Exogenous Melatonin Enhances Cd Tolerance and Phytoremediation Efficiency by Ameliorating Cd-Induced Stress in Oilseed Crops: A Review

Abstract

Heavy metal pollution is of increasing global concern as it adversely impacts different spheres including pedosphere, hydrosphere, biosphere, and humansphere. Cadmium (Cd) contamination of agriculture soil has become a thoughtful challenge across the globe due to uncontrolled anthropogenic activities, limiting agricultural productivity and disturbing eco-environment sustainability. Sustainable mitigation strategies are, therefore, required for effective Cd decontamination. Biostimulation is a promising method to improve and stimulate life processes of plants in general and oilseed crops in particular, without posing threats to eco-environment sustainability and safer food production. Melatonin (N-acetyl-5-methoxytryptamine), a recognized pleiotropic molecule, has emerged as a research highlight regarding its role as an effective natural plant growth promoter, a broad spectrum antioxidant, and more specifically an efficient biostimulator to improve Cd-induced tolerance of plants for phytoremediation purposes and simultaneous biofuel production. This review provides a novel insight to understand the multifunctional role of melatonin in improving Cd phytoremediation capacity together with reducing phytotoxicity in oilseed crops, which was not previously assessed. Among oilseed crops, Brassicaceae spp. have unique stress adaptive mechanisms with exceptionally high Cd accumulating capabilities, apart from the diverse agri-horticultural importance. Interestingly, the discovery of dynamic approaches regarding melatonin application helped to better understand the basic mechanisms involved in melatonin-mediated responses against Cd stress in oilseed crops. Our review will provide a useful basis for the development of alternative strategies to genetically engineer low-Cd-content oilseed crops, with the aim of improving eco-friendly crop production and ensuring food safety. Furthermore, future researches will be supposed to provide more genetic evidences about Cd-induced melatonin regulation in higher plants.