Lorenz Hiltner, a pioneer in rhizosphere microbial ecology and soil bacteriology research

Lorenz Hiltner is recognized as the first scientist to coin the term rhizosphere in 1904. His scientific career and achievements are summarized in this essay. Most of his research he performed in the Bavarian Agriculture-Botanical Institute (later named the Bavarian Institute of Plant Growth and Plant Protection) in Munich, where he was the director from 1902 to 1923. Beginning with intensive and thorough investigations on the germination and growth of different crop plants (legumes and non-legumes) Hiltner became convinced, that root exudates of different plants support the development of different bacterial communities. His definition of the rhizosphere in the year 1904 centered on the idea, that plant nutrition is considerably influenced by the microbial composition of the rhizosphere. Hiltner observed bacterial cells even inside the rhizodermis of healthy roots. In analogy with fungal root symbionts, Hiltner named the bacterial community that is closely associated with roots bacteriorhiza. In his rhizosphere concept, Hiltner also envisioned, that beneficial bacteria are not only attracted by the root exudates but that there are also uninvited guests, that adjust to the specific root exudates. Based on his observations he hypothesized that the resistance of plants towards pathogenesis is dependent on the composition of the rhizosphere microflora. He even had the idea, that the quality of plant products may be dependent on the composition of the root microflora. In addition to his scientific achievements, Hiltner was very dedicated to applied work. Together with F. Nobbe he had the first patent on Rhizobium inoculants (Nitragin). He continuously improved formulations and the effectivity of the Rhizobium preparations and he also initiated seed dressing with sublimate for plant protection of seedlings. Thus, Hiltner tightly linked breakthroughs in basic research to improved rhizosphere management practices. In addition, he wrote a pioneering monograph on plant protection for everybody's practical use. His emphasis on understanding microbes in the context of their micro-habitat, the rhizosphere, made him a pioneer in microbial ecology. Even now, in the era of genome and postgenome analysis with our better understanding of plant nutrition and soil bacteriology, his ideas and contributions are as fresh as they were more than 100 years ago.