Diverse roles of tocopherols in response to abiotic and biotic stresses and strategies for genetic biofortification in plants

Tocopherols are a group of lipid-soluble compounds and they are essential for human nutrition. Tocopherols are also well known as powerful antioxidants associated with ascorbate and glutathione, which protect plants from oxygen toxicity. However, an increasing amount of research suggests that tocopherols play roles in stress resistance far beyond their antioxidative activity. This work focuses on the tocopherol biosynthetic pathway, the activities of their different forms, and their roles in response to abiotic and biotic stresses. Here, we made an overview of three major tocopherol-related retrograde signaling pathways which were mediated by plant hormones, reactive oxygen species (ROS), and microRNAs (miRNAs), respectively. Furthermore, the available information on the progress of genetic engineering of tocopherol in plants was summarized. Finally, we identified questions that need to be addressed and discussed the prospective methods for tocopherol biofortification in plants. This review highlights the multifaceted role of tocopherols, which will facilitate further elucidating the functions of genes and direction of crop breeding.