Resumo

The rhizosphere is the region of soil in close contact with plant roots, constituting a hot spot for microbial activity due to the high and constant supply of easily exploitable nutrients. These characteristics make the rhizosphere a high competitive environment, where bacterial groups are selected by their specificities for consuming plant exudates as well as their ability to strive against other microbes. Burkholderia and Pseudomonas are two abundant bacterial genera on sugarcane crop soils, and previous studies suggest that they are members of sugarcane rhizosphere microbiome, participating in beneficial activities for plant health. The aim of this study was to assess the hypothesis that these two bacterial genera are differently selected by the rhizosphere compared to bulk soil. To this end, total soil DNA was isolated from bulk soil and rhizosphere of 6 random plants in a sugarcane field (Piracicaba, SP). Amplification with specific primers for each genus followed by quantitative PCR (qPCR) and denaturant gradient gel electrophoresis (DGGE) analyses allowed investigation of the differences in bacterial numbers per soil gram, and shifts on the structure of bacterial communities by comparing rhizosphere against bulk soil. Quantitation results (copies of the 16S rRNA gene per g of soil) showed that both genera are significantly enriched in the rhizosphere (p < 0.01); with Pseudomonas community shifting from 2.5x10⁴ in the bulk soil to 8.7x10⁵ in the rhizosphere, while values for Burkholderia were 4.0x10⁴ and 2.2x10⁵ for bulk soil and rhizosphere, respectively. The results of DGGE, based on the multivariate analysis (NMDS by Bray-Curtis similarity) showed that the rhizosphere have a homogenizing effect in the community of each genus, evidenced by the clustering of rhizosphere samples in comparison to the wider dispersion of samples from the bulk soil. Together, these results show a clear selection effect of sugarcane rhizosphere in the targeted bacterial genera, suggesting that the plant recruit members of these groups from the total pool of bulk soil.