AsIII-oxidizing and Cd-tolerant plant growth-promoting bacteria synergistically reduce arsenic translocation, toxicity and accumulation in KDML105 rice

Arsenic (As) contamination in rice grains, a staple food for the global human population, is a pervasive problem that leads to serious health risks. In this study, various combinations of AsIII-oxidizing bacteria (Pseudomonas stutzeri 4.25, 4.27, and 4.44) and Cd-tolerant bacteria (Cupriavidus taiwanensis KKU2500-3 and Delftia acidovorans KKU2500-12) affected the growth of KDML105 rice in pots containing As-contaminated soil under greenhouse conditions. All strains produced inorganic and organic sulfides and exhibited plant growthpromoting traits in vitro. The KKU2500-12/4.25, KKU2500-3/4.25, KKU2500-3/4.27 and KKU2500-3/4.44 combinations increased the oxidation rate of AsIII to AsV. Noninoculated and coinoculated rice was grown in soils collected from As-contaminated paddy fields containing non-detected As or low (29.13 mg/kg) or high (45.11 mg/kg) total As levels. Arsenic affected rice growth and productivity and As accumulation in rice tissues and grains. Bacterial inoculation significantly increased rice growth and yield and decreased the unfilled-grain percentage. KKU2500-3/4.44 was the most effective combination in decreasing As toxicity and accumulation, resulting in non-detected As or a 50 % reduction in As in rice grains grown in low- or high-As soils, respectively. The counts of inoculated bacteria on the root surface and interior were relatively high. Thus, bacteria reduced As availability in the rhizosphere and subsequently decreased As uptake by roots. The bacteria enhanced sulfide accumulation in roots and likely induced the formation of insoluble, nontoxic arsenic sulfide. Various Asx forms and compounds were likely induced by bacteria within roots and reduced AsIII translocation from roots to shoots, leaves and grains, resulting in decreased As toxicity during rice growth and decreased As accumulation in grains. A possible role for the inoculated bacteria in the serial root-xylem-phloem-leaf-phloem-grain translocation of AsIII is proposed. These findings demonstrate the potential for the use of these bacteria in fields with low levels of As contamination (below 29.13 mg/kg).