Improved multiplex TaqMan qPCR assay with universal internal control offers reliable and accurate detection of Clavibacter michiganensis

Clavibacter michiganensis Reframed: The Story of How the Genomics Era Made a New Face for an Old Enemy

OBJECTIVE

Bacterial wilt and canker of tomato caused by the gram-positive corynebacterial species Clavibacter michiganensis is an economically important disease threatening the tomato industry in both open-air and greenhouse productions around the world. The disease occurs in many countries, with a particular importance in regions characterised by high temperature and water scarcity. Management of bacterial canker has been a major problem since its original description in 1909. This is due in part to the seedborne nature of the pathogen, allowing the bacterium to be transmitted over long distances via infected seeds, as well as a lack of effective treatment to clean seeds. Detection of the pathogen from seeds is difficult due to high competition on culture media with diverse members of the seed-associated microbiota. Identification of the pathogen can also be difficult owing to the presence of different colony variants on culture media. In this review, we provide a historical perspective and an updated overview on the aetiology, epidemiology and management strategies of the bacterial canker disease. We also gathered recent molecular findings in the pathogenicity mechanisms and bioecology of C. michiganensis to boost management of the bacterial canker disease in the 21st century tomato industry.

TAXONOMY

Class: Actinobacteria; Order: Micrococcales; Family: Microbacteriaceae; Genus: Clavibacter; Species: Clavibacter michiganensis.

DISEASE SYMPTOMS

Interveinal leaf chlorosis leading to necrotic areas. Canker on stems and lateral branches of the plant. Discolouration of vascular and pith tissues to dark yellow or brown. Small and early ripened fruits or discolouration of the placenta from white to yellow in the interior part of the ripening fruits.

HOST RANGE

Tomato (Solanum lycopersicum) is the main host of the pathogen while natural infection has also been reported on eggplant, pepper and wild nightshade plants.

SYNONYMS (HISTORICAL/NON-PREFERRED SCIENTIFIC NAMES)

Aplanobacter michiganensis; Pseudomonas michiganense; Pseudomonas michiganensis; Bacterium michiganense; Phytomonas michiganensis; Mycobacterium michiganense; Erwinia michiganensis (=michiganense); Corynebacterium michiganense; Corynebacterium michiganense pv. michiganense; Corynebacterium michiganense subsp. michiganense; Clavibacter michiganensis subsp. michiganensis.

MICROBIOLOGICAL PROPERTIES

The bacterium produces domed, round and shiny mucoid colonies on general culture media. Colonies are usually yellow-pigmented, while pink-pigmented strains are occasionally observed. Cells are gram-positive, aerobic, non-motile, non-spore-producing curved rods (coryneform).

DISTRIBUTION

Present in all continents.

PHYTOSANITARY CATEGORIZATION

EPPO A2 List no. 50, EU 2019/2072 RNQP Annex IV. See EPPO (https://gd.eppo.int/taxon/CORBMI/categorization) and CABI (https://www.cabidigitallibrary.org/doi/10.1079/cabicompendium.15338) databases for further country-specific categorisations. EPPO code: CORBMI.

Development and validation of genome-informed and multigene-based qPCR and LAMP assays for accurate detection of Dickeya solani: a critical quarantine pathogen threatening the potato industry

Dickeya solani one of the most aggressive pectinolytic phytopathogens, causes blackleg disease in potatoes, resulting in significant economic losses and adversely impacting one of the world's most important food crops. The diagnostics methods are critical in monitoring the latent infection for international trade of potato seed tubers and in implementation of control strategies. Our study employed a whole-genome comparative approach, identifying unique target gene loci (LysR and TetR family of transcriptional regulators gene regions) and designing loop-mediated isothermal amplification (LAMP) and a multi-gene-based multiplex TaqMan qPCR assays for specific detection and differentiation of D. solani. Both methods underwent meticulous validation with extensive inclusivity and exclusivity panels, exhibiting 100% accuracy and no false positives or negatives. The LAMP method demonstrated the detection limit of 100 fg and 1 CFU per reaction using pure genomic DNA and crude bacterial cell lysate, respectively. The qPCR detection limit was 1 pg, 100 fg and 10 fg with quadplex, triplex, and singleplex, respectively. None of the assays were impacted by any inhibitory or competitive effects after adding host DNA (in qPCR) or crude lysate (in LAMP). The assays proved robust and reproducible in detecting the target pathogen in infected samples, with the LAMP assay being field-deployable due to its simplicity and rapid results acquisition within approximately 9 min. The reproducibility was confirmed by performing the assay in two independent laboratories. These assays offer a robust, rapid, and reliable solution for routine testing, with applications in phytosanitary inspection, disease diagnosis, and epidemiological studies.IMPORTANCEDickeya solani, one of the most aggressive soft rot causing bacteria and a quarantine pathogen, poses a severe threat to food security by causing substantial economic losses to the potato industry. Accurate and timely detection of this bacterium is vital for monitoring latent infections, particularly for international trade of potato seed tubers, and for implementing effective control strategies. In this research, we have developed LAMP and multi-gene-based multiplex TaqMan qPCR assays for specific detection of D. solani. These assays, characterized by their precision, rapidity, and robustness, are crucial for distinguishing D. solani from related species. Offering unparalleled sensitivity and specificity, these assays are indispensable for phytosanitary inspection and epidemiological monitoring, providing a powerful tool enabling management of this threatening pathogen.

A New Multiplex TaqMan qPCR for Precise Detection and Quantification of Clavibacter michiganensis in Seeds and Plant Tissue

Bacterial canker of tomato caused by Clavibacter michiganensis (Cm) is one of the most devastating bacterial diseases affecting the tomato industry worldwide. As the result of Cm colonization of the xylem, the susceptible host shows typical symptoms of wilt, marginal leaf necrosis, stem cankers, and ultimately plant death. However, what makes Cm an even more dangerous pathogen is its ability to infect seeds and plants without causing symptoms. Unfortunately, there are no resistant cultivars or effective chemical or biological control methods available to growers against Cm. Its control relies heavily on prevention. The implementation of a rapid and accurate detection tool is imperative to monitor the presence of Cm and prevent its spread. In this study, we developed a specific and sensitive multiplex TaqMan qPCR assay to detect Cm and distinguish it from related bacterial species that affect tomato plants. Two Cm chromosomal virulence-related genes, rhuM and tomA, were used as specific targets. The plant internal control tubulin alpha-3 was included in each of the multiplexes to improve the reliability of the assay. Specificity was evaluated with 37 bacterial strains including other Clavibacter spp. and related and unrelated bacterial pathogens from different geographic locations affecting a wide variety of hosts. Results showed that the assay is able to discriminate Cm strains from other related bacteria. The assay was validated on tissue and seed samples following artificial infection, and all tested samples accurately detected the presence of Cm. The tool described here is highly specific, sensitive, and reliable for the detection of Cm and allows the quantification of Cm in seeds, roots, stems, and leaves. The diagnostic assay can also be adapted for multiple purposes such as seed certification programs, surveillance, biosafety, the effectiveness of control methods, border protection, and epidemiological studies.[Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Development of a multiplex TaqMan qPCR targeting unique genomic regions for the specific and sensitive detection of Pectobacterium species and P. parmentieri

AIM

The newly defined species P. parmentieri has emerged as an aggressive pathogen that causes soft rot and blackleg diseases on potato and has been widely disseminated across the globe, jeopardizing the productivity and potato food safety. The implementation of a fast and accurate detection tool is imperative to control, monitor and prevent further spread of these pathogens. The objective of this work was to develop a specific and sensitive multiplex TaqMan qPCR to detect P. parmentieri and distinguish it from all known Pectobacterium species. A universal internal control (UIC) was included to enhance the reliability of the assay.

METHODS AND RESULTS

A comparative genomics approach was used to identify O-acetyltransferase and the XRE family transcriptional regulator as specific targets for primers/probe design for the detection of the Pectobacterium genus and P. parmentieri, respectively. Specificity was assessed with 35 and 25 strains included inclusivity and exclusivity panels, respectively, isolated from different geographic locations and sources. The assay specifically detected all 35 strains of Pectobacterium sp. and all 15 P. parmentieri strains. No cross-reactivity was detected during assay validation. Our assay detected up to 10 fg genomic DNA and 1 CFU ml^-1 bacterial culture. No change in the detection threshold (1 CFU ml^-1 ) was observed in spiked assays after adding host tissue to the reactions. The assay was validated with naturally and artificially infected host tissues and soil rhizosphere samples. All infected plant samples containing the target pathogens were accurately amplified.

CONCLUSION

The presented multiplex TaqMan qPCR diagnostic assay is highly specific, sensitive, reliable for the detection of Pectobacterium species and P. parmentieri with no false positives or false negatives.

SIGNIFICANCE AND IMPACT

The developed assay can be adopted for multiple purposes such as seed certification programs, surveillance, biosecurity, microbial forensics, quarantine, border protection, inspections, and epidemiology.

Adsorption of Recombinant Human β-Defensin 2 and Two Mutants on Mesoporous Silica Nanoparticles and Its Effect against Clavibacter michiganensis subsp. michiganensis

Solanum lycopersicum L. is affected among other pests and diseases, by the actinomycete Clavibacter michiganensis subsp. michiganensis (Cmm), causing important economic losses worldwide. Antimicrobial peptides (AMPs) are amphipathic cationic oligopeptides with which the development of pathogenic microorganisms has been inhibited. Therefore, in this study, we evaluate antimicrobial activity of mesoporous silica nanoparticles (MSN5.4) loaded with human β-defensin-2 (hβD2) and two mutants (TRX-hβD2-M and hβD2-M) against Cmm. hβD2, TRX-hβD2-M and hβD2-M presented a half-maximum inhibitory concentration (IC₅₀) of 3.64, 1.56 and 6.17 μg/mL, respectively. MSNs had average particle sizes of 140 nm (SEM) and a tunable pore diameter of 4.8 up to 5.4 nm (BJH). AMPs were adsorbed more than 99% into MSN and a first release after 24 h was observed. The MSN loaded with the AMPs inhibited the growth of Cmm in solid and liquid media. It was also determined that MSNs protect AMPs from enzymatic degradation when the MSN/AMPs complexes were exposed to a pepsin treatment. An improved AMP performance was registered when it was adsorbed in the mesoporous matrix. The present study could expand the applications of MSNs loaded with AMPs as a biological control and provide new tools for the management of phytopathogenic microorganisms.