BI- AND TRICYCLOALTERNARENES PRODUCED BY THE PHYTOPATHOGENIC FUNGUS ALTERNARIA ALTERNATA
B LIEBERMANN1, RP NUSSBAUM1, R ELLINGER1 and W GUENTHER2
1Institute of Pharmacy, and 2Institute of Organic and Macromolecular Chemistry, University of Jena, 07743 Jena, Germany
Background and objectives
The deuteromycete Alternaria alternata (Fries) Keissler is both a plant pathogen and a post-harvest decay organism of vegetables and fruits. The fungus produces a broad spectrum of secondary metabolites, which act either as host-specific or as non-specific phytotoxins. Host-specific toxins are particularly involved in phytopathogenesis.
Kono et al.  isolated from the culture filtrate of A. citri (a pathotype of A. altemata) several ACTG toxins that cause damage on particular Citrus species. At this time, ACTG toxins were regarded as host-specific. We have now isolated a multitude of related compounds, named bicycloalternarenes (BCAS) and tricycloalternarenes (TCAS) , respectively, from a strain of A. alternata that does not orginate from Citrus but from Brassica sinensis.
Materials and methods
The strain was cultivated in phosphate-deficient medium or modified Richard's solution (still culture, 28°C, 7-14 days). BCAs and TCAs were obtained from the culture filtrate by the usual isolation procedures (continuous ether and solid-phase extraction, chromatography on Sephadex G-15 and silica gel, preparative HPLC chromatography). Structure elucidation was performed by IR-, UV-, MS- and NMR-spectroscopy.
Results and conclusions
A total of 11 BCAs were isolated, among them nine novel compounds and two substances previously reported as host-specific ACTG major toxins from A. citri. Furthermore, 19 TCAs were identified, among them 15 new substances and three compounds known as host-specific ACTG minor toxins from A. citri. In our opinion, bi- and tricycloalternarenes cannot be denoted as host-specific toxins, since the strain used orginates from Brassica sinensis and has been in culture for 25 years in order to produce other bioactive substances. Moreover, BCA 2 induces effects similar to those shown by jasmonic acid in a bioassay with Spirodela polyrhiza.
BCAs are found as genuine compounds in the slightly alkaline culture filtrate. Upon acidification a ring closure occurs rapidly, and two isomeric TCAs orginate from one BCA. This reaction occurs in vitro but certainly also in the plant-pathogen interaction. In this way the BCAs which are strongly active against Citrus species can be detoxified to weakly active TCAS. The structures of both compounds are indicative of a mixed terpenoid. Investigation of the biosynthesis of BCAs is under way.
1. Kono Y, Gardner J, Suzuki Y, Takeuchi S, 1986. Agricultural and Biological Chemistry 50, 1597-1606.
2. Liebennann B, Ellinger R, Guenther W et al., 1997. Phytochemistry 46, 297-303.