Abstract
The large-scale use of the herbicide glyphosate leads to growing ecotoxicological and human health concerns. Microbe-assisted phytoremediation arises as a good option to remove, contain, or degrade glyphosate from soils and waterbodies, and thus avoid further spreading to non-target areas. To achieve this, availability of plant-colonizing, glyphosate-tolerant and -degrading strains is required and at the same time, it must be linked to plant-microorganism interaction studies focusing on a substantive ability to colonize the roots and degrade or transform the herbicide. In this work, we isolated bacteria from a chronically glyphosate-exposed site in Argentina, evaluated their glyphosate tolerance using the minimum inhibitory concentration assay, their in vitro degradation potential, their plant growth-promotion traits, and performed whole genome sequencing to gain insight into the application of a phytoremediation strategy to remediate glyphosate contaminated agronomic soils. Twenty-four soil and root-associated bacterial strains were isolated. Sixteen could grow using glyphosate as the sole source of phosphorous. As shown in MIC assay, some strains tolerated up to 10000 mg kg–1 of glyphosate. Most of them also demonstrated a diverse spectrum of in vitro plant growth-promotion traits, confirmed in their genome sequences. Two representative isolates were studied for their root colonization. An isolate of Ochrobactrum haematophilum exhibited different colonization patterns in the rhizoplane compared to an isolate of Rhizobium sp. Both strains were able to metabolize almost 50% of the original glyphosate concentration of 50 mg l–1 in 9 days. In a microcosms experiment with Lotus corniculatus L, O. haematophilum performed better than Rhizobium, with 97% of glyphosate transformed after 20 days. The results suggest that L. corniculatus in combination with to O. haematophilum can be adopted for phytoremediation of glyphosate on agricultural soils. An effective strategy is presented of linking the experimental data from the isolation of tolerant bacteria with performing plant-bacteria interaction tests to demonstrate positive effects on the removal of glyphosate from soils.
Highlights
Glyphosate (N-phosphonomethyl glycine) is a broad-spectrum systemic herbicide, generally known as the active compound of the commercial product Roundup
This paper describes the whole process from the isolation of sixteen glyphosate tolerant bacterial strains from Lotus pastureland plots and characterization of their plant growth promotion potential, glyphosate tolerance and root colonization, to the assessment of the glyphosate degradation abilities of the two best performing ones
P30BS-XVII and two strains of Rhizobium, P16RR-IX and P44RR-XXIV. Another six strains tolerated glyphosate concentrations up to 7500 mg kg−1, while the remaining strains were inhibited by lower concentrations
Summary
Glyphosate (N-phosphonomethyl glycine) is a broad-spectrum systemic herbicide, generally known as the active compound of the commercial product Roundup. It has been widely used in agriculture since the mid-seventies. To alleviate the effects of the non-target application as well as the off-site runoff, in situ degradation or transformation of glyphosate should be boosted. To this end, much attention has been directed toward exploiting plant-microbe interactions, known as microbe assisted phytoremediation, to remediate pesticides polluted soils (Glick, 2003; Kuiper et al, 2004; Vangronsveld et al, 2009)
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