Detection and Characterization of Endobacteria in the Fungal Endophyte Piriformospora indica

Abstract

The nonpathogenic Alphaproteobacterium Rhizobium radiobacter (syn. Agrobacterium tumefaciens) (RrF4) is a subculture of the endobacterium R. radiobacter, which is intricately associated with its host, the beneficial plantcolonizing fungal basidiomycete Piriformospora indica. RrF4 is genetically very similar to the well-studied plant pathogenic R. radiobacter biovar I strain C58 (genomovar G8). Highly similar genetic content of RrF4 and C58 denotes the high potential for RrF4 to directly interact with plants. The failure to cure P. indica from its endobacterium still hampers a conclusive prediction of the bacterium's extended role in the interaction of the fungus with a broad spectrum of host plants. However, beneficial activities shown in cereals and the Brassicaceae Arabidopsis thaliana were hardly distinguishable when induced either by RrF4 or P. indica. We discuss here the various strategies employed to detect, characterize, and eventually elucidate the endobacterium's role in the tripartite symbiosis with its fungal host and a plant.