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Begomoviruses (family Geminiviridae ) are known for causing devastating diseases in fruit, fibre, pulse, and vegetable crops throughout the world. Begomoviruses are transmitted in the field exclusively through insect vector whitefly ( Bemisia tabaci ), and the frequent outbreaks of begomoviruses are attributed largely due to the abundance of whitefly in the agri-ecosystem. Begomoviruses being phloem-borne were known not be transmitted through seeds of the infected plants. The recent findings of seed transmission of begomoviruses brought out a new dimension of begomovirus perpetuation and dissemination. The first convincing evidence of seed transmission of begomoviruses was known in 2015 for sweet potato leaf curl virus followed by several begomoviruses, like bhendi yellow vein mosaic virus, bitter gourd yellow mosaic virus, dolichos yellow mosaic virus, mungbean yellow mosaic virus, mungbean yellow mosaic India virus, pepper yellow leaf curl Indonesia virus, tomato leaf curl New Delhi virus, tomato yellow leaf curl virus, tomato yellow leaf curl Sardinia virus, and okra yellow mosaic Mexico virus. These studies brought out two perspectives of seed-borne nature of begomoviruses: (i) the presence of begomovirus in the seed tissues derived from the infected plants but no expression of disease symptoms in the progeny seedlings and (ii) the seed infection successfully transmitted the virus to cause disease to the progeny seedlings. It seems that the seed transmission of begomovirus is a feature of a specific combination of host-genotype and virus strain, rather than a universal phenomenon. This review comprehensively describes the seed transmitted begomoviruses reported in the last 9 years and the possible mechanism of seed transmission. An emphasis is placed on the experimental results that proved the seed transmission of various begomoviruses, factors affecting seed transmission and impact of begomovirus seed transmission on virus circulation, outbreak of the disease, and management strategies.

Tomato brown rugose fruit virus (ToBRFV) is a highly infectious virus, that is becoming a threat to tomato production worldwide. In this work we evaluated the localization of ToBRFV particles in tomato seeds, its seed transmission rate and efficacy of disinfection, and the effects of different thermal- and chemical-based treatments on ToBRFV-infected seeds’ germination. Analyses demonstrated that ToBRFV was located in the seed coat, sometime in the endosperm, but never in the embryo; its transmission from infected seeds to plantlets occurs by micro-lesions during the germination. The ToBRFV seed transmission rate was 2.8% in cotyledons and 1.8% in the third true leaf. Regarding the different disinfection treatments, they returned 100% of germination at 14 days post-treatment (dpt), except for the treatment with 2% hydrochloric acid +1.5% sodium hypochlorite for 24 h, for which no seed germinated after 14 dpt. All treatments have the ability to inactivate ToBRFV, but in six out of seven treatments ToBRFV was still detectable by RT-qPCR. These results raise many questions about the correct way to carry out diagnosis at customs. To our knowledge, this is the first study on the effective localization of ToBRFV particles in seeds.

Tomato spotted wilt orthotospovirus (TSWV) severely damaged agricultural production in many places around the world. It is generally believed that TSWV transmits among plants via their insect vector. In this study, we provide evidence on the seed-borne transmission of TSWV in pepper (Capsicum annuum L.) plants. RT-PCR, RT-qPCR, and transmission electron microscopy data demonstrate the seed transmission ability of TSWV in peppers. Endosperm, but not the embryo, is the abundant virus-containing seed organ. TSWV can also be detected in the second generation of newly germinated seedlings from virus-containing seed germination experiments. Our data are useful for researchers, certification agencies, the seed industry, and policy makers when considering the importance of TSWV in vegetable production all over the world.

Mustard is a commercial oilseed crop worldwide infected by a highly infectious turnip mosaic virus (TuMV). In the experimental field at ICAR-IARI, New Delhi, in 2022, a 100% incidence of TuMV infection was observed in brown, black and yellow mustard. A very low aphid population suggested the possibility of seed transmission. Earlier, the virus genome was characterized by high throughput sequencing and it was a recombinant of World-B and Asian-BR isolates. The presence of TuMV in immature seeds was confirmed in eight field-grown genotypes via RT-PCR using CP-specific primers designed from the same genome sequence. TuMV was found to be localized in embryo and cotyledon, indicating its true seed-borne nature. Presence of TuMV was also confirmed by RT-PCR in the grow out plants from seeds of field grown eight infected genotypes and 9 genotypes collected from seed stock, that were grown in an aphid-free growth chamber. Further, out of 24 seedlings of Pusa Gold (seed stock) and Pusa Karishma (seeds from field grown plants), 20 and 17 seedlings were found infected with TuMV, respectively. The internally seed-borne nature of the virus leads to its early establishment at the seedling stage, leading to stunting and leaf-puckering symptoms in the progeny plants. This study is the first evidence of seed embryo infection and seedling transmission of TuMV of all the three species of mustard plants (brown, black and yellow mustard). Seed transmission of TuMV in mustard genotypes have implications for the seed exchange programme of mustard seeds.

Background Seed transmission of plant viruses can be important due to the role it plays in their dissemination to new areas and subsequent epidemics. Seed transmission largely depends on the ability of a virus to replicate in reproductive tissues and survive during the seed maturation process. It occurs through the infected embryo or mechanically through the contaminated seed coat. Alfalfa ( Medicago sativa L.) is an important legume forage crop worldwide, and except for a few individual seedborne viruses infecting the crop, its seed virome is poorly known. The goal of this research was to perform initial seed screenings on alfalfa germplasm accessions maintained by the USDA ARS National Plant Germplasm System in order to identify pathogenic viruses and understand their potential for dissemination. Methods For the detection of viruses, we used high throughput sequencing combined with bioinformatic tools and reverse transcription-polymerase chain reactions. Results Our results suggest that, in addition to common viruses, alfalfa seeds are infected by other potentially pathogenic viral species that could be vertically transmitted to offspring. Conclusions To the best of our knowledge, this is the first study of the alfalfa seed virome carried out by HTS technology. This initial screening of alfalfa germplasm accessions maintained by the NPGS showed that the crop’s mature seeds contain a broad range of viruses, some of which were not previously considered to be seed-transmitted. The information gathered will be used to update germplasm distribution policies and to make decisions on the safety of distributing germplasm based on viral presence.

Banana bunchy top disease is caused by banana bunchy top virus (BBTV). BBTV is transmitted locally by aphids ( Pentalonia spp.), but the long-distance spread is through the movement of infected planting materials. This study investigated potential alternative hosts of BBTV in ornamental Musa and related species in the Zingiberales in the Philippines. Artificial inoculation of BBTV, molecular detection and transmission assay were used to evaluate 15 plant test species. The potential for seed transmission of BBTV through Canna indica seeds was also investigated. Seed samples were validated and quantified for BBTV presence using molecular tools, and then grown for transmission assay. Typical symptoms of BBTV in bananas, including dark green streak on the midrib and petiole and rosetting were observed on inoculated Musa coccinea (banana blossom), M. velutina (velutina), M laterita . (bronze banana) and Canna indica (Bandera Espanola). PCR assays confirmed BBTV infection in these symptomatic test plants, as well as in Curcuma longa (turmeric) which exhibited large chlorotic blotches on the leaf. BBTV was detected from both seeds and germinated seedlings of artificially inoculated and field-collected C. indica samples. This study identified M. laterita as a new host of BBTV. The susceptibility to BBTV of M. coccinea, M. velutina , C. indica , and C. longa was also confirmed. The study also provided the first evidence of seed transmission of BBTV. C indica is an ornamental plant popularly used for landscaping in the Philippines and seeds were shown to be an efficient mode of transmission of the virus with rates up to 34%. The discovery of natural infection in ornamental plants and seeds poses a risk to the banana industry and responsible propagation and appropriate quarantine protocols must be implemented.

Evidence for seed transmission of phytoplasmas has grown in several pathosystems including coconut ( Cocos nucifera ). Bogia coconut syndrome (BCS) is a disease associated with the lethal yellowing syndrome associated with the presence of ‘ Candidatus Phytoplasma noviguineense’ that affects coconut, betel nut ( Areca catechu ) and bananas ( Musa spp.) in Papua New Guinea. Coconut and betel nut drupes were sampled from BCS-infected areas in Papua New Guinea, dissected, the extracted nucleic acid was used in polymerase chain reaction (PCR), and loop mediated isothermal amplification (LAMP) used to check for presence of phytoplasma DNA. In a second study, drupes of both plant species were collected from multiple field sites and grown in insect-proof cages. Leaf samples taken at 6 months were also tested with PCR and LAMP. The studies of dissected coconut drupes detected phytoplasma DNA in several tissues including the embryo. Drupes from betel nut tested negative. Among the seedlings, evidence of possible seed transmission was found in both plant species. The results demonstrate the presence of ‘ Ca . P. noviguineense’ in coconut drupes and seedlings, and in seedlings of betel nut; factors that need to be considered in ongoing management and containment efforts.

Since the first report of the tobamovirus tomato brown rugose fruit virus (ToBRFV) in 2014, it has become globally distributed. Its rapid spread has been primarily attributed to seed-borne transmission. Here, the seed-borne nature of ToBRFV transmission was investigated in different cultivars of tomato, bell pepper, and eggplant. In situ hybridization to localize the virus in reproductive organs of ToBRFV-infected tomato plants revealed that the virus was not present in shoot apices, flower buds, or in ovules during flower opening, indicating the virus may be restricted to the outer integument and transported in the vascular bundles during seed development. However, during early fruit development, the virus was present in the integuments in the ovule. Seeds of tomato cultivars with or without tobamovirus resistance gene Tm-2 2 transmitted the virus to the progeny seedlings at rates that reflected the ineffectiveness of the gene against ToBRFV. Seeds of bell peppers transmitted ToBRFV at higher rates than tomato seeds, but a bell pepper cultivar that has resistance gene L 3 was not systemically infected, and its seeds did not harbor the virus. Three eggplant cultivars were systemically infected with ToBRFV but without showing any obvious symptoms, and even though ToBRFV was present in their seeds, the seedlings were not infected. ToBRFV was detected in the seed coats of contaminated tomato and bell pepper seeds, but not in eggplant seed coats. These results indicate mechanistic differences in seed-borne transmission among the three Solanaceae crops.

Tall fescue (Lolium arundinaceum (Schreb.) Darbysh.) is a cool-season perennial grass widely grown for forage and turf. Tall fescue lives in association with a fungal endophyte that helps the grass overcome abiotic and biotic stressors. The endophyte is asexual and transmits vertically from the tall fescue plant to the next generation through the seed. Producers of endophyte-infected tall fescue must have endophyte infection in at least 70% of their seed. Therefore, endophyte seed transmission is vital in breeding and seed production. Transfer of endophytes from their native host to different backgrounds of elite tall fescue cultivars can lead to a low seed transmission of the endophyte to the seed. This study screened 23 previously uncharacterized endophyte strains for transmissibility when artificially inoculated into continental and Mediterranean-type host tall fescue. We found no correlation between the rate of successful inoculation and the seed transmission rate of the endophyte in the new host. Nor did the seed transmission rate of the endophyte strains in their native host correlate with the seed transmission rate of the endophyte in the new host. Five strains exhibited seed transmission above 70% in both Mediterranean and Continental host backgrounds and will be characterized further for potential use in cultivar development.

AimsSeeds are vectors of a diversified microbiota including plant pathogens. To better understand transmission of common bacterial blight (CBB) agents to bean seeds, we analyzed the role of non-pathogenic xanthomonads on seed transmission efficiency and investigated the location of Xanthomonas citri pv. fuscans (Xcf) into seeds and plantlets.MethodsCompetition between CBB and NP strains was initially assessed in vitro and then extended in planta to monitor the impact of co-inoculation on Xcf seed transmission. Moreover, location of Xcf strains in seeds and seedlings was visualized using a combination of gfp-tagged strain and DOPE-FISH/CSLM.ResultsWhereas CBB agent growth was inhibited in vitro by some seed-borne non-pathogenic xanthomonads strains, these strains did not transmit efficiently to seed through floral pathway and did not affect Xcf seed transmission. Xcf cells were observed entering seed through vascular elements and parenchyma of funiculus, but also micropyle and testa. Xcf cells were observed, moreover, among other bacteria on radicle surfaces, especially tip, in cotyledons, and plumules.ConclusionsCBB agents are more efficient than non-pathogenic xanthomonads in using the floral route to colonize seeds. CBB agents are located within different niches in the seed tissues up to the embryonic axis.