The rhizosphere and root exudates of maize seedlings drive plasmid mobilization in soil

Plasmid mobilization is important for environments. However, the effects of rhizospheres and root exudates on plasmid mobilization have received little attention. To address this problem, mobilization of the pSUP106 plasmid by a TOL-like plasmid from Pseudomonas fluorescens P13 into rifampicin-resistant P13 cells was investigated by biparental and triparental matings in the rhizosphere soils of maize seedlings and bulk soils. Mobilization frequencies were higher in the rhizosphere than in bulk soils. Plasmid mobilization frequencies decreased with vertical and horizontal distance away from plant roots. Furthermore, plasmid mobilization frequencies increased with growing contents of root exudates and artificial root exudate (ARE) of maize seedlings for 1 or 5 d of incubation time; however, donor and recipient population remained basically stable for all contents. Individual components of the exudates failed to promote mobilization of pSUP106 but a combination of sugars and amino acids of the exudates did. Removal of individual components from the ARE reduced the plasmid mobilization-promoting activity but simultaneous removal of several components destroyed the plasmid mobilization. All the data suggest that the rhizosphere of maize seedlings promoted the mobilization of the pSUP106 plasmid by the TOL-like plasmid and that root exudate of maize seedlings was the important driver of plasmid mobilization. The individual components of root exudates were necessary to increase plasmid mobilization but their combination was important for plasmid mobilization.