Adaptation of Bacillus thuringiensis to plant colonization affects differentiation and toxicity

et al. 2021

Preprint

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The soil ubiquitous Bacillus subtilis is known to promote plant growth and protect plants against disease. These characteristics make B. subtilis highly relevant in an agricultural perspective, fueling the interest in studying B. subtilis-plant interactions. Here, we employ an experimental evolution approach to explore adaptation of B. subtilis to Arabidopsis thaliana roots. B. subtilis rapidly adapts to the plant root environment, as evidenced by improved root colonizers observed already after 12 consecutive transfers between seedlings in a hydroponic setup. Further phenotypic characterization of evolved isolates from transfer 30 revealed that increased root colonization was associated with robust biofilm formation in response to the plant polysaccharide xylan. Additionally, several evolved isolates across independent populations were impaired in motility, a redundant trait in the selective environment. Interestingly, two evolved isolates outcompeted the ancestor during competition on the root but suffered a fitness disadvantage in non-selective environment, demonstrating an evolutionary cost of adaptation to the plant root. Finally, increased root colonization by a selected evolved isolate was also demonstrated in the presence of resident soil microbes. Our findings provide novel insights into how a well-known PGPR rapidly adapts to an ecologically relevant environment and reveal evolutionary consequences that are fundamental to consider when evolving strains for biocontrol purposes.

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

Experimental evolution ofBacillus subtilisonArabidopsis thalianaroots reveals fast adaptation and improved root colonization in the presence of soil microbes

Nordgaard

Blake

et al. 2021

Preprint

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“…Previously, we have investigated the evolution of B. thuringiensis 407 biofilms on plants by repeated selection for root-associated biofilm cycles 30 . The bead biofilm model provides a simpler, abiotic selection system to reveal how adaption to the biofilm life cycle influences bacterial evolution.…”

Section: Discussionmentioning

confidence: 99%

“…Similarly, reduced spatial heterogeneity or hampered motility can also select for higher matrix production 27 . Recent works started to exploit Bacilli to understand how colonization of a plant host influence bacterial evolution [28][29][30] .…”

Section: Introductionmentioning

confidence: 99%

Adaptation and phenotypic diversification ofBacillus thuringiensis407 biofilm are accompanied by a fuzzy spreader morphotype

Yang

Strube

et al. 2021

Preprint

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Bacillus cereus group (Bacillus cereus sensu lato) has a diverse ecology, including various species that produce biofilms on abiotic and biotic surfaces. While genetic and morphological diversification enable the adaptation of multicellular communities, this area remains largely unknown in the Bacillus cereus group. In this work, we dissected the experimental evolution of Bacillus thuringiensis 407 Cry- during continuous recolonization of plastic beads. We observed the evolution of a distinct colony morphotype that we named fuzzy spreader (FS) variant. Most multicellular traits of the FS variant displayed higher competitive ability versus the ancestral strain, suggesting an important role for diversification in the adaptation of B. thuringiensis to the biofilm lifestyle. Further genetic characterization of FS variant revealed the disruption of a guanylyltransferase gene by an insertion sequence (IS) element, which could be similarly observed in the genome of a natural isolate. The evolved FS and the deletion mutant in the guanylyltransferase gene (Bt407ΔrfbM) displayed similarly altered aggregation and hydrophobicity compared to the ancestor strain, suggesting that adaptation process highly depends on the physical adhesive forces.

“…We established an experimental evolution approach inspired by the bead transfer model of Poltak & Cooper (2011) to study B. subtilis evolution on plant roots, similar to the setup exploited for Bacillus thuringiensis (Lin et al ., 2020). Therefore, every other day, a previously colonized plant seedling was transferred to a new sterile seedling, allowing only bacterial cells that attach to the new root to continue in the experiment, enabling selection for a continuous cycle of dispersal, chemotaxis towards the plant root, and biofilm formation on the root.…”

Section: Methodsmentioning

confidence: 99%

Diversification ofB. subtilisduring experimental evolution onA. thalianaand the complementarity in root colonization of evolved subpopulations

Blake

Nordgaard

et al. 2021

Preprint

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The soil bacterium Bacillus subtilis is known to suppress pathogens as well as promote plant growth. However, in order to fully exploit the potential as natural fertilizer, we need a better understanding of the interactions between B. subtilis and plants. Here, B. subtilis was examined for root colonization through experimental evolution on Arabidopsis thaliana. The populations evolved rapidly, improved in root colonization and diversified into three distinct morphotypes. In order to better understand the adaptation that had taken place, single evolved isolates from the final transfer were randomly selected for further characterization, revealing changes in growth and pellicle formation in medium supplemented with plant polysaccharides. Intriguingly, certain evolved isolates showed improved root colonization only on the plant species they evolved on, but not on another plant species, namely tomato, suggesting A. thaliana specific adaption paths. Finally, synergism in plant root colonization was observed for a mix of all three morphotypes, as the mix performed better than the sum of its constituents in monoculture. Our results suggest, that genetic diversification occurs in an ecological relevant setting on plant roots and proves to be a stable strategy for root colonization.