Selenium in plants: Boon or bane?

Selenium (Se) is yet to be confirmed as essential for plants, but it has proven to be an essential trace element for humans and other animals. However, it is well documented that Se is a beneficial element for plants. Recent studies show that Se treatment at a low concentration exerts positive effects on plant growth, development, and yield. Thus, Se may act as a vital element by altering several physiological and biochemical processes. Remarkably, Se also acts as a plant antioxidant and pro-oxidant, and confers tolerance against different abiotic stresses, including salinity, drought, extreme temperature, and toxic metals/metalloids stresses. Its protective mechanism involves the enhancement of photosynthetic pigment synthesis, photosynthetic rate, gas exchange, accumulation of osmoprotectants, and secondary metabolites. Another important role of Se is the enhancement of the antioxidant defense system, which reduces reactive oxygen species (ROS) accumulation and thus oxidative stress. Moreover, Se also induces and modulates stress-responsive proteins/genes. Notably, an excessive Se concentration in growing media shows phytotoxicity associated with the overproduction of ROS as pro-oxidative Se activity, inhibition of the biosynthesis of photosynthetic pigments, and suppression of growth, developmental, and physiological processes. On the other hand, as a phytofortifier, Se may improve the nutritional quality of food crops and fodder. Moreover, plants can play important roles in overwhelming the Se deficiency and toxicity in various areas of the world; thus, a comprehensive mechanism of Se metabolism is crucial for effective Se-biofortification. Therefore, this review focuses on the recent advancement in the beneficial and harmful physiological role of Se in plants under abiotic stresses.