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

Abstract

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.

Arabidopsis thaliana induces multigenerational stress tolerance against biotic and abiotic stressors and memorization of host colonization in Bacillus subtilis

“…It has long been known that bacterial cells that have experienced different environmental histories may respond differently to current conditions 66 . In addition, genetic diversification of B. subtilis into morphotypes also plays an important role in evolution during plant root colonization 25 . Considering the complex nature of plant-bacillus interactions, history-dependent behavioral differences may be physically necessary consequences of the prior interaction, as they may stabilize symbiosis by providing ongoing benefits to the interacting bacteria.…”

“…The plant can also induce long-term adaptations: Bacteria that were repeatedly cultured with A. thaliana evolved rapidly, to generate morphotypes with improved root colonization. In this experimental system, Bacillus subtilis cells diversified into three distinct morphotypes altered in their growth and pellicle formation in a medium supplemented with plant polysaccharides 18 .…”

“…Plants (representing biotic hosts and their surfaces) were reported to induce long-term adaptations in bacteria through a genetic selection of distinct morphotypes: B. subtilis repeatedly cultured with A. thaliana evolved rapidly to generate morphotypes with improved root colonization. In this experimental system, the bacterial cells diversified into three distinct morphotypes altered in their growth and pellicle formation in a medium supplemented with plant polysaccharides 25 (Blake et al, 2021). Whether symbiosis or interaction with biotic surfaces induces more transient multigenerational responses and whether these long-term adaptations provide quantifiable fitness advantage remain unknown.…”

Genetic Circuit Design in Rhizobacteria

“…Alternatively, experimental evolution strategies could be used to select for highcolonizing strains that persist in the rhizosphere over multiple inoculation/selection cycles. This approach has evolved Pseudomonas and Bacillus species with elevated root colonization through mutations in genes encoding global regulators, biofilm development, and motility machinery [107][108][109][110]. Refining genetic design rules for root colonization should further our ability to spatiotemporally control circuit functioning and transfer colonization phenotypes into potentially beneficial bacteria that do not natively colonize plants.…”

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

“…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] .…”