Cellular destruction, phytohormones and growth modulating enzymes production by Bacillus subtilis strain BC8 impacted by fungicides

Abstract

In vitro experiments were performed to ascertain the impact of kitazin, hexaconazole, metalaxyl and carbendazim on growth behaviour, enzymatic profile, ultrastructure, cell permeability and bioactive molecules of phosphate-solubilizing bacterium. Strain BC8 isolated from Brassica oleracea rhizosphere was characterized and identified as Bacillus subtilis by 16S rDNA sequencing (Accession no. MG028650) technique. Strain BC8 was unambiguously chosen due to its high tolerance capability to various fungicides and substantial production of plant growth regulators. The biomarker enzymatic assays (lipid peroxidation, lactate dehydrogenase) and oxidative stress (catalase) induced by fungicides exhibited significant (p < 0.05) toxicity of fungicides toward strain BC8. Fungicides caused the cellular/ultrastructural damage and reduced the viability of strain BC8 as clearly revealed under scanning (SEM), high resolution transmission (HR-TEM) and confocal laser scanning (CLSM) microscopy. As the concentration of fungicides increased, a gradual drop in the plant growth promoting traits of B. subtilis strain BC8 was observed. Kitazin at 2400 μg mL−1, hexaconazole at 1500 μg mL−1, metalaxyl at 1200 μg mL−1 and carbendazim at 1200 μg mL−1decreased the IAA production by 35 (48.3 μg mL−1), 27 (51.5 μg mL−1), 39 (43.6 μg mL−1) and 47% (37.3 μg mL−1), respectively, over control (71.3 μg mL−1), while, α-ketobutyrate was declined by 51 (29.6), 56 (26.2), 61 (22.8) and 68 (19)%, respectively, over untreated control (59.9 mg protein−1 h−1). Also, with increase in the concentration of fungicides there was a significant decrease in plant nutrient (P); the maximum being (19.6 μg mL−1) observed at 1500 μg mL−1 hexaconazole with consequent drop in pH (from pH 6.4 to 4.2). The current findings clearly suggest that despite injury, B. subtilis maintained secreting active biomolecules and this property makes this organism truly indispensable for enhancing crop production under fungicide stressed conditions.