Grafting on a Non-Transgenic Tolerant Tomato Variety Confers Resistance to the Infection of a Sw5-Breaking Strain of Tomato spotted wilt virus via RNA Silencing

Evaluation of Grafting for Management of Southern Blight in Processing Tomatoes in California

Options for managing southern blight of processing tomato (caused by Athelia rolfsii) in California are limited. The objectives of this study were to: (i) evaluate grafting with the resistant rootstock Maxifort for southern blight management in processing tomato and (ii) evaluate increasing the height of the graft union to further reduce incidence of southern blight in grafted plants. We evaluated two cultivars (Heinz 5608 or Heinz 8504) and a grafting factor with three levels (grafted to Maxifort rootstock with standard scion height, grafted to Maxifort rootstock at a tall height, and nongrafted) in a field study with natural inoculum or in inoculated greenhouse experiments. Southern blight severity was low in both greenhouse experiments in 2018 and 2019, and no consistent trends were observed. In field experiments in 2018 and 2019, mean incidence in nongrafted plots was 6.2 to 17.0 times higher when compared with either the standard or tall grafted treatments. Southern blight was numerically lower in tall grafted plots compared with standard, but the magnitude was small and not statistically significant. Based on our studies, grafting can reduce losses of processing tomato in California to southern blight, but increasing the height of the graft union does not offer a tangible benefit.

Rootstock-induced scion resistance against tobacco mosaic virus is associated with the induction of defence-related transcripts and graft-transmissible mRNAs

Grafting is a common horticultural practice used to confer desirable traits between rootstock and scion, including disease resistance. To investigate graft-conferred resistance against viral diseases a novel heterografting system was developed using Nicotiana benthamiana scions grafted onto different tomato rootstocks. N. benthamiana is normally highly susceptible to tobacco mosaic virus (TMV) infection. However, specific tomato rootstock varieties were found to confer a range of resistance levels to N. benthamiana scions inoculated with TMV. Conferred resistance was associated with delays in virus accumulation and the reduction in virus spread. RNA sequencing analysis showed the enrichment of transcripts associated with disease resistance and plant stress in N. benthamiana scions grafted onto resistance-inducing tomato rootstocks. Genome sequencing of resistance- and nonresistance-conferring rootstocks was used to identify mobile tomato transcripts within N. benthamiana scions. Within resistance-induced N. benthamiana scions, enriched mobile tomato transcripts were predominantly associated with defence, stress, and abscisic acid signalling when compared to similar scions grafted onto nonresistance-inducing rootstock. Combining these findings suggests that graft-induced resistance is modulated by rootstock scion transcriptional responses and rootstock-specific mobile transcripts.

Comparative Analysis of Bioactive Compounds in Two Globe Artichoke Ecotypes Sanitized and Non-Sanitized from Viral Infections

Globe artichoke ecotypes sanitized from plant pathogen infections are characterized by high vegetative vigor, productivity, and quality of capitula. The recent availability on the market of these plants has renewed the interest of farmers and pharmaceutical industries in the crop. Globe artichoke exhibits interesting nutraceutical properties due to the high content of health-promoting bioactive compounds (BACs), such as polyphenols, that could be extracted from waste biomass. The production of BACs depends on several factors including the plant portion considered, the globe artichoke variety/ecotype, and the physiological status of the plants, linked to biotic and abiotic stresses. We investigated the influence of viral infections on polyphenol accumulation in two Apulian late-flowering ecotypes "Locale di Mola tardivo" and "Troianella", comparing sanitized virus-free material (S) vs. naturally virus-infected (non-sanitized, NS) plants. Transcriptome analysis of the two ecotypes highlighted that differentially expressed genes (DEGs), in the two tested conditions, were mainly involved in primary metabolism and processing of genetic/environmental information. The up-regulation of the genes related to the biosynthesis of secondary metabolites and the analysis of peroxidase activity suggested that their modulation is influenced by the phytosanitary status of the plant and is ecotype-dependent. Conversely, the phytochemical analysis showed a remarkable decrease in polyphenols and lignin accumulation in S artichokes compared to NS plants. This unique study analyzes the potential of growing vigorous, sanitized plants, in order to have high amounts of 'soft and clean' biomass, finalized for BAC extraction for nutraceutical purposes. This, in turn, opens new perspectives for a circular economy of sanitized artichokes, in line with the current phytosanitary standards and sustainable development goals.

Long distance signalling and epigenetic changes in crop grafting

Humans have used grafting for more than 4000 years to improve plant production, through physically joining two different plants, which can continue to grow as a single organism. Today, grafting is becoming increasingly more popular as a technique to increase the production of herbaceous horticultural crops, where rootstocks can introduce traits such as resistance to several pathogens and/or improving the plant vigour. Research in model plants have documented how long-distance signalling mechanisms across the graft junction, together with epigenetic regulation, can produce molecular and phenotypic changes in grafted plants. Yet, most of the studied examples rely on proof-of-concept experiments or on limited specific cases. This review explores the link between research findings in model plants and crop species. We analyse studies investigating the movement of signalling molecules across the graft junction and their implications on epigenetic regulation. The improvement of genomics analyses and the increased availability of genetic resources has allowed to collect more information on potential benefits of grafting in horticultural crop models. Ultimately, further research into this topic will enhance our ability to use the grafting technique to exploit genetic and epigenetic variation in crops, as an alternative to traditional breeding.

Grafting to Manage Infections of the Emerging Tomato Leaf Curl New Delhi Virus in Cucurbits

Tomato leaf curl New Delhi virus (ToLCNDV) is an emerging begomovirus (Geminiviridae family) listed in the EPPO Alert List 2, present in the Mediterranean area and in Italy, where it was reported in 2015 in Sicilian courgette. The virus is widespread in cucurbits where it causes up to 100% production losses. In 2018, ToLCNDV was isolated in Apulia (southern Italy) in commercial fields of zucchini squash and since then its recurrent outbreaks generated justified concern among growers. Thus, a sustainable and environmentally friendly approach must be adopted. Genetic resistances have been identified in Cucurbita moschata and Luffa cylindrica but, compared to genetic resistance, grafting could provide a faster and more flexible solution because the graft wounding induces tolerance rather than resistance against airborne virus infection. Compared to tolerance, the up-regulation of resistance genes requires energy resources mobilized at the expense of primary metabolism, plant growth, and development. Results of screening among twenty-one local cucurbit cvs. ecotypes and accessions to evaluate tolerance levels against rub-inoculation of ToLCNDV led to the identification of potential rootstocks to attain suitable levels of tolerance against the virus in commercial cucurbit varieties. Cucurbit plants were challenged by a ToLCNDV isolated in Apulia denoted ToLCNDV-Le and evaluated for disease symptoms development and viral DNA accumulation up to 28 days after inoculation. On the basis of disease symptoms developed, plants were classified as tolerant, moderately tolerant, moderately susceptible, and susceptible. Cucumis melo cv. Barattiere did not show any detectable disease symptoms and very low levels of viral DNA accumulation was recorded; thus, it was used as rootstock for some of the remaining cucurbit genotypes that were used as scions. The tolerance trait was transmitted to the otherwise susceptible and moderately susceptible cucurbit genotypes grafted onto the cv. Barattiere. The results of this study suggest practical implications of the approach described.

Micrografting: An Old Dog Plays New Tricks in Obligate Plant Pathogens

Micrografting, which was developed almost 50 years ago, has long been used for virus eradication, micropropagation, regeneration, rejuvenation, and graft compatibility. Recently, micrografting has been used for studies of long-distance trafficking and signaling of molecules between scions and rootstocks. The graft transmissiveness of obligate plant pathogens, such as viruses, viroids, and phytoplasmas, facilitated the use of micrografting to study biological indexing and pathogen transmission, pathogen-induced graft incompatibility, and screening for the pathogen resistance during the past 20 years. The present study provides comprehensive information on the latter subjects. Finally, prospects are proposed to direct further studies.

Viruses of Economic Impact on Tomato Crops in Mexico: From Diagnosis to Management-A Review

Tomato is the most economically important vegetable crop worldwide and the second most important for Mexico. However, viral diseases are among the main limiting factors that affect the productivity of this crop, causing total losses in some cases. This review provides key information and findings on the symptoms, distribution, transmission, detection, and management of diseases caused by viruses of major importance in tomato crops in Mexico. Currently, about 25 viruses belonging to nine different families have been reported infecting tomato in Mexico, but not all of them cause economically significant diseases. Viruses of economic importance include tomato brown rugose fruit virus (ToBRFV), tomato spotted wilt virus (TSWV), tomato yellow leaf curl virus (TYLCV), pepino mosaic virus (PepMV), and tomato marchitez virus (ToMarV). The topics discussed here will provide updated information about the status of these plant viruses in Mexico as well as diverse management strategies that can be implemented according to the specific circumstances of each viral pathosystem. Additionally, a list of tomato-affecting viruses not present in Mexico that are continuous threats to the crop health is included.

Grafting: a potential method to reveal the differential accumulation mechanism of secondary metabolites

Plant secondary metabolites make a great contribution to the agricultural and pharmaceutical industries. Their accumulation is determined by the integrated transport of target compounds and their biosynthesis-related RNA, protein, or DNA. However, it is hard to track the movement of these biomolecules in vivo. Grafting may be an ideal method to solve this problem. The differences in genetic and metabolic backgrounds between rootstock and scion, coupled with multiple omics approaches and other molecular tools, make it feasible to determine the movement of target compounds, RNAs, proteins, and DNAs. In this review, we will introduce methods of using the grafting technique, together with molecular biological tools, to reveal the differential accumulation mechanism of plant secondary metabolites at different levels. Details of the case of the transport of one diterpene alkaloid, fuziline, will be further illustrated to clarify how the specific accumulation model is shaped with the help of grafting and multiple molecular biological tools.

Grafting for managing vegetable crop pests

Nematode and disease resistant rootstocks have been developed for many vegetable crops including tomato, eggplant, melon, watermelon, and cucumber and are being utilized by an increasing number of growers. Grafting commercially desirable vegetable scions on nematode and disease resistant rootstocks has been significantly stimulated by the need for an alternative to banned soil fumigation with methyl bromide, which had been the primary method for managing soil-borne nematodes, diseases, and weeds. Rootstocks resistant to root-knot nematodes (Meloidogyne spp.) and diseases including Fusarium wilt, Fusarium crown and root rot, Verticillium wilt, bacterial wilt, Southern blight, and sudden wilt have been developed and many are available commercially. New technologies such as transcriptomics, identification of differentially expressed genes, transgene rootstocks, and RNAi silencing are being used in the development of vegetable rootstocks which are resistant to pests, salt tolerant, and heat and cold tolerant. Overall, grafting has proven to be a successful and environmentally safe method for managing root-knot nematodes and soil-borne diseases by reducing infection, disease development, and inoculum build-up in the soil, which is especially important for growth of healthy subsequent crops. © 2021 Society of Chemical Industry.

The Role of Grafting in the Resistance of Tomato to Viruses

Grafting is routinely implemented in modern agriculture to manage soilborne pathogens such as fungi, oomycetes, bacteria, and viruses of solanaceous crops in a sustainable and environmentally friendly approach. Some rootstock/scion combinations use specific genetic resistance mechanisms to impact also some foliar and airborne pathogens, including arthropod or contact-transmitted viruses. These approaches resulted in poor efficiency in the management of plant viruses with superior virulence such as the strains of tomato spotted wilt virus breaking the Sw5 resistance, strains of cucumber mosaic virus carrying necrogenic satellite RNAs, and necrogenic strains of potato virus Y. Three different studies from our lab documented that suitable levels of resistance/tolerance can be obtained by grafting commercial tomato varieties onto the tomato ecotype Manduria (Ma) rescued in the framework of an Apulian (southern Italy) regional program on biodiversity. Here we review the main approaches, methods, and results of the three case studies and propose some mechanisms leading to the tolerance/resistance observed in susceptible tomato varieties grafted onto Ma as well as in self-grafted plants. The proposed mechanisms include virus movement in plants, RNA interference, genes involved in graft wound response, resilience, and tolerance to virus infection.

Suppression of RNA-dependent RNA polymerase 6 in tomatoes allows potato spindle tuber viroid to invade basal part but not apical part including pluripotent stem cells of shoot apical meristem

RNA-dependent RNA polymerase 6 (RDR6) is one of the key factors in plant defense responses and suppresses virus or viroid invasion into shoot apical meristem (SAM) in Nicotiana benthamiana. To evaluate the role of Solanum lycopersicum (Sl) RDR6 upon viroid infection, SlRDR6-suppressed (SlRDR6i) ‘Moneymaker’ tomatoes were generated by RNA interference and inoculated with intermediate or lethal strain of potato spindle tuber viroid (PSTVd). Suppression of SlRDR6 did not change disease symptoms of both PSTVd strains in ‘Moneymaker’ tomatoes. Analysis of PSTVd distribution in shoot apices by in situ hybridization revealed that both PSTVd strains similarly invade the basal part but not apical part including pluripotent stem cells of SAM in SlRDR6i plants at a low rate unlike a previous report in N. benthamiana. In addition, unexpectedly, amount of PSTVd accumulation was apparently lower in SlRDR6i plants than in control tomatoes transformed with empty cassette in early infection especially in the lethal strain. Meanwhile, SlRDR6 suppression did not affect the seed transmission rates of PSTVd. These results indicate that RDR6 generally suppresses PSTVd invasion into SAM in plants, while suppression of RDR6 does not necessarily elevate amount of PSTVd accumulation. Additionally, our results suggest that host factors such as RDR1 other than RDR6 may also be involved in the protection of SAM including pluripotent stem cells from PSTVd invasion and effective RNA silencing causing the decrease of PSTVd accumulation during early infection in tomato plants.

The Tomato spotted wilt virus (TSWV) Genome is Differentially Targeted in TSWV-Infected Tomato (Solanum lycopersicum) with or without Sw-5 Gene

Tospoviruses cause significant losses to a wide range of agronomic and horticultural crops worldwide. The type member, Tomato spotted wilt tospovirus (TSWV), causes systemic infection in susceptible tomato cultivars, whereas its infection is localized in cultivars carrying the Sw-5 resistance gene. The response to TSWV infection in tomato cultivars with or without Sw-5 was determined at the virus small RNA level in the locally infected leaf. Predicted reads were aligned to TSWV reference sequences. The TSWV genome was found to be differentially processed among each of the three-viral genomic RNAs—Large (L), Medium (M) and Small (S)—in the Sw-5(+) compared to Sw-5(−) genotypes. In the Sw-5(+) cultivar, the L RNA had the highest number of viral small-interfering RNAs (vsiRNAs), whereas in the Sw-5(−) cultivar the number was higher in the S RNA. Among the three-viral genomic RNAs, the distribution of hotspots showed a higher number of reads per million reads of vsiRNAs of 21 and 22 nt class at the 5′ and 3′ ends of the L and the S RNAs, with less coverage in the M RNA. In the Sw-5(−) cultivar, the nature of the 5′ nucleotide-end in the siRNAs varied significantly; reads with 5′-adenine-end were most abundant in the mock control, whereas cytosine and uracil were more abundant in the infected plants. No such differences were seen in case of the resistant genotype. Findings provided insights into the response of tomato cultivars to TSWV infection.

Grafting alters tomato transcriptome and enhances tolerance to an airborne virus infection

Grafting of commercial tomato varieties and hybrids on the tomato ecotype Manduria resulted in high levels of tolerance to the infection of Sw5 resistance-breaking strains of tomato spotted wilt virus and of severe cucumber mosaic virus strains supporting hypervirulent satellite RNAs that co-determine stunting and necrotic phenotypes in tomato. To decipher the basis of such tolerance, here we used a RNAseq analysis to study the transcriptome profiles of the Manduria ecotype and of the susceptible variety UC82, and of their graft combinations, exposed or not to infection of the potato virus Y recombinant strain PVY^C-to. The analysis identified graft- and virus-responsive mRNAs differentially expressed in UC82 and Manduria, which led to an overall suitable level of tolerance to viral infection confirmed by the appearance of a recovery phenotype in Manduria and in all graft combinations. The transcriptome analysis suggested that graft wounding and viral infection had diverging effects on tomato transcriptome and that the Manduria ecotype was less responsive than the UC82 to both graft wounding and potyviral infection. We propose that the differential response to the two types of stress could account for the tolerance to viral infection observed in the Manduria ecotype as well as in the susceptible tomato variety UC82 self-grafted or grafted on the Manduria ecotype.

Grafting alters tomato transcriptome and enhances tolerance to an airborne virus infection

Grafting of commercial tomato varieties and hybrids on the tomato ecotype Manduria resulted in high levels of tolerance to the infection of Sw5 resistance-breaking strains of tomato spotted wilt virus and of severe cucumber mosaic virus strains supporting hypervirulent satellite RNAs that co-determine stunting and necrotic phenotypes in tomato. To decipher the basis of such tolerance, here we used a RNAseq analysis to study the transcriptome profiles of the Manduria ecotype and of the susceptible variety UC82, and of their graft combinations, exposed or not to infection of the potato virus Y recombinant strain PVY^C-to. The analysis identified graft- and virus-responsive mRNAs differentially expressed in UC82 and Manduria, which led to an overall suitable level of tolerance to viral infection confirmed by the appearance of a recovery phenotype in Manduria and in all graft combinations. The transcriptome analysis suggested that graft wounding and viral infection had diverging effects on tomato transcriptome and that the Manduria ecotype was less responsive than the UC82 to both graft wounding and potyviral infection. We propose that the differential response to the two types of stress could account for the tolerance to viral infection observed in the Manduria ecotype as well as in the susceptible tomato variety UC82 self-grafted or grafted on the Manduria ecotype.

Morphological and Chemical Profile of Three Tomato (Solanum lycopersicum L.) Landraces of A Semi-Arid Mediterranean Environment

Puglia (Southern Italy), particularly rich in tomato agro-biodiversity, can be considered a typical region of the semi-arid Mediterranean environments. In this study, three local varieties of tomato (Manduria, Giallo di Crispiano and Regina) were characterized by using morphological descriptors according to international standards. Chemical (isoprenoids, ascorbic acid, total phenols, sugars and mineral content) and agronomic assessment were carried out to highlight the specific traits of these local varieties well adapted to a semi-arid environment. Data of morphological traits according to the “International Union for the Protection of New Varieties of Plants” (UPOV) guideline evidenced a clear distinctness among all three landraces, especially as regards fruits. Results also highlighted that a great part of variation in chemical traits was almost exclusively due to genotypes, while in a few cases observed differences resulted from the interaction between genotype and harvest time. The results of the present study may represent the first step toward the recognition of “conservation variety” status for Regina, Giallo di Crispiano and Manduria tomato landraces. At the same time, both quality traits and agronomic performance of these tomato genotypes suggest the possibility of their cultivation in other semi-arid environments also considering their quality traits, in view of a sustainable production.

Dual resistance of transgenic plants against Cymbidium mosaic virus and Odontoglossum ringspot virus

Taxonomically distinct Cymbidium mosaic potexvirus (CymMV) and Odontoglossum ringspot tobamovirus (ORSV) are two of the most prevalent viruses worldwide; when co-infecting orchids, they cause synergistic symptoms. Because of the huge economic loss in quality and quantity in the orchid industry with virus-infected orchids, virus-resistant orchids are urgently needed. To date, no transgenic resistant lines against these two viruses have been reported. In this study, we generated transgenic Nicotiana benthamiana expressing various constructs of partial CymMV and ORSV genomes. Several transgenic lines grew normally and remained symptomless after mixed inoculation with CymMV and ORSV. The replication of CymMV and ORSV was approximately 70-90% lower in protoplasts of transgenic lines than wild-type (WT) plants. Of note, we detected extremely low or no viral RNA or capsid protein of CymMV and ORSV in systemic leaves of transgenic lines after co-infection. Grafting experiments further revealed that CymMV and ORSV trafficked extremely inefficiently from co-infected WT stocks to transgenic scions, presumably due to RNA-mediated interference. This study reports the first successful creation of dual resistant transgenic lines against CymMV and ORSV. Our studies shed light on the commercial development of transgenic orchid production to combat the global viral threat.

Host-Induced Silencing of Pathogenicity Genes Enhances Resistance to Fusarium oxysporum Wilt in Tomato

This study presents a novel approach of controlling vascular wilt in tomato by RNAi expression directed to pathogenicity genes of Fusarium oxysporum f. sp. lycopersici. Vascular wilt of tomato caused by Fusarium oxysporum f. sp. lycopersici leads to qualitative and quantitative loss of the crop. Limitation in the existing control measures necessitates the development of alternative strategies to increase resistance in the plants against pathogens. Recent findings paved way to RNAi, as a promising method for silencing of pathogenicity genes in fungus and provided effective resistance against fungal pathogens. Here, two important pathogenicity genes FOW2, a Zn(II)2Cys6 family putative transcription regulator, and chsV, a putative myosin motor and a chitin synthase domain, were used for host-induced gene silencing through hairpinRNA cassettes of these genes against Fusarium oxysporum f. sp. lycopersici. HairpinRNAs were assembled in appropriate binary vectors and transformed into tomato plant targeting FOW2 and chsV genes, for two highly pathogenic strains of Fusarium oxysporum viz. TOFOL-IHBT and TOFOL-IVRI. Transgenic tomatoes were analyzed for possible attainment of resistance in transgenic lines against fungal infection. Eight transgenic lines expressing hairpinRNA cassettes showed trivial disease symptoms after 6-8 weeks of infection. Hence, the host-induced posttranscriptional gene silencing of pathogenicity genes in transgenic tomato plants has enhanced their resistance to vascular wilt disease caused by Fusarium oxysporum.

Resistance to Tospoviruses in Vegetable Crops: Epidemiological and Molecular Aspects

During the past three decades, the economic impact of tospoviruses has increased, causing high yield losses in a variety of crops and ornamentals. Owing to the difficulty in combating thrips vectors with insecticides, the best way to limit/prevent tospovirus-induced diseases involves a management strategy that includes virus resistance. This review briefly presents current tospovirus taxonomy, diversity, molecular biology, and cytopathology as an introduction to a more extensive description of the two main resistance genes employed against tospoviruses: the Sw5 gene in tomato and the Tsw in pepper. Natural and experimental resistance-breaking (RB) isolates allowed the identification of the viral avirulence protein triggering each of the two resistance gene products; epidemiology of RB isolates is discussed to reinforce the need for allelic variants and the need to search for new/alternative resistance genes. Ongoing efforts for alternative resistance strategies are described not only for Tomato spotted wilt virus (TSWV) in pepper and tomato but also for other vegetable crops heavily impacted by tospoviruses.