Changes in membrane potential of intact soybean root elongation zone cells induced by Azospirillum brasilense

Abstract

Inoculation of soybean seedlings with Azospirillum brasilense significantly reduced the membrane potential of root cells and made it less negative. The effect was most pronounced in the cells of the elongation zone. Three A. brasilense strains reduced the membrane potential of elongation zone cells to a similar extent. Dead cells or cells of an associative nonbeneficial Pseudomonas species caused no effect. Reduction of membrane potential of root cells was directly related to the physiological status of the bacterial cells. Active bacteria reduced the membrane potential. Stressed bacteria, i.e., bacteria exposed to either starvation, anaerobic conditions, or a high level of streptomycin prior to inoculation, failed to affect membrane potential. Continuous perfusion of the roots immediately after inoculation, while maintaining the bacterial cells surrounding the roots, cancelled the effect of live bacteria on membrane potential. Regardless of bacterial cell treatment or effect on membrane potential of root cells, root colonization levels of all A. brasilense strains were similar throughout all experiments. In addition, all strains positively affect the growth of soybean seedlings. It is suggested that inoculation of soybean plants with live A. brasilense strains, possessing an active metabolism, reduced the membrane potential of root cells probably through the release of an, as yet, unidentified bacterial signal(s). Key words: Azospirillum, beneficial bacteria, Glycine max, membrane potential, plant–bacteria interaction, rhizosphere bacteria, soybean.