Markers to distinguish Erwinia amylovora strains and to follow bacterial invasion in plant tissue

Several markers were applied to distinguish individual Erwinia amylovora strains during simultaneous colonization of plants. E. amylovora strains were labelled with the Green Fluorescent Protein (GFP) and visualized in plant tissue by fluorescence microscopy. Inoculation of the intercostal region of apple leaves caused movement of the bacteria in the apoplast followed by invasion into the vascular system. After inoculation of apple leaf tips migration of the bacteria was observed through the xylem vessel and outbreaks into the adjacent intercellular space of the parenchyma. Non-pathogenic mutants did not move from the inoculation site. Plant resistance enhancers caused a decrease in the migration rate. The migration rate in the central vein of apple leaves corresponded to disease ratings from symptom formation on shoots of various apple cultivars. Other markers for strain differentiation were the size of short sequence DNA repeats and streptomycin-resistance. Also, these marked strains were also compared in plant tissue colonization. Quantitative PCR was performed with a light cycler and fluctuation was found for leaf colonization.