COLONIZATION OF WHEAT ROOTS BY AN EXOPOLYSACCHARIDE-PRODUCING PANTOEA-AGGLOMERANS STRAIN AND ITS EFFECT ON RHIZOSPHERE SOIL AGGREGATION

The effect of bacterial secretion of an exopolysaccharide (EPS) on rhi zosphere soil physical properties was investigated by inoculating stra in NAS206, which was isolated from the rhizosphere of wheat (Triticum durum L,) growing in a Moroccan vertisol and was identified as Pantoea aglomerans. Phenotypic identification of this strain with the Biotype -100 system was confirmed by amplified ribosomal DNA restriction analy sis, After inoculation of wheat seedlings with strain NAS206, coloniza tion increased at the rhizoplane and in root-adhering soil (RAS) but n ot in bulk soil. Colonization further increased under relatively dry c onditions (20% soil water content; matric potential, -0.55 MPa). By me ans of genetic fingerprinting using enterobacterial repetitive interge nic consensus PCR, we were able to verify that colonies counted as str ain NAS206 on agar plates descended from inoculated strain NAS206, The intense colonization of the wheat rhizosphere by these EPS-producing bacteria was associated with significant soil aggregation, as shown by increased ratios of RAS dry mass to root tissue (RT) dry mass (RAS/RT ) and the improved water stability of adhering soil aggregates. The ma ximum effect of strain NAS206 on both the RAS/RT ratio and aggregate s tability was measured at 24% average soil,vater content (matric potent ial, -0.20 MPa). Inoculated strain NAS206 improved RAS macroporosity ( pore diameter, 10 to 30 mu m) compared to the noninoculated control, p articularly when the soil was nearly water saturated (matric potential , -0.05 MPa). Our results suggest that P. agglomerans NAS206 can play an important role in the regulation of the water content (excess or de ficit) of the rhizosphere of wheat by improving soil aggregation.