No Evidence for Seed Transmission of Tomato Yellow Leaf Curl Sardinia Virus in Tomato

Emerging evidence of seed transmission of begomoviruses: implications in global circulation and disease outbreak

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.

Further Molecular Diagnosis Determines Lack of Evidence for Real Seed Transmission of Tomato Leaf Curl New Delhi Virus in Cucurbits

Begomoviruses (family Geminiviridae) cause serious diseases in many crop families. Since 2013, the Spanish isolate of tomato leaf curl New Delhi virus (ToLCNDV) has been a limiting factor for cucurbits production in the Mediterranean basin, forcing farmers to adapt new management and control techniques. Although it is well-known that begomoviruses are naturally transmitted by the whitefly Bemisia tabaci, the capacity of these viruses to be vertically transmitted through seeds remains controversial. Clarifying the potential ToLCNDV seed transmission is essential to understand the epidemiology of this threating-for-cucurbits virus and to design appropriate control strategies. We assessed ToLCNDV distribution in the leaves, flowers and seeds of the infected plants of susceptible Cucumis melo accessions and toleration to the infected genotypes of Cucurbita moschata by conventional and quantitative PCR. We analyzed whether the viral particle was transmitted to offspring. We also evaluated ToLCNDV presence in commercial seeds of cucurbits (zucchini (Cucurbita pepo), melon (C. melo), cucumber (Cucumis sativus) and watermelon (Citrullus lanatus)) and in their progenies. As the assayed seedlings remained symptomless, we increased the reliability and accuracy of detection in these samples by searching for replicative forms of ToLCNDV by combining Southern blot hybridization and rolling-circle amplification (RCA). However, integral genomic DNA was not identified in the plants of offspring. Although the seedborne nature of ToLCNDV was confirmed, our results do not support the transmission of this virus from contaminated seeds to progeny.

Begomoviruses: what is the secret(s) of their success?

Begomoviruses constitute an extremely successful group of emerging plant viruses transmitted by whiteflies of the Bemisia tabaci complex. Hosts include important vegetable, root, and fiber crops grown in the tropics and subtropics. Factors contributing to the ever-increasing diversity and success of begomoviruses include their predisposition to recombine their genomes, interaction with DNA satellites recruited throughout their evolution, presence of wild plants as a virus reservoir and a source of speciation, and extreme polyphagia and continuous movement of the insect vectors to temperate regions. These features as well as some controversial issues (replication in the insect vector, putative seed transmission, transmission by insects other than B. tabaci, and expansion of the host range to monocotyledonous plants) will be analyzed in this review.

Begomovirus Tomato Leaf Curl New Delhi Virus Is Seedborne but Not Seed Transmitted in Melon

Seed transmission can be of considerable relevance to the dissemination of plant viruses in nature and for their prevalence and perpetuation. Long-distance spread of isolates of the begomovirus species Tomato leaf curl New Delhi virus (genus Begomovirus, family Geminiviridae) has recently occurred from Asia to the Middle East and the Mediterranean Basin. Here, we investigated the possible transmission by melon (Cucumis melo L.) seeds of a tomato leaf curl New Delhi virus (ToLCNDV) isolate of the "Spain" strain widely distributed in the Mediterranean area as an alternative mechanism for long-distance spread. PCR amplification detection of ToLCNDV in floral parts and mature seeds of melon plants reveals that this virus is seedborne. "Seedborne" is defined as the ability of a virus to be carried through seeds, which does not necessarily lead to transmission to the next generation. Treatment with a chemical disinfectant significantly reduced the detectable virus associated with melon seeds, suggesting ToLCNDV contamination of the external portion of the seed coat. Also, when the internal fraction of the mature seed (seed cotyledons + embryo) was analyzed by quantitative PCR amplification, ToLCNDV was detectable at low levels, suggesting the potential for viral contamination or infection of the internal portions of seed. However, grow-out studies conducted with melon progeny plants germinated from mature seeds collected from ToLCNDV-infected plants and evaluated at early (1-leaf) or at late (20-leaf) growth stages did not support the transmission of ToLCNDV from seeds to offspring.