This study aimed to investigate the potential of Rhodopseudomonas palustris C1 and Rubrivivax benzoatilyticus C31 to ameliorate As toxicity and to reduce As uptake in rice. Strain C1 was superior to strain C31 for siderophore production. The mixed culture (1: 1) was most effective in reducing the toxicity of As species [As(III) and/or As(V), each 30 mg/l] by yielding maximal germination index that related to α- and β-amylase activities in two Thai rice cultivars (HomNil: HN and PathumThani 1: PT). Arsenic toxicity to the seed germination followed the order: mixed As species > As(III) > As(V); and the toxicity was reduced in inoculated sets, particularly with a mixed culture. The mixed culture significantly enhanced rice growth under As stress in both rice cultivars as indicated by an increase in the production of chlorophyll a and b, and also supporting the non-enzymatic (carotenoids, lipid oxidation, and nitric oxide) and enzymatic (superoxide dismutase, ascorbate peroxidase, catalase, and glutathione reductase) activities. These were concomitant with productions of 5-aminolevulinic acid, indole-3-acetic acid, exopolymeric substances, and siderophores which significantly reduced As accumulation in treated rice. It can be concluded that the mixed culture has great potential to ameliorate rice from As toxicity by preventing As species entry into rice for enhancing rice growth and also for reducing As accumulation to produce safe rice from rice grown in contaminated paddy fields.
Keywords: Arsenic; Contamination; Phototrophic bacteria; Plant growth-promoting substances; Protection mechanisms; Rice.