Effect of different environmental conditions on bacterial chemotaxis toward fungal spores

Abstract

The chemotactic response of four common soil bacteria, Agrobacterium radiobacter, Bacillus subtilis, Pseudomonas fluorescens, and Xanthomonas malvacearum, was observed in natural soil to conidia of Cochliobolus sativus, chlamydospores of Fusarium oxysporum f.sp. ciceri, sclerotia of Macrophomina phaseolina, and oospores of Phytophthora drechsleri f.sp. cajani. All bacteria migrated through soil toward fungal spores. Chemical analysis of fungal exudates demonstrated the presence of various amino acids and sugars that served as chemoattractants. The effect of temperature, pH, soil water matric potential, and soil texture on bacterial chemotaxis toward fungal spores was also investigated. In general, the response of bacteria to different types of fungal spores was significantly greater (P = 0.05) at higher water matric potential (0 and −5 kPa) than at lower soil water potential (−10 and −20 kPa). The chemotactic response was greatest in sandy soil, followed by sandy loam and clay loam soil. High temperature (40 °C) was not favourable for bacterial chemotaxis. The chemotactic response decreased by 4 to 28 times when soil pH decreased from 7 to 5 or increased from 7 to 9. The relative concentration response of the exudate of fungal spores and the motility or chemotaxis of bacteria were assessed. A significant positive correlation (P = 0.05; r = 0.89–0.92) was recorded between motility and relative concentrations of exudate. The swimming speed and tumbling frequency of Pseudomonas fluorescens cells was observed at different pH values and temperatures. The swimming speed of bacteria increased with increasing temperature, but decreased with an increase in pH from 5 to 7. The tumbling frequency had a sharp peak at 30 °C and pH 7. Bacteria were able to stimulate the release of exudate from the fungal spores. The amount of exudation was also affected by temperature, pH, and soil texture. A positive significant correlation (P = 0.05; r = 0.79–0.85) was recorded between exudation and a progressive increase in temperature. The amount of exudation also increased with an increase in soil pH from 5 to 8, but further increases in pH decreased the rate of exudation. In general, fungal spores exuded the greatest amounts of carbon compounds in sandy soil, followed by sandy loam and clay loam soil. Exudation in sandy soil was often greatest in the presence of A. radiobacter cells, while the greatest exudation in sandy loam and clay loam soils was induced by B. subtilis cells.Key words: chemotaxis, chemoattractants, fungal spores, motility.