Endophytic lifestyle of an insect pathogen causes isolate-specific differences in the development of the host plant but not in its herbivores

The fungus Beauveria bassiana is a soil-borne pathogen of insects and a commercial biopesticide. However, only in recent years it has been established that B. bassiana can infect plants in which they live as an endophyte without causing disease symptoms. To date, little is known how plants respond to B. bassiana infection, although in some cases fungal colonisation may cause enhanced insect resistance. In this study we assessed the effects of two B. bassiana isolates (FRh2 and BG11) on the development of Arabidopsis thaliana Col-1 and the performance of a leaf-chewing (Plutella xyllostella) and a phloem-sucking herbivore (Myzus persicae) when feeding on endophyte-infected plants. Both fungal strains were isolated in New Zealand and inoculated by root-dipping for systemic infection. Our results showed that the B. bassiana isolates differed in their impacts on the plant’s phenology measured on the BBCH scale. When inoculated with BG11, A. thaliana developmental time was attenuated leading e.g. to delayed flowering when compared to FRh2 and controls.  Also germination rate of F1 seeds was lower in BG11 plants. However, plants colonized by this strain showed significantly increased biomasses. Despite these apparent changes in host plant phenology, both herbivore species were not affected in neither growth (P. xylostella) nor offspring production (M. persicae) suggesting that the endophytic lifestyle of the studied B. bassiana isolates does not translate into higher plant resistance. Preliminary data show that the fungus did not lose its pathogenicity during plant colonisation since infection of P. xylostella with re-isolated B. bassiana caused significant mortality. The ecological role of endophytic entomopathogens and their value for plant protection needs further research.