Mechanisms and kinetics of plant microbe interactions that reduce arsenic uptake and loading in rice seedlings

Arsenite (AsIII)-oxidizing and plant growth-promoting bacteria (PGPB) grown in 0-50 µM AsIII hydroponic solution were coinculated with KDML105 rice seedlings, and the kinetics of arsenic (As) uptake and plant responses were examined. The bacteria reduced oxidative stress and improved the growth parameters of the rice seedlings. The levels of enzymatic and nonenzymatic antioxidant compounds (particularly phytochelatins) and sulfide were significantly greater in the coinoculated (RB) than in the uninoculated (R) seedlings. Among the bacterial combinations, coinoculated KKU2500-3/4.44 coinoculation yielded the best results and reduced As accumulation by 14-25% in roots, 38-60% in shoots, and 17-67% in leaves compared with that in the R seedlings. The dose-dependent assay revealed clear reductions in As uptake and transport, as represented by reduced Vmax and increased Km values. The Vmax values obtained for the net and apoplastic uptake of the RB seedlings were 138.75 and 138.75 nmol/g FW/h, which were lower than those of the R seedlings (146.15 and 139.88 nmol/g FW/h, respectively). The Km values obtained for net and apoplastic uptake of the RB seedlings were 78.20 and 95.59 nM, which were greater than those of the R seedlings (69.90 and 85.80 nM, respectively). These results indicated that the coinoculated bacteria blocked the uptake and loading of As from the roots to the xylem by enhancing antioxidant defense and thiol metabolism. Thus, the coinoculation of seedlings with Cupriavidus taiwanensis KKU2500-3 and Pseudomonas stutzeri 4.44 might promote rice cultivation under a wide range of As levels and should be investigated in paddy fields.