Genome-wide association study of cassava brown streak disease resistance in cassava germplasm conserved in South America.

Cassava (Manihot esculenta Crantz) is a vital carbohydrate source for over 800 million people globally, yet its production in East Africa is severely affected by cassava brown streak disease (CBSD). Genebanks, through ex-situ conservation, play a pivotal role in preserving crop diversity, providing crucial resources for breeding resilient and disease-resistant crops. This study genotyped 234 South American cassava accessions conserved at the CIAT genebank, previously phenotyped for CBSD resistance by an independent group, to perform a genome-wide association analysis (GWAS) to identify genetic variants associated with CBSD resistance. Our GWAS identified 35 single nucleotide polymorphism (SNP) markers distributed across various chromosomes, associated with disease severity or the presence/absence of viral infection. Markers were annotated within or near genes previously identified with functions related to pathogen recognition and immune response activation. Using the SNP candidates, we screened the world's largest cassava collection for accessions with a higher frequency of favorable genotypes, proposing 35 accessions with potential resistance to CBSD. Our results provide insights into the genetics of CBSD resistance and highlight the importance of genetic resources to equip breeders with the raw materials needed to develop new crop varieties resistant to pests and diseases.

Sampling strategies for genotyping common bean (Phaseolus vulgaris L.) Genebank accessions with DArTseq: a comparison of single plants, multiple plants, and DNA pools.

Introduction: Genotyping large-scale gene bank collections requires an appropriate sampling strategy to represent the diversity within and between accessions. Methods: A panel of 44 common bean (Phaseolus vulgaris L.) landraces from the Alliance Bioversity and The Alliance of Bioversity International and the International Center for Tropical Agriculture (CIAT) gene bank was genotyped with DArTseq using three sampling strategies: a single plant per accession, 25 individual plants per accession jointly analyzed after genotyping (in silico-pool), and by pooling tissue from 25 individual plants per accession (seq-pool). Sampling strategies were compared to assess the technical aspects of the samples, the marker information content, and the genetic composition of the panel. Results: The seq-pool strategy resulted in more consistent DNA libraries for quality and call rate, although with fewer polymorphic markers (6,142 single-nucleotide polymorphisms) than the in silico-pool (14,074) or the single plant sets (6,555). Estimates of allele frequencies by seq-pool and in silico-pool genotyping were consistent, but the results suggest that the difference between pools depends on population heterogeneity. Principal coordinate analysis, hierarchical clustering, and the estimation of admixture coefficients derived from a single plant, in silico-pool, and seq-pool successfully identified the well-known structure of Andean and Mesoamerican gene pools of P. vulgaris across all datasets. Conclusion: In conclusion, seq-pool proved to be a viable approach for characterizing common bean germplasm compared to genotyping individual plants separately by balancing genotyping effort and costs. This study provides insights and serves as a valuable guide for gene bank researchers embarking on genotyping initiatives to characterize their collections. It aids curators in effectively managing the collections and facilitates marker-trait association studies, enabling the identification of candidate markers for key traits.

Identifying genetically redundant accessions in the world's largest cassava collection.

Crop diversity conserved in genebanks facilitates the development of superior varieties, improving yields, nutrition, adaptation to climate change and resilience against pests and diseases. Cassava (Manihot esculenta) plays a vital role in providing carbohydrates to approximately 500 million people in Africa and other continents. The International Center for Tropical Agriculture (CIAT) conserves the largest global cassava collection, housing 5,963 accessions of cultivated cassava and wild relatives within its genebank. Efficient genebank management requires identifying and eliminating genetic redundancy within collections. In this study, we optimized the identification of genetic redundancy in CIAT's cassava genebank, applying empirical distance thresholds, and using two types of molecular markers (single-nucleotide polymorphism (SNP) and SilicoDArT) on 5,302 Manihot esculenta accessions. A series of quality filters were applied to select the most informative and high-quality markers and to exclude low-quality DNA samples. The analysis identified a total of 2,518 and 2,526 (47 percent) distinct genotypes represented by 1 to 87 accessions each, using SNP or SilicoDArT markers, respectively. A total of 2,776 (SNP) and 2,785 (SilicoDArT) accessions were part of accession clusters with up to 87 accessions. Comparing passport and historical characterization data, such as pulp color and leaf characteristic, we reviewed clusters of genetically redundant accessions. This study provides valuable guidance to genebank curators in defining minimum genetic-distance thresholds to assess redundancy within collections. It aids in identifying a subset of genetically distinct accessions, prioritizing collection management activities such as cryopreservation and provides insights for follow-up studies in the field, potentially leading to removal of duplicate accessions.

Genome Analysis and Pathobiology of Cassava-Infecting Torradoviruses Containing a Putative Maf/HAM1 Pyrophosphatase Domain.

Next generation sequencing has been used to identify and characterize the full genome sequence of a cassava-infecting torradovirus, revealing the presence of a Maf/HAM1 domain downstream of the RNA-dependent RNA-polymerase (RdRp) domain in RNA1 in all isolates sequenced. A similar domain is also found in unrelated potyvirids infecting Euphorbiaceae hosts in the Americas and cassava in Africa. Even though cassava torrado-like virus (CsTLV) could not be mechanically transmitted to a series of herbaceous hosts, it can be efficiently transmitted by bud graft-inoculation to different cassava landraces. Our bioassays show that CsTLV has a narrow host range. Crystal-like structures of isometric virus-like particles were observed in cells of plants with single infection by CsTLV, and consistently induced chlorotic leaf spots and affected root yields significantly. Moreover, CsTLV infection induces changes in the accumulation of total sugars in storage roots. Field surveys indicated the presence of CsTLV in the main cassava growing regions of Colombia, and the occurrence of two different cassava-infecting torradovirus species. Profiles of small RNAs of 21 to 24 nucleotides in length, derived from CsTLV RNAs targeted by cassava RNA silencing defense mechanisms, are also reported.

Global Cropland Connectivity: A Risk Factor for Invasion and Saturation by Emerging Pathogens and Pests.

The geographic pattern of cropland is an important risk factor for invasion and saturation by crop-specific pathogens and arthropods. Understanding cropland networks supports smart pest sampling and mitigation strategies. We evaluate global networks of cropland connectivity for key vegetatively propagated crops (banana and plantain, cassava, potato, sweet potato, and yam) important for food security in the tropics. For each crop, potential movement between geographic location pairs was evaluated using a gravity model, with associated uncertainty quantification. The highly linked hub and bridge locations in cropland connectivity risk maps are likely priorities for surveillance and management, and for tracing intraregion movement of pathogens and pests. Important locations are identified beyond those locations that simply have high crop density. Cropland connectivity risk maps provide a new risk component for integration with other factors-such as climatic suitability, genetic resistance, and global trade routes-to inform pest risk assessment and mitigation.

Complete Genome Sequence of Rice hoja blanca tenuivirus Isolated from a Susceptible Rice Cultivar in Colombia.

We describe here the complete genome of Rice hoja blanca tenuivirus The sequenced isolate was obtained by insect vector transmission from a symptomatic rice sample grown in Colombia. Sequence data from the four RNA components were obtained by deep sequencing (Illumina), and infections were confirmed by enzyme-linked immunosorbent assay and Sanger sequencing.

Resolution of cassava-infecting alphaflexiviruses: Molecular and biological characterization of a novel group of potexviruses lacking the TGB3 gene.

Several potexviruses (Family Alphaflexiviridae) have been reported infecting cassava (Manihot esculenta Crantz) in the Americas. They were isolated from severely diseased plants during the last 30-40 years and include: Cassava common mosaic virus (CsCMV), Cassava Caribbean mosaic virus (CsCaMV), Cassava Colombian symptomless virus (CsCSV) and Cassava virus X (CsVX). However, their definitive classification as distinct species remains unresolved for several reasons, including the lack of sequence data and unavailability of samples from original isolates. This complicates disease diagnostics, cassava germplasm exchange certification, evaluation of virus cleaning protocols and epidemiological studies. Furthermore, a recently detected novel alphaflexivirus, indicates that cassava-infecting potexviruses may be more diverse. To solve the identity of these viruses, we started indexing samples from different parts of Colombia using different sets of PCR primers, antisera available and inoculation to indicator plants. Results show that there are three major phylogenetic groups of potexviruses infecting cassava, and they correspond to CsCMV, CsVX and the newly identified Cassava new alphaflexivirus (CsNAV). Bioassays and sequence analysis established that isolates of CsNAV and CsVX cause latent infections in different cassava landraces, they are not efficiently transmitted to the indicator plant Nicotiana benthamiana and they lack the gene 3 of the conserved potexviral 'triple gene block' (TGB). In contrast, all isolates of CsCMV (which have a characteristic potexvirus genome arrangement) caused Cassava Common Mosaic Disease (CCMD) in single infections and were efficiently transmitted to N. benthamiana. Although phylogenetic analysis of the replicase sequence placed CsNAV and CsVX as members of the Potexvirus genus, their distinct genome arrangement and biological characteristics suggest they can be considered as members of a separate taxonomic group.

Complete genome sequence of bean leaf crumple virus, a novel begomovirus infecting common bean in Colombia.

A copy of the complete genome of a novel bipartite begomovirus infecting common bean (Phaseolus vulgaris L.) in Colombia was obtained by rolling-circle amplification (RCA), cloned, and sequenced. The virus is associated with leaf crumple symptoms and significant yield losses in Andean and Mesoamerican beans. Such symptoms have been reported increasingly in Colombia since at least 2002, and we detected the virus in leaf material collected since 2008. Sequence analysis showed that the virus is a member of a distinct species, sharing 81% and 76% nucleotide (nt) sequence identity (in DNA-A and DNA-B, respectively) to other begomoviruses infecting common bean in the Americas. The data obtained support the taxonomic status of this virus (putatively named 'bean leaf crumple virus', BLCrV) as a member of a novel species in the genus Begomovirus.

Enhanced production of human influenza virus in PBS-12SF cells with a reduced interferon response.

Influenza is one of the most important infectious diseases in humans. The best way to prevent severe illness caused by influenza infection is vaccination. Cell culture-derived influenza vaccines are being considered in addition to the widely used egg-based system in order to support the increasing seasonal demand and to be prepared in case of a pandemic. Cell culture based systems offer increased safety, capacity, and flexibility with reduced downstream processing relative to embryonated eggs. We have previously reported a chick embryo cell line, termed PBS-12SF, that supports replication of human and avian influenza A viruses to high titers (>10(7) PFU/ml) without the need for exogenous proteases or serum proteins. Viral infections in cells are limited by the Interferon (IFN) response typified by production of type I IFNs that bind to the IFNα/ß receptor and activate an antiviral state. In this study, we investigated how neutralizing the interferon (IFN) response in PBS-12SF cells, via shRNA-mediated knock-down of IFNAR1 mRNA expression, affects influenza virus production. We were successful in knocking down ∼90% of IFNAR1 protein expression by this method, resulting in a significant decrease in the response to recombinant chIFNα stimulation in PBS-12SF cells as shown by a reduction in expression of interferon-responsive genes when compared to control cells. Additionally; IFNAR1-knock-down cells displayed enhanced viral HA production and released more virus into cell culture supernatants than parental PBS-12SF cells.

Unraveling complex viral infections in cassava (Manihot esculenta Crantz) from Colombia.

In the Americas, different disease symptoms have been reported in cassava including leaf mosaics, vein clearings, mottles, ring spots, leaf distortions and undeveloped and deformed storage roots. Some viruses have been identified and associated with these symptoms while others have been reported in symptomless plants or latent infections. We observed that reoviruses associated with severe root symptoms (RS) of Cassava Frogskin Disease (CFSD) are not associated with leaf symptoms (LS) observed in the cassava indicator plant 'Secundina'. Neither were these LS associated with the previously characterized Cassava common mosaic virus, Cassava virus X, Cassava vein mosaic virus or phytoplasma, suggesting the presence of additional pathogens. In order to explain LS observed in cassava we used a combination of biological, serological and molecular tests. Here, we report three newly described viruses belonging to the families Secoviridae, Alphaflexiviridae and Luteoviridae found in cassava plants showing severe RS associated with CFSD. All tested plants were infected by a mix of viruses that induced distinct LS in 'Secundina'. Out of the three newly described viruses, a member of family Secoviridae could experimentally induce LS in single infection. Our results confirm the common occurrence of complex viral infections in cassava field-collected since the 1980s.

Hepatitis C virus impairs the induction of cytoprotective Nrf2 target genes by delocalization of small Maf proteins.

The expression of a variety of cytoprotective genes is regulated by short cis-acting elements in their promoters, called antioxidant response elements (AREs). A central regulator of ARE-mediated gene expression is the NF-E2-related factor 2 (Nrf2). Nrf2/ARE-regulated genes are crucial for the maintenance of cellular integrity. Hepatitis C virus inhibits the induction of ARE-regulated genes, but neither induction nor inhibition of ARE-regulated gene expression affects HCV replication directly. In HCV-replicating cells the core protein triggers the delocalization of sMaf proteins from the nucleus to the replicon complex. Here sMafs bind to NS3. The extranuclear sMaf proteins prevent Nrf2 from entry in the nucleus and thereby inhibit the induction of Nrf2/ARE-regulated genes. This results in the decreased expression of cytoprotective genes. Consistent with this finding, the elimination of ROI is impaired in HCV-replicating cells as demonstrated by elevated protein oxidation or 8-OH-dG formation, reflecting DNA damage. In conclusion, these data identified a novel mechanism of Nrf2 regulation and suggest that the HCV-dependent inhibition of Nrf2/ARE-regulated genes confers to the HCV-associated pathogenesis by elevation of intracellular ROI that affect integrity of the host genome and regenerative processes.

Hepatitis B virus induces expression of antioxidant response element-regulated genes by activation of Nrf2.

The expression of a variety of cytoprotective genes is regulated by short cis-acting elements in their promoters, called antioxidant response elements (AREs). A central regulator of ARE-mediated gene expression is the NF-E2-related factor 2 (Nrf2). Human hepatitis B virus (HBV) induces a strong activation of Nrf2/ARE-regulated genes in vitro and in vivo. This is triggered by the HBV-regulatory proteins (HBx and LHBs) via c-Raf and MEK. The Nrf2/ARE-mediated induction of cytoprotective genes by HBV results in a better protection of HBV-positive cells against oxidative damage as compared with control cells. Furthermore, there is a significantly increased expression of the Nrf2/ARE-regulated proteasomal subunit PSMB5 in HBV-positive cells that is associated with a decreased level of the immunoproteasome subunit PSMB5i. In accordance with this finding, HBV-positive cells display a higher constitutive proteasome activity and a decreased activity of the immunoproteasome as compared with control cells even after interferon α/γ treatment. The HBV-dependent induction of Nrf2/ARE-regulated genes might ensure survival of the infected cell, shape the immune response to HBV, and thereby promote establishment of the infection.

Localization determines function: N-terminally truncated NS5A fragments accumulate in the nucleus and impair HCV replication.

BACKGROUND/AIMS: The Hepatitis C Virus (HCV) nonstructural protein 5A (NS5A) is an essential part of the ER-localized HCV-replicon complex. Although NS5A harbours a conserved NLS in its C-terminal domain, NS5A is associated with the cytoplasmic face of the ER by an amphipathic helix close to its N-terminus. METHODS: Intracellular distribution of NS5A in HCV replicating cells was analyzed by confocal microscopy and subcellular fractionation. The effect on HCV replication was analyzed using the JFH-1-based infection/replication system. RESULTS: During viral life cycle N-terminally truncated NS5A fragments are caspase-dependent formed that lack the ER-attachment signal and are localized within the nucleus. These N-terminally truncated fragments inhibit HCV replication. If their formation is blocked by inhibition of caspases HCV replication is increased. The C-terminal domain of NS5A binds to c-Raf and thereby localizes it to the replicon complex. This interaction is essential for HCV replication. The N-terminally truncated NS5A fragments are still able to bind c-Raf. However, due to their nuclear localization they withdraw c-Raf from the replicon complex into the nucleus resulting in an impaired HCV replication. CONCLUSIONS: Formation of N-terminally truncated NS5A fragments could represent a mechanism to regulate HCV replication by withdrawal of essential factors from the replicon complex.

Hepatitis C virus infection sensitizes human hepatocytes to TRAIL-induced apoptosis in a caspase 9-dependent manner.

Apoptosis of infected cells represents a key host defense mechanism against viral infections. The impact of apoptosis on the elimination of hepatitis C virus (HCV)-infected cells is poorly understood. The TRAIL has been implicated in the death of liver cells in hepatitis-infected but not in normal liver cells. To determine the impact of TRAIL on apoptosis of virus-infected host cells, we studied TRAIL-induced apoptosis in a tissue culture model system for HCV infection. We demonstrated that HCV infection sensitizes primary human hepatocytes and Huh7.5 hepatoma cells to TRAIL induced apoptosis in a dose- and time-dependent manner. Mapping studies identified the HCV nonstructural proteins as key mediators of sensitization to TRAIL. Using a panel of inhibitors targeting different apoptosis pathways, we demonstrate that sensitization to TRAIL is caspase-9 dependent and mediated in part via the mitochondrial pathway. Sensitization of hepatocytes to TRAIL-induced apoptosis by HCV infection represents a novel antiviral host defense mechanism that may have important implications for the pathogenesis of HCV infection and may contribute to the elimination of virus-infected hepatocytes.