Impact of Glyphosate and Glyphosate-Based Herbicides on Phyllospheric Methylobacterium

Abstract

Symbiotic Methylobacterium comprise a significant portion of the phyllosphere, and are known to benefit host plant growth, development, and confer tolerance to stress factors. The near ubiquitous use of the broad-spectrum herbicide, glyphosate, in farming operations globally has necessitated a more expansive evaluation of the impacts of the agent itself and formulations containing glyphosate on important components of the plant phyllosphere, including Methylobacterium. This study provides an investigation of the sensitivity of 18 strains of Methylobacterium to glyphosate and two commercially available glyphosate-based herbicides (GBH). Nearly all strains of Methylobacterium showed signs of sensitivity to the popular GBH formulations WeatherMax® and Transorb® in a modified Kirby Bauer experiment. However, exposure to pure forms of glyphosate did not show a significant effect on growth for any strain in both the Kirby Bauer test and in liquid broth, until polysorbate-20 (Tween20) was added as a surfactant. Artificially increasing membrane permeability through the introduction of polysorbate-20 caused a 78–84% reduction in dry cell biomass relative to controls containing glyphosate or high levels of surfactant only (0–9% and 6–37% reduction respectively). The observable high sensitivity of Methylobacterium to the tested GBH formulations, including complete bactericidal activity in liquid broth at concentrations as low as 0.05% w/v (500 µg/L), was shown by the inability to recover culture viability when transferred to fresh media. To better understand the compatibility of important phyllospheric bacteria with commercial herbicides, this study endeavours to uncover the feasibility of synthetic products with biofertilizers and explores possible mechanisms by which toxicity may be induced in Methylobacteria.