The application of nanotechnology in agriculture has received much attention in order to improve crop yield, quality and food safety. In the present study, a Cd-tolerant endophytic fungus Colletotrichum fructicola KL19 was first ever reported to produce SeNPs, and the production conditions were optimized using the Box-Behnken design in the Response Surface Methodology (RSM-BBD), achieving a peak yield of 1.06 mM under optimal conditions of 2.62 g/20 mL biomass, 4.56 mM Na2SeO3, and pH 6.25. Following this, the properties of the biogenic SeNPs were elucidated by using TEM, DLS, and FTIR, in which the 144.8 nm spherical-shaped SeNPs were stabilized by different functional groups with a negative zeta potential of -18.3 mV. Furthermore, strain KL19 and SeNPs (0, 5, 10, 20 and 50 mg/L) were inoculated in the root zone of small-leaf spinach (Spinacia oleracea L.) seedlings grown in the soil with 33.74 mg/kg Cd under controlled conditions for seven weeks. Impressively, compared with Cd stress alone, the strain KL19 and 5 mg/L SeNPs treatments significantly (p < 0.05) exhibited a reduction in Cd contents (0.62 and 0.50 folds) within the aboveground parts of spinach plants and promoted plants' growth by improving the leaf count (0.92 and 1.36 folds), fresh weight (2.94 and 3.46 folds), root dry weight (4.00 and 5.60 folds) and root length (0.14 and 0.51 folds), boosting total chlorophyll synthesis (0.38 and 0.45 folds), enhancing antioxidant enzymes (SOD, POD) activities, and reducing the contents of reactive oxygen species (MDA, H2O2) in small-leaf spinach under Cd stress. Overall, this study revealed that utilizing endophytic fungus C. fructicola or its derived SeNPs could mitigate reactive oxygen species generation by enhancing antioxidant enzyme activity as well as diminish the absorption and accumulation of Cd in small-leaf spinach, promoting plant growth under Cd stress.

Background and aimsCadmium (Cd) contamination poses a potential threat to plant growth and human health. In this study, we aimed to determine the effect of selenium nanoparticles (SeNPs) on Cd and selenium (Se) uptake and accumulation in bok choy, and investigate the detoxification mechanism of SeNPs on bok choy under Cd stress.MethodsA pot culture was performed in Cd-contaminated soil with soil applied and foliar-sprayed SeNPs, including SLow, SHigh, FLow, FHigh, and corresponding control treatment. The soil available Cd content, Cd and Se fractions in soil, elements accumulation, subcellular Cd/Se distribution, MDA content, SOD activity, and Fourier transformed infrared spectroscopy (FTIR) were evaluated.ResultsSoil applied SeNPs significantly reduced Cd concentration by 25.9-42.4%, and Cd uptake rate by 33.4-37.8%. Further, soil applied SeNPs had no significant effect on available Cd but did affect Se fractions in soil. Additionally, soil applied SeNPs increased Se concentration by 3.1 - 6.3 times in bok choy and caused a higher Se concentration in root than in shoot, with the residual and organic matter-bound Se mainly affecting Se accumulation in shoot. However, foliar-sprayed SeNPs had no significant effect on Cd uptake but increased Se accumulation by 2.4 - 33.0 times in bok choy. Soil applied and foliar-sprayed SeNPs prompted the distribution of Cd in cell wall and in soluble component in shoot, respectively, which reduced the damage of Cd on organelle.ConclusionSoil applied SeNPs was an effective method for reducing Cd accumulation and improving Se biofortification and mineral elements accumulation in bok choy.