Plants are host to a large amount of pathogenic bacteria. Fire blight, caused by the bacterium Erwinia amylovora, is an important disease in Rosaceae. Pathogenicity of E. amylovora is greatly influenced by the production of exopolysaccharides, such as amylovoran, and the use of the type III secretion system, which enables bacteria to penetrate host tissue and cause disease. When infection takes place, plants have to rely on the ability of each cell to recognize the pathogen and the signals emanating from the infection site in order to generate several defence mechanisms. These mechanisms consist of physical barriers and the production of antimicrobial components, both in a preformed and an inducible manner. Inducible defence responses are activated upon the recognition of elicitor molecules by plant cell receptors, either derived from invading microorganisms or from pathogen-induced degradation of plant tissue. This recognition event triggers a signal transduction cascade, leading to a range of defence responses [reactive oxygen species (ROS), plant hormones, secondary metabolites, ...] and redeployment of cellular energy in a fast, efficient and multiresponsive manner, which prevents further pathogen ingress. This review highlights the research that has been performed during recent years regarding this specific plantpathogen interaction between Erwinia amylovora and Rosaceae, with a special emphasis on the pathogenicity and the infection strategy of E. amylovora and the possible defence mechanisms of the plant against this disease.

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Bacterial systems are widely applied as production platforms for proteins of biopharmaceutical or therapeutic interest and industrial enzymes. Among these prokaryotic systems, streptomycetes are attractive host cells because several strains of these Gram-positive bacteria have a high innate secretion capacity and extensive knowledge on their fermentation is available. A survey of the literature and our own experience suggests that several proteins are secreted to commercially acceptable levels. However, many heterologous proteins, most often of eukaryotic origin, are currently only poorly secreted by this host, indicating the need for further optimization of Streptomyces as a production host. In this review, the considerable efforts and strategies made in recent years aimed at improving streptomycetes as a host for the production of recombinant proteins will be discussed.