Microscopic analysis of plant-bacterium interactions using auto fluorescent proteins

Plant growth promoting rhizobacteria (PGPR) include bacteria that fix nitrogen (e.g., Rhizobiaceae, Herbaspirillum, Azoarcus), produce phytohormones (e.g., Azospirillum) and provide protection against fungal and/or bacterial pathogens (e.g., Pseudomonas, Bacillus, Streptomyces). Interactions between PGPR and plants can be divided into different steps which include initial attraction, attachment, proliferation and colonization e.g., of roots, stem, leaves and flowers. At the genetic level the expression of many bacterial genes are altered during these processes. In addition to the interaction with the plant, PGPR interact and compete with the endogenous microflora, consisting of other bacteria, fungi and/or mycorrhizal fungi. In the case of biocontrol bacterial strains, a direct interaction with the pathogen is often required to suppress the disease. Microscopic analyses of plant growth promoting rhizobacteria (PGPR) in their natural environment and in specific during their interaction(s) with the host plant(s) and/or their target organism(s) is essential for the elucidation of their functioning and the successful application of commercial inoculants. With the discovery and development of auto fluorescent proteins (AFPs) as markers and the development of highly sophisticated fluorescence microscopes such as confocal laser scanning microscopes, a new dimension has been created for studying PGPR in their natural environment. This paper will give a short overview on available tools, the application of AFPs in PGPR research and some future perspectives. Several recent reviews will give the reader an option for further reading (Bloemberg and Lugtenberg 2004; Chalfie and Kain 2005; Larrainzar et al. 2005; Rediers et al. 2005; Bloemberg and Camacho 2006).