Revisiting the role of phenylpropanoids in plant defense against UV-B stress

Abstract

Ultraviolet-B (UV-B) radiation from the past few decades has been widely studied due to the substantial depletion of the stratospheric ozone (O3) layer, causing morphological, physiological, cytological, and biochemical alterations in plants. As a potent abiotic stress factor, enhanced reactive oxygen species (ROS) generation has been flagged as a key factor under UV-B stress. UV-B adversely affects plant growth and development. Plants have evolved defense strategies under UV-B stress to counteract these detrimental effects. The most common protective response is an accumulation of secondary metabolites. These provide photoprotection by acting as UV-B absorbing compounds through quenching ROS and reactive nitrogen species (RNS).UV-B-induced metabolites share a common origin in the phenylpropanoid biosynthetic pathway. These phenylpropanoids induced via UV Resistance Locus 8 (UVR8) signaling are transcriptionally regulated by MYB transcription factors (TFs) and are efficiently transported to different cellular and membrane-limited compartments. The accumulation of metabolites like flavonoids, anthocyanins, lignins and tannins in plants marks the importance of its metabolism in UV-B tolerance mechanism. Plant species responded differently to increased UV-B exposure in terms of phenylpropanoid concentration. Apart from UV-B screening agents, these are also potent ROS scavengers due to their structural composition and less reactive nature. The dynamic balance between ROS accumulation under UV-B stress and its metabolism via phenylpropanoids holds the key to the stress tolerance attributes of a plant. The present review focuses on the UV-B-induced secondary metabolites in plants, their biosynthesis, and defense strategies that will help in the elucidation of medicinally important bioactive compounds as well as in the development of UV-B tolerant plants.