Abstract
BackgroundThe ubiquitous signaling molecule melatonin (N-acetyl-5-methoxytryptamine) (MT) plays vital roles in plant development and stress tolerance. Selenium (Se) may be phytotoxic at high concentrations. Interactions between MT and Se (IV) stress in higher plants are poorly understood. The aim of this study was to evaluate the defensive roles of exogenous MT (0 μM, 50 μM, and 100 μM) against Se (IV) (0 μM, 50 μM, 100 μM, and 200 μM) stress based on the physiological and biochemical properties, thiol biosynthesis, and antioxidant system of Brassica napus plants subjected to these treatments.ResultsSe (IV) stress inhibited B. napus growth and biomass accumulation, reduced pigment content, and lowered net photosynthetic rate (Pn) and PSII photochemical efficiency (Fv/Fm) in a dose-dependent manner. All of the aforementioned responses were effectively alleviated by exogenous MT treatment. Exogenous MT mitigated oxidative damage and lipid peroxidation and protected the plasma membranes from Se toxicity by reducing Se-induced reactive oxygen species (ROS) accumulation. MT also alleviated osmotic stress by restoring foliar water and sugar levels. Relative to standalone Se treatment, the combination of MT and Se upregulated the ROS-detoxifying enzymes SOD, APX, GR, and CAT, increased proline, free amino acids, and the thiol components GSH, GSSG, GSH/GSSG, NPTs, PCs, and cys and upregulated the metabolic enzymes γ-ECS, GST, and PCS. Therefore, MT application attenuates Se-induce oxidative damage in plants. MT promotes the accumulation of chelating agents in the roots, detoxifies Se there, and impedes its further translocation to the leaves.ConclusionsExogenous MT improves the physiological traits, antioxidant system, and thiol ligand biosynthesis in B. napus subjected to Se stress primarily by enhancing Se detoxification and sequestration especially at the root level. Our results reveal better understanding of Se-phytotoxicity and Se-stress alleviation by the adequate supply of MT. The mechanisms of MT-induced plant tolerance to Se stress have potential implications in developing novel strategies for safe crop production in Se-rich soils.Graphical abstract
Highlights
The ubiquitous signaling molecule melatonin (N-acetyl-5-methoxytryptamine) (MT) plays vital roles in plant development and stress tolerance
➣ Excessive Se inhibits the plant growth, biomass accumulation and impairs photosynthesis ➣ Se causes osmotic stress and modulates the thiol metabolism ➣ Se induces oxidative injuries by desynchronizing the reactive oxygen species (ROS)-detoxifying enzyme activities ➣ Exogenous MT protects the physio-biochemical traits by scavenging Se-oxidative damages ➣ MT enhances plant tolerance by inducing thiols accumulation to sequester Se in roots
Based on our findings, a schematic diagram was plotted to highlight the Se-induced toxic effects in Brassica napus plants mitigated by exogenous MT (Fig. 6)
Summary
The ubiquitous signaling molecule melatonin (N-acetyl-5-methoxytryptamine) (MT) plays vital roles in plant development and stress tolerance. Selenium (Se) may be phytotoxic at high concentrations. Interactions between MT and Se (IV) stress in higher plants are poorly understood. The naturally occurring metalloid selenium (Se) is an essential micronutrient/trace element for human and certain animals. Over the past few decades, Se levels have been rising in agricultural soils and could be toxic to plants, humans, and animals [3]. Selenite (IV) and selenate (VI) are the mains forms of Se available for plant uptake in soils. In response to oxidative stress, plants produce antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR). Plants produce thiol ligands such as non-protein thiols (NPTs), cysteine (cys), reduced glutathione (GSH), oxidized glutathione (GSSG), and phytochelatins (PCs) to chelate and detoxify metals and metalloids [12,13,14]
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