Insights into the maternal pathway for Cucumber green mottle mosaic virus infection of cucurbit seeds

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.

Genome Characterisation of the CGMMV Virus Population in Australia—Informing Plant Biosecurity Policy

The detection of cucumber green mottle mosaic (CGMMV) in the Northern Territory (NT), Australia, in 2014 led to the introduction of strict quarantine measures for the importation of cucurbit seeds by the Australian federal government. Further detections in Queensland, Western Australia (WA), New South Wales and South Australia occurred in the period 2015–2020. To explore the diversity of the current Australian CGMMV population, 35 new coding sequence complete genomes for CGMMV isolates from Australian incursions and surveys were prepared for this study. In conjunction with published genomes from the NT and WA, sequence, phylogenetic, and genetic variation and variant analyses were performed, and the data were compared with those for international CGMMV isolates. Based on these analyses, it can be inferred that the Australian CGMMV population resulted from a single virus source via multiple introductions.

Ability of Non-Hosts and Cucurbitaceous Weeds to Transmit Cucumber Green Mottle Mosaic Virus

Cucumber green mottle mosaic virus (CGMMV) is a Tobamovirus of economic importance affecting cucurbit crops and Asian cucurbit vegetables. Non-host crops of CGMMV, including capsicum (Capsicum annum), sweetcorn (Zea mays), and okra (Abelmoschus esculentus), were tested for their susceptibility to the virus, with field and glasshouse trials undertaken. After 12 weeks post-sowing, the crops were tested for the presence of CGMMV, and in all cases, no CGMMV was detected. Commonly found within the growing regions of cucurbits and melons worldwide are weeds, such as black nightshade (Solanum nigrum), wild gooseberry (Physalis minima), pigweed (Portulaca oleracea), and Amaranth species. Several weeds/grasses were tested for their ability to become infected with CGMMV by inoculating weeds directly with CGMMV and routinely testing over a period of eight weeks. Amaranthus viridis was found to be susceptible, with 50% of the weeds becoming infected with CGMMV. To further analyse this, six Amaranth samples were used as inoculum on four watermelon seedlings per sample and tested after eight weeks. CGMMV was detected in three of six watermelon bulk samples, indicating that A. viridis is a potential host/reservoir for CGMMV. Further research into the relationship between CGMMV and weed hosts is required. This research also highlights the importance of proper weed management to effectively manage CGMMV.

Evolution of cucurbit-infecting tobamoviruses: Recombination and codon usage bias

Currently, there are seven cucurbit-infecting tobamoviruses comprising cucumber green mottle mosaic virus (CGMMV), Kyuri green mottle mosaic virus (KGMMV), cucumber fruit mottle mosaic virus (CFMMV), zucchini green mottle mosaic virus (ZGMMV), cucumber mottle virus (CMoV), watermelon green mottle mosaic virus (WGMMV), and Trichosanthes mottle mosaic virus (TrMMV). To gain more insights into their evolution, recombination analyses were conducted. Four CGMMV isolates and one KGMMV isolate were suggested to be recombinants. And there was an interspecies recombination event between CGMMV and ZGMMV. Phylogenetic incongruence was also observed for CGMMV and KGMMV. A probable ancestral pattern was inferred for the gene junction region between RdRp and MP. Codon usage bias analysis revealed that the viral genes had additional influence independent of compositional constraint. In codon preference, the seven viruses were both similar to and different from the host cucumber (Cucumis sativus). Moreover, the viruses were not deficient in CpG and UpA dinucleotides.

Garlic Potyviruses Are Translocated to the True Seeds through the Vegetative and Reproductive Systems of the Mother Plant

Garlic lost its ability to produce true seeds millennia ago, and today non-fertile commercial cultivars are propagated only vegetatively. Garlic viruses are commonly carried over from one generation of vegetative propagules to the other, while nematodes and arthropods further transmit the pathogens from infected to healthy plants. A recent breakthrough in the production of true (botanical) garlic seeds resulted in rapid scientific progress, but the question of whether viruses are transmitted via seeds remains open and is important for the further development of commercial seed production. We combined morpho-physiological analysis, fluorescence in situ hybridization (FISH), and PCR analysis to follow potyvirus localization and translocation within garlic fertile plants and seeds. Spatial distribution was recorded in both vegetative and reproductive organs. We conclude that garlic potyviruses are translocated to the seeds from the infected mother plant during flower development and post-fertilization, while pollen remains virus-free and does not contribute to seed infection. Therefore, the main practical goal for virus-clean seed production in garlic is the careful maintenance of virus-free mother plants. Although garlic pollen is free of potyviral infection, the male parents' plants also need to be protected from contamination, since viral infection weakens plants, reducing flowering ability and pollen production.

Genome and Phylogenetic Analysis of Cucumber Green Mottle Mosaic Virus Global Isolates and Validation of a Highly Sensitive RT-qPCR Assay

The last two decades have seen exponential growth in the international movement of seeds for annual food crops, from a gross U.S. import value of $349 million in 1999 to $1.05 billion in 2019. This has led to the proportionate growth of seedborne pathogens dispersed with seed stocks. One such viral pathogen is cucumber green mottle mosaic virus (CGMMV), a tobamovirus that infects cucurbit crops such as melon, watermelon, cucumber, pumpkin, and squash. The first CGMMV introduction to California occurred in 2013, with subsequent annual outbreaks or detections since then. Here, we describe the use of next-generation sequencing to characterize the full genomes of 25 CGMMV isolates collected between 2013 and 2020 in California, either from CGMMV field detections or seed lots identified as CGMMV positive. We sequenced an additional 31 CGMMV isolates collected in Europe, Israel, and southeast Asia that were provided by industry collaborators. We also performed an in silico nucleotide database search in GenBank for full genome CGMMV sequences to include in all in silico analyses. Based on conserved regions within the coat protein gene, we then developed a quantitative reverse-transcription PCR assay for the sensitive and specific detection of CGMMV in seed and plant samples. Finally, based on our sequence and phylogenetic analysis, our data support that CGMMV has been introduced multiple times into California.

Investigating the Longevity and Infectivity of Cucumber green mottle mosaic virus in Soils of the Northern Territory, Australia

Cucumber green mottle mosaic virus (CGMMV) is a Tobamovirus of economic importance affecting cucurbit crops and Asian cucurbit vegetables. CGMMV was detected in the Northern Territory (NT) in September 2014, the first record for Australia, with 26 properties confirmed as of May 2016. Research was undertaken to determine virus longevity in soils in the NT and investigate the use of disinfectants to remove viable CGMMV from the soil. An in-field trial at 12 months post-quarantine at four properties, and bioassays from collected soils indicate that CGMMV remained viable in at least two of the properties 12 months after plant hosts were removed from the ground. The infectivity of CGMMV from soil was also investigated in two trials with 140 watermelon seeds and 70 watermelon plants sown into CGMMV infested soils with or without the application of the disinfectants Virkon^TM (2%) and Bleach (1%). Watermelons grown in soil, not treated with the Virkon^TM or Bleach, showed CGMMV infection rates of 4% and 2.5% respectively. When Virkon^TM or Bleach was applied, no positive CGMMV detections were observed in the watermelons. This research highlights the importance of proper management of infested properties and the need for on-farm biosecurity to manage CGMMV.

New early phenotypic markers for cucumber green mottle mosaic virus disease in cucumbers exposed to fluctuating extreme temperatures

Studies of early stages of cucumber green mottle mosaic virus (CGMMV) disease have been recently focused on plant molecular responses. However, extreme diurnal environmental temperatures, characteristic of global climate changes, could affect plant susceptibility and disease phenotype progression. Our studies of CGMMV disease progression, under simulated extreme temperature waves, have revealed two new disease initiation phenotypes that developed gradually, preceding severe symptom manifestations of post-recovery CGMMV systemic infections. 'Early post-recovery stage' bright yellow islands (BYIs) with defined boundaries amid asymptomatic leaf blades were first emerging followed by 'late post-recovery stage' BYIs with diffused boundaries. A deduced CGMMV disease progression scheme, postulating BYI symptom occurrence time-windows, revealed BYIs in field grown cucumber plants exposed to extreme diurnal temperatures. Profiling ontology of cucumber differentially expressed genes in BYIs vs the associated dark-green surrounding tissues disclosed activation of jasmonic acid (JA) pathway in 'early post-recovery stage' BYIs. JA signaling was inactivated in 'late post-recovery stage' BYIs concomitant with increasing expressions of JA signaling inhibitors and downregulation of JA responsive phenylpropanoid pathway. Our results disclosed a new phenotypic description of CGMMV disease initiation, characteristic of cucumbers grown under extreme environmental temperature fluctuations. The BYI phenotypes could define a time-window for CGMMV disease management applications.