Antagonistic Evaluation of Chromobacterium sp. JH7 for Biological Control of Ginseng Root Rot Caused by Cylindrocarpon destructans

Taxonomy-guided selection of Paraburkholderia busanensis sp. nov.: a versatile biocontrol agent with mycophagy against Colletotrichum scovillei causing pepper anthracnose

IMPORTANCE

Traditional control methods for postharvest diseases rely on fungicides, which cause human health and environmental concerns. This study introduces a taxonomy-guided strategy for selecting biocontrol agents. By focusing on Paraburkholderia group, which harbors diverse plant-beneficial strains, the inadvertent selection of harmful strains was circumvented, thereby obviating the need for laborious in vitro screening assays. A highly promising candidate, strain P39, has been identified, exhibiting remarkable biocontrol activity against Colletotrichum scovillei. Through comprehensive genomic, physiological, and biochemical analyses, P39 was characterized as a novel species within the Paraburkholderia genus and designated Paraburkholderia busanensis. Moreover, these findings deepen our understanding of bacterial-fungal interactions, as they elucidate a potential pathway for the utilization of fungal chitin, thereby enhancing our understanding of bacterial mycophagy. P. busanensis is a promising source of antifungal volatiles and putative novel secondary metabolites.

Identification of 2,4-diacetylphloroglucinol production in the genus Chromobacterium

The compound 2,4-diacetylphloroglucinol (DAPG) is a broad-spectrum antibiotic that is primarily produced by Pseudomonas spp. DAPG plays an important role in the biocontrol disease suppressing activity of Pseudomonas spp. In the current study, we report the discovery of the DAPG biosynthetic cluster in strains of Chromobacterium vaccinii isolated from Brazilian aquatic environments and the distribution of the biosynthetic cluster in the Chromobacterium genus. Phylogenetic analysis of the phlD protein suggests the biosynthetic cluster probably entered the genus of Chromobacterium after a horizontal gene transfer event with a member of the Pseudomonas fluorescens group. We were able to detect trace amounts of DAPG in wild type cultures and confirm the function of the cluster with heterologous expression in Escherichia coli. In addition, we identified and verified the presence of other secondary metabolites in these strains. We also confirmed the ability of C. vaccinii strains to produce bioactive pigment violacein and bioactive cyclic depsipeptide FR900359. Both compounds have been reported to have antimicrobial and insecticidal activities. These compounds suggest strains of C. vaccinii should be further explored for their potential as biocontrol agents.

A Broad-Specificity Chitinase from Penicillium oxalicum k10 Exhibits Antifungal Activity and Biodegradation Properties of Chitin

Penicillium oxalicum k10 isolated from soil revealed the hydrolyzing ability of shrimp chitin and antifungal activity against Sclerotinia sclerotiorum. The k10 chitinase was produced from a powder chitin-containing medium and purified by ammonium sulfate precipitation and column chromatography. The purified chitinase showed maximal activity toward colloidal chitin at pH 5 and 40 °C. The enzymatic activity was enhanced by potassium and zinc, and it was inhibited by silver, iron, and copper. The chitinase could convert colloidal chitin to N-acetylglucosamine (GlcNAc), (GlcNAc)2, and (GlcNAc)3, showing that this enzyme had endocleavage and exocleavage activities. In addition, the chitinase prevented the mycelial growth of the phytopathogenic fungi S. sclerotiorum and Mucor circinelloides. These results indicate that k10 is a potential candidate for producing chitinase that could be useful for generating chitooligosaccharides from chitinous waste and functions as a fungicide.

Antagonistic and Plant Growth-Promoting Effects of Bacillus velezensis BS1 Isolated from Rhizosphere Soil in a Pepper Field

Pepper (Capsicum annuum L.) is an important agricultural crop worldwide. Recently, Colletotrichum scovillei, a member of the C. acutatum species complex, was reported to be the dominant pathogen causing pepper anthracnose disease in South Korea. In the present study, we isolated bacterial strains from rhizosphere soil in a pepper field in Gangwon Province, Korea, and assessed their antifungal ability against C. scovillei strain KC05. Among these strains, a strain named BS1 significantly inhibited mycelial growth, appressorium formation, and disease development of C. scovillei. By combined sequence analysis using 16S rRNA and partial gyrA sequences, strain BS1 was identified as Bacillus velezensis, a member of the B. subtilis species complex. BS1 produced hydrolytic enzymes (cellulase and protease) and iron-chelating siderophores. It also promoted chili pepper (cv. Nockwang) seedling growth compared with untreated plants. The study concluded that B. velezensis BS1 has good potential as a biocontrol agent of anthracnose disease in chili pepper caused by C. scovillei.

Paraburkholderia panacisoli sp. nov., a potentially antagonistic bacterium against the root rot fungal pathogen Cylindrocarpon destructans, isolated from ginseng cultivation soil

A new bacterium, designated DCY113^T, was isolated from ginseng cultivation soil in Gochang-gun, South Korea, and its taxonomic position identified by the polyphasic approach. 16S rRNA gene sequence analysis determined that this isolate belongs to the genus Paraburkholderia, and was closest to P. dipogonis DL7^T (98.6%), P. phytofirmans PsJN^T (98.5%), P. kirstenboschensis Kb15^T (98.4%) and P. aromaticivorans BN^T (98.1%). Strain DCY113^T is Gram-reaction negative, strictly aerobic, rod-shaped, non-motile, and catalase and oxidase positive. The predominant isoprenoid quinone of DCY113^T was ubiquinone Q-8. The major cellular fatty acids were C_16:0, cyclo-C_17:0 and the Summed feature 8 (C_18:1ω7c and/or C_18:1ω6c). The major polar lipids were diphosphatidylglycerol (DPG), phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and an unknown amino lipid (AL1). The G+C content of the genomic DNA was 62.2 mol%. Average nucleotide identity (ANI) between strain DCY113^T and the related Paraburkholderia type strains were below the threshold value for species delineation. This low DNA relatedness in combination with phylogenetic and phenotypic tests indicates that strain DCY113^T cannot be assigned to any recognized species. Strain DCY113^T was also found to have antifungal activity against the pathogenic fungi Cylindrocarpon destructans. In conclusion, this study found DCY113^T to be a novel species within the genus Paraburkholderia, for which the name P. panacisoli is proposed. The type strain is DCY113^T (= KCTC 52951^T = JCM 32098^T).

A thermostable chitinase from the antagonistic Chromobacterium violaceum that inhibits the development of phytopathogenic fungi

The biocontrol activity of some soil strains of Chromobacterium sp. against pathogenic fungi has been attributed to secreted chitinases. The aim of this work was to characterize biochemically a recombinant chitinase (CvChi47) from C. violaceum ATCC 12472 and to investigate its effects on phytopathogenic fungi. CvChi47 is a modular enzyme with 450 amino acid residues, containing a type I signal peptide at the N-terminal region, followed by one catalytic domain belonging to family 18 of the glycoside hydrolases, and two type-3 chitin-binding domains at the C-terminal end. The recombinant enzyme was expressed in Escherichia coli as a His-tagged protein and purified to homogeneity. The native signal peptide of CvChi47 was used to direct its secretion into the culture medium, from where the recombinant product was purified by affinity chromatography on chitin and immobilized metal. The purified protein showed an apparent molecular mass of 46 kDa, as estimated by denaturing polyacrylamide gel electrophoresis, indicating the removal of the signal peptide. CvChi47 was a thermostable protein, retaining approximately 53.7% of its activity when heated at 100 °C for 1 h. The optimum hydrolytic activity was observed at 60 °C and pH 5. The recombinant chitinase inhibited the conidia germination of the phytopathogenic fungi Fusarium oxysporum and F. guttiforme, hence preventing mycelial growth. Furthermore, atomic force microscopy experiments revealed a pronounced morphological alteration of the cell surface of conidia incubated with CvChi47 in comparison to untreated cells. Taken together, these results show the potential of CvChi47 as a molecular tool to control plant diseases caused by these Fusarium species.

Structure-function characterization of an insecticidal protein GNIP1Aa, a member of an MACPF and β-tripod families

The crystal structure of the Gram-negative insecticidal protein, GNIP1Aa, has been solved at 2.5-Å resolution. The protein consists of two structurally distinct domains, a MACPF (membrane attack complex/PerForin) and a previously uncharacterized type of domain. GNIP1Aa is unique in being a prokaryotic MACPF member to have both its structure and function identified. It was isolated from a Chromobacterium piscinae strain and is specifically toxic to Diabrotica virgifera virgifera larvae upon feeding. In members of the MACPF family, the MACPF domain has been shown to be important for protein oligomerization and formation of transmembrane pores, while accompanying domains define the specificity of the target of the toxicity. In GNIP1Aa the accompanying C-terminal domain has a unique fold composed of three pseudosymmetric subdomains with shared sequence similarity, a feature not obvious from the initial sequence examination. Our analysis places this domain into a protein family, named here β-tripod. Using mutagenesis, we identified functionally important regions in the β-tripod domain, which may be involved in target recognition.

Enhanced Tolerance of Chinese Cabbage Seedlings Mediated by Bacillus aryabhattai H26-2 and B. siamensis H30-3 against High Temperature Stress and Fungal Infections

Two rhizobacteria Bacillus aryabhattai H26-2 and B. siamensis H30-3 were evaluated whether they are involved in stress tolerance against drought and high temperature as well as fungal infections in Chinese cabbage plants. Chinese cabbage seedlings cv. Ryeokgwang (spring cultivar) has shown better growth compared to cv. Buram-3-ho (autumn cultivar) under high temperature conditions in a greenhouse, whilst there was no difference in drought stress tolerance of the two cultivars. In vitro growth of B. aryabhattai H26-2 and B. siamensis H30-3 were differentially regulated under PEG 6000-induced drought stress at different growing temperatures (30, 40 and 50°C). Pretreatment with B. aryabhattai H26-2 and B. siamensis H30-3 enhanced the tolerance of Chinese cabbage seedlings to high temperature, but not to drought stress. It turns out that only B. siamensis H30-3 showed in vitro antifungal activities and in planta crop protection against two fungal pathogens Alternaria brassicicola and Colletotrichum higginsianum causing black spots and anthracnose on Chinese cabbage plants cv. Ryeokgwang, respectively. B. siamensis H30-3 brings several genes involved in production of cyclic lipopeptides in its genome and secreted hydrolytic enzymes like chitinase, protease and cellulase. B. siamensis H30-3 was found to produce siderophore, a high affinity iron-chelating compound. Expressions of BrChi1 and BrGST1 genes were up-regulated in Chinese cabbage leaves by B. siamensis H30-3. These findings suggest that integration of B. aryabhattai H26-2 and B. siamensis H30-3 in Chinese cabbage production system may increase productivity through improved plant growth under high temperature and crop protection against fungal pathogens.

Ornithinimicrobium panacihumi sp. nov., Antagonistic Bacteria Against Root Rot Fungal Pathogens, Isolated from Cultivated Ginseng Soil

A Gram-positive bacterium (DCY118^T) was isolated from ginseng-cultivated soil in Gochang-gun, Republic of Korea. This isolate was assigned to the genus Ornithinimicrobium and is closely related to Ornithinimicrobium kibberense K22-20^T (98.8%), O. pekingense DSM 21552^T (98.5%), O. algicola JC311^T (98.2%), and O. humiphilum DSM 12362^T (97.9%) based on 16S rRNA gene sequence analysis. However, strain DCY118^T showed < 55% DNA-DNA homology with closely related reference strains. Cells were non-motile, non-sporulating, catalase- and oxidase-positive, aerobic, short rods, and cocci, and produced light-yellow, circular, and smooth colonies on TSA medium. MK-8(H₄) was the predominant menaquinone. The major cellular fatty acids were iso-C_15:0, anteiso-C_15:0, and C_16:0. The polar lipid profile consisted of diphosphatidylglycerol (DPG), phosphatidylglycerol (PG), phosphatidylinositol (PI), an unknown phospholipid (PL1), an unknown amino lipid (AL1), and unidentified polar lipids (L1-5). The genomic DNA G+C content was 71.1 mol%. The peptidoglycan contained L-ornithine as the diagnostic diamino acid. Whole-cell sugars were composed of glucose, arabinose, and xylose. Overall, data collected from phenotypic and genotypic tests during this study indicated that strain DCY118^T could not be assigned to a recognized species. Strain DCY118^T showed antagonistic activity against the fungal pathogens causing root rot in ginseng, i.e., Fusarium solani (KACC 44891^T) and Cylindrocarpon destructans (KACC 44660^T). The results from this study confirm the DCY118^T strain as a new species within the genus Ornithinimicrobium, for which the name Ornithinimicrobium panacihumi is proposed. The type strain is DCY118^T (=KCTC 39962^T=JCM 32156^T).