Biological Control Efficacy and Action Mechanism of Klebsiella pneumoniae JCK-2201 Producing Meso-2,3-Butanediol Against Tomato Bacterial Wilt

A Point Mutation in Cassette Relieves the Repression Regulation of CcpA Resulting in an Increase in the Degradation of 2,3-Butanediol in Lactococcus lactis

In lactic acid bacteria, the global transcriptional regulator CcpA regulates carbon metabolism by repressing and activating the central carbon metabolism pathway, thus decreasing or increasing the yield of certain metabolites to maximize carbon flow. However, there are no reports on the deregulation of the inhibitory effects of CcpA on the metabolism of secondary metabolites. In this study, we identified a single-base mutant strain of Lactococcus lactis N8-2 that is capable of metabolizing 2,3-butanediol. It has been established that CcpA dissociates from the catabolite responsive element (cre) site due to a mutation, leading to the activation of derepression and expression of the 2,3-butanediol dehydrogenase gene cluster (butB and butA). Transcriptome analysis and quantitative polymerase chain reaction (Q-PCR) results showed significant upregulation of transcription of butB and butA compared to the unmutated strain. Furthermore, micro-scale thermophoresis experiments confirmed that CcpA did not bind to the mutated cre. Furthermore, in a bacterial two-plasmid fluorescent hybridization system, it was similarly confirmed that the dissociation of CcpA from cre eliminated the repressive effect of CcpA on downstream genes. Finally, we investigated the differing catalytic capacities of the 2,3-butanediol dehydrogenase gene cluster in L. lactis N8-1 and L. lactis N8-2 for 2,3-butanediol. This led to increased expression of butB and butA, which were deregulated by CcpA repression. This is the first report on the elimination of the deterrent effect of CcpA in lactic acid bacteria, which changes the direction of enzymatic catalysis and alters the direction of carbon metabolism. This provides new perspectives and strategies for metabolizing 2,3-butanediol using bacteria in synthetic biology.

Insight into the soil bacterial community succession of Nicotiana benthamiana in response to Tobacco mosaic virus

Background

Tobacco mosaic virus (TMV) is one famous plant virus responsible for substantial economic losses worldwide. However, the roles of bacterial communities in response to TMV in the tobacco rhizosphere remain unclear.

Methods

We explored the soil physicochemical properties and bacterial community succession of the healthy (YTH) and diseased (YTD) plants with TMV infection by 16S rRNA gene sequencing and bioinformatics analysis.

Results

We found that soil pH in the YTD group was significantly lower than in the YTH group, and the soil available nutrients were substantially higher. The bacterial community analysis found that the diversity and structure significantly differed post-TMV disease onset. With TMV inoculated, the alpha diversity of the bacterial community in the YTD was markedly higher than that in the YTH group at the early stage. However, the alpha diversity in the YTD group subsequently decreased to lower than in the YTH group. The early bacterial structure of healthy plants exhibited higher susceptibility to TMV infection, whereas, in the subsequent stages, there was an enrichment of beneficial bacterial (e.g., Ramlibacter, Sphingomonas, Streptomyces, and Niastella) and enhanced energy metabolism and nucleotide metabolism in bacteria.

Conclusion

The initial soil bacterial community exhibited susceptibility to TMV infection, which might contribute to strengthening resistance of Tobacco to TMV.

Isolation and draft genome sequence of Enterobacter asburiae strain i6 amenable to genetic manipulation

Enterobacter asburiae is a species of Gram-negative bacteria that is found in soil, water, and sewage. E. asburiae is generally considered to be an opportunistic pathogen, but has also been reported as a plant growth-promoting bacterium (PGPB), which may have beneficial effects on plant growth and development. However, genetic analysis of E. asburiae has been limited, possibly due to its redundant enzymes that digest exogenous DNA in the cell. Here, an E. asburiae strain i6 was isolated from soil in Nara, Japan. This strain was amenable to transformation and the one-step gene inactivation method based on λ Red recombinase. The transformation efficiency of the i6 strain with the 10 kb plasmid DNA pCF430 was at least four orders of magnitude higher than that of the previously sequenced E. asburiae strain ATCC 35953, which could not be transformed with the same plasmid DNA. A draft genome sequence of the i6 strain was determined and deposited into the database, allowing several factors that may determine transformation efficiency to be perturbed and tested. Together with the amenability of the i6 strain to genetic manipulation, the information from the i6 genome will facilitate characterization and fine-tuning of the beneficial and detrimental traits of this species.

Bacterial wilt affects the structure and assembly of microbial communities along the soil-root continuum

BACKGROUND

Beneficial root-associated microbiomes play crucial roles in enhancing plant growth and suppressing pathogenic threats, and their application for defending against pathogens has garnered increasing attention. Nonetheless, the dynamics of microbiome assembly and defense mechanisms during pathogen invasion remain largely unknown. In this study, we aimed to investigate the diversity and assembly of microbial communities within four niches (bulk soils, rhizosphere, rhizoplane, and endosphere) under the influence of the bacterial plant pathogen Ralstonia solanacearum.

RESULTS

Our results revealed that healthy tobacco plants exhibited more diverse community compositions and more robust co-occurrence networks in root-associated niches compared to diseased tobacco plants. Stochastic processes (dispersal limitation and drift), rather than determinism, dominated the assembly processes, with a higher impact of drift observed in diseased plants than in healthy ones. Furthermore, during the invasion of R. solanacearum, the abundance of Fusarium genera, a known potential pathogen of Fusarium wilt, significantly increased in diseased plants. Moreover, the response strategies of the microbiomes to pathogens in diseased and healthy plants diverged. Diseased microbiomes recruited beneficial microbial taxa, such as Streptomyces and Bacilli, to mount defenses against pathogens, with an increased presence of microbial taxa negatively correlated with the pathogen. Conversely, the potential defense strategies varied across niches in healthy plants, with significant enrichments of functional genes related to biofilm formation in the rhizoplane and antibiotic biosynthesis in the endosphere.

CONCLUSION

Our study revealed the varied community composition and assembly mechanism of microbial communities between healthy and diseased tobacco plants along the soil-root continuum, providing new insights into niche-specific defense mechanisms against pathogen invasions. These findings may underscore the potential utilization of different functional prebiotics to enhance plants' ability to fend off pathogens.

Potential of algal-based products for the management of potato brown rot disease

BACKGROUND

Ralstonia solanacearum causes potato brown rot disease, resulting in lower crop's production and quality. A sustainable and eco-friendly method for controlling the disease is required. Algae's bioactive chemicals have shown the potential to enhance plant defenses. For the first time, the efficacy of foliar application of Acanthophora spicifera and Spirulina platensis seaweed extracts, along with the utilization of dried algal biomasses (DABs) of Turbinaria ornata and a mixture of Caulerpa racemosa and Cystoseira myrica (1:1)on potato yield and brown rot suppression were investigated under field conditions. Field experiments were conducted in three locations: Location 1 (Kafr Shukr district, Kaliobeya governorate), Location 2 (Moneira district, Kaliobeya governorate), and Location 3 (Talia district, Minufyia governorate). Locations 1 and 2 were naturally infested with the pathogen, while location 3 was not. The study evaluated potato yield, plant nutritive status and antioxidants, soil available nitrogen-phosphorus-potassium (N-P-K), and organic matter percentage. Additionally, the shift in soil microbial diversity related to R. solanacearum suppression was examined for the most effective treatment.

RESULTS

The results revealed that seaweed extracts significantly increased potato yield at all locations, which correlated with higher phosphorus absorption, while T. ornate DAB increased potato yield only at location 2, accompanied by noticeable increases in soil nitrogen and plant phosphorus. The mixed DABs of C. racemosa and C. myrica demonstrated greater disease suppression than foliar applications. The disease-suppressive effect of the mixed DABs was accompanied by significant increases in flavonoids and total antioxidant capacity (TAC). Moreover, the application of mixed DABs increased soil bacterial biodiversity, with a higher abundance of oligotrophic marine bacterial species such as Sphingopyxis alaskensis and growth-promoting species like Glutamicibacter arilaitensis, Promicromonospora sp., and Paenarthrobacter nitroguajacolicus in all three locations compared to the untreated control. Klebsiella sp., Pseudomonas putida, and P. brassicacearum abundances were increased by the mixed DABs in Location 1. These species were less abundant in locations 2 and 3, where Streptomyces sp., Bacillus sp., and Sphingobium vermicomposti were prevalent.

CONCLUSIONS

The results demonstrated that the used seaweed extracts improved potato yield and phosphorous absorption, while the mixed DABs potentially contributed in disease suppression and improved soil microbial diversity.

Assessment of the Biocontrol Potential of Bacillus velezensis WL-23 against Kiwifruit Canker Caused by Pseudomonas syringae pv. actinidiae

Kiwifruit canker disease, caused by Pseudomonas syringae pv. actinidiae (Psa), is the main threat to kiwifruit production worldwide. Currently, there is no safe and effective disease prevention method; therefore, biological control technologies are being explored for Psa. In this study, Bacillus velezensis WL-23 was isolated from the leaf microbial community of kiwifruit and used to control kiwifruit cankers. Indoor confrontation experiments showed that both WL-23 and its aseptic filtrate had excellent inhibitory activity against the main fungal and bacterial pathogens of kiwifruit. Changes in OD₆₀₀, relative conductivity, alkaline proteinase, and nucleic acid content were recorded during Psa growth after treatment with the aseptic filtrate, showing that Psa proliferation was inhibited and the integrity of the cell membrane was destroyed; this was further verified using scanning electron microscopy and transmission electron microscopy. In vivo, WL-23 promoted plant growth, increased plant antioxidant enzyme activity, and reduced canker incidence. Therefore, WL-23 is expected to become a biological control agent due to its great potential to contribute to sustainable agriculture.

Microbial interactions and metabolisms in response to bacterial wilt and black shank pathogens in the tobacco rhizosphere

Background

Tobacco bacterial wilt (TBW) and black shank (TBS) are responsible for substantial economic losses worldwide; however, microbial interactions and metabolisms in response to TBW and TBS pathogens in the tobacco rhizosphere remain unclear.

Methods

We explored and compared the response of rhizosphere microbial communities to these two plant diseases with the incidences in moderate and heavy degrees by sequencing of 16S rRNA gene amplicons and bioinformatics analysis.

Results and discussions

We found that the structure of rhizosphere soil bacterial communities was significantly (p < 0.05) changed from the incidences of TBW and TBS, which also led to decreased Shannon diversity and Pielou evenness. Compared with the healthy group (CK), the OTUs with significantly (p < 0.05) decreased relative abundances were mostly affiliated with Actinobacteria (e.g., Streptomyces and Arthrobacter) in the diseased groups, and the OTUs with significantly (p < 0.05) increased relative abundances were mainly identified as Proteobacteria and Acidobacteria. Also, molecular ecological network analysis showed that the nodes (<467) and links (<641) were decreased in the diseased groups compared with the control group (572; 1056), suggesting that both TBW and TBS weakened bacterial interactions. In addition, the predictive functional analysis indicated that the relative abundance of genes related to the biosynthesis of antibiotics (e.g., ansamycins and streptomycin) was significantly (p < 0.05) decreased due to incidences of TBW and TBS, and antimicrobial tests showed that some Actinobacteria strains (e.g., Streptomyces) and their secreted antibiotics (e.g., streptomycin) could effectively inhibit the growth of these two pathogens.