Effect of Enterobacter sp. EG16 on Selenium biofortification and speciation in pak choi (Brassica rapa ssp. chinensis).

• Mutual effects of Se and PGPR on Se-enriched biofortification were explored. • The combination of low-dose PGPR and exogenous Se promoted plant growth. • Se content in plant's edible parts significantly increased with rising doses of PGPR and Se. • SeMet, SeMeCys and SeCys2 were the main Se forms in plant's edible parts. • Optimal combinations of PGPR and Se for Se biofortification were obtained by Two-way ANOVA. Using plant growth promoting rhizobacteria (PGPR) to boost Se uptake and accumulation in crops is a promising recent approach to biofortification. Most studies, however, have focused on a certain dose of PGPR or Se, while the synergistic effects of simultaneously changing the concentration of both exogenous Se and PGPR on plant growth and Se bioaccumulation are rarely explored. In this study, the combined treatments of 0-2 mg L−1 Se and (3.83-11.3) ×107 CFU mL−1 PGPR strain Enterobacter sp. EG16 were used for the hydroponic experiments of pak choi (Brassica rapa) to investigate the mutual effects on plant growth and Se absorption. The growth of pak choi, in terms of plant height, number of leaves and root tips, was promoted by combining 7.65×107 CFU mL−1 EG16 and no more than 1 mg L−1 of Se. Growth was significantly inhibited when the concentration of EG16 was raised to 11.3×107 CFU mL−1. Chlorophyll content, superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities were also boosted under the same combination of Se and EG16 concentrations. In addition, the Se content of aboveground part and intact plants significantly increased with rising concentrations of EG16 and Se. Selenomethionine (SeMet), methylselenocysteine (SeMeCys) and selenocystine (SeCys2) were the main organo-Se compounds detected in pak choi, accounting for almost 90% when supplied with selenite. Among these compounds, SeMet accounted for a high proportion and SeCys2 for a relatively low percentage in most treatments. The two forms were inversely related, probably because the transformation was driven by EG16. On the other hand, the proportion of SeMeCys was significantly affected by different concentrations of exogenous Se and EG16. Selenate was also detected, indicative of selenite oxidation. In terms of both plant biomass and nutritional quality, the combined formulation of 1 mg L−1 Se and 7.65×107 CFU mL−1 of EG16 is a promising means of producing Se-enriched vegetables. [ABSTRACT FROM AUTHOR]