1Institut fur Mikrobiologie, Technische Universitat Braunschweig, Spiehnannstrasse 7, D-38106 Braunschweig, Germany; 2Fachbereich Chemie und Chemietechnik der Universitat-GH-Paderbom, Warburger Strasse 100, D-33098 Paderbom, Germany

Background and objectives
Despite the increasing importance of combinatorial chemistry, fungal isolates continue to be screened for novel secondary metabolites to meet the demand for new compounds by the pharmaceutical and agrochemical industries. In contrast to the synthetic substances, those produced by living organisms can be assumed to have been selected to play a role in nature. This assumption means that the secondary metabolites produced by fungi should vary with their ecological niche. We have screened fungal isolates from soils and plants and compared their ability to produce novel secondary metabolites in culture.

Results and conclusions
Fungi were isolated from soils, both extreme and temperate habitats, and from diseased and healthy plants onto antibiotic agar media. Plant tissues were surface sterilized to obtain fungi growing within both healthy and diseased plants. Of the 5500 fungal strains screened for their antibacterial, antifungal and herbicidal activity, 87% of the endophytic isolates inhibited growth of at least one of the test organisms, whereas only 53% of the phytopathogens, 57% of the epiphytes and 70% of the soil isolates were active in one of the tests. We also found that whereas 57% of the endophytes were herbicidally active, only 27% of the phytopathogens and 18% of the soil isolates were. These results not only show that, as a group the highest proportion of endophytic strains were biologically active, but also substantiate the hypothesis that the production of secondary metabolites varies with ecological niche.

In order to obtain a maximum proportion of novel secondary metabolites, the following criteria were applied for the choice of the fungal strains and their cultivation for the isolation of the active substances. (1) Although not suitable for industrial throughput screening, culture was on semi-solid agar media at room temperature, where fungi produce more metabolites than in liquid culture. (2) The taxa of the fungal isolates were, if possible, determined; these were compared with entries in a data bank of secondary metabolites to exclude redundant discoveries of compounds. (3) The metabolic profile of the fungal culture extract as revealed by thin-layer chromatography (TLC) yielded either a large number and /or high concentrations of the substances. Bioassays of the TLCs revealed herbicidal or fungicidal metabolites.

As a group, the endophytes yielded the highest proportion of novel structures: 50% of the structures determined from the endophytic culture extracts were new compared with only 33% of those from cultures of soil isolates. In addition, 95% of the metabolites isolated from endophytic cultures were herbicidally active. Not only do these results show that endophytes are a good source of biologically active novel compounds, but they also suggest that they are synthesized in situ. Possible roles that these metabolites might play in vivo are: (1) a weakening of the host tissue and/or defense to better enable infection, (2) an increase in the membrane permeability of the host, permitting a better nutrient supply for the endophyte, (3) in a senescent or weakened host, degradation of host tissue allowing the endophyte to grow as a nectrotroph.