Beneficial bacteria, particularly the Pseudomonas spp., are known to produce a plethora of bioactive compounds, including cyclic lipopeptides (CLPs), which are well known for their antimicrobial activities. Yet, the roles of CLPs in plant-bacterial interactions, especially in natural environments, are not fully understood. This thesis includes four manuscripts that elucidate the important role of CLP viscosin in bacterial colonization, microbiome assembly, and drought stress.
In the first manuscript, the study employs viscosin-producing Pseudomonas fluorescens SBW25 as well as viscosin-producing mutant strains inoculated with wheat to elucidate the impact of viscosin on bacterial root colonization and rhizoplane microbiome assembly in different wheat genotypes. Through quantitative PCR analysis and 16S and 18S rRNA gene amplicon sequencing, it was shown that viscosin improves colonization potential and has a significant effect on the assembly of both bacterial and protist communities at the root-soil interphase. This effect was shown to be plant cultivar dependent and pinpoints the importance of microbe-microbe interactions in colonization of plant roots.
The second manuscript addresses the critical issue of drought stress, which poses formidable challenges to agriculture. While plant drought resistance has been extensively studied through genetic modification, the potential of utilizing high-quality bacteria within natural soil ecosystems for drought tolerance remains underexplored. In this manuscript, the colonization of inoculant bacteria on the rhizoplane prior to, during, and post drought was evaluated using quantitative PCR (qPCR), with plants being inoculated with CLP-viscosin producer P. fluorescens SBW25 and viscosindeficient mutant strains. We also analyzed plant responses to inoculants under drought conditions. In addition, this study examined the diversity and composition of nonribosomal peptide synthetase (NRPS) synthesis-related gene clusters and bacterial communities in the rhizosplane through the sequencing of 16S rRNA and NRPS synthesis-related adenylation (A) domain amplicons. The findings reveal that SBW25 exhibited superior colonization of the plant rhizoplane relative to the mutant strain during drought stress, and its abundance positively coincided with the induction of drought tolerance enzymes. Furthermore, drought stress increases bacterial diversity without altering the diversity within the A domain. Inoculation treatments further enhance bacterial diversity under drought stress. Both inoculation and drought stress had a significant impact on the bacterial community structure and the composition of the A domain.
The third manuscript characterizes the root volatiles of two wheat cultivars, Heerup and Sheriff, and analyzes the modulation of these emissions in response to drought stress. The results showed that the Heerup and Sheriff cultivars exhibited significant differences in the amount and type of volatiles released. In addition, drought stress altered the emission of volatile compounds from the roots of the Sheriff cultivar. Some alkenes, namely dotriacontane, hexatriacontane, 11-decyltetracosane, and tritetracontane, showed a significant increase in emissions under drought stress. This has implications for understanding plant communication and defense mechanisms, especially under abiotic stress conditions.
The fourth manuscript focuses on understanding fundamental interactions at the plant-soil interface, which is critical for leveraging natural microbial communities to bolster crop resilience. Through 16S amplicon sequencing data from rhizocompartments of four wheat cultivars, it is evident that both rhizocompartments and cultivar selection significantly impact community composition.
In summary, this thesis describes the effects of rhizocompartments and cultivar on microbial communities, revealing the role of synthetic viscosin in the colonization potential of different wheat cultivars, their impact on microbiome assembly. Also the effect of viscosin on colonization capacity and on bacterial community and bacterial secondary metabolite composition under drought stress. The effect of drought stress on plant root volatiles was also described. These results provide insights for the effective use of bacterial metabolites for potential applications in sustainable agriculture.