We examined whether long-term Cd exposure leads to beneficial changes in the cultivable endophytic bacteria present in the seeds of Agrostis capillaris. Therefore the cultivable seed endophytes of Agrostis capillaris growing on a long-term Cd/Ni-contaminated plot (Cd/Ni seeds) were compared with those originating from a non-contaminated plot (control seeds). We observed plant- and contaminant-dependent effects on the population composition between control and Cd/Ni seeds. Also differences in phenotypic characteristics were found: endophytes from Cd/Ni seeds exhibited more ACC deaminase activity and production of siderophores and IAA, while endophytes from control seeds, very surprisingly, showed more metal tolerance. Finally, the 3 most promising seed endophytes were selected based on their metal tolerance and plant growth promoting potential, and inoculated in Agrostis capillaris seedlings. In case of non-exposed plants, inoculation resulted in a significantly improved plant growth; after inoculation of Cd-exposed plants an increased Cd uptake was achieved without affecting plant growth. This indicates that inoculation of Agrostis with its seed endophytes might be beneficial for its establishment during phytoextraction and phytostabilisation of Cd-contaminated soils.

Exploiting the potential of bacteria in phytoremediation for the removal of organic and inorganic pollutants from soils and (ground)water holds great promise. Besides bacteria, mycorrhizal fungi and free-living saprotrophs are well known for their strong degradative capacities and plant growth promotion effects, which makes them of high interest for use in different bioremediation strategies. To further increase the efficiency and successes of phytoremediation, interactions between plants and their associated microorganisms, both bacteria and fungi, should be further investigated, in addition to the close interactions between bacteria and fungi. Benefitting from an increased understanding of microbial community structure and assembly allows us to better understand how the holobiont can be modified to improve pollutant degradation and plant growth. In this review, we present an overview of insights in plant-bacteria-fungi interactions and the opportunities of exploiting these tripartite interactions to enhance the effectiveness of phytoremediation of organic pollutants.

Plants on contaminated mining soils often show a reduced growth due to nutrient depletion as well as trace elements (TEs) toxicity. Since those conditions threat plant's survival, plant growth-promoting rhizobacteria (PGPRs), such as rhizobia, might be of crucial importance for plant colonization on TE-contaminated soils. Native rhizobia from mining soils are promising candidates for bioaugmented phytoremediation of those soils as they are adapted to the specific conditions. In this work, rhizobia from Zn- and Cd-contaminated mining soils were in vitro screened for their PGP features [organic acids, indole-3-acetic acid (IAA), and siderophore (SID) production; 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity; and Ca(PO) solubilization] and Zn and Cd tolerance. In addition, some type and reference rhizobia strains were included in the study as well. The in vitro screening indicated that rhizobia and other native genera have great potential for phytoremediation purposes, by exerting, besides biological N fixation, other plant growth-promoting traits. Leucaena leucocephala-Mesorhizobium sp. (UFLA 01-765) showed multielement tolerance and an efficient symbiosis on contaminated soil, decreasing the activities of antioxidative enzymes in shoots. This symbiosis is a promising combination for phytostabilization.

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Chlorendic acid is a recalcitrant, highly chlorinated organic pollutant for which no microbial degrader has yet been identified. To address this knowledge gap, fungi were isolated from bulk soil, rhizosphere, and roots of the common bent ( Agrostis capillaris ) and the hybrid poplar [ Populus deltoides × ( Populus trichocarpa × P. deltoides ) cv. Grimminge], both of which grow on a chlorendic acid polluted site in Belgium. Isolates were taxonomically identified and phenotypically screened for chlorendic acid degradation. Several fungal isolates could degrade chlorendic acid in liquid media up to 45%. The chlorendic acid degrading fungal isolates produced higher levels of hydroxyl radicals when exposed to the pollutant when compared to non-exposed controls, suggesting that the oxidative degradation of chlorendic acid occurs through production of Fenton-mediated hydroxyl radicals. In addition, the isolated Ascomycete Penicillium sp. 1D-2a degraded 58% of the original chlorendic acid concentration in the soil after 28 days. This study demonstrates that the presence of fungi in a chlorendic acid polluted soil can degrade this highly chlorinated organic pollutant. These results indicate that recalcitrant, seemingly non-biologically degradable organic pollutants, such as chlorendic acid, can be remediated by using bioremediation, which opens new perspectives for in situ bioremediation.