Editorial

Abstract

The rhizosphere of plants is one of the most fascinating microbial habitats for basic and applied studies in the field of environmental microbiology, as it is shaped by the soil, the plant and the microorganisms. The importance of this compartment for plant growth and soil microbiology had already been realized in the very pioneer times of microbiology in the late 19th century. In 1888, Hellriegel and Wilfarth proofed the special case of nitrogen nutrition of legumes through their root nodule bacteria, which Beijerinck finally isolated in 1889. Also Winogradskyi's studies on chemolithotrophy and nitrifying bacteria occurred in this time. As agricultural practice was in close contact to the new achievements of basic sciences, the challenge quickly arose whether at all, and how, these discoveries of the – at that time – very young science ‘soil bacteriology’ could be applied in the field. In 1885, the young scientist Lorenz Hiltner graduated from the University of Erlangen in Frankonia, Germany, and focused his scientific interests in plant sciences and soil microbiology. He joined the ‘Plant Physiological Research Station Tharandt’ near Dresden, Saxonia, headed by Professor Dr F. Nobbe. In his doctoral thesis, he worked on aspects of pathogen–root interactions. In 1887–88 Hiltner was certainly fascinated by the findings of Hellriegel and Wilfarth about the special case of nitrogen nutrition in legumes. Together with Professor Nobbe, intensive studies were conducted about the nature of the symbiotic interaction of nodule bacteria and legume roots. Apparently during these studies, Hiltner became aware of the importance of the ecological interactions in the root zone. Together with his teacher he developed the first inoculant based on root nodule bacteria for agricultural practice which they called ‘Nitragin’ in 1890. After several improvements and the discovery of the specificity of the Rhizobium–legume symbiosis, the inoculant worked very reliably and was successfully introduced into agricultural practice. In 1902, Dr Lorenz Hiltner (1862–1923) became the founding director of the Royal Bavarian Agriculture-Botanical Institute in Munich and also Professor of Soil Bacteriology at the Technical University of Munich. In his presentation at a meeting of the German Agricultural Society in 1904 in Eisenach, Thuringia, Germany, he clearly stated the general importance of plant–microbe interactions in the root zone for plant nutrition as well as for plant health. The international ‘Rhizosphere Congress 2004’ held in Munich, 12–17 September 2004, commemorated the 100 year anniversary of Lorenz Hiltner's definition of the Rhizosphere concept in 1904. As rhizosphere research is a true interdisciplinary field of biological and soil sciences, the presentations at the conference were dealing with new scientific findings and cutting-edge technologies in soil chemistry and physics as well as modelling and the molecular biological and functional genomic approaches of plant–microbe interactions. This special ‘Rhizosphere Issue’ of Environmental Microbiology presents some of the microbiological contributions at this conference but also research papers independent of this conference. The scope of the papers is very broad and concerns, for example, community studies of rhizosphere bacteria in different plant systems or detailed studies of the molecular interaction of microbes and plant roots, including signalling with quorum-sensing molecules or plant hormones. The special issue deals with phytopathogenic as well as symbiotic or plant growth-promoting bacteria and fungi. In addition, features of root endophytic bacteria as well as endofungal bacteria in ectomycorrhizae are presented. The understanding of the functional diversity and the underlying mechanisms of interactions between rhizosphere microbes and roots are the basis for further implementation and improvements of rhizosphere biotechnologies in agriculture and bioremediation.