Anthocyanins: Biotechnological targets for enhancing crop tolerance to salinity stress

Salinity is a major abiotic stress factor affecting crop plants' growth, development, and yield in many areas of the world, which may greatly influence the world's food security in the future. It is therefore a must to understand and unmask the crop salinity tolerance mechanisms to find biomarkers that can be utilized in developing crop species resistant to high salinity. Anthocyanins are a class of secondary metabolites that are produced as one of the final products of the flavonoid pathway. Anthocyanins have been shown to accumulate in tolerant species/genotypes under saline conditions providing important roles under stress. Anthocyanin accumulation under salinity stress is hence correlated with plant salinity tolerance, and anthocyanin synthesis downregulation at least in part has boosted the accumulation of ROS and increased the plant sensitivity to stress. In addition, evidence indicates anthocyanins have many human health benefits, pointing out the high demand for anthocyanins. Further, elevated anthocyanin production under high salinity has been shown to be related to increased expression of anthocyanin biosynthesis and regulatory genes, suggesting these genes may be candidates for the improvement of anthocyanin accumulation and crop salinity tolerance. Genetic and nongenetic approaches have successfully improved anthocyanin accumulation and promoted crop tolerance to high salinity. In this review, we provide a comprehensive view of the state-of-the-art literature on anthocyanins, discussing recent evidence related to anthocyanin accumulation and its roles in crop salinity tolerance, as well as anthocyanin synthesis, regulation, transport, and genetic manipulation. Strategies for enhancing anthocyanin levels under saline stress are also pointed out. It is noteworthy that the development of anthocyanin-enriched crops will not only enhance the salinity tolerance and productivity of horticultural and agronomic crops but also will increase valuable natural bioactive compounds that provide several critical health benefits for humans.