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

Abstract

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.

Exopolysaccharides produced by the symbiotic nitrogen-fixing bacteria of leguminosae

“…The inoculation of wheat (Triticum durum L.) with the associative bacterium Paenibacillus polymyxa, selected by its ability to fix N 2 , resulted in a 57 % increase of the soil mass that adhered to the roots and an increased frequency of aggregates with sizes between 0.2 and 2 mm due to EPS production (levan), which contributed to the aggregation of this soil (Gouzou et al, 1993;Bezzate et al, 2000). The same effect on wheat was also observed after inoculation with other associative species.…”

“…Furthermore, they are not pathogenic and produce large amounts of EPS. In figure 3 we illustrate the appearance of colonies characterized by aggregation process is related to several factors, such as the physical properties, climatic conditions and biological activities in the soil (Materechera et al, 1994;Bezzate et al, 2000).…”

Effect of exo-polysaccharides producing bacterial inoculation on growth of roots of wheat (Triticum aestivum L.) plants grown in a salt-affected soil

“…A higher insoluble soil saccharides content of RS of the inoculated than un-inoculated wheat plants was thus indication of an enhanced EPS synthesis activity in the rootzone. An increased mass of soil aggregated around roots of the inoculated than un-inoculated wheat plants and a highly significant positive correlation (r=0.866, p<0.01) of water insoluble soil saccharides with RS/root ratio signified the implication of the bacterial EPS in aggregating soil around roots over other soil microbial secretions or plant mucilages (Watt, et al, 1993;Alami, et al, 2000;Bezzate, et al, 2000). Origin of the EPS, however, could not be ascribed specifically to the inoculated EPSproducing bacterial isolates since the chemical structure and quantity of the individual component of the soil saccharides were not determined.…”

Biofertilising, plant-stimulating and biocontrol potentials of maize plant growth promoting rhizobacteria isolated in central and northern Benin

“…EPS bind to Na + cations and, in particular, decrease its content, thereby helping to reduce salt stress in plants (Ashraf et al, 2004). EPS-producing PGPR strains induce tolerance to soil salinity, promote the growth of soybean (Glycine maxima) plants (Bezzate et al, 2000) and limit the uptake of Na + by wheat roots (Ashraf et al, 2004). Sandhya et al (2009) argue that EPS participate in the formation of bacterial aggregates and consequently improve soil aeration, water infiltration and root growth.…”