Degradation of Triazole Fungicides by Plant Growth-Promoting Bacteria from Contaminated Agricultural Soil

Abstract

The widespread application of triazole fungicides (TFs) in agricultural practices can result in the considerable accumulation of active compound residues in the soil and a subsequent negative impact on the soil microbiota and crop health. In this study, we isolated three TF-degrading bacterial strains from contaminated agricultural soils and identified them as Klebsiella sp., Pseudomonas sp., and Citrobacter sp. based on analysis of morphological characteristics and 16S rRNA gene sequences. The strains used three common TFs, namely hexaconazole, difenoconazole, and propiconazole, as their only sources of carbon and energy for growth in a liquid mineral salt medium, with high concentrations (~ 500 mg/l) of each TF. In addition to the ability to degrade fungicides, the isolates also exhibited plant growth-promoting characteristics, such as nitrogen fixation, indole acetic acid production, phosphate dissolution, and cellulose degradation. The synergistic combination of three bacterial isolates significantly improved plant growth and development with an increased survival rate (57%), and achieved TF degradation ranging from 85.83 to 96.59% at a concentration of approximately 50 mg/kg of each TF within 45 days in the soil-plant system. Based on these findings, the three strains and their microbial consortium show promise for application in biofertilizers, to improve soil health and facilitate optimal plant growth.

Keywords: Triazole fungicides; biodegradation; microbial consortium; plant growth promotion; soil microorganisms.

Fig. 1. Degradation of three TFs compounds: hexaconazole (A), propiconazole (B) and difenoconazole (C), during the growth of three bacterial strains (D5-2, D9-1, and D10-3).

Negative controls (DC) lacked bacterial strains in the culture medium. Different letters indicate statistically significant differences (p < 0.05). The error bars represent the SD and the values are the means of three replicates.

Fig. 2. Growth experiment with Green Lollo Rossa lettuce in soil supplemented with bacterial mixtures and TFs (150 mg/kg) after 45 DAP.

Treatments: (A) NT1 without TFs and bacterial mixtures, (B) NT2 with bacterial mixtures, (C) NT3 with TFs, and (D) NT4 with both TFs and bacterial mixtures.

Fig. 3. Degradation of TFs in treatment NT3 and NT4 with an initial total TF concentration of 150 mg/kg soil, respectively.

Values are the means of three replicates, and error bars represent SD. Different letters indicate statistically significant differences (p < 0.05).