Abstract
Rhizobium radiobacter (syn. Agrobacterium tumefaciens, syn. “Agrobacterium fabrum”) is an endofungal bacterium of the fungal mutualist Piriformospora (syn. Serendipita) indica (Basidiomycota), which together form a tripartite Sebacinalean symbiosis with a broad range of plants. R. radiobacter strain F4 (RrF4), isolated from P. indica DSM 11827, induces growth promotion and systemic resistance in cereal crops, including barley and wheat, suggesting that R. radiobacter contributes to a successful symbiosis. Here, we studied the impact of endobacteria on the morphology and the beneficial activity of P. indica during interactions with plants. Low numbers of endobacteria were detected in the axenically grown P. indica (long term lab-cultured, lcPiri) whereas mycelia colonizing the plant root contained increased numbers of bacteria. Higher numbers of endobacteria were also found in axenic cultures of P. indica that was freshly re-isolated (riPiri) from plant roots, though numbers dropped during repeated axenic re-cultivation. Prolonged treatments of P. indica cultures with various antibiotics could not completely eliminate the bacterium, though the number of detectable endobacteria decreased significantly, resulting in partial-cured P. indica (pcPiri). pcPiri showed reduced growth in axenic cultures and poor sporulation. Consistent with this, pcPiri also showed reduced plant growth promotion and reduced systemic resistance against powdery mildew infection as compared with riPiri and lcPiri. These results are consistent with the assumption that the endobacterium R. radiobacter improves P. indica’s fitness and thus contributes to the success of the tripartite Sebacinalean symbiosis.
Highlights
Plant-microbe interactions have promoted the development and evolution of land plants
To exclude the possibility that the hyphal wall constituted a barrier for Fluorescence in situ hybridization (FISH) probes and conditioned the low number of fluorescent signals, the FISH method was applied on crushed hyphae (Figure 1C) and fungal protoplasts, which confirmed the low abundance of endobacteria
While the endobacterium detected in the mycelium was coccoid-shaped and much smaller in size, isolated radiobacter strain F4 (RrF4) cells detected by FISH showed rod-shaped cells with a mean size of 1.2–2.0 μm in length and 0.7–0.9 μm in width (Figure 1D)
Summary
Plant-microbe interactions have promoted the development and evolution of land plants. Rhizobacteria and mycorrhizal fungi are typical examples of mutualistic microbes. The biological nitrogen fixing Rhizobium spp. converts stable nitrogen gas into the biologically useful form ammonia as nitrogen source for the leguminous family (Long, 1989; Parniske, 2000; Oldroyd et al, 2011). To their nutritional benefits, arbuscular mycorrhizae fungi (AMF) induce resistance in plants against necrotrophic pathogens and insects through jasmonic acid and ethylene (JA-ET) dependent signaling pathways (Liu et al, 2007; Pozo and Azcón-Aguilar, 2007). Beneficial effects on plants have a high relevance in natural and agricultural ecosystems because of the reduced need of industrial fertilizer in agricultural soils (Yang et al, 2009; Weyens et al, 2009)
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