Short exposure to Azospirillum brasilense Cd inoculation enhanced proton efflux of intact wheat roots

Inoculation of wheat seedlings with Azospirillum brasilense Cd significantly increased the proton efflux of the roots 5 h after inoculation compared with noninoculated plants. Removal of the bacterial cells from the root surface 2, 4, or 10 h after inoculation did not affect proton extrusion, which remained similar to the proton efflux of inoculated roots with a permanent A. brasilense Cd population. Proton efflux from roots after short exposure to A. brasilense Cd was directly related to the inoculation level and to the physiological status of the bacterial cells. Active bacteria, at an optimal level for inoculation (105–107 cfu∙mL−1), produced the most proton extrusion from roots. Stressed bacteria, i.e., bacteria exposed to starvation, 45 °C, anaerobic conditions, or a high level of streptomycin before inoculation, induced smaller but not statistically significant increases in proton efflux. Inoculation with dead A. brasilense Cd cells, cell-wall fragments, or several associative nonbeneficial rhizosphere bacteria belonging to other genera did not enhance the proton efflux compared with noninoculated plants. Continuous perfusion of the nutrient solution of plants while maintaining the bacterial cells in the root vicinity eliminated the proton efflux enhancement caused by A. brasilense Cd. Bacteria, regardless of species or stress conditions, in the absence of plants produced negligible proton efflux. It is suggested that short exposures of wheat roots to live A. brasilense Cd enhanced the proton efflux of roots, probably through the release of an as yet unidentified bacterial signal. Active metabolism of the bacterium is required for this enhancement, which is of a triggering nature. Key words: Azospirillum, beneficial bacteria, plant–bacteria interaction, proton efflux, rhizosphere bacteria, Triticum aestivum, wheat.