Abstract
Exopolysaccharides (EPSs) are sugar-based polymers that are synthesized and secreted by many microorganisms. It has been suggested that microbial EPSs are involved in the establishment of symbiotic and pathogenic associations of microbes with their hosts. In the legume-rhizobia symbiosis, the recognition of compatible rhizobia EPS is essential for root nodules symbiosis (RNS) and previous studies showed that perception of EPS was important for endophytic nodule colonization. Albeit current studies suggest that microbial EPSs are species or even strain-specific, the capacity and type of EPS produced by most microorganisms remain largely unknown. Consequently, it is difficult to dissect the impact of bacterial EPS on root-associated microbiota establishment from the microbial side.
However, the availability of Lotus mutants impaired in bacterial EPS recognition provides an opportunity to dissect the roles of bacterial EPS in the establishment of root-associated microbiota. In this study, I have performed genome mining for the commensal bacteria of Lotus japonicus from LJSPHERE and identified bacterial genes predicted to encode enzymes involved in EPS production. Next, in vitro assays were used to characterize the EPS phenotype of Rhizobiales and Burkholderiales strains. These studies revealed that root commensals varied in their genetic and phenotypic ability to produce and secrete EPS. Next, I took a plant genetics-based approach to characterize the impact of EPS on root and rhizosphere microbiota assembly. Wild type and mutants of the model legume Lotus japonicus have been grown in natural soil and then bacterial and fungal communities from their roots and rhizosphere were characterized. We then employed gnotobiotic reconstitution experiments using whole-genome sequenced bacteria from the LjSPHERE culture collection to explore whether the culture-independent patterns observed in natural soil can be recapitulated. Overall, the data from these two experimental systems showed a broad overlap, and enabled dissection of the impact of plant genes and the perception of bacterial EPS for association with specific members of microbiota. These studies provide a stepping stone for further studies aimed at understanding how microbial EPS enable root and rhizosphere colonization.
However, the availability of Lotus mutants impaired in bacterial EPS recognition provides an opportunity to dissect the roles of bacterial EPS in the establishment of root-associated microbiota. In this study, I have performed genome mining for the commensal bacteria of Lotus japonicus from LJSPHERE and identified bacterial genes predicted to encode enzymes involved in EPS production. Next, in vitro assays were used to characterize the EPS phenotype of Rhizobiales and Burkholderiales strains. These studies revealed that root commensals varied in their genetic and phenotypic ability to produce and secrete EPS. Next, I took a plant genetics-based approach to characterize the impact of EPS on root and rhizosphere microbiota assembly. Wild type and mutants of the model legume Lotus japonicus have been grown in natural soil and then bacterial and fungal communities from their roots and rhizosphere were characterized. We then employed gnotobiotic reconstitution experiments using whole-genome sequenced bacteria from the LjSPHERE culture collection to explore whether the culture-independent patterns observed in natural soil can be recapitulated. Overall, the data from these two experimental systems showed a broad overlap, and enabled dissection of the impact of plant genes and the perception of bacterial EPS for association with specific members of microbiota. These studies provide a stepping stone for further studies aimed at understanding how microbial EPS enable root and rhizosphere colonization.
| Original language | English |
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| Publisher | Århus Universitet |
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| Number of pages | 192 |
| Publication status | Published - Mar 2022 |