Rhizosphere Colonization: Molecular Determinants from Plant-Microbe Coexistence Perspective

Inoculation of plants with beneficial microbes can be traced back for centuries. Although bacteria were not proven to exist until in 1683 von Leeuwenhoek discovered microscopic ‘animals’ under the lens of his microscope, their utilisation to stimulate plant growth in agriculture has been exploited since ancient times. Theophrastus (372-287 BC) suggested the mixing of different soils as a means of ‘remedying defects and adding heart to the soil’ (Vessey 2003). Colonisation of plant root system is the very first step in nearly all interactions between plants and soil borne microbes. The importance of this compartment for plant growth and soil microbiology had already been realised in the very pioneer times of microbiology in the late 19th century. In 1888, Hellriegel and Wilfarth proved the special case of nitrogen nutrition of legumes through their root nodule bacteria, which Beijerinck finally isolated in 1889. In 1887–1888 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 (Nobbe and Hiltner 1896). 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. 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. Region of contact between root and soil where soil is affected by roots was designated as “rhizosphere” by Hiltner (1904).