Bacterial diversity and soil enzyme activity in diseased and disease free apple rhizosphere soils

A culture-independent survey of the bacterial diversity in rhizosphere soils of diseased (scab) and disease-free apple trees was conducted to assess the role of bacteria in disease suppression. Community DNA was extracted from soil samples and amplified by PCR using primers specific for bacterial 16S rRNA gene sequences. Clone libraries were constructed with the PCR products and analysed based on amplified rDNA restriction analysis (ARDRA) patterns. The phylotypes and their frequency distribution in both libraries indicated that the phylotype did not represent a single group. Rarefaction curve, and Shannon and Simpson diversity indices exhibited insignificant diversity differences between the samples in terms of bacterial community composition, whereas more chitinase and beta-1,3 glucanase activities were recorded in samples from disease-free trees than from diseased trees. Based on the operational taxonomic units identified in ARDRA, 80 representative clones were selected from the libraries and partially sequenced. Sequence similarity searches with the resulting sequences identified the dominance of uncultured bacteria to the extent of 70% and 72.5% in disease free and diseased rhizospheres, respectively. The foregoing studies conclude a possible role for enhanced microbial activity in terms of enzyme production in tree (apple) health, although no distinct partitioning of composition or significant diversity of bacterial communities inhabiting diseased and disease-free rhizosphere soils was observed at any given time.