Insights into the early transcriptomic response against watermelon mosaic virus in melon.

BACKGROUND: Watermelon mosaic virus (WMV) is one of the most prevalent viruses affecting melon worldwide. Recessive resistance to WMV in melon has previously been reported in the African accession TGR-1551. Moreover, the genomic regions associated to the resistance have also been described. Nevertheless, the transcriptomic response that might infer the resistance to this potyvirus has not been explored. RESULTS: We have performed a comparative transcriptomic analysis using mock and WMV-inoculated plants of the susceptible cultivar "Bola de oro" (BO) and a resistant RIL (Recombinant inbred line) derived from the initial cross between "TGR-1551" and BO. In total, 616 genes were identified as differentially expressed and the weighted gene co-expression network analysis (WGCNA) detected 19 gene clusters (GCs), of which 7 were differentially expressed for the genotype x treatment interaction term. SNPs with a predicted high impact on the protein function were detected within the coding regions of most of the detected DEGs. Moreover, 3 and 16 DEGs were detected within the QTL regions previously described in chromosomes 11 and 5, respectively. In addition to these two specific genomic regions, we also observde large transcriptomic changes from genes spread across the genome in the resistant plants in response to the virus infection. This early response against WMV implied genes involved in plant-pathogen interaction, plant hormone signal transduction, the MAPK signaling pathway or ubiquitin mediated proteolysis, in detriment to the photosynthetic and basal metabolites pathways. Moreover, the gene MELO3C021395, which coded a mediator of RNA polymerase II transcription subunit 33A (MED33A), has been proposed as the candidate gene located on chromosome 11 conferring resistance to WMV. CONCLUSIONS: The comparative transcriptomic analysis presented here showed that, even though the resistance to WMV in TGR-1551 has a recessive nature, it triggers an active defense response at a transcriptomic level, which involves broad-spectrum resistance mechanisms. Thus, this study represents a step forward on our understanding of the mechanisms underlaying WMV resistance in melon. In addition, it sheds light into a broader topic on the mechanisms of recessive resistances.

Advanced Genetic Studies on Powdery Mildew Resistance in TGR-1551.

Cucurbits powdery mildew (CPM) is one of the main limiting factors of melon cultivation worldwide. Resistance to races 1, 2, and 5 has been reported in the African accession TGR-1551, whose resistance is controlled by a dominant-recessive epistasis. The dominant and recessive quantitative trail loci (QTL) have previously been located in chromosomes 5 and 12, respectively. We used several densely genotyped BC3 families derived from the cross between TGR-1551 and the susceptible cultivar 'Bola de Oro' to finely map these resistance regions. The further phenotyping and genotyping of the selected BC5, BC5S1, BC5S2, BC4S1, BC4xPS, and (BC4xPS) S1 offspring allowed for the narrowing of the candidate intervals to a 250 and 381 kb region in chromosomes 5 and 12, respectively. Moreover, the temperature effect over the resistance provided by the dominant gene has been confirmed. High resolution melting markers (HRM) were tightly linked to both resistance regions and will be useful in marker-assisted selection programs. Candidate R genes with variants between parents that caused a potential modifier impact on the protein function were identified within both intervals. These candidate genes provide targets for future functional analyses to better understand the resistance to powdery mildew in melons.

Carotenoid fortification of zucchini fruits using a viral RNA vector.

BACKGROUND: Carotenoids are health-promoting metabolites in livestock and human diets. Some important crops have been genetically modified to increase their content. Although the usefulness of transgenic plants to alleviate nutritional deficiencies is obvious, their social acceptance has been controversial. RESULTS: Here, we demonstrate an alternative biotechnological strategy for carotenoid fortification of edible fruits in which no transgenic DNA is involved. A viral RNA vector derived from zucchini yellow mosaic virus (ZYMV) was modified to express a bacterial phytoene synthase (crtB), and inoculated to zucchini (Cucurbita pepo L.) leaves nurturing pollinated flowers. After the viral vector moved to the developing fruit and expressed crtB, the rind and flesh of the fruits developed yellow-orange rather than green color. Metabolite analyses showed a substantial enrichment in health-promoting carotenoids, such as α- and ß-carotene (provitamin A), lutein and phytoene, in both rind and flesh. CONCLUSION: Although this strategy is perhaps not free from controversy due to the use of genetically modified viral RNA, our work does demonstrate the possibility of metabolically fortifying edible fruits using an approach in which no transgenes are involved.

Grafting Snake Melon [Cucumis melo L. subsp. melo Var. flexuosus (L.) Naudin] in Organic Farming: Effects on Agronomic Performance; Resistance to Pathogens; Sugar, Acid, and VOC Profiles; and Consumer Acceptance.

The performance of snake melon [Cucumis melo var. flexuosus (L.)] in organic farming was studied under high biotic and salt stress conditions. Soilborne diseases (mainly caused by Macrophomina phaseolina and Neocosmospora falciformis), combined with virus incidence [Watermelon mosaic virus (WMV), Zucchini yellow mosaic virus (ZYMV), and Tomato leaf curl New Delhi virus (ToLCNDV)] and Podosphaera xanthii attacks, reduced yield by more than 50%. Snake melon susceptibility to M. phaseolina and Monosporascus cannonballus was proved in pathogenicity tests, while it showed some degree of resistance to Neocosmospora keratoplastica and N. falciformis. On the contrary, salt stress had a minor impact, although a synergic effect was detected: yield losses caused by biotic stress increased dramatically when combined with salt stress. Under biotic stress, grafting onto the melon F1Pat81 and wild Cucumis rootstocks consistently reduced plant mortality in different agroecological conditions, with a better performance compared to classic Cucurbita commercial hybrids. Yield was even improved under saline conditions in grafted plants. A negative effect was detected, though, on consumer acceptability, especially with the use of Cucurbita rootstocks. Cucumis F1Pat81 rootstock minimized this side effect, which was probably related to changes in the profile of sugars, acids, and volatiles. Grafting affected sugars and organic acid contents, with this effect being more accentuated with the use of Cucurbita rootstocks than with Cucumis. In fact, the latter had a higher impact on the volatile organic compound profile than on sugar and acid profile, which may have resulted in a lower effect on consumer perception. The use of Cucumis rootstocks seems to be a strategy to enable organic farming production of snake melon targeted to high-quality markets in order to promote the cultivation of this neglected crop.

Melon Genome Regions Associated with TGR-1551-Derived Resistance to Cucurbit yellow stunting disorder virus.

Cucurbit yellow stunting disorder virus (CYSDV) is one of the main limiting factors of melon cultivation worldwide. To date, no commercial melon cultivars resistant to CYSDV are available. The African accession TGR-1551 is resistant to CYSDV. Two major quantitative trait loci (QTLs) have been previously reported, both located near each other in chromosome 5. With the objective of further mapping the gene or genes responsible of the resistance, a recombinant inbred line (RIL) population derived from the cross between TGR-1551 and the susceptible cultivar 'Bola de Oro' was evaluated for resistance to CYSDV in five different assays and genotyped in a genotyping by sequencing (GBS) analysis. The major effect of one of the two QTLs located on chromosome 5 was confirmed in the multienvironment RIL assay and additionally verified through the analysis of three segregating BC1S1 populations derived from three resistant RILs. Furthermore, progeny test using the offspring of selected BC3 plants allowed the narrowing of the candidate interval to a 700 kb region. The SNP markers identified in this work will be useful in marker-assisted selection in the context of introgression of CYSDV resistance in elite cultivars.

Resistance to Tomato Yellow Leaf Curl Virus in Tomato Germplasm.

Tomato yellow leaf curl virus (TYLCV) is a virus species causing epidemics in tomato (Solanum lycopersicum) worldwide. Many efforts have been focused on identification of resistance sources by screening wild tomato species. In many cases, the accession numbers were either not provided in publications or not provided in a consistent manner, which led to redundant screenings. In the current study, we summarized efforts on the screenings of wild tomato species for TYLCV resistance from various publications. In addition, we screened 708 accessions from 13 wild tomato species using different inoculation assays (i.e., whitefly natural infection and Agrobacterium-mediated inoculation) from which 138 accessions exhibited no tomato yellow leaf curl disease (TYLCD) symptoms. These symptomless accessions include 14 accessions from S. arcanum, 43 from S. chilense, 1 from S. chmielewskii, 28 from S. corneliomulleri, 5 from S. habrochaites, 4 from S. huaylasense, 2 from S. neorickii, 1 from S. pennellii, 39 from S. peruvianum, and 1 from S. pimpinellifolium. Most of the screened S. chilense accessions remained symptomless. Many symptomless accessions were also identified in S. arcanum, S. corneliomulleri, and S. peruvianum. A large number of S. pimpinellifolium accessions were screened. However, almost all of the tested accessions showed TYLCD symptoms. Further, we studied allelic variation of the Ty-1/Ty-3 gene in few S. chilense accessions by applying virus-induced gene silencing and allele mining, leading to identification of a number of allele-specific polymorphisms. Taken together, we present a comprehensive overview on TYLCV resistance and susceptibility in wild tomato germplasm, and demonstrate how to study allelic variants of the cloned Ty-genes in TYLCV-resistant accessions.

Fruit quality assessment of watermelons grafted onto citron melon rootstock.

BACKGROUND: The grafting of watermelons (Citrullus lanatus) is a common technique that increases yield under stressful soil conditions. The most common rootstocks for watermelons are Cucurbita hybrids. However, they often have a negative impact on fruit quality. Exploiting novel Citrullus germplasm such as citron melon (Citrullus lanatus var. citroides) is an alternative to avoid these quality problems. RESULTS: Citron melon has been validated as watermelon rootstock, comparing its effects on watermelon quality to those of Cucurbita hybrids. Larger fruits with thicker rinds were observed in fruits from plants grafted onto both citron and Cucurbita rootstocks. The citron melon had no significant effect on flesh sugars or acid profiles compared to non-grafted watermelons, except for an increase in glucose and malic acid content, which also occurred in the Cucurbita rootstocks. The aroma profile of fruits produced on citron melon was similar to that of the non-grafted and self-grafted controls. The citron rootstock did not display the increased levels of (Z)-6-nonen-1-ol (a compound associated with pumpkin-like odors) found in fruits produced with Cucurbita hybrids. CONCLUSION: The low impact of citron melon rootstock on fruit quality, along with the enhanced resistance against nematodes, make the citron a promising alternative to Cucurbita rootstocks. © 2016 Society of Chemical Industry.

Assessing the genetic variation of Ty-1 and Ty-3 alleles conferring resistance to tomato yellow leaf curl virus in a broad tomato germplasm.

Tomato yellow leaf curl virus (TYLCV) hampers tomato production worldwide. Our previous studies have focussed on mapping and ultimately cloning of the TYLCV resistance genes Ty-1 and Ty-3. Both genes are derived from Solanum chilense and were shown to be allelic. They code for an RNA-dependent RNA polymerase (RDR) belonging to the RDRγ type defined by a DFDGD catalytic domain. In this study, we first fine-mapped the TYLCV resistance in S. chilense LA1932, LA1960 and LA1971. Results showed that chromosomal intervals of the causal genes in these TYLCV-resistant accessions overlap and cover the region where Ty-1/Ty-3 is located. Further, virus-induced gene silencing was used to silence Ty-1/Ty-3 in tomato lines carrying TYLCV resistance introgressed from S. chilense LA1932, LA1938 and LA1971. Results showed that silencing Ty-1/Ty-3 compromised the resistance in lines derived from S. chilense LA1932 and LA1938. The LA1971-derived material remained resistant upon silencing Ty-1/Ty-3. Further, we studied the allelic variation of the Ty-1/Ty-3 gene by examining cDNA sequences from nine S. chilense-derived lines/accessions and more than 80 tomato cultivars, landraces and accessions of related wild species. The DFDGD catalytic domain of the Ty-1/Ty-3 gene is conserved among all tomato lines and species analysed. In addition, the 12 base pair insertion at the 5-prime part of the Ty-1/Ty-3 gene was found not to be specific for the TYLCV resistance allele. However, compared with the susceptible ty-1 allele, the Ty-1/Ty-3 allele is characterized by three specific amino acids shared by seven TYLCV-resistant S. chilense accessions or derived lines. Thus, Ty-1/Ty-3-specific markers can be developed based on these polymorphisms. Elevated transcript levels were observed for all tested S. chilenseRDR alleles (both Ty-1 and ty-1 alleles), demonstrating that elevated expression level is not a good selection criterion for a functional Ty-1/Ty-3 allele.