Bipartite Begomovirus Research Articles

Complete sequence of a new bipartite begomovirus infecting Sida sp. in Northeastern Brazil.

In Brazil, non-cultivated plants, especially weeds, are infected with a diversity of begomoviruses and often show striking golden mosaic symptoms. In the present study, leaves showing these symptoms were collected from Sida sp. plants in Guadalupe, Piaui State, Northeastern Brazil, in 2015 and 2016. PCR tests with degenerate primers revealed the presence of begomovirus DNA-A and DNA-B components. Restriction enzyme digestion of rollingcircle-amplified DNA revealed fragments totaling ~5.2 kb, indicating infection by a bipartite begomovirus. The DNA-A and DNA-B components have a genome organization typical of New World (NW) bipartite begomoviruses and a common region of 220 nucleotides (nt) with 96% identity, indicating these are cognate components. Comparisons performed with the DNA-A sequence revealed the highest nt sequence identity (84%) with that of sida angular mosaic virus (SiAMV), whereas those performed with the DNA-B sequence revealed highest identity (77%) with that of sida chlorotic vein virus (SiCVV). In phylogenetic analyses, the DNA-A sequence was placed in a strongly supported clade with SiAMV and SiCVV from Piaui, whereas the DNA-B sequence was placed in a clade with SiCVV and corchorus mottle virus. Based on the current ICTV criteria for the demarcation of begomovirus species (<91% nt sequence identity for the DNA-A component), this is a member of a new species for which the name "Sida yellow golden mosaic virus" is proposed.

Ty-1, a universal resistance gene against geminiviruses that is compromised by co-replication of a betasatellite.

SummaryTomato yellow leaf curl virus (TYLCV), a begomovirus, causes large yield losses and breeding for resistance is an effective way to combat this viral disease. The resistance gene Ty‐1 codes for an RNA‐dependent RNA polymerase and has recently been shown to enhance transcriptional gene silencing of TYLCV. Whereas Ty‐1 was earlier shown to also confer resistance to a bipartite begomovirus, here it is shown that Ty‐1 is probably generic to all geminiviruses. A tomato Ty‐1 introgression line, but also stable transformants of susceptible tomato cv. Moneymaker and Nicotiana benthamiana (N. benthamiana) expressing the Ty‐1 gene, exhibited resistance to begomoviruses as well as to the distinct, leafhopper‐transmitted beet curly top virus, a curtovirus. Stable Ty‐1 transformants of N. benthamiana and tomato showed fewer symptoms and reduced viral titres on infection compared to wild‐type plants. TYLCV infections in wild‐type N. benthamiana plants in the additional presence of a betasatellite led to increased symptom severity and a consistent, slightly lowered virus titre relative to the high averaged levels seen in the absence of the betasatellite. On the contrary, in Ty‐1 transformed N. benthamiana viral titres increased in the presence of the betasatellite. The same was observed when these Ty‐1‐encoding plants were challenged with TYLCV and a potato virus X construct expressing the RNA interference suppressor protein βC1 encoded by the betasatellite. The resistance spectrum of Ty‐1 and the durability of the resistance are discussed in light of antiviral RNA interference and viral counter defence strategies.

Frequent occurrence of Mungbean yellow mosaic India virus in tomato leaf curl disease affected tomato in Oman

Next generation sequencing (NGS) of DNAs amplified by rolling circle amplification from 6 tomato (Solanum lycopersicum) plants with leaf curl symptoms identified a number of monopartite begomoviruses, including Tomato yellow leaf curl virus (TYLCV), and a betasatellite (Tomato leaf curl betasatellite [ToLCB]). Both TYLCV and ToLCB have previously been identified infecting tomato in Oman. Surprisingly the NGS results also suggested the presence of the bipartite, legume-adapted begomovirus Mungbean yellow mosaic Indian virus (MYMIV). The presence of MYMIV was confirmed by cloning and Sanger sequencing from four of the six plants. A wider analysis by PCR showed MYMIV infection of tomato in Oman to be widespread. Inoculation of plants with full-length clones showed the host range of MYMIV not to extend to Nicotiana benthamiana or tomato. Inoculation to N. benthamiana showed TYLCV to be capable of maintaining MYMIV in both the presence and absence of the betasatellite. In tomato MYMIV was only maintained by TYLCV in the presence of the betasatellite and then only at low titre and efficiency. This is the first identification of TYLCV with ToLCB and the legume adapted bipartite begomovirus MYMIV co-infecting tomato. This finding has far reaching implications. TYLCV has spread around the World from its origins in the Mediterranean/Middle East, in some instances, in live tomato planting material. The results here may suggest that begomoviruses which do not commonly infect tomato, such as MYMIV, could be spread as a passenger of TYLCV in tomato.

Breeding for Enhancing Legumovirus Resistance in Mungbean: Current Understanding and Future Directions

Yellow mosaic disease (YMD) affects several types of leguminous crops, including the Vigna species, which comprises a number of commercially important pulse crops. YMD is characterized by the formation of a bright yellow mosaic pattern on the leaves; in severe forms, this pattern can also be seen on stems and pods. This disease leads to tremendous yield losses, even up to 100%, in addition to deterioration in seed quality. Symptoms of this disease are similar among affected plants; YMD is not limited to mungbean (Vigna radiata L. Wilczek) and also affects other collateral and alternate hosts. In the last decade, rapid advancements in molecular detection techniques have been made, leading to an improved understanding of YMD-causing viruses. Three distinct bipartite begomoviruses, namely, Mungbean Yellow Mosaic India Virus (MYMIV), Mungbean Yellow Mosaic Virus (MYMV), and Horsegram Yellow Mosaic Virus (HgYMV), are known to cause YMD in Vigna spp. Vigna crops serve as an excellent protein source for vegetarians worldwide; moreover, they aid in improving soil health by fixing atmospheric nitrogen through a symbiotic association with Rhizobium bacteria. The loss in the yield of these short-duration crops due to YMD, thus, needs to be checked. This review highlights the discoveries that have been made regarding various aspects of YMD affecting mungbean, including the determination of YMD-causing viruses and strategies used to develop high-yielding YMD-resistant mungbean varieties that harness the potential of related Vigna species through the use of different omics approaches.

Occurrence and distribution of begomoviruses infecting tomatoes, peppers and cucurbits in Thailand

Incidence of begomoviruses infecting tomatoes, peppers and cucurbits was investigated in major production fields in the north, northeastern, central and southern regions of Thailand between January 2015 and June 2017. Of 585 samples subjected to begomovirus detection by polymerase chain reaction (PCR), 57% were positive for begomovirus infection. Nucleotide sequence analysis of coat protein (CP) and replication initiation protein (Rep) genes identified 15 previously known begomovirus species, 7 of which were reported infecting these crops in Thailand for the first time. A candidate novel bipartite begomovirus designated Melon yellow mosaic virus was also identified in melons from a northeastern province of Thailand. The predominant begomovirus species were different depending on the host plant (Tomato yellow leaf curl Thailand virus [TYLCTHV] for tomato, Pepper yellow leaf curl Thailand virus [PepYLCTHV] for pepper, Tomato leaf curl New Delhi virus [ToLCNDV] for loofah, bitter gourd, cucumber and melon, and Squash leaf curl China virus [SLCCNV] for pumpkin and wax gourd) and location. The update on diversity and distribution of begomoviruses obtained from this study provides useful information for further epidemiological study and support development of effective disease management through selective use of resistant cultivars suitable for particular plants and locations. Additionally, these data can be applied for development of diagnostic assays to specifically detect begomoviruses for continuous monitoring of viral diversity in these economic crops.

Caracterización molecular de un nuevo begomovirus aislado de cinco especies de arvenses colectadas en cultivos de tomate en Valle del Cauca

Las arvenses son hospederos alternos de begomovirus (Geminiviridae), los cuales facilitan su persistencia y propagación a cultivos de interés agronómico, como el tomate. El objetivo de esta investigación fue obtener el genoma completo de un begomovirus bipartita encontrado en Amaranthus dubius, Rivina humilis, Rhynchosia minima, Desmodium sp. y Caesalpinia sp., las cuales fueron colectadas en cultivos de tomate en Ginebra y Cerrito, Valle del Cauca. El genoma del begomovirus fue obtenido utilizando amplificación por círculo rodante y digestión con las enzimas EcoRI y EcoRV, las cuáles cortan el componente genómico A y B, respectivamente. Estos fragmentos fueron clonados, secuenciados y analizados. Finalmente, se verificó la presencia de este begomovirus en todas las arvenses mediante PCR específico. Se obtuvieron tres clonas EcoRI y cinco clonas EcoRV. Los fragmentos que portan los componentes A y B presentan un tamaño de 2 584 y 2 543 nt, respectivamente. El análisis de secuencia de nucleótidos del genoma begomoviral A con otros begomovirus previamente reportados, mostró la mayor identidad (90,9 %) con el virus del mosaico dorado de Rhynchosia de Yucatán. Tomando como base el criterio de demarcación actual para las especies de Begomovirus establecido por el Comité Internacional de Taxonomía de Virus, el geminivirus aislado de las arvenses A. dubius, R. humilis, R. minima, Desmodium sp. y Caesalpinia sp., constituye una nueva especie begomoviral. Con base en la sintomatología observada en campo, se propone el nombre de Virus del mosaico dorado de Rhynchosia de Colombia para designar a esta nueva especie.

Complete genome sequence of a new bipartite begomovirus infecting tomato in Brazil.

A novel bipartite begomovirus infecting begomovirus-resistant tomato plants was detected via Illumina sequencing analysis, and its genome sequence was confirmed by Sanger sequencing. The DNA-A (2627 nt) and DNA-B (2587 nt) have a genome organization that is typical of New World bipartite begomoviruses, sharing 82.5% identity with tomato golden leaf distortion virus and 75.1% identity with sida chlorotic vein virus. Based on the current classification criteria for begomoviruses, this isolate should be considered a member of a new species, and the name "tomato interveinal chlorosis virus-2" (ToICV2) is proposed for this virus.

AC4 protein of tomato leaf curl Palampur virus is an RNA silencing suppressor and a pathogenicity determinant

Plants deploy RNA silencing as a natural defence against invading viruses involving sequence-specific degradation of the viral RNAs. As a counter-defence strategy, viruses encode suppressor proteins that simultaneously target different steps of the silencing machinery. Tomato leaf curl Palampur virus (ToLCPalV) is a bipartite begomovirus in Geminiviridae family. It is responsible for significant reduction in the crop yield and quality. DNA-A of the virus encodes for six proteins whereas DNA-B codes for two proteins. In this study, all viral genes were screened for their role in suppression of green fluorescent protein (GFP) silencing in Nicotiana tabacum cv. Xanthi, employing agrobacterium based co-infiltration assay. The assay identified AC4 as a potential suppressor of RNA silencing. In addition, AC4 expression also suppressed virus-induced gene silencing (VIGS) of the phytoene desaturase (PDS) gene in N. benthamiana. Potato virus X (PVX) mediated transient expression of the AC4 in N. benthamiana showed enhanced symptoms that include downward leaf curling, leaf puckering and tissue necrosis. Further, N. benthamiana lines stably expressing AC4 showed severe developmental abnormalities. Mutational analysis suggested that glycine at 2nd position is essential for AC4 pathogenicity. Collectively, these findings demonstrate the role of ToLCPalV AC4 in viral pathogenesis, disease establishment and suppression of gene silencing.

A Bipartite Begomovirus Infecting Boerhavia erecta (Family Nyctaginaceae) in the Dominican Republic Represents a Distinct Phylogenetic Lineage and has a High Degree of Host Specificity.

Boerhavia erecta plants in and around agricultural fields in the Azua Valley of the southeastern Dominican Republic often show striking golden mosaic symptoms. Leaf samples from B. erecta plants showing these symptoms were collected in 2012 and 2013, and PCR tests with degenerate primers revealed begomovirus DNA-A and DNA-B components. The complete sequences of the DNA-A and DNA-B components of four isolates show a high degree of sequence identity (>96%) and a genome organization typical of New World (NW) bipartite begomoviruses. Sequence comparisons and phylogenetic analyses revealed that these isolates composed a new phylogenetic lineage of NW bipartite begomoviruses. The most closely related begomovirus is Merremia mosaic virus, a weed-infecting species from Puerto Rico. Because DNA-A sequence identities are well below the 91% threshold, these isolates represent a new begomovirus species, for which the name Boerhavia golden mosaic virus (BoGMV) is proposed. Infectious cloned BoGMV DNA-A and DNA-B components induced golden mosaic symptoms in agroinoculated B. erecta plants, thereby fulfilling Koch's postulates for this disease. Agroinoculation and mechanical transmission experiments revealed that BoGMV has an unusually narrow host range, limited to members of the family Nyctaginaceae and not including the permissive host Nicotiana benthamiana. The inability of BoGMV to infect N. benthamiana was due to a deficiency in cell-to-cell movement but not to a unique amino acid residue in the movement protein.

Occurrence and geographic distribution of Squash leaf curl virus in the Baja California Peninsula, Mexico

Squash leaf curl (SLC) disease causes serious damage in a great diversity of horticultural crops worldwide and is due to infection by different ssDNA bipartite begomoviruses (Family Geminiviridae). Symptoms of leaf yellowing, stunting and curling have been observed in squash crops in the southern region of the Baja California (BC) Peninsula in the northwestern region of Mexico for at least a decade. The damage estimated for all localities showed disease incidence of 73.3% to 97.7% and severity of 49%. Infected plants were collected in three localities in addition to adult whiteflies for species determination. Squash leaf curl virus (SLCuV) was detected in both infected plants and whiteflies by PCR using specific primers that amplified a 601 bp fragment. The full-length DNA-A sequence was 99.1% identical to sequences in GenBank. Phylogenetic studies using the core CP region showed that the BC isolate grouped within a well-defined clade together with SLCuV from the Mediterranean Middle East and USA. The molecular analysis in individual adult whiteflies determined that all populations belonged to Bemisia tabaci (MEAM1) (biotype B) species. No evidence of the Mediterranean (MED) species (biotype Q) was found. The SLC disease has prevailed in squash in the region in several growing cycles and in geographically separated localities. To the best of our knowledge, this is the first report of SLCuV associated with squash plants in BCS, Mexico.

Pepper yellow leaf curl Aceh virus: a novel bipartite begomovirus isolated from chili pepper, tomato, and tobacco plants in Indonesia.

During 2017, leaf samples of chili pepper (Capsicum annuum), tomato (Solanum lycopersicum), and tobacco (Nicotiana tabacum) plants exhibiting yellowing and curling symptoms were collected from Aceh province, Indonesia. These samples were used to isolate and sequence viral genomic DNA. Six isolates with complete DNA-A and DNA-B sequences of begomovirus were obtained, all of which showed >99% sequence identity to the others. DNA-A sequences shared the highest nucleotide sequence identity (89.3%-89.7%) with monopartite pepper yellow leaf curl Indonesia virus 2 (PepYLCIV2) and the second-highest sequence identity (87.3%-87.4%) with bipartite pepper yellow leaf curl Indonesia virus (PepYLCIV). The DNA-B sequences shared the highest nucleotide sequence identity (95%-97.5%) with PepYLCIV. Results of recombination analysis indicated that the novel begomovirus was a recombinant. In accordance with the guidelines for begomovirus species demarcation, these isolates should be assigned to a new species, and we have proposed the name ''pepper yellow leaf curl Aceh virus'' (PepYLCAV) for this virus.

Fine-tuning expression of begomoviral movement and nuclear shuttle proteins confers cell-to-cell movement to mastreviral replicons in Nicotiana benthamiana leaves.

Geminiviruses are a group of small plant viruses responsible for devastating crop damage worldwide. The emergence of agricultural diseases caused by geminiviruses is attributed in part to their high rates of recombination, leading to complementary function between viral components across species and genera. We have developed a mastreviral reporter system based on bean yellow dwarf virus (BeYDV) that replicates to high levels in the plant nucleus, expressing very high levels of GFP. To investigate the potential for complementation of movement function by other geminivirus genera, the movement protein (MP) and nuclear shuttle protein (NSP) from the bipartite begomovirus Bean dwarf mosaic virus (BDMV) were produced and characterized in Nicotiana benthamiana leaves. While overexpression of MP and NSP strongly inhibited GFP expression from the mastreviral reporter and caused adverse plant symptoms, optimizing the expression levels of MP and NSP allowed functional cell-to-cell movement. Hybrid virus vectors were created that express BDMV MP and NSP from mastreviral replicons, allowing efficient cell-to-cell movement comparable to native BDMV replicons. We find that the expression levels of MP and NSP must be fine-tuned to provide sufficient MP/NSP for movement without eliciting the plant hypersensitive response or adversely impacting gene expression from viral replicons. The ability to confer cell-to-cell movement to mastrevirus replicons depended strongly on replicon size: 2.1-2.7 kb replicons were efficiently moved, while 3 kb replicons were inhibited, and 3.9 kb replicons were very strongly inhibited. Optimized expression of MP/NSP from the normally phloem-limited Abutilon mosaic virus (AbMV) allows efficient movement in non-phloem cells.

Molecular characterization and phylogenetic analysis of tomato leaf curl Palampur virus, a bipartite begomovirus, associated with Cucumis sativus L. in Pakistan.

Leaf samples of Cucumis Sativus L. (C. sativus) (Family; Cucurbitaceae) showing vein thickening, mild leaf curling and leaf enations were collected from the farmer's field. Amplification of the full-length viral molecules was performed through rolling circle amplification (RCA). Cloning of the full-length viral molecules was done through standard cloning procedure followed by sequencing. Sequence similarity analysis and phylogenetic studies showed that the virus associated with leaf curling and enations in C. sativus was a bipartite begomovirus, where DNA-A and DNA-B showed highest nucleotide sequence homology of 98% and 97% to tomato leaf curl Palampur virus (ToLCPMV) from India. Attempts to isolate betasatellites and alphasatellites through PCR using RCA product as template, did not result in any amplification. A maximum likelihood phylogenetic tree grouped DNA-A and B components with other isolates from India. SDT was used to find the pairwise identity scores of different sequences of ToLCPMV present in the database. Phylogenetic analysis showed that sequences of ToLCPMV DNA-A and B components in this study share high degree of homology with existing viruses and are isolates of ToLCPMV-India. Infectious molecules of both components (Accessions, MG252783 and MG252784, respectively) were constructed for infectivity analysis to fulfill the Koch's postulate. Infectivity analysis revealed that ToLCPMV DNA-A is infectious to model host plant Nicotiana benthamiana and viral accumulation was confirmed through Southern blot analysis. Accumulation of DNA-B was confirmed through PCR. Infectivity analysis was also conducted using the original host, C. sativus, but plants were unable to survive the agroinoculation. To our knowledge this is the first report of ToLCPMV associated with C. sativus L. in Pakistan.

Nanopore sequencing of a novel bipartite New World begomovirus infecting cowpea.

A new bipartite begomovirus (family Geminiviridae) was detected on cowpea (Vigna unguiculata) plants exhibiting bright golden mosaic symptoms on leaves under field conditions in Brazil. Complete consensus sequences of DNA-A and DNA-B components of an isolate of the virus (PE-088) were obtained by nanopore sequencing and confirmed by Sanger sequencing. The genome components presented the typical genomic organization of New World (NW) begomoviruses. Pairwise sequence comparisons revealed low levels of identity with other begomovirus species previously reported infecting cowpea around the world. Phylogenetic analysis using complete sequences of DNA-A components revealed that the closest relatives of PE-088 (85-87% nucleotide sequence identities) were three legume-infecting begomoviruses from Brazil: bean golden mosaic virus, macroptilium common mosaic virus and macroptilium yellow vein virus. According to the current classification criteria, PE-088 represents a new species in the genus Begomovirus, tentatively named as cowpea bright yellow mosaic virus (CoBYMV).

Two new begomoviruses infecting tomato and Hibiscus sp. in the Amazon region of Brazil.

Two begomoviruses were isolated in the northern Brazilian state of Pará, infecting non-cultivated Hibiscus sp. and cultivated tomato (Solanum lycopersicum). The complete genomes (DNA-A and DNA-B) of the two viruses showed the typical organization of New World bipartite begomoviruses. Based on the species assignment criteria in the genus Begomovirus, each virus is a member of a new species. The virus from Hibiscus is most closely related to sida yellow mosaic Yucatan virus, while the tomato virus is most closely related to abutilon mosaic Brazil virus and corchorus mottle virus. Recombination events were detected in the DNA-A of the tomato virus, but not in the Hibiscus virus genome. We propose the names "hibiscus golden mosaic virus" (HGMV) and "tomato chlorotic leaf curl virus" (ToCLCV) for the viruses reported in this study.

Molecular characterization and infectivity analysis of a bipartite begomovirus associated with cotton leaf curl Multan betasatellite naturally infecting Rumex nepalensis in northern India

Rumex nepalensis (Nepal Dock) is an Indian traditional medicinal herb of the Western Himalayas. During a survey in 2013, typical begomovirus-like symptoms were observed on naturally grown R. nepalensis in the Bandla region of Palampur, Himachal Pradesh, India. PCR-based detection identified a bipartite begomovirus and a betasatellite in all the symptomatic plant samples. Complete sequence characterization established their identity as tomato leaf curl Palampur virus (ToLCPalV) and cotton leaf curl Multan betasatellite (CLCuMuB), respectively. Infectious clones of the virus and betasatellite were agro-infiltrated on both natural (R. nepalensis) as well as experimental (Nicotiana benthamiana) hosts. At 25 days post infiltration (dpi), N. benthamiana developed typical virus symptoms in DNA-A + DNA-B and DNA-A + DNA-B + CLCuMuB-infiltrated plants, whereas the plants infiltrated with DNA-A alone did not show any diseased phenotype. However, in combination with CLCuMuB, mild symptoms were observed. Agro-infiltrated R. nepalensis plants did not show visible symptoms in any of the above combinations. PCR and Southern blot analysis confirmed the replication and systemic spread of ToLCPalV and CLCuMuB in both R. nepalensis and N. benthamiana infiltrated plants. The present report shows R. nepalensis as a new natural host of the bipartite ToLCPalV associated with CLCuMuB. It also demonstrates the efficient trans-replication compatibility of CLCuMuB by a bipartite ToLCPalV in both of its natural and experimental hosts.

Tomato Twisted Leaf Virus: A Novel Indigenous New World Monopartite Begomovirus Infecting Tomato in Venezuela.

Begomoviruses are one of the major groups of plant viruses with an important economic impact on crop production in tropical and subtropical regions. The global spread of its polyphagous vector, the whitefly Bemisia tabaci, has contributed to the emergence and diversification of species within this genus. In this study, we found a putative novel begomovirus infecting tomato plants in Venezuela without a cognate DNA-B component. This begomovirus was genetically characterized and compared with related species. Furthermore, its infectivity was demonstrated by agroinoculation of infectious clones in tomato (Solanum lycopersicum) and Nicotiana benthamiana plants. The name Tomato twisted leaf virus (ToTLV) is proposed. ToTLV showed the typical genome organization of the DNA-A component of New World bipartite begomoviruses. However, the single DNA component of ToTLV was able to develop systemic infection in tomato and N. benthamiana plants, suggesting a monopartite nature of its genome. Interestingly, an additional open reading frame ORF was observed in ToTLV encompassing the intergenic region and the coat protein gene, which is not present in other closely related begomoviruses. A putative transcript from this region was amplified by strand-specific reverse transcription-PCR. Along with recent studies, our results showed that the diversity of monopartite begomoviruses from the New World is greater than previously thought.

Molecular identification and sequence analysis of bipartite Begomovirus infecting Horsegram legume in India

The molecular diversity of Begomovirus infecting Horsegram yellow Mosaic viruses (HgYMV1 and HgYMV2), from two main horsegram growing farms near Bangalore, Karnataka State, South India was investigated. The viral DNA was amplified from horsegram plants exhibiting mild and severe symptoms by polymerase chain reaction, and complete genome of the HgYMV were identified by their sequence analysis. Isolates of HgYMV1 and HgYMV2 were found to be associated with severe symptom phenotype from HgYMV. HgYMV was most closely related to Mungbean yellow mosaic indian viruse (MYMIV) and Mungbean yellow mosaic virus (MYMV) at 81.8 to 84.8 % nucleotide identity, based on DNA-A and DNA-B component sequences. HgYMV was distantly related to Dolicos yellow mosaic virus from Asia (DoYMV-Ban and DoYMV-DB) and partially to Cowpea golden mosaic virus from Nigeria (CPGMV-[NG]) at 64 and 62 % DNA nucleotide identity. Analysis of the DNA-B sequence of HgYMV revealed a DNA-B component identical to those of Bean golden yellow mosaic virus BGYMV isolates described. Furthermore, the DNA-B component for extant BGYMV isolates and strains were also the closest relatives for the HgYMV1 DNA-B components at 48.7 % nucleotide identity. Therefore, HgYMV could be considered to be a new species of the genus Begomovirus (family Geminiviridae). Keywords: Begomovirus, Horsegram, yellow Mosaic viruses , DNA sequencing

Ty-6, a major begomovirus resistance gene on chromosome 10, is effective against Tomato yellow leaf curl virus and Tomato mottle virus

Key messageTy-6 is a major resistance gene on chromosome 10 of tomato that provides resistance against monopartite and bipartite begomoviruses and complements resistance conferred by the known Ty-3 and ty-5 genes.Resistance to monopartite and bipartite begomoviruses is an important breeding objective for cultivated tomato. Several begomovirus resistance genes have been introgressed from related Solanum species and are available for breeding purposes. In the present study, we mapped an additional locus, Ty-6, to chromosome 10 of tomato. Ty-6 is effective against both monopartite Tomato yellow leaf curl virus (TYLCV) and bipartite Tomato mottle virus (ToMoV). Gene action is incomplete dominance, with an intermediate resistance response when Ty-6 is heterozygous. Analysis of populations segregating for Ty-6 along with Ty-3 or ty-5 indicates that the highest level of resistance against TYLCV is attained when Ty-6 is combined with an additional resistance allele. Our results also demonstrate that ty-5 is ineffective against ToMoV. Although multiple SNPs linked to Ty-6 were identified and can be used for breeding purposes, none of these were consistently polymorphic between Ty-6 and ty-6 breeding lines. Further research is underway to generate resequencing data for several Ty-6 inbred lines for the discovery of additional sequence polymorphisms that can be used for fine mapping and characterizing the Ty-6 locus.

Analysis of a new begomovirus unveils a composite element conserved in the CP gene promoters of several Geminiviridae genera: Clues to comprehend the complex regulation of late genes.

A novel bipartite begomovirus, Blechum interveinal chlorosis virus (BleICV), was characterized at the genome level. Comparative analyses revealed that BleICV coat protein (CP) gene promoter is highly divergent from the equivalent region of other begomoviruses (BGVs), with the single exception of Tomato chino La Paz virus (ToChLPV) with which it shares a 23-bp phylogenetic footprint exhibiting dyad symmetry. Systematic examination of the homologous CP promoter segment of 132 New World BGVs revealed the existence of a quasi-palindromic DNA segment displaying a strongly conserved ACTT-(N7)-AAGT core. The spacer sequence between the palindromic motifs is constant in length, but its sequence is highly variable among viral species, presenting a relaxed consensus (TT)GGKCCCY, which is similar to the Conserved Late Element or CLE (GTGGTCCC), a putative TrAP-responsive element. The homologous CP promoter region of Old World BGVs exhibited a distinct organization, with the putative TATA-box overlapping the left half of the ACTT-N7 composite element. Similar CP promoter sequences, dubbed “TATA-associated composite element” or TACE, were found in viruses belonging to different Geminiviridae genera, hence hinting unsuspected evolutionary relationships among those lineages. To get cues about the TACE function, the regulatory function of the CLE was explored in distinct experimental systems. Transgenic tobacco plants harboring a GUS reporter gene driven by a promoter composed by CLE multimers expressed high beta-glucuronidase activity in absence of viral factors, and that expression was increased by begomovirus infection. On the other hand, the TrAP-responsiveness of a truncated CP promoter of Tomato golden mosaic virus (TGMV) was abolished by site-directed mutation of the only CLE present in it, whereas the artificial addition of one CLE to the -125 truncated promoter strongly enhanced the transactivation level in tobacco protoplasts. These results indicate that the CLE is a TrAP-responsive element, hence providing valuable clues to interpret the recurrent association of the CLE with the TACE. On the basis of the aforesaid direct evidences and the insights afforded by the extensive comparative analysis of BleICV CP promoter, we propose that the TACE might be involved in the TrAP-mediated derepression of CP gene in vascular tissues.