Inoculation of wild type or EPS mutant Bacillus subtilis does not alter rhizocompetence, soil EPS, and tomato growth in the absence of stress

Biofilm formation on plant roots is traditionally assumed an important trait for plant growth promoting rhizobacteria, but its importance in the interplay between soil, plant roots and the rhizosphere bacterial and archaeal community is not well understood. In a greenhouse experiment, tomato plants grown in a sandy soil were inoculated with Bacillus subtilis NCIB 3610 wild type or a ΔepsA-O mutant strain with defective biofilm formation. Strain-dependent effects on rhizocompetence, bacterial/archaeal community composition, soil EPS contents and plant growth were hypothesized. High-throughput sequencing of 16S rRNA genes amplified from community DNA showed that inoculation-dependent shifts of the bacterial and archaeal community compositions occurred mainly in the rhizoplane and rhizosphere between one or eight days after inoculation. Contrary to our hypothesis, the mutant had a similar rhizocompetence as the wild type according to selective plating. The EPS-saccharide content in the soil and rhizosphere did not differ among inoculation treatments. Plants inoculated with the mutant had larger roots compared to the wild type and control treatments – likely due to inoculation-dependent bacterial and archaeal community shifts. Thus, this study shows that the inoculants′ ability to produce biofilms was not linked with rhizocompetence, EPS contents in soil and plant growth. It sheds light on new aspects regarding the relevance of biofilm formation for rhizocompetence and plant growth promotion under optimal plant growth conditions.