Disruption of the Paenibacillus polymyxa levansucrase gene impairs its ability to aggregate soil in the wheat rhizosphere

Inoculation of wheat roots with Paenibacillus (formerly Bacillus) polymyxa CF43 increases the mass of root-adhering soil. We tested the role of levan, a fructosyl polymer produced by strain CF43, in the aggregation of soil adhering to wheat roots. The P. polymyxa gene homologous to the Bacillus subtilis sacB gene encoding levansucrase was cloned and sequenced. The corresponding gene product synthesises high molecular weight levan. A P. polymyxa mutant strain, SB03, whose sacB gene is disrupted, was constructed using heterogramic conjugation. Effects of wheat inoculation with the wild type and the mutant strain were compared using two different cultivated silt loam soils in four independent pot experiments. Roots of wheat plantlets inoculated with CF43 or SB03 were colonized after 7-14 days at the same level, and root and shoot masses were not significantly different from those of the non-inoculated control plants. The ratio of root-adhering soil dry mass to root tissue dry mass was significantly higher for plants inoculated with strain CF43 than for those inoculated with mutant strain SB03: + 30% in Orgeval soil and + 100% in Dieulouard soil. Thus the levan produced by P. polymyxa is implicated in the aggregation of root-adhering soil on wheat.