Studies of plant colonisation by closely related Bacillus amyloliquefaciens biocontrol agents using strain specific quantitative PCR assays

Genomic content of wheat has a higher influence than plant domestication status on the ability to interact with Pseudomonas plant growth-promoting rhizobacteria

Plant evolutionary history has had profound effects on belowground traits, which is likely to have impacted the ability to interact with microorganisms, but consequences on root colonization and gene expression by plant growth-promoting rhizobacteria (PGPR) remain poorly understood. Here, we tested the hypothesis that wheat genomic content and domestication are key factors determining the capacity for PGPR interaction. Thus, 331 wheat representatives from eight Triticum or Aegilops species were inoculated under standardized conditions with the generalist PGPR Pseudomonas ogarae F113, using an autofluorescent reporter system for monitoring F113 colonization and expression of phl genes coding for the auxinic inducing signal 2,4-diacetylphloroglucinol. The interaction with P. ogarae F113 was influenced by ploidy level, presence of genomes AA, BB, DD, and domestication. While root colonization was higher for hexaploid and tetraploid species, and phl expression level higher for hexaploid wheat, the diploid Ae. tauschii displayed higher phl induction rate (i.e., expression:colonisation ratio) on roots. However, a better potential of interaction with F113 (i.e., under non-stress gnotobiotic conditions) did not translate, after seed inoculation, into better performance of wheat landraces in non-sterile soil under drought. Overall, results showed that domestication and especially plant genomic content modulate the PGPR interaction potential of wheats.

Species-Level Monitoring of Key Bacteria in Fermentation Processes Using Single-Nucleotide Resolved Nucleic Acid Assays Based on CRISPR/Cas12

Microorganisms can determine the flavor and quality of fermented food, such as Baijiu, which is produced via Daqu fermentation. Therefore, monitoring key microorganisms during fermentation is important for ensuring high-quality fermented food. Here, we report a single-nucleotide resolved nucleic acid assay based on the CRISPR/Cas12 system, enabling the quantification of Bacillus amyloliquefaciens, a key microorganism in Daqu fermentation at the species level. The assay employs an amplification-refractory mutation system derived from PCR to analyze minor genetic differences between different Bacillus species. The utilization of CRISPR/Cas12 further guaranties the specificity of identifying the PCR amplicon and enables the quantification of Bacillus amyloliquefaciens via end-measurement fluorescence. Compared to conventional qPCR, the assay allows for species-level detection of bacteria, thus enabling the precise detection of the Bacillus strain that yields high-level 2,3,5,6-tetramethylpyrazine. The assay promises the precise monitoring of bacterial growth and contribution to flavor during Daqu fermentation, thus facilitating fermented food quality control.

Development of a Real-Time Quantitative PCR Assay for the Specific Detection of Bacillus velezensis and Its Application in the Study of Colonization Ability

Bacillus velezensis is a widely used biocontrol agent closely related to B. amyloliquefaciens, and the two species cannot be distinguished by universal primers that are currently available. The study aimed to establish a rapid, specific detection approach for B. velezensis. Many unique gene sequences of B. velezensis were selected through whole genome sequence alignment of B. velezensis strains and were used to design a series of forward and reverse primers, which were then screened by PCR and qPCR using different Bacillus samples as templates. The colonization ability of B. velezensis ZF2 in different soils and different soil environmental conditions was measured by qPCR and a 10-fold dilution plating assay. A specific primer pair targeting the sequence of the D3N19_RS13500 gene of B. velezensis ZF2 was screened and could successfully distinguish B. velezensis from B. amyloliquefaciens. A rapid specific real-time qPCR detection system for B. velezensis was established. B. velezensis ZF2 had a very strong colonization ability in desert soil, and the optimal soil pH was 7-8. Moreover, the colonization ability of strain ZF2 was significantly enhanced when organic matter from different nitrogen sources was added to the substrate. This study will provide assistance for rapid specificity detection and biocontrol application of B. velezensis strains.

Development of a Strain-Specific Quantification Method for Monitoring Bacillus amyloliquefaciens TF28 in the Rhizospheric Soil of Soybean

Bacillus amyloliquefaciens TF28 can be used to control soybean root disease. To assess its commercial potential as a biocontrol agent, it is necessary to develop a strain-specific quantification method to monitor its colonization dynamics in the rhizospheric soil of soybean under field conditions. Based on genomic comparison with the same species in NCBI databases, a strain-unique gene ukfpg was used as molecular marker to develop strain-specific PCR assay. Among three primer pairs, only primer pairs (F2/R2) could specifically differentiate TF28 from other strains of B. amyloliquefaciens with the detection limit of 10 fg and 100 CFU/g for DNA extracted from pure culture and dry soil, respectively. Then, a colony count coupled with PCR assay was used to monitor the population of TF28 in the rhizospheric soil of soybean in the field. The results indicated that TF28 successfully colonized in the rhizospheric soil of soybean. The colonization population of TF28 changed dynamically within the 120-day growth period with high population at the branching (V6) and flowering stages (R2). This study provides an efficient method to quantitatively monitor the colonization dynamics of TF28 in the rhizospheric soil of soybean in the field and demonstrates the potential of TF28 as a biocontrol agent for commercial development.

Genetic, Epigenetic and Phenotypic Diversity of Four Bacillus velezensis Strains Used for Plant Protection or as Probiotics

Bacillus velezensis strains are applied as ecologically safe biopesticides, plant growth promoting rhizobacteria (PGPR), and in veterinary probiotics. They are abundant in various environments including soil, plants, marine habitats, the intestinal micro-flora, etc. The mechanisms underlying this adaptive plasticity and bioactivity are not well understood, nor is it clear why several strains outperform other same species isolates by their bioactivities. The main objective of this work was to demonstrate versatility of bioactivities and lifestyle strategies of the selected B. velezensis strains suitable to serve as model organisms in future studies. Here, we performed a comparative study of newly sequenced genomes of four B. velezensis isolates with distinct phenotypes and isolation origin, which were assessed by RNA sequencing under the effect of root exudate stimuli and profiled by epigenetic modifications of chromosomal DNA. Among the selected strains, UCMB5044 is an oligotrophic PGPR strain adapted to nutrient poor desert soils. UCMB5113 and At1 are endophytes that colonize plants and require nutrient rich media. In contrast, the probiotic strain, UCMB5007, is a copiotroph, which shows no propensity to colonize plants. PacBio and Illumina sequencing approaches were used to generate complete genome assemblies, tracing epigenetic modifications, and determine gene expression profiles. All sequence data was deposited at NCBI. The strains, UCMB5113 and At1, show 99% sequence identity and similar phenotypes despite being isolated from geographically distant regions. UCMB5007 and UCMB5044 represent another group of organisms with almost identical genomes but dissimilar phenotypes and plant colonization propensity. The two plant associated strains, UCMB5044 and UCMB5113, share 398 genes putatively associated with root colonization, which are activated by exposure to maize root exudates. In contrast, UCMB5007 did not respond to root exudate stimuli. It was hypothesized that alterations in the global methylation pattern and some other epigenetic modifications enable adaptation of strains to different habitats and therefore may be of importance in terms of the biotechnological applicability of these bacteria. Contrary, the ability to grow on root exudates as a sole source of nutrients or a strong antagonism against phytopathogens showed by the strains in vitro cannot be considered as good predictors of PGPR activities.

Bacillus velezensis 5113 Induced Metabolic and Molecular Reprogramming during Abiotic Stress Tolerance in Wheat

Abiotic stresses are main limiting factors for agricultural production around the world. Plant growth promoting rhizobacteria (PGPR) have been shown to improve abiotic stress tolerance in several plants. However, the molecular and physiological changes connected with PGPR priming of stress management are poorly understood. The present investigation aimed to explore major metabolic and molecular changes connected with the ability of Bacillus velezensis 5113 to mediate abiotic stress tolerance in wheat. Seedlings treated with Bacillus were exposed to heat, cold/freezing or drought stress. Bacillus improved wheat survival in all stress conditions. SPAD readings showed higher chlorophyll content in 5113-treated stressed seedlings. Metabolite profiling using NMR and ESI-MS provided evidences for metabolic reprograming in 5113-treated seedlings and showed that several common stress metabolites were significantly accumulated in stressed wheat. Two-dimensional gel electrophoresis of wheat leaves resolved more than 300 proteins of which several were differentially expressed between different treatments and that cold stress had a stronger impact on the protein pattern compared to heat and drought. Peptides maps or sequences were used for database searches which identified several homologs. The present study suggests that 5113 treatment provides systemic effects that involve metabolic and regulatory functions supporting both growth and stress management.

Understanding the ontogeny and succession of Bacillus velezensis and B. subtilis subsp. subtilis by focusing on kimchi fermentation

Bacillus subtilis and B. velezensis are frequently isolated from various niches, including fermented foods, water, and soil. Within the Bacillus subtilis group, B. velezensis and B. subtilis subsp. subtilis have received significant attention as biological resources for biotechnology-associated industries. Nevertheless, radical solutions are urgently needed to identify microbes during their ecological succession to accurately confirm their action at the species or subspecies level in diverse environments, such as fermented materials. Thus, in this study, previously published genome data of the B. subtilis group were compared to exploit species- or subspecies-specific genes for use as improved qPCR targets to detect B. velezensis and B. subtilis subsp. subtilis in kimchi samples. In silico analyses of the selected genes and designed primer sequences, in conjunction with SYBR Green real-time PCR, confirmed the robustness of this newly developed assay. Consequently, this study will allow for new insights into the ontogeny and succession of B. velezensis and B. subtilis subsp. subtilis in various niches. Interestingly, in white kimchi without red pepper powder, neither B. subtilis subsp. subtilis nor B. velezensis was detected.

Analysis of plant growth-promoting properties of Bacillus amyloliquefaciens UCMB5113 using Arabidopsis thaliana as host plant

MAIN CONCLUSION

This study showed that Bacillus amyloliquefaciens UCMB5113 colonizing Arabidopsis roots changed root structure and promoted growth implying the usability of this strain as a novel tool to support sustainable crop production. Root architecture plays a crucial role for plants to ensure uptake of water, minerals and nutrients and to provide anchorage in the soil. The root is a dynamic structure with plastic growth and branching depending on the continuous integration of internal and environmental factors. The rhizosphere contains a complex microbiota, where some microbes can colonize plant roots and support growth and stress tolerance. Here, we report that the rhizobacterium Bacillus amyloliquefaciens subsp. plantarum UCMB5113 stimulated the growth of Arabidopsis thaliana Col-0 by increased lateral root outgrowth and elongation and root-hair formation, although primary root elongation was inhibited. In addition, the growth of the above ground tissues was stimulated by UCMB5113. Specific hormone reporter gene lines were tested which suggested a role for at least auxin and cytokinin signaling during rhizobacterial modulation of Arabidopsis root architecture. UCMB5113 produced cytokinins and indole-3-acetic acid, and the formation of the latter was stimulated by root exudates and tryptophan. The plant growth promotion effect by UCMB5113 did not appear to depend on jasmonic acid in contrast to the disease suppression effect in plants. UCMB5113 exudates inhibited primary root growth, while a semi-purified lipopeptide fraction did not and resulted in the overall growth promotion indicating an interplay of many different bacterial compounds that affect the root growth of the host plant. This study illustrates that beneficial microbes interact with plants in root development via classic and novel signals.