Physiological effects of soybean inoculation with Rhodococcus sp AW3 in the presence of arsenite: a microcosm study

WEVAR OLLER AL; CRISTIANI, M; TALANO MA; AGOSTINI E

La Falda- Cordoba

Workshop; II LATINAMERICAN WORKSHOP ON PGPR; 2014

Introduction In the last years, metal-tolerant plant-microbe associations have attracted much attention due to the potential of microorganisms for bioaccumulating/mobilizating/immobilizating metals from polluted environment and consequently enhancing metal uptake and plant growth. Synergistic use of plants and PGPR metal-tolerant microbes has been useful for cleanup of metalliferous soils. Arsenic (As) is a highly toxic metalloid widely distributed in the environment, including farming soils. In Argentina, particularly in rural areas, As-contaminated aquifers are commonly used for irrigation, producing phytotoxicity and long-term effects on crop yield thus delaying sustainable agricultural development. So, rhizostabilization and rhizoremediation are presented as interesting strategies for As mitigation. The aim of this work was to evaluate if Rhodococcus sp. AW3, a highly As tolerant bacterium isolated from the rhizosphere of soybean plants grown in soils from Córdoba, could be useful to increase soybean biomass and influence As accumulation or stabilization. Materials and methods Microcosm assays were carried out to evaluate the effect of Rhodococcus sp. AW3 inoculation on soybean plantlets growth under As stress. Soybean DM4670 seeds superficially disinfected and 6 day germinated were placed in glass pots containing volcanic sand moisturized with 100 mL of a 50 µM sodium arsenite (As+3) (Sigma) solution or distilled water (control). Then, each seed was inoculated with 1 mL of a Rhodococcus sp. AW3 overnight culture in YEM medium or with 1 mL of sterilized YEM medium (control). The pots were incubated under controlled conditions in a cultivation chamber and irrigated with distilled sterilized water every two days. Soybean plantlets were harvested after 45 d of incubation, evaluated for physiological parameters, divided in roots and aerial part, frozen and kept at -80 °C for total protein, chlorophyll and malondialdehyde (MDA) concentration determination. A portion of the pool of roots and aerial part tissues were oven dried and used for total As determination by a graphite furnace atomic absorption spectrophotometric (GF-AAS) technique after a wet acid digestion with nitric acid. Results The used concentration of As+3 adversely affected biomass production, which was evidenced by a decrease of approximately 50% in length and fresh weight (FW) of the aerial part, both in inoculated as non-inoculated soybean plants. On the other hand, the length and FW of the roots were negatively affected, but to a lesser extent. Inoculation with AW3 resulted in a slight positive effect in the roots FW. Total protein content and chlorophyll concentration, both a and b, increased in the soybean plants inoculated with AW3 and under As stress in comparison with non-inoculated plants. MDA concentration was higher in the aerial parts than in the roots in all treatments. A significant increase in MDA content was observed in the aerial part of plants under As stress, both inoculated and non-inoculated. Arsenic was accumulated mostly in roots, while only a small percentage was translocated to the aerial parts, in both inoculated and non-inoculated plants. Conclusions Inoculation of soybean seedlings with Rhodococcus sp. AW3 under As +3 stress resulted in improved chlorophyll and protein content, although biomass production was not enhanced and metal-induced stress was not alleviated. Soybean accumulated As mainly in the roots, suggesting that it is an excluder of As, at least under the studied conditions.