Bacterial cell-cell inter-species signaling in the plant endosphere

Wang, Yixu (2023). Bacterial cell-cell inter-species signaling in the plant endosphere. PhD thesis The Open University.



The plant microbiome comprises a diverse microbial community from various reservoirs, including the rhizosphere, phyllosphere, and endosphere. These microorganisms establish complex associations with plants and play crucial roles in promoting plant productivity and health in natural environments. The endosphere, consisting of the inner plant tissues, is colonized by microbial endophytes, including bacteria and fungi, which have been found in all thus far analyzed plant species. Plants lacking endophytes would exhibit reduced resistance to pathogens and heightened susceptibility to abiotic stress conditions.

This thesis is organized as follows: Chapter I is a general introduction for introducing bacterial endophytes, the endosphere microbiome, and cell-cell- interactions in microbial communities. Chapter Ⅱ focuses on the development of multi-strain consortia using beneficial endophytic bacteria isolated from rice roots. These consortia were tested for co-existence in vitro and in planta and some demonstrated enhanced plant-growth promotion (PGP) through synergistic interactions. The identification of two 5-strain consortia and a 2-strain consortium exhibiting positive interactions opens new possibilities for agricultural applications. Utilizing multi-strain consortia offers increased functional diversity, leading to improved plant growth and biocontrol, presenting a promising strategy beyond single-strain inoculants. Chapter III explores metabolic interactions between the two-bacterial strain consortia of Pseudomonas fulva AG1028 and Bacillus megaterium AG190, which have been demonstrated to display plant growth-promoting effects in Chapter II. Metabolome analysis reveals distinct metabolite profiles for each strain, indicating their involvement in different metabolic pathways. Co-culturing these strains enhances the production of differential metabolites, suggesting the existence of a cooperative metabolic network. Additionally, the study highlights the potential role of B. megaterium AG190 in regulating the biofilm formation of P. fulva AG1028. The observed enhanced swarming motility and enriched bacterial chemotaxis in the co-culture emphasize the cooperative behavior between the strains. Understanding the mechanisms and implications of these metabolic interactions holds promise for harnessing their potential to promote plant growth. Chapter IV investigates the presence and distribution of AHL-quorum sensing (QS) systems among proteobacterial endophytes and uses Pseudomonas fluorescens L111 as a bacterial-study model. The study demonstrates the involvement of quorum sensing in both rhizosphere and endosphere colonization by P. fluorescens L111. However, the presence or absence of AHLs does not significantly affect the microbiome composition, indicating the role/influence of other factors. Nevertheless, AHL-QS systems play a role in root colonization and potentially impact bacterial population dynamics during rice growth. Finally, Chapter V is a general discussion of all experimental chapters unifying results and providing future directions.

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