have evolved in their metabolism, an effective system of secondary metabolites (SMs) such as phenolics that play critical roles in plant's growth and development under normal and stress conditions. Phenolics perform a wide range of functions and bioactivity in plants. These have multiple protection modes ranging from toxicity and light/UV tolerance to signal transduction activities and are one of the most abundant and diverse classes of SMs found across the plant kingdom. Although phenolic compounds have little involvement in plant growth, they are important in how plants interact with their environment. The metabolic engineering of genes related to phenylpropanoid and flavonoid pathways responsible for phenolics biosynthesis has attracted a lot of attention in biotechnology and plant science for securing stress resistance in crop plants. This review discusses how phenolic compounds interact in plants under harsh environmental conditions and provides a current and updated research related to phenolics with a focus on improving plant stress tolerance to achieve food security in agriculture. Alteration in phenolics could potentially impact feed, fodder, and fuel for the coming generation.