Biological control of apple scab and fire blight by the application of the non-pathogenic bacterium Pseudomonas fluorescens Bk3 to the leaf surface

Summary

The biological control of plant diseases by application of antagonistic microorganisms to
the plant phyllosphere is an alternative strategy to prevent the frequent treatment of plants
by pesticides. Microbiological antagonists can firstly interact directly against the pathogen
by releasing antimicrobial compounds and/or secondly induce the plant resistance of the
host plant by expression of pathogenesis-related proteins (PR proteins). The focus of our
study is on the interaction of the non-pathogenic bacterium Pseudomonas fluorescens Bk3
to the plant phyllosphere of Malus domestica cv. Holsteiner Cox. After application of P.
fluorescens Bk3 to the phyllosphere of M. domestica cv. Holsteiner Cox we observed
dramatic changes in the protein composition of the apoplast of the host plant. Sequencing
of the induced proteins by ESI-Q-ToF mass spectrometry and homology search identified
these additional proteins as pathogenesis related proteins (PR) like ß-1,3- glucanase,
thaumatin-like protein, chitinase and hevein-like protein. To confirm these findings, a
suppressive subtractive hybridization with total RNA from leaves before and after
inoculation of P. fluorescens Bk3 to the leaves of the host plant was performed. It revealed
an increased expression level of many PR and stress related genes.
The induction of PR proteins and plant defence genes in host plants after application of
non-pathogenic bacterial antagonists to the plant phylloshere can presumably prevent or
reduce successful infections by plant pathogens.