Effect of arsenic on the plant growth-promoting bacteria Azospirillum brasilense Az39

ARMENDARIZ, AL; TALANO MA; MEDINA MI; AGOSTINI E

La Falda- Cordoba

Workshop; II LATINAMERICAN WORKSHOP ON PGPR; 2014

Introduction Arsenic (As) is frequently found at high concentrations in Argentinean agriculture soils and groundwater. Particularly in soils, rhizospheric microorganisms play a crucial role since their metabolic abilities can affect As bioavailability, and consequently As phytotoxicity. The selection of symbiotic or free-living plant growth promoting rhizobacteria (PGPR) strains with remediation capabilities has been emphasized, because they can contribute with plant tolerance to As and/or limit the metalloid incorporation in their tissues. The aim of the present work was to study the effect of As on Azospirillum brasilense Az39, a PGPR strain with ability to increase the productivity of grain crops such as soybean, particularly: a) its growth under different As concentrations, b) exopolysaccharides (EPS) production, c) biofilm formation, d) motility and As metabolism, since they could be considered as tolerance strategies. Materials and methods To analyze the effect of As on A. brasilense Az39 growth, the bacterium was grown in LB liquid medium supplemented with 25 µM, 500 µM and 5 mM of arsenate (AsV) and arsenite (AsIII). For that, DO620 nm values were registered during 26 h and also FCU/ml were determined. The effect of As on EPS production was determined through Anthrone method from bacterial cultures grown during 24 h in LB medium supplemented with 25 and 500 µM of both salts of As. Biofilm analysis was carried out in tubes by a spectrophotometric method using violet crystal (Abs570nm). Motility (swarming and swimming) was analyzed in LB diluted 1/10 solid medium (0.3 or 0.5 % agar, respectively) supplemented with 25 and 500 µM of AsV or AsIII. The results were expressed as ring diameter of motility (cm). After 25 µM As treatment, total residual As, species (AsV and AsIII) concentration in the culture medium as well as As accumulation in bacterial pellet were measured by a graphite furnace atomic absorption spectrophotometric method (GF-AAS). Results A. brasilense Az39 was able to grow in all tested conditions even at high AsIII concentration (500 µM), with the exception of 5 mM AsIII where growth was completely inhibited. EPS and biofilm production showed statistically significant increase under 500 µM AsIII, coinciding with the condition at which growth and motility (swarming and swimming) were seriously affected. When A. brasilense Az39 was grown in the presence of AsV, it was able to reduce around 36% of initial AsV. Moreover, it showed higher efficiency for AsIII oxidation (around 53%) and was able to accumulate an 8.5% of the total As in cell biomass under this condition. Conclusion A. brasilense Az39 motility and biofilm/EPS production was not affected at the tested As concentrations with the exception of AsIII 500 µM, which correlates with the observed high tolerance of this strain to As. Therefore, the high AsIII tolerance, ability for AsIII oxidation and As accumulation of A. brasilense Az39 suggests that this strain would be able to efficiently colonize As contaminated soils. In this way, inoculation with A. brasilense Az39 would positively contribute to promoting growth of soybean and/or other plants, commonly inoculated with B. japonicum E109, which is seriously affected under As stress and/or other plant species.