Quantification and Localization of Bacteria in Plant Tissues Using Quantitative Real-Time PCR and Online Emission Fingerprinting

In order to quantify and localize specific bacterial target genes in plant tissue, this project has generated relevant new insights in the combined application of quantitative real-time PCR in parallel with the in situ PCR + probe-hybridization and online emission fingerprinting using LSM 510 META. After designing an Enterobacter radicincitans species-specific probe, introduced bacterial cells were monitored in growing plant parts and their colonization behaviour was examined in relation to the native bacterial community. For this purpose, the plant growth-promoting rhizobacterial (PGPR) strain Enterobacter radicincitans was applied to Brassica oleracea plants in increasing inoculum concentrations 10⁷, 10⁸ and 10⁹ cells per plant. Inoculation of 10⁹ E. radicincitans cells per plant to Brassica oleracea leaves and roots resulted in significant increases of root, leaf and tuber growth. Total bacterial cell numbers were estimated using quantitative real-time PCR to be between 10⁷ and 10⁹ cells g⁻¹ fresh leaf weight and about 10⁸ cells g⁻¹ fresh root weight of Brassica oleracea plants. Using quantitative real-time PCR, a significant colonization of Brassica oleracea leaves and roots with E. radicincitans cells was measured. Roots were colonized with a density of 10⁷ cells g⁻¹ fresh root weight up to at least 14 days after inoculation. That is equivalent to a proportion of E. radicincitans 16S rDNA-gene copy numbers compared to the total bacterial communities of about 10–16%. Online emission fingerprinting established that the introduced bacteria proliferated on and inside the root and that they colonized the intercellular spaces of the root cortex layer. Hence, E. radicincitans was able to successfully compete with the native bacterial population.