Nordgaard, M. | bioRxiv Science

bioRxiv2023

Parallel genetic adaptation of Bacillus subtilis to different plant species

Plant growth-promoting rhizobacteria benefit plants by stimulating their growth or protecting them against phytopathogens. Rhizobacteria must colonise and persist on plant roots to exert their benefits. However, little is known regarding the processes by which rhizobacteria adapt to different plant species, or behave under alternating host plant regimes. Here, we used experimental evolution and whole-population whole-genome sequencing to analyse how Bacillus subtilis evolves on Arabidopsis thaliana and tomato seedlings, and under an alternating host plant regime, in a static hydroponic setup. We observed parallel evolution across multiple levels of biological organisation in all conditions, which was greatest for the two heterogeneous, multi-resource spatially-structured environments at the genetic level. Species-specific adaptation at the genetic level was also observed, possibly caused by the selection stress imposed by different host plants. Furthermore, a trade-off between motility and biofilm development was supported by mutational changes in motility- and biofilm-related genes. Finally, we identified several condition-specific and common targeted genes in different environments by comparing three different B. subtilis biofilm adaptation settings. The results demonstrate a common evolutionary pattern when B. subtilis is adapting to the plant rhizosphere in similar conditions, and reveal differences in genetic mechanisms between different host plants. These findings will likely support strain improvements for sustainable agriculture. Data summarySequencing data associated with this article are available in the CNGB Sequence Archive (CNSA) [1] of the China National GeneBank DataBase (CNGBdb) [2] under accession numbers CNP0002416 and CNP0003952. Strain data for the DK1042 ancestor are available under accession number CNP0002416. Impact statementFor rhizobacteria to benefit plant growth and protect against phytopathogens, bacteria must colonise and persist on plant roots. Understanding how rhizobacteria adapt to different plant species will assist strain development in sustainable agriculture. To explore the rhizobacterial adaptation process for different plant species and alternating host regimes, B. subtilis was experimentally evolved on A. thaliana or tomato roots, or an alternating host regime. Both parallel and species-specific adaptation was revealed at the genetic level. Analysis of the trade-off between motility and biofilm formation revealed several condition-specific and commonly targeted genes based on experimentally evolving B. subtilis biofilms.

microbiology↗

bioRxiv2021

Experimental evolution of Bacillus subtilis on Arabidopsis thaliana roots reveals fast adaptation and improved root colonization in the presence of soil microbes

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. We found that B. subtilis rapidly adapted to the plant root environment, as evidenced by improved root colonizers observed already after 12 consecutive transfers between seedlings in a hydroponic setup. In addition, two selected evolved isolates from independent populations from transfer 30 outcompeted the ancestor during root colonization. Re-sequencing of single evolved isolates and endpoint populations revealed mutations in genes related to different bacterial traits. Further, phenotypic characterization of evolved isolates from transfer 30 showed 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 suffered a fitness disadvantage in a 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 plant growth-promoting rhizobacterium rapidly adapts to an ecologically relevant environment and reveal evolutionary consequences that are fundamental to consider when evolving strains for biocontrol purposes.

microbiology↗