Characterization and molecular mechanism of a naturally occurring metsulfuron-methyl resistant strain of Pseudomonas aeruginosa

Nicosulfuron application in agricultural soils drives the selection towards NS-tolerant microorganisms harboring various levels of sensitivity to nicosulfuron

The action mode of sulfonylurea herbicides is the inhibition of the acetohydroxyacid synthase (AHAS) required for the biosynthesis of amino acids valine and isoleucine in plants. However, this enzyme is also present in a range of non-targeted organisms, among which soil microorganisms are known for their pivotal role in ecosystem functioning. In order to assess microbial toxicity of sulfonylurea herbicide nicosulfuron (NS), a tiered microcosm (Tier I) to field (Tier II) experiment was designed. Soil bacteria harboring AHAS enzyme tolerant to the herbicide nicosulfuron were enumerated, isolated, taxonomically identified, and physiologically characterized. Results suggested that application of nicosulfuron drives the selection towards NS-tolerant bacteria, with increasing levels of exposure inducing an increase in their abundance and diversity in soil. Tolerance to nicosulfuron was shown to be widespread among the microbial community with various bacteria belonging to Firmicutes (Bacillus) and Actinobacteria (Arthrobacter) phyla representing most abundant and diverse clusters. While Arthrobacter bacterial population dominated community evolved under lower (Tier II) nicosulfuron selection pressure, it turns out that Bacillus dominated community evolved under higher (Tier I) nicosulfuron selection pressure. Different NS-tolerant bacteria likewise showed different levels of sensitivity to the nicosulfuron estimated by growth kinetics on nicosulfuron. As evident, Tier I exposure allowed selection of populations able to better cope with nicosulfuron. One could propose that sulfonylureas-tolerant bacterial community could constitute a useful bioindicator of exposure to these herbicides for assessing their ecotoxicity towards soil microorganisms.

Expression, characterization, and site-directed mutation of a multiple herbicide-resistant acetohydroxyacid synthase (rAHAS) from Pseudomonas sp. Lm10

A multiple herbicide-resistant acetohydroxyacid synthase (rAHAS) gene was cloned from Pseudomonas sp. Lm10. Sequence analysis showed that the rAHAS regulatory subunit was identical to that of Pseudomonas putida KT2440 (sensitive AHAS, sAHAS), whereas six different sites [H134→N (rAHAS→sAHAS), A135→P, S136→T, I210→V, F264→Y, and S486→W] were found in the catalytic subunit. The rAHAS and sAHAS were over expressed, purified and characterized. rAHAS showed higher resistance to four kinds of AHAS-inhibitor herbicides than sAHAS. The resistance factor of rAHAS was 56.0-fold, 12.6-fold, 6.5-fold, and 9.2-fold as compared with sAHAS when metsulfuron-methyl, imazethapyr, flumetsulam, and pyriminobac-methyl used as inhibitor, respectively. The specific activity of rAHAS was lower than that of sAHAS and the K (m) value of rAHAS for pyruvate was approximately onefold higher than the corresponding value for sAHAS. Data from site-directed mutagenesis demonstrated that alteration at A135, F264, and S486 resulted in resistance reduction, while the mutation at H134, S136, and I210 has little effect on the resistance. A135 was mainly responsible for resistance to imidazolinone; F264 conferred resistance to sulfonylurea and triazolopyrimidine sulfonamide; and S486 showed multiple herbicides resistance to the four herbicides.