Identification of differentially expressed genes in Malus domestica after application of the non-pathogenic bacterium Pseudomonas fluorescens Bk3 to the phyllosphere

Biological control of plant diseases by the application of antagonistic micro-organisms to the plant phyllosphere is only marginally understood. Suppression subtractive hybridization (SSH) was used for the identification of genes expressed after application of the non-pathogenic bacterium Pseudomonas fluorescens Bk3 to the phyllosphere of the apple scab-susceptible cultivar Malus domestica cv. Holsteiner Cox. In total, 157 expressed sequence tag (EST) clones were obtained. The sequencing of 113 ESTs which have a significantly elevated transcript level and the comparison of the obtained sequences with databases revealed similarities to different classes of pathogenesis-related proteins, for example, RNase-like PR10 protein and endochitinase, or similarities to proteins expressed under stress conditions that could have a protective function, for example, a germin-like protein, glutathione S-transferase, thioredoxin-like proteins, and heat shock proteins. In addition, several transcripts were identified that code for proteins which have a crucial role at different stages of pathogen recognition and in signalling pathways or an as yet unknown function in plant defence. The results show that a number of transcripts encoding proteins/enzymes which are known to be up-regulated after pathogen infection are also up-regulated after the application of a non-pathogenic bacterium to a M. domestica cultivar. The expression of these proteins might increase the plant resistance towards pathogen infection and damage.