Plant Growth-Promoting and Arsenic Accumulation Reduction Effects of Two Endophytic Bacteria Isolated from Brassica napus

Infertile and heavy-metal-contaminated croplands are widely distributed and threaten global food security. Plant growth-promoting bacteria provide an option for alleviating the agricultural problems. In this study, the endophytic bacteria Rahnella victoriana B38 (Accession No. OK658118) and Bacillus paramycoides N38 (Accession No. OK658562) were isolated from Brassica napus. Their plant growth-promoting effects and mechanisms were investigated by gnotobiotic and pot experiments, and extracellular metabolites analysis. The results revealed that both B38 and N38 significantly increased shoot and root length in B. napus and fresh weight, root length, pod number, and total pod weight of Arabidopsis thaliana (p < 0.05). In addition, they promoted Arabidopsis growth under arsenic (As) stress by significantly increasing fresh weight and root length and decreasing the As content by 55.81% and 30.47%, respectively (p < 0.05). Thus, these bacteria were able to protect Arabidopsis from As stress by reducing As accumulation in plants. B38 and N38 release gamma-aminobutyric acid (GABA) and enzymes including proteases, cellulases, and lipases, which can increase soil nutrient availability. Substances related to plant biomass increasing and As-stress alleviation, including indoleacetic acid (IAA), GABA, putrescine, quinic acid, trans-zeatin, N-acetylglutamic, 3-indolepropionic acid, putrescine, and lathyrine were also detected. Therefore, they could represent ideal microbial biofertilizer candidates for increasing crop yields and alleviating As accumulation in plants.