Defence Reactions in Roots Elicited by Endofungal Bacteria of the Sebacinalean Symbiosis

Abstract

The Alphaproteobacterium Rhizobium radiobacter F4 (RrF4) was orig-inally detected as an endofungal bacterium associated with the endophytic basid-iomycete Piriformospora indica that forms a beneficial symbiosis with a wide rangeof green plants. While attempts to cureP. indicafromRrF4 repeatedly failed, thebacterium could be isolated and grown in pure culture. In contrast to some otherendofungal bacteria, the genome size ofRrF4 is not reduced. Instead, it shows ahigh degree of similarity to the plant pathogenic R.radiobacter(formerly:Agrobacterium tumefaciens) C58, except vibrant differences in both the tumor-inducing (pTi) and the accessor (pAt) plasmids, which can explain the loss ofRrF4’s pathogenicity. Similar to its fungal host,RrF4 colonizes plant roots withouthost preference and forms aggregates of attached cells and dense biofilms at the rootsurface of maturation zones.RrF4-colonized plants show increased biomass andenhanced resistance against bacterial and fungal leaf pathogens. Resistance medi-ated byRrF4 is dependent on the plant’s jasmonate-based induced systemic resis-tance (ISR) pathway while the systemic acquired resistance (SAR) pathway is non-operative as shown by genetic analysis. Based on these findings we concluded thatRrF4- andP. indica-induced pattern of defence gene expression are similar.However, in clear contrast toP. indica, but similar to plant growth promotingrhizobacteria (PGPR),RrF4 colonized not only the root outer cortex but spreadbeyond the endodermis into the stele. Based on our findingsRrF4 is an efficientplant growth promoting bacterium.