Brassinosteroids mitigate cadmium toxicity in cowpea plants

Anthropogenic activities and improper uses of phosphate fertilizers have led to an increase in cadmium concentrations in agricultural soils. Brassinosteroids are steroid hormones that are rapidly assimilated and metabolised with beneficial roles in physiological and biochemical processes in plants. Our aim was to ascertain whether exogenous treatment with 24-epibrassinolide (EBR) can mitigate the Cd toxicity, and whether this substance can reduce the Cd accumulation in plant tissues. Furthermore, the dose response to EBR was determined following exposure to Cd in Vigna unguiculata. The experiment was a completely randomised factorial design with two concentrations of Cd (0 and 500 mu M) and three concentrations of EBR (0, 50, and 100 nM). Spraying plants exposed to Cd with EBR significantly reduced the concentrations of Cd and increased nutrient contents in all tissues. The EBR treatment caused significant enhancements in leaf, root, and total dry matter. Foliar application of EBR reduced the negative effects of Cd toxicity on chlorophyll fluorescence and gas exchange parameters. Pretreatment with EBR also increased contents of pigments in plants exposed to Cd, compared with the identical treatments without EBR. Cd elevated contents of oxidant compounds, inducing cell damages, while EBR significantly decreased the concentrations of these compounds. We confirmed that EBR mitigated the negative effects related to Cd toxicity, reduced the absorption and transport of Cd, and increased the contents of essential elements. In plants exposed to Cd, the most apparent dose response was found for 100 nM EBR, with beneficial repercussions on growth, gas exchange, primary photosynthetic processes, and photosynthetic pigments, which were intrinsically connected to lower production of oxidant compounds and cell damage.