Improved Methods for Treatment of Phytopathogenic Biofilms: Metallic Compounds as Anti-Bacterial Coatings and Fungicide Tank-Mix Partners

Characterization of the Fusarium circinatum biofilm environmental response role

The capacity to form biofilms is a common trait among many microorganisms present on Earth. In this study, we demonstrate for the first time that the fatal pine pitch canker agent, Fusarium circinatum, can lead a biofilm-like lifestyle with aggregated hyphal bundles wrapped in extracellular matrix (ECM). Our research shows F. circinatum's ability to adapt to environmental changes by assuming a biofilm-like lifestyle. This was demonstrated by varying metabolic activities exhibited by the biofilms in response to factors like temperature and pH. Further analysis revealed that while planktonic cells produced small amounts of ECM per unit of the biomass, heat- and azole-exposed biofilms produced significantly more ECM than nonexposed biofilms, further demonstrating the adaptability of F. circinatum to changing environments. The increased synthesis of ECM triggered by these abiotic factors highlights the link between ECM production in biofilm and resistance to abiotic stress. This suggests that ECM-mediated response may be one of the key survival strategies of F. circinatum biofilms in response to changing environments. Interestingly, azole exposure also led to biofilms that were resistant to DNase, which typically uncouples biofilms by penetrating the biofilm and degrading its extracellular DNA; we propose that DNases were likely hindered from reaching target cells by the ECM barricade. The interplay between antifungal treatment and DNase enzyme suggests a complex relationship between eDNA, ECM, and antifungal agents in F. circinatum biofilms. Therefore, our results show how a phytopathogen's sessile (biofilm) lifestyle could influence its response to the surrounding environment.

Antibacterial activities of coumarin-3-carboxylic acid against Acidovorax citrulli

Coumarin-3-carboxylic acid (3-CCA), previously screened from natural coumarins, was found to possess strong antibacterial activity against Acidovorax citrulli (Ac). In order to further evaluate the activity of this compound against plant bacterial pathogens and explore its potential value as a bactericidal lead compound, the activity of 3-CCA against 14 plant pathogenic bacteria in vitro and in vivo was tested. Results showed that 3-CCA exhibited strong in vitro activities against Ac, Ralstonia solanacearum, Xanthomonas axonopodis pv. manihotis, X. oryzae pv. oryzae, and Dickeya zeae with EC50 values ranging from 26.64 μg/mL to 40.73 μg/mL. Pot experiment results showed that 3-CCA had powerful protective and curative effects against Ac. In addition, the protective efficiency of 3-CCA was almost equivalent to that of thiodiazole copper at the same concentration. The results of SEM and TEM observation and conductivity tests showed that 3-CCA disrupted the integrity of the cell membrane and inhibited polar flagella growth. Furthermore, 3-CCA resulted in reductions in motility and extracellular exopolysaccharide (EPS) production of Ac while inhibiting the biofilm formation of Ac. These findings indicate that 3-CCA could be a promising natural lead compound against plant bacterial pathogens to explore novel antibacterial agents.

Antibacterial activity and action mechanism of flavonoids against phytopathogenic bacteria

As the most difficult to control in plant disease, phytopathogenic bacteria cause huge losses to agricultural products and economy worldwide. However, the commercially available bactericides are few and enhance pathogen resistance. To alleviate this situation, 50 flavonoids were evaluated for their antibacterial activities and mechanism of action against two intractable plant bacterial pathogens. The results of bioassays showed that most of the flavonoids exhibited moderate inhibitory effects against Xanthomonas oryzae (Xo) and Xanthomonas axonopodis pv citri (Xac). Remarkably, kaempferol showed excellent antibacterial activity against Xo in vitro (EC₅₀ = 15.91 μg/mL) and quercetin showed the best antibacterial activity against Xac in vitro (EC₅₀ = 14.83 μg/mL), which was better than thiodiazole copper (EC₅₀ values against Xo and Xac were 16.79 μg/mL, 59.13 μg/mL, respectively). Subsequently, in vivo antibacterial activity assay further demonstrated kaempferol exhibited a stronger control effect on bacterial infections than thiodiazole copper. Then, the preliminary antibacterial mechanism of kaempferol was investigated by ultrastructural observations, transcriptomic, qRT-PCR analysis and biochemical index determination. These results showed that kaempferol mainly exerted bacteriostatic effects at the molecular level by affecting bacterial energy metabolism, reducing pathogenicity, and leading to disruption of cellular integrity, leakage of contents and cell death eventually.

Bactericidal Efficacy of Oxidized Silver against Biofilms Formed by Curtobacterium flaccumfaciens pv. flaccumfaciens

Bacterial wilt is a re-emerging disease on dry bean and can affect many other crop species within the Fabaceae. The causal agent, Curtobacterium flaccumfaciens pv. flaccumfaciens (CFF), is a small, Gram-positive, rodshaped bacterium that is seed-transmitted. Infections in the host become systemic, leading to wilting and economic loss. Clean seed programs and bactericidal seed treatments are two critical management tools. This study characterizes the efficacies of five bactericidal chemicals against CFF. It was hypothesized that this bacterium was capable of forming biofilms, and that the cells within biofilms would be more tolerant to bactericidal treatments. The minimum biocide eradication concentration assay protocol was used to grow CFF biofilms, expose the biofilms to bactericides, and enumerate survivors compared to a non-treated control (water). Streptomycin and oxysilver bisulfate had EC95 values at the lowest concentrations and are likely the best candidates for seed treatment products for controlling seed-borne bacterial wilt of bean. The results showed that CFF formed biofilms during at least two phases of the bacterial wilt disease cycle, and the biofilms were much more difficult to eradicate than their planktonic counterparts. Overall, biofilm formation by CFF is an important part of the bacterial wilt disease cycle in dry edible bean and antibiofilm bactericides such as streptomycin and oxysilver bisulfate may be best suited for use in disease management.

Curtobacterium, A Foliar Pathogen Isolated from Maize in Central Argentina

Plant pathogens, such as fungi, bacteria, and viruses, can cause serious damage to crops and significantly reduce yield and quality. Bacterial diseases of agronomic crops, however, have been little studied. The present study aims to isolate and identify bacteria recovered from symptomatic maize (Zea mays) leaves collected from field samples in the province of Cordoba, Argentina. Bacterial strains were identified using whole-cell matrix-assisted laser-desorption-ionization-time-off light mass spectrometry and 16S rDNA sequencing. Members of the genera Exiguobacterium and Curtobacterium were dominant in the studied vegetal material. Two strains (RC18-1/2 and RC18-3/1) were selected for further studies. The pathogenicity test showed that plants inoculated with Curtobacterium sp. RC18-1/2 exhibited the same symptoms as those previously detected in the field. To our knowledge, this study provides the first evidence about the isolation of a Curtobacterium pathogenic strain in maize. Effective crop disease management will require the use of integrated strategies, such as resistant cultivars and/or biocontrol agents.

Plant-associated fungal biofilms-knowns and unknowns

Nearly all microbes, including fungi, grow firmly attached to surfaces as a biofilm. Yet, attention toward fungal interactions with plants and the environment is dedicated to free-floating (planktonic) cells. Fungal biofilms are generally thought to configure interactions across and among plant populations. Despite this, plant fungal biofilm research lags far behind the research on biofilms of medically important fungi. The deficit in noticing and exploring this research avenue could limit disease management and plant improvement programs. Here, we provide the current state of knowledge of fungal biofilms and the different pivotal ecological roles they impart in the context of disease, through leveraging evidence across medically important fungi, secondary metabolite production, plant beneficial functions and climate change. We also provide views on several important information gaps potentially hampering plant fungal biofilm research, and propose a way forward to address these gaps.

Regioselective Direct C-H Trifluoromethylation of Pyridine

A highly efficient and regioselective direct C-H trifluoromethylation of pyridine based on an N-methylpyridine quaternary ammonium activation strategy has been developed. A variety of trifluoromethylpyridines can be obtained in good yield and excellent regioselectivity by treating the pyridinium iodide salts with trifluoroacetic acid in the presence of silver carbonate in N,N-dimethylformamide. The protocol features good functional group compatibility, easily available starting materials, and operational simplicity. Controlled experiments showed that the reaction may involve a nucleophilic trifluoromethylation mechanism.

Bacterial wilt of dry beans caused by Curtobacterium flaccumfaciens pv. flaccumfaciens: A new threat from an old enemy

Bacterial wilt and tan spot of dry beans (family Fabaceae), caused by Curtobacterium flaccumfaciens pv. flaccumfaciens, is an important emerging disease threatening the edible legume industry around the globe. The management of bacterial wilt has been a major problem since its original description in 1922. This is in part due to the seedborne nature of the pathogen allowing the bacterium to be transmitted long distances via infected seeds, as well as a lack of detailed molecular information concerning the pathogenicity repertoires and virulence determinates of the pathogen. Identification can also be difficult owing to the presence of five different colony colour variants (i.e., yellow, orange, pink, purple, and red) on culture media. In this review, we provide an overview of the aetiology, epidemiology, and management strategies of bacterial wilt disease. First, a comprehensive and comparative symptomology of the disease on different dry bean species is described. Then, the taxonomic history of the causal agent and utility of high-throughput sequencing-based approaches in the precise characterization of the pathogen is explained. Furthermore, we provide an updated outline on the global distribution of the pathogen, highlighting expansion of the causal agent into the areas with no history of the disease until the beginning of the current century. Finally, because there are limited options for use of conventional pesticides against the pathogen, we highlight the use of integrated pest management strategies, for example quarantine inspections, resistant cultivars, and crop sanitation, to combat the risk of bacterial wilt disease in the dry bean industry.

DISEASE SYMPTOMS

Interveinal chlorosis on leaflets leading to necrotic areas and systemic wilt. Seed discolouration to yellow, orange, pink, or purple is seen in white-seeded cultivars.

HOST RANGE

Causes bacterial wilt and tan spot disease on edible dry beans in the Fabaceae family, including common bean (Phaseolus vulgaris), cowpea (Vigna unguiculata), mungbean (Vigna radiata), soybean (Glycine max), as well as a number of weed species.

TAXONOMIC STATUS OF THE PATHOGEN

Bacteria; phylum Actinobacteria; order Actinomycetales; suborder: Micrococcineae; family Microbacteriaceae; genus Curtobacterium; species Curtobacterium flaccumfaciens.

SYNONYMS

Corynebacterium flaccumfaciens subsp. flaccumfaciens; Corynebacterium flaccumfaciens pv. flaccumfaciens, Corynebacterium flaccumfaciens, Phytomonas flaccumfaciens, Bacterium flaccumfaciens.

MICROBIOLOGICAL PROPERTIES

Multicoloured (yellow, orange, pink, purple, and red), gram-positive, aerobic, curved rod, nonspore-forming, polar flagellated, motile cells.

DISTRIBUTION

Widespread in America (Brazil, Canada, and the USA), Australia, and Iran. Restricted occurrence in Africa and Europe.

PHYTOSANITARY CATEGORIZATION

EPPO A2 list no. 48, EU Annex II⁄B.