Molecular Epidemiology of Ralstonia solanacearum Species Complex Strains Causing Bacterial Wilt of Potato in Uganda

Molecular Epidemiology of Ralstonia pseudosolanacearum Phylotype I Strains in the Southwest Indian Ocean Region and Their Relatedness to African Strains

The increasing requirement for developing tools enabling fine strain traceability responsible for epidemics is tightly linked with the need to understand factors shaping pathogen populations and their environmental interactions. Bacterial wilt caused by the Ralstonia solanacearum species complex (RSSC) is one of the most important plant diseases in tropical and subtropical regions. Sadly, little, outdated, or no information on its epidemiology is reported in the literature, although alarming outbreaks are regularly reported as disasters. A large set of phylotype I isolates (n = 2,608) was retrieved from diseased plants in fields across the Southwest Indian Ocean (SWIO) and Africa. This collection enabled further assessment of the epidemiological discriminating power of the previously published RS1-MLVA14 scheme. Thirteen markers were validated and characterized as not equally informative. Most had little infra-sequevar polymorphism, and their performance depended on the sequevar. Strong correlation was found with a previous multilocus sequence typing scheme. However, 2 to 3% of sequevars were not correctly assigned through endoglucanase gene sequence. Discriminant analysis of principal components (DAPC) revealed four groups with strong phylogenetic relatedness to sequevars 31, 33, and 18. Phylotype I-31 isolates were highly prevalent in the SWIO and Africa, but their dissemination pathways remain unclear. Tanzania and Mauritius showed the greatest diversity of RSSC strains, as the four DAPC groups were retrieved. Mauritius was the sole territory harboring a vast phylogenetic diversity and all DAPC groups. More research is still needed to understand the high prevalence of phylotype I-31 at such a large geographic scale.

Potato bacterial wilt in Ethiopia: history, current status, and future perspectives

Background

Potato is an essential food staple and a critical tuber crop for rural livelihoods in Ethiopia, where many pathogenic pests are threatening production. Bacterial wilt, also known as brown rot of potato, ranks among the diseases that most affect many potato farmers in Ethiopia and the disease losses dramatically threatening the vibrant potato sector even in the highlands of the country where it has been uncommon so far.

Methodology

To devise a strategy towards boosting potato productivity in Ethiopia where food insecurity is most prevalent, production constraints should be investigated and properly addressed. Hence, we have used existing reviews and reports on the subjects, such as textbooks, and proceeding and conference abstracts in Plant Protection Society of Ethiopia; Web of Science; Google Scholar; Research Gate and CIP's database to document most relevant information on the occurrence, distribution, and disease management of bacterial wilt in Ethiopia.

Results

Provision of comprehensive information on potato bacterial wilt occurrence, distribution, and management techniques are crucial for potato growers, researchers and stakeholders engaged on potato industry. In this review, we provided insights on the history, status, and future perspectives of potato bacterial wilt in Ethiopia.

Conclusions

Awareness of potato bacterial wilt and integrated disease management approaches could bring a fundamental impact to the farming community mostly to smallholder farmers in developing countries. This document compiled such imperative information targeting bacterial wilt management techniques to ensure food security.

Ralstonia wenshanensis sp. nov., a novel bacterium isolated from a tobacco field in Yunnan, China

A Gram-negative, aerobic, motile with paired polar flagella and rod-shaped bacterium strain (56D2^T) was isolated from tobacco planting soil in Yunnan, PR China. Major fatty acids were C_{16  :  1} ω7c (summed feature 3), C_{16  :  0} and C_{18  :  1}  ω7c (summed feature 8). The polar lipid profile of strain 56D2^T consisted of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, one unidentified aminophospholipid and one unidentified glycolipid. Moreover, strain 56D2^T contained ubiquinone Q-8 as the sole respiratory quinone. 16S rRNA gene sequence analysis showed that strain 56D2^T was closely related to members of the genus Ralstonia and the two type strains with the highest sequence identities were R. mannitolilytica LMG 6866^T (98.36 %) and R. pickettii K-288^T (98.22 %). The 16S rRNA gene sequence identities between strain 56D2^T and other members of the genus Ralstonia were below 98.00 %. Genome sequencing revealed a genome size of 5.87 Mb and a G+C content of 63.7 mol%. The average nucleotide identity values between strain 56D2^T and R. pickettii K-288^T, R. mannitolilytica LMG 6866 ^T and R. insidiosa CCUG 46789^T were less than 95 %, and the in silico DNA-DNA hybridization values (yielded by formula 2) were less than 70 %. Based on these data, we conclude that strain 56D2^T represents a novel species of the genus Ralstonia, for which the name Ralstonia wenshanensis sp. nov. is proposed. The type strain of Ralstonia wenshanensis sp. nov. is 56D2^T (=CCTCC AB 2021466^T=GDMCC 1.2886^T=JCM 35178^T).

Ralstonia Strains from Potato-Growing Regions of Kenya Reveal Two Phylotypes and Epidemic Clonality of Phylotype II Sequevar 1 Strains

Bacterial wilt, caused by the Ralstonia solanacearum species complex (RSSC), is the most destructive potato disease in Kenya. Studies were conducted to (i) determine the molecular diversity of RSSC strains associated with bacterial wilt of potato in Kenya, (ii) generate an RSSC distribution map for epidemiological inference, and (iii) determine whether phylotype II sequevar 1 strains exhibit epidemic clonality. Surveys were conducted in 2018 and 2019, in which tubers from wilting potato plants and stem samples of potential alternative hosts were collected for pathogen isolation. The pathogen was phylotyped by multiplex PCR and 536 RSSC strains typed at a sequevar level. Two RSSC phylotypes were identified, phylotype II (98.4%, n = 506 [sequevar 1 (n = 505) and sequevar 2 (n = 1)]) and phylotype I (1.6%, n = 30 [sequevar 13 (n = 9) and a new sequevar (n = 21)]). The phylotype II sequevar 1 strains were haplotyped using multilocus tandem repeat sequence typing (TRST) schemes. The TRST scheme identified 51 TRST profiles within the phylotype II sequevar 1 strains with a modest diversity index (HGDI = 0.87), confirming the epidemic clonality of RSSC phylotype II sequevar 1 strains in Kenya. A minimum spanning tree and mapping of the TRST profiles revealed that TRST27 '8-5-12-7-5' is the primary founder of the clonal complex of RSSC phylotype II sequevar 1 and is widely distributed via latently infected seed tubers. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Isolation and Characterization of Novel Lytic Bacteriophage vB_RsoP_BMB50 infecting Ralstonia solanacearum

Ralstonia solanacearum is a soil-borne phytopathogen, and it can cause bacterial wilt disease in a variety of key crops around the world, thus resulting in enormous financial losses. However, there is a lack of effective, green, and safe prevention and control measures against increasingly devastating bacterial wilt disease. Bacteriophages (phages) are considered as potential biocontrol agents against bacterial wilt disease. Although many phages infecting R. solanacearum have been isolated, so far, these Ralstonia phages are still insufficient to deal with the diversity of the bacteria of R. solanacearum. In this study, a novel lytic bacteriophage vB_RsoP_BMB50 infecting multiple R. solanacearum was isolated from tomato fields in Dalian, China. Transmission electron microscopy and genomics analysis indicated that vB_RsoP_BMB50 belonged to the subfamily Okabevirinae, Autographiviridae family, and order Caudovirales, and it comprised a double-stranded DNA with a full length of 43,665 bp and a mean G+C content of 61.79%, containing 53 open reading frames (ORFs). This novel phage exhibited a large burst size, high temperature stability (4-50 °C), and strong pH tolerance (pH 5-10). Comparative analyses and phylogenetic analyses revealed that vB_RsoP_BMB50 represented a novel Ralstonia phage genus since it exhibited a low sequence similarity to other phages in the GenBank database. Due to its broad lytic spectrum, high thermal stability, and strong pH tolerance, vB_RsoP_BMB50 is considered as an effective candidate biocontrol agent against bacterial wilt disease caused by R. solanacearum.

Phylogenetic Distribution of Ralstonia solanacearum Species Complex Populations in Potato in Kenya

Ralstonia solanacearum is a pathogen causing bacterial wilt disease of potato, resulting in 70% potato production losses in Kenya. A study was conducted to determine the diversity of R. solanacearum species complex strains within the main potato-growing regions of Kenya. Potato tubers were collected in different potato-growing regions of Kenya from visibly wilted potato plants as well as samples of tomato, irrigation water, and cultures for pathogen isolation. Genomic DNA was isolated from 135 purified cultures of RSSC isolates and PCR-amplified using multiplex and sequevar primers targeting the endoglucanase (egl) partial gene sequences. Pathogenicity tests using R. solanacearum strain (phylotype II sequevar I) were done on the cultivars Kenya Karibu, Shangi, Chulu, Wanjiku, and MoneyMaker. Phylogenetic analysis of the partial egl gene identified two genospecies, R. pseudosolanacearum sp. nov. (1.5%) and R. solanacearum (98.5%). All R. solanacearum strains clustered in sequevar I and were distributed in all the potato-growing regions surveyed. The cultivars were grown in a greenhouse for two cycles in a randomized complete block design and inoculated with R. solanacearum strain. The severity scores were assessed and the area under the disease progress curve (AUDPC) was determined. All the cultivars tested for pathogenicity exhibited wilting symptoms at varying intervals after infection, with none showing complete resistance to R. solanacearum. Cultivar Shangi exhibited minimum disease severity and progression of 41.14% and AUDPC of 1041.7, respectively, while 'Kenya Karibu' was the most susceptible with a high progression rate of 68.24% and AUDPC of 1897.5, respectively. 'MoneyMaker', 'Chulu', and 'Wanjiku' showed no significant difference in disease severity, depicting a simultaneous rate of infection among them. These findings provide valuable information to better understand the pathogen genetic diversity in Kenya and how it spreads.

A cysteine-rich receptor-like protein kinase CaCKR5 modulates immune response against Ralstonia solanacearum infection in pepper

BACKGROUND

Cysteine-rich receptor-like kinases (CRKs) represent a large subfamily of receptor-like kinases and play vital roles in diverse physiological processes in regulating plant growth and development.

RESULTS

CaCRK5 transcripts were induced in pepper upon the infection of Ralstonia solanacearum and treatment with salicylic acid. The fusions between CaCRK5 and green fluorescence protein were targeted to the plasma membrane. Suppression of CaCRK5 via virus-induced gene silencing (VIGS) made pepper plants significantly susceptible to R. solanacearum infection, which was accompanied with decreased expression of defense related genes CaPR1, CaSAR8.2, CaDEF1 and CaACO1. Overexpression of CaCRK5 increased resistance against R. solanacearum in Nicotiana benthamiana. Furthermore, electrophoretic mobility shift assay and chromatin immunoprecipitation coupled with quantitative real-time PCR analysis revealed that a homeodomain zipper I protein CaHDZ27 can active the expression of CaCRK5 through directly binding to its promoter. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) analyses suggested that CaCRK5 heterodimerized with the homologous member CaCRK6 on the plasma membrane.

CONCLUSIONS

Our data revealed that CaCRK5 played a positive role in regulating immune responses against R. solanacearum infection in pepper.

Molecular Diversity and Pathogenicity of Ralstonia solanacearum Species Complex Associated With Bacterial Wilt of Potato in Rwanda

Bacterial wilt (BW), caused by Ralstonia solanacearum species complex (RSSC), leads to substantial potato yield losses in Rwanda. Studies were conducted to (i) determine the molecular diversity of RSSC strains associated with BW of potato, (ii) generate an RSSC distribution map for epidemiological inferences, and (iii) test the pathogenicity of predominant RSSC phylotypes on six commercial potato cultivars. In surveys conducted in 2018 and 2019, tubers from wilting potato plants were collected for pathogen isolation. DNA was extracted from 95 presumptive RSSC strain colonies. The pathogen was phylotyped by multiplex PCR and typed at sequevar level. Phylotype II sequevar 1 strains were then haplotyped using multilocus tandem repeat sequence typing (TRST) schemes. Pathogenicity of one phylotype II strain and two phylotype III strains were tested on cultivars Kinigi, Kirundo, Victoria, Kazeneza, Twihaze, and Cruza. Two RSSC phylotypes were identified, phylotype II (95.79%, n = 91) and phylotype III (4.21%, n = 4). This is the first report of phylotype III strains from Rwanda. Phylotype II strains were identified as sequevar 1 and distributed across potato growing regions in the country. The TRST scheme identified 14 TRST haplotypes within the phylotype II sequevar 1 strains with moderate diversity index (HGDI = 0.55). Mapping of TRST haplotypes revealed that a single TRST '8-5-12-7-5' haplotype plays an important epidemiological role in BW of potato in Rwanda. None of the cultivars had complete resistance to the tested phylotypes; the level of susceptibility varied among cultivars. Cultivar Cruza, which is less susceptible to phylotype II and III strains, is recommended when planting potatoes in the fields with history of BW.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Contrasting genetic diversity and structure among Malagasy Ralstonia pseudosolanacearum phylotype I populations inferred from an optimized Multilocus Variable Number of Tandem Repeat Analysis scheme

The Ralstonia solanacearum species complex (RSSC), composed of three species and four phylotypes, are globally distributed soil-borne bacteria with a very broad host range. In 2009, a devastating potato bacterial wilt outbreak was declared in the central highlands of Madagascar, which reduced the production of vegetable crops including potato, eggplant, tomato and pepper. A molecular epidemiology study of Malagasy RSSC strains carried out between 2013 and 2017 identified R. pseudosolanacearum (phylotypes I and III) and R. solanacearum (phylotype II). A previously published population biology analysis of phylotypes II and III using two MultiLocus Variable Number of Tandem Repeats Analysis (MLVA) schemes revealed an emergent epidemic phylotype II (sequevar 1) group and endemic phylotype III isolates. We developed an optimized MLVA scheme (RS1-MLVA14) to characterize phylotype I strains in Madagascar to understand their genetic diversity and structure. The collection included isolates from 16 fields of different Solanaceae species sampled in Analamanga and Itasy regions (highlands) in 2013 (123 strains) and in Atsinanana region (lowlands) in 2006 (25 strains). Thirty-one haplotypes were identified, two of them being particularly prevalent: MT007 (30.14%) and MT004 (16.44%) (sequevar 18). Genetic diversity analysis revealed a significant contrasting level of diversity according to elevation and sampling region. More diverse at low altitude than at high altitude, the Malagasy phylotype I isolates were structured in two clusters, probably resulting from different historical introductions. Interestingly, the most prevalent Malagasy phylotype I isolates were genetically distant from regional and worldwide isolates. In this work, we demonstrated that the RS1-MLVA14 scheme can resolve differences from regional to field scales and is thus suited for deciphering the epidemiology of phylotype I populations.

NAC transcription factor involves in regulating bacterial wilt resistance in potato

Bacterial wilt (BW) is a serious disease that affects potato (Solanum tuberosum L.) production. Although resistance to this disease has been reported, the underlying mechanism is unknown. In this study, we identified a NAC family transcription factor (StNACb4) from potato and characterised its structure, function, expression, its localisation at the tissue and its role in BW resistance. To this end, the transgenic Nicotiana benthamiana Domin lines were generated in which the expression of NACb4 was constitutively upregulated or suppressed using RNAi. Different tobacco mutants were stained after inoculating with Ralstonia solanacearum to observe the cell death and callose deposition. The results indicated that StNACb4 could be upregulated under the induction of R. solanacearum, and salicylic acid, abscisic acid and methyl jasmonate could also induce the expression of StNACb4. Tissue localisation analysis indicated that its expression was tissue specific, and it was mainly in the phloem of the vascular system of stems and leaves. NbNACb4 gene silencing can enhance the sensitivity of tobacco to R. solanacearum; on the contrary, StNACb4 gene overexpression can enhance the tolerance of tobacco to R. solanacearum. Meanwhile, StNACb4 gene overexpression can induce cell death and callose deposition in tobacco. The upregulated expression of StNACb4 can also activate the StPR10 gene expression. Our results provide important new insights into the regulatory mechanisms of bacterial wilt resistance in potato.