Abstract
Lead (Pb) and cadmium (Cd) are major environmental pollutants, and the accumulation of these elements in soils and plants is of great concern in agricultural production due to their toxic effects on crop growth. Also, these elements can enter into the food chain and severely affect human and animal health. Bioaugmentation with plant growth-promoting bacteria (PGPB) can contribute to an environmentally friendly and effective remediation approach by improving plant survival and promoting element phytostabilization or extraction under such harsh conditions. We isolated and characterised Pb and Cd-tolerant root-associated bacteria from Helianthus petiolaris growing on a Pb/Cd polluted soil in order to compose inoculants that can promote plant growth and also ameliorate the phytostabilization or phytoextraction efficiency. One hundred and five trace element-tolerant rhizospheric and endophytic bacterial strains belonging to eight different genera were isolated from the aromatic plant species Helianthus petiolaris. Most of the strains showed multiple PGP-capabilities, ability to immobilise trace elements on their cell wall, and promotion of seed germination. Bacillus paramycoides ST9, Bacillus wiedmannii ST29, Bacillus proteolyticus ST89, Brevibacterium frigoritolerans ST30, Cellulosimicrobium cellulans ST54 and Methylobacterium sp. ST85 were selected to perform bioaugmentation assays in greenhouse microcosms. After 2 months, seedlings of sunflower (H. annuus) grown on polluted soil and inoculated with B. proteolyticus ST89 produced 40% more biomass compared to the non-inoculated control plants and accumulated 20 % less Pb and 40% less Cd in the aboveground plant parts. In contrast, B. paramycoides ST9 increased the bioaccumulation factor (BAF) of Pb three times and of Cd six times without inhibiting plant growth. Our results indicate that, depending on the strain, bioaugmentation with specific beneficial bacteria can improve plant growth and either reduce trace element mobility or enhance plant trace element uptake.
Highlights
Trace element pollution of agricultural soils and waters has been dramatically increased during the last few decades [1]
Some bacteria genera isolated in our study (i.e., Bacillus, Mhetylobacterium, Cellulosimicrobium and Rhizobium) have already been reported in studies made on other plants species harvested from trace elements polluted soils [36,37,38,39,40,41]
Most earlier reports found that bacterial density/diversity decreases from outside to the root interior [42,43], while in our study the number of cultivable trace element tolerant strains isolated from the root endosphere of H. petiolaris plants was almost double that isolated from the no earlier reports about the isolation of endophytic and rhizospheric bacteria from this plant species
Summary
Trace element pollution of agricultural soils and waters has been dramatically increased during the last few decades [1]. There has been an increasing ecological and global public health concern associated with environmental contamination by these elements [2]. Trace elements such as Pb and Cd are non-essential for living organisms, carcinogenic at low concentrations and are usually originating from mining, smelting, electroplating, petrochemical production [3], and military activities [4]. Phytoextraction comprises the process of plant root uptake, root-to-shoot translocation, shoot accumulation and detoxification to concentrate pollutants in harvestable biomass [10,11].
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