Rhizosphere fungal communities are influenced by Senecio jacobaea pyrrolizidine alkaloid content and composition

The contents and the compositions of the pyrrolizidine alkaloid (PA) complex of ragwort (Senecio jacobaea L.) were examined as potential drivers of fungal community structure in the rhizosphere. S. jacobaea plants within the coastal sand dune reserve of Meijendel (the Netherlands) were assayed for concentration and composition of PAs in roots. Rhizosphere soil was collected from pre-flowering plants, which differed up to 8-fold in PA production, and represented both jacobine and senecionine/seneciphylline chemotypes. Bulk soil samples from the same site were also collected for comparative examination. A culture-independent approach, involving direct DNA isolation, PCR of fungal 18S rRNA genes, and denaturing gradient gel electrophoresis (DGGE), was applied to compare the fungal communities of plants with different PA contents, as well as differences between bulk and rhizosphere samples. Cluster analysis of PCR-DGGE profiles revealed no clear evidence for PA-induced selection of specific fungal communities. However, canonical variance analysis showed that fungal communities associated with high-PA jacobine chemotypes could be discriminated from low PA samples and from the senecionine/seneciphylline chemotypes. The diversity of DGGE banding patterns, both in terms of band number and evenness, showed a trend toward lower diversity in the rhizosphere of high-PA plants as compared to low-PA plants and bulk soil. These results indicate that PA chemotypes of S. jacobaea differ in their influence on soil-borne fungal communities, with jacobine-containing plants exerting a greater selection in the rhizosphere than plants containing senecionine/seneciphylline.