Biochemical and physiological peculiarities of the interactions between Sinorhizobium meliloti and Sorghum bicolor in the presence of phenanthrene

The effect of phenanthrene, a polycyclic aromatic hydrocarbon (PAH) at concentrations of 0, 10, and 100 mg/kg and the bacterium Sinorhizobium meliloti P221 on root exudation of Sorghum bicolor L. Moench was studied in laboratory vegetative experiments. Inoculation of the bacterium promoted plant resistance to the pollutant stress and increased their acclimation rate and biomass formation. The ability of this microorganism to produce a phytohormone, indolyl-3-acetic acid, and to degrade phenanthrene, resulted in morphological changes of the plant root system and in the changed intensity of root exudation. In root exudates of sorghum, enzyme activities towards the metabolites formed during microbial degradation of PAH were revealed, which is indicative of a direct involvement of plants in PAH degradation in the rhizosphere as well as of the coupled plant-microbial metabolism in the course of xenobiotic degradation in the root zone. In phenanthrene-contaminated soil, sorghum was found to support selectively the development of the S. meliloti P221 population.