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.

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Seed endophyte inoculation can enhance the plant tolerance to pollutants, which allows plant cultivation on trace element (TE) polluted soils. Methylobacterium sp. CP3 and Kineococcus endophyticus CP19 were tested in vitro for their tolerance to Zn and Cd and their plant growth promotion traits. The in vivo effects of bioaugmentation with individual strains or both strains were tested using two poplar cultivars, Populus deltoides x (P. trichocarpa x P. maximowiczii) ‘Dender’ and ‘Marke’, grown in TE polluted soil for six weeks. Methylobacterium sp. was found to grow on media enriched with 0.4 and 0.8 mM Cd, and both endophytes tolerated 0.6 and 1 mM Zn, due to the presence of genes involved in Zn and Cd tolerance and transport. Methylobacterium sp. showed an extracellular ion sequestration mechanism. Production of indole-3-acetic acid by Methylobacterium sp. and K. endophyticus, as well as phosphorus solubilization by Methylobacterium sp. were observed. Bioaugmentation with both endophytes increased the shoot length of Populus ‘Marke’ and enhanced the Mg uptake in both cultivars. Inoculation with Methylobacterium sp. reduced the bioaccumulation of Zn in ‘Marke’, conferring it an excluder strategy. Methylobacterium sp. and K. endophyticus seemed to improve the plant nutritional status, which can alleviate abiotic stress.