Molecular diversity of yam virus Y and identification of banana mild mosaic virus isolates infecting yam (Dioscorea spp.).

Two members of the family Betaflexiviridae associated with yam (Dioscorea spp.) have been described so far: yam latent virus (YLV) and yam virus Y (YVY). However, their geographical distribution and molecular diversity remain poorly documented. Using a nested RT-PCR assay, we detected YVY in D. alata, D. bulbifera, D. cayenensis, D. rotundata, and D. trifida in Guadeloupe, and in D. rotundata in Côte d'Ivoire, thus extending the known host range of this virus and geographical distribution. Using amplicon sequencing, we determined that the molecular diversity of YVY in the yam samples analyzed in this work ranged between 0.0 and 29.1% and that this diversity is partially geographically structured. We also identified three isolates of banana mild mosaic virus (BanMMV) infecting D. alata in Guadeloupe, providing the first evidence for BanMMV infection in yam.

Epidemiology of Yam Viruses in Guadeloupe: Role of Cropping Practices and Seed-Tuber Supply.

The epidemiology of yam viruses remains largely unexplored. We present a large-scale epidemiological study of yam viruses in Guadeloupe based on the analysis of 1124 leaf samples collected from yams and weeds. We addressed the prevalence of cucumber mosaic virus (CMV), Cordyline virus 1 (CoV1), Dioscorea mosaic associated virus (DMaV), yam asymptomatic virus 1 (YaV1), yam mosaic virus (YMV), yam mild mosaic virus (YMMV), badnaviruses, macluraviruses and potexviruses, and the key epidemiological drivers of these viruses. We provide evidence that several weeds are reservoirs of YMMV and that YMMV isolates infecting weeds cluster together with those infecting yams, pointing to the role of weeds in the epidemiology of YMMV. We report the occurrence of yam chlorotic necrosis virus (YCNV) in Guadeloupe, the introduction of YMMV isolates through the importation of yam tubers, and the absence of vertical transmission of YaV1. We identified specific effects on some cropping practices, such as weed management and the use of chemical pesticides, on the occurrence of a few viruses, but no crop-related factor had a strong or general effect on the overall epidemiology of the targeted viruses. Overall, our work provides insights into the epidemiology of yam viruses that will help design more efficient control strategies.

Viruses of Yams (Dioscorea spp.): Current Gaps in Knowledge and Future Research Directions to Improve Disease Management.

Viruses are a major constraint for yam production worldwide. They hamper the conservation, movement, and exchange of yam germplasm and are a threat to food security in tropical and subtropical areas of Africa and the Pacific where yam is a staple food and a source of income. However, the biology and impact of yam viruses remains largely unknown. This review summarizes current knowledge on yam viruses and emphasizes gaps that exist in the knowledge of the biology of these viruses, their diagnosis, and their impact on production. It provides essential information to inform the implementation of more effective virus control strategies.

Risk Assessment of Infectious Endogenous Banana Streak Viruses in Guadeloupe.

Infectious alleles of endogenous banana streak viruses (eBSVs) are present in the genome of all banana interspecific cultivars, including plantains and cooking types. Activation of these infectious eBSV alleles by biotic and abiotic stresses leads to spontaneous infections by cognate viruses and raises concerns about their ability to promote outbreaks of banana streak viruses under field cultivation conditions. We undertook a comprehensive risk assessment study of infectious eBSV alleles of species BSOLV, BSGFV and BSIMV in banana interspecific cultivars in Guadeloupe, a tropical island of the Caribbean where bananas are grown for export and local markets. We carried out a prevalence survey of BSOLV, BSGFV and BSIMV species in a range of cultivars grown in Guadeloupe. Our results suggest that BSOLV and BSGFV infections arise from the activation of infectious eBSVs rather than vector-borne transmission and point to a correlation between altitude and infection rates in interspecific hybrids with AAB genotypes. We studied the dynamics of activation of infectious eBSOLV and eBSGFV alleles by tissue culture and field cultivation in a range of cultivars. We showed that tissue culture and field cultivation trigger distinct activation pathways, resulting in distinct activation patterns. We also showed that activation decreased over time during cell culture and field cultivation and that BSV infections arising from the activation of infectious eBSV alleles cause symptomless infections in the most cultivated plantain in Guadeloupe, French Clair. Overall, our study shows that the risk of BSV outbreaks resulting from the activation of infectious eBSVs in plantain originating from vegetative multiplication is negligible in Guadeloupe.

Molecular characterization of Cordyline virus 1 isolates infecting yam (Dioscorea spp).

Cordyline virus 1 (CoV1) is a velarivirus that has so far only been reported in ornamental Ti plants (Cordyline fruticosa). Using high-throughput sequencing, we identified CoV1 infection in yam accessions from Vanuatu. Using a specific RT-PCR assay, we found that CoV1 is also present and highly prevalent in Dioscorea alata, D. cayenensis, and D. trifida in Guadeloupe. Phylogenetic analysis showed that CoV1 isolates infecting yam in Guadeloupe display a low level of molecular diversity. These data provide insights into the transmission of CoV1 in yam in Guadeloupe.

Host range and molecular variability of the sadwavirus dioscorea mosaic associated virus.

Dioscorea mosaic associated virus (DMaV) is a member of the genus Sadwavirus, family Secoviridae, that is associated with mosaic symptoms in Dioscorea rotundata in Brazil. The genome of a DMaV isolate detected in D. trifida in Guadeloupe was sequenced by high-throughput sequencing. Using an RT-PCR-based detection assay, we found that DMaV infects D. alata, D. bulbifera, D. cayenensis-rotundata, D. esculenta, and D. trifida accessions conserved in Guadeloupe and Côte d'Ivoire and displays a very high level of molecular diversity in a relatively small region of the genome targeted by the assay. We also provide evidence that DMaV is also present in D. rotundata in Benin and in D. alata in Nigeria.

Critical Evaluation of Cross-Sectoral Collaborations to Inform the Implementation of the "One Health" Approach in Guadeloupe.

In Guadeloupe, a French overseas territory located in the Eastern Caribbean, infectious and non-infectious diseases, loss of biodiversity, natural disasters and global change threaten the health and well-being of animals, plants, and people. Implementing the "One Health" (OH) approach is crucial to reduce the archipelago's vulnerability to these health threats. However, OH remains underdeveloped in Guadeloupe, hampering efficient and effective intersectoral and transdisciplinary collaborations for disease surveillance and control. A multidisciplinary research group of volunteer researchers working in Guadeloupe, with collective expertise in infectious diseases, undertook a study to identify key attributes for OH operationalization by reviewing past and current local collaborative health initiatives and analyzing how much they mobilized the OH framework. The research group developed and applied an operational OH framework to assess critically collaborative initiatives addressing local health issues. Based on a literature review, a set of 13 opinion-based key criteria was defined. The criteria and associated scoring were measured through semi-directed interviews guided by a questionnaire to critically evaluate four initiatives in animal, human, plant, and environmental health research and epidemiological surveillance. Gaps, levers, and prospects were identified that will help health communities in Guadeloupe envision how to implement the OH approach to better address local health challenges. The methodology is simple, generic, and pragmatic and relies on existing resources. It can be transposed and adapted to other contexts to improve effectiveness and efficiency of OH initiatives, based on lessons-learned of local past or current multi-interdisciplinary and intersectoral initiatives.

Detection and diversity of viruses infecting African yam (Dioscorea rotundata) in a collection and F1 progenies in Côte d'Ivoire shed light to plant-to-plant viral transmission.

Yam (Dioscorea spp.) is a major staple food whose production is hampered by viral diseases. However, the prevalence, diversity, transmission, and impact of yam-infecting viruses remain poorly documented. This study reports on the symptomatology, prevalence, and molecular diversity of eight viruses in 38 D. rotundata accessions from a germplasm collection and 206 F1 hybrid progenies maintained in Côte d'Ivoire. Mean severity scores as assessed from leaf symptoms ranged from 2 to 4 in the germplasm collection and from 1 to 3 in F1 hybrids, respectively. Dioscorea mosaic-associated virus (DMaV), potexviruses, and yam mosaic virus (YMV) were detected by PCR-based diagnosis tools in single and mixed infections in both the D. rotundata collection and F1 progenies, whereas badnaviruses were detected only in the germplasm collection. In contrast, cucumber mosaic virus (CMV), yam macluraviruses, yam asymptomatic virus 1 (YaV1), and yam mild mosaic virus (YMMV) could not be detected. No correlation could be established between severity scores and indexing results. Phylogenetic analysis performed on partial viral sequences amplified from infected samples unveiled the presence of two putative novel viral species belonging to genera Badnavirus and Potexvirus and provided evidence for plant-to-plant transmission of YMV, DMaV, and yam potexviruses.

Molecular Viral Diagnosis and Sanitation of Yam Genetic Resources: Implications for Safe Yam Germplasm Exchange.

Yam (Dioscorea spp.) is an important crop in tropical and subtropical regions. Many viruses have been recently identified in yam, hampering genetic conservation and safe international exchanges of yam germplasm. We report on the implementation of reliable and cost-effective PCR-based detection tools targeting eight different yam-infecting viruses. Viral indexing of the in vitro yam collection maintained by the Biological Resources Center for Tropical Plants (BRC-TP) in Guadeloupe (French West Indies) unveiled a high prevalence of potyviruses, badnaviruses, Dioscorea mosaic associated virus (DMaV) and yam asymptomatic virus 1 (YaV1) and a high level of coinfections. Infected yam accessions were subjected to a combination of thermotherapy and meristem culture. Sanitation levels were monitored using PCR-based and high-throughput sequencing-based diagnosis, confirming the efficacy and reliability of PCR-based detection tools. Sanitation rates were highly variable depending on viruses. Sixteen accessions were successfully sanitized, paving the way to safe yam germplasm exchanges and the implementation of clean seed production programs worldwide.

ICTV Virus Taxonomy Profile: Caulimoviridae.

Caulimoviridae is a family of non-enveloped reverse-transcribing plant viruses with non-covalently closed circular dsDNA genomes of 7.1-9.8 kbp in the order Ortervirales. They infect a wide range of monocots and dicots. Some viruses cause economically important diseases of tropical and subtropical crops. Transmission occurs through insect vectors (aphids, mealybugs, leafhoppers, lace bugs) and grafting. Activation of infectious endogenous viral elements occurs in Musa balbisiana, Petunia hybrida and Nicotiana edwardsonii. However, most endogenous caulimovirids are not infectious. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Caulimoviridae, which is available at ictv.global/report/caulimoviridae.

Yam asymptomatic virus 1, a novel virus infecting yams (Dioscorea spp.) with significant prevalence in a germplasm collection.

A novel virus infecting yams (Dioscorea spp.), tentatively named "yam asymptomatic virus 1" (YaV1), was characterized and sequenced from an asymptomatic D. alata plant from Vanuatu. Sequence comparisons and phylogenetic analysis showed that YaV1 is a novel ampelovirus and has the smallest genome among "subgroup 1" members. RT-PCR-based screening of a yam germplasm collection conserved in Guadeloupe showed that YaV1 is prevalent in D. alata, D. bulbifera, D. cayennensis subsp. rotundata, D. esculenta and D. trifida accessions but causes no apparent symptoms. Additional phylogenetic analysis revealed a low variability of YaV1 in Guadeloupe in a limited part of the genome, and suggested the occurrence of plant-to-plant transmission.

The genome sequence of Dioscorea bacilliform TR virus, a member of the genus Badnavirus infecting Dioscorea spp., sheds light on the possible function of endogenous Dioscorea bacilliform viruses.

The complete genome sequence of Dioscorea bacilliform TR virus (DBTRV) was determined. The closest relatives of DBTRV are Dioscorea bacilliform AL virus (DBALV) and Dioscorea bacilliform RT virus 1 (DBRTV1). Specific primers were designed and used to determine the prevalence of DBTRV in a yam germplasm collection. It was found that this virus infects Dioscorea alata and D. trifida plants in Guadeloupe and French Guyana. DTRBV was not detected in any of the tested D. cayenensis-rotundata accessions. In silico analysis provided evidence for the presence of DBTRV-like endogenous sequences in the genome of D. cayenensis-rotundata, pointing to a possible role of these sequences in antiviral defense.

The genome of African yam (Dioscorea cayenensis-rotundata complex) hosts endogenous sequences from four distinct Badnavirus species.

Several endogenous viral elements (EVEs) have been identified in plant genomes, including endogenous pararetroviruses (EPRVs). Here, we report the first characterization of EPRV sequences in the genome of African yam of the Dioscorea cayenensis-rotundata complex. We propose that these sequences should be termed 'endogenous Dioscorea bacilliform viruses' (eDBVs). Molecular characterization of eDBVs shows that they constitute sequences originating from various parts of badnavirus genomes, resulting in a mosaic structure that is typical of most EPRVs characterized to date. Using complementary molecular approaches, we show that eDBVs belong to at least four distinct Badnavirus species, indicating multiple, independent, endogenization events. Phylogenetic analyses of eDBVs support and enrich the current taxonomy of yam badnaviruses and lead to the characterization of a new Badnavirus species in yam. The impact of eDBVs on diagnosis, yam germplasm conservation and movement, and breeding is discussed.

The T-DNA oncogene A4-orf8 from Agrobacterium rhizogenes A4 induces abnormal growth in tobacco.

The related orf8 and iaaM T-DNA genes from Agrobacterium are each composed of two distinct parts. The 5' parts (called Norf8 or NiaaM) encode a 200-amino-acid (aa) sequence with homology to various T-DNA oncoproteins such as RolB, RolC, and 6b. The 3' parts (Corf8 or CiaaM) encode a 550-aa sequence with homology to IaaM proteins from Pseudomonas and Pantoea spp. Whereas iaaM genes encode flavin adenine dinucleotide (FAD)-dependent tryptophan 2-monooxygenases that catalyze the synthesis of indole-3-acetamide (IAM), A4-orf8 from Agrobacterium rhizogenes A4 does not. Plants expressing a 2x35S-A4-Norf8 construct accumulate soluble sugars and starch. We now have regenerated plants that express the full-size 2x35S-A4-orf8 and the truncated 2x35S-A4-Corf8 gene. 2x35S-A4-Corf8 plants accumulate starch and show reduced growth like 2x35S-A4-Norf8 plants but, in addition, display a novel set of characteristic growth modifications. These consist of leaf hypertrophy and hyperplasia (blisters); thick, dark-green leaves; thick stems; and swollen midveins. Mutations in the putative FAD-binding site of A4-Orf8 did not affect the blister syndrome. Plants expressing 2x35S-A4-Corf8 had a normal phenotype but contained less starch and soluble sugars than did wild-type plants. When 2x35S-A4-Corf8 plants were crossed to starch-accumulating 2x35S-A4-Norf8 plants with reduced growth, A4-Corf8 partially restored growth and reduced starch accumulation. A4-Corf8xA4-Norf8 crosses did not lead to the blister syndrome, suggesting that this requires physical linkage of the A4-NOrf8 and A4-COrf8 sequences.

The rolB-like part of the Agrobacterium rhizogenes orf8 gene inhibits sucrose export in tobacco.

Many Agrobacterium T-DNA genes belong to the highly diverse rolB family. The mode of action of most of these genes is still unknown. rolB-like sequences also are present at the 5' ends of the T-DNA-located iaaM genes and the iaaM homolog orf8, whereas iaaM genes from Pseudomonas and Erwinia spp. lack such sequences. iaaM genes encode tryptophan monooxygenases; these enzymes convert tryptophan into indole-3-acetamide, a precursor of indole-3-acetic acid. Tobacco plants expressing the rolB-like part of the A4 orf8 gene (2x35S-A4-Norf8 plants) accumulate glucose, fructose, sucrose, and starch and resemble sucrose transporter (NtSUT1) antisense plants. Different lines of evidence indicate that 2x35S-A4-Norf8 plants export less sucrose from source leaves. Glucose, fructose, sucrose, and starch accumulate in source leaves during sink-source transition, whereas sink tissues like petioles and midveins contain lower levels than normal. Petiole exudation experiments demonstrate a significant decrease in export of label after 14C-sucrose infiltration and after 14CO2 labeling. Grafting of stunted homozygous 2x35S-A4-Norf8 plants onto wild-type rootstocks restores growth, indicating that unloading is not affected. Growth of 2x35S-A4-Norf8 seedlings is inhibited on naphthalene acetic acid-containing media, suggesting a link between sucrose transport and auxin sensitivity.