Abstract
The Bradyrhizobium sp. DOA9 strain isolated from a paddy field has the ability to nodulate a wide spectrum of legumes. Unlike other bradyrhizobia, this strain has a symbiotic plasmid harboring nod, nif, and type 3 secretion system (T3SS) genes. This T3SS cluster contains all the genes necessary for the formation of the secretory apparatus and the transcriptional activator (TtsI), which is preceded by a nod-box motif. An in silico search predicted 14 effectors putatively translocated by this T3SS machinery. In this study, we explored the role of the T3SS in the symbiotic performance of DOA9 by evaluating the ability of a T3SS mutant (ΩrhcN) to nodulate legumes belonging to Dalbergioid, Millettioid, and Genistoid tribes. Among the nine species tested, four (Arachis hypogea, Vigna radiata, Crotalaria juncea, and Macroptilium atropurpureum) responded positively to the rhcN mutation (ranging from suppression of plant defense reactions, an increase in the number of nodules and a dramatic improvement in nodule development and infection), one (Stylosanthes hamata) responded negatively (fewer nodules and less nitrogen fixation) and four species (Aeschynomene americana, Aeschynomene afraspera, Indigofera tinctoria, and Desmodium tortuosum) displayed no phenotype. We also tested the role of the T3SS in the ability of the DOA9 strain to endophytically colonize rice roots, but detected no effect of the T3SS mutation, in contrast to what was previously reported in the Bradyrhizobium SUTN9-2 strain. Taken together, these data indicate that DOA9 contains a functional T3SS that interferes with the ability of the strain to interact symbiotically with legumes but not with rice.
Highlights
The establishment of the legume–rhizobium symbiosis relies on the exchange of diffusible signal molecules between the two partners
This genetic organization is similar to that found in other Bradyrhizobium or Sinorhizobium strains and a very high level of AA identity was observed with the corresponding homologous proteins
As described in other rhizobia, we could assume that this main cluster is under the dual control of the transcriptional activator (TtsI) and NodD since a ttsI homolog was found at the vicinity of the main operon preceded by a nod-box (Figure 1A)
Summary
The establishment of the legume–rhizobium symbiosis relies on the exchange of diffusible signal molecules between the two partners. Besides NFs, other bacterial components are important for the successful establishment of the symbiosis, including exopolysaccharides (EPS), lipopolysaccharides (LPS), capsular polysaccharides (KPS), and cyclic β-glucans (Gibson et al, 2008; Janczarek et al, 2015). These bacterial compounds can act as a symbiotic signal or permit to avoid plant immune responses (Gourion et al, 2015; Kawaharada et al, 2015). Some rhizobia use a type 3 secretion system (T3SS), which shares homology with the T3SS apparatus of plant and animal pathogenic bacteria, to translocate some putative effectors into the host cell, named nodulation outer proteins (Nops). Secreted Nop proteins can be recognized by plant receptor proteins [resistant (R) proteins] leading to effector-triggered immunity, which blocks infection and nodulation (Yang et al, 2010; Yasuda et al, 2016)
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