Synergistic effects of exogenous melatonin and zinc oxide nanoparticles in alleviating cobalt stress in Brassica napus: insights from stress-related markers and antioxidant machinery

Abstract

Cobalt (Co) toxicity severely hinders plant growth, highlighting the imperative to devise strategies that promote plant growth and inhibit Co accumulation in plants. Melatonin (MT) and zinc oxide nanoparticles (ZnO NPs) have emerged as key contributors to promoting sustainable crop production and enhancing stress resistance. However, the mechanisms underlying the ameliorative effects of foliar applications of the growth regulators MT and/or ZnO NPs on Co accumulation and its associated phytotoxicity in Brassica napus remain poorly understood. To address this knowledge gap, the present study was carried out to investigate the protective roles of MT and/or ZnO NPs in promoting growth and enhancing tolerance against Co stress in B. napus. Treatment of Co (300 μM) alone significantly reduced the leaf fresh weight (33%), dry weight (49%), full plant height (51%), antioxidant enzyme activity and stomatal conductance. However, the application of MT (50 μM) with ZnO NPs (25 μM) resulted in a substantial decline in the accumulation of Co concentration and contents of MDA (33% in leaves and 26% in roots), H₂O₂ (41% in leaves and 35% in roots) and O₂˙⁻ (26% in leaves and 39% in roots). Additionally, combined application of MT + ZnO NPs markedly enhanced the activities of antioxidant enzymes as well as the levels of thiol compounds such as GSH, NPTs, PCs and Cys. Furthermore, the investigation of genes responsible for encoding the extrinsic proteins of photosystem II showed a marked upregulation in the expression levels of the BnPsbA, BnPsbB, BnPsbC, and BnPsbD genes. Remarkably, the synergistic application of MT and ZnO NPs proved more effective in promoting plant growth than their individual applications. Further, this study investigated the synergistic mechanisms by which MT and ZnO NPs, both individually and in combination, enhance enzymatic activities, improve photosynthetic capacity, and optimize nutrient accumulation in plants. Thus, this research provides insights into the biochemical and physiological processes underlying the ameliorative effects of these compounds on plant stress tolerance.