Restricted Spread of Tomato spotted wilt virus in Thrips-Resistant Pepper

Impact of Host Resistance to Tomato Spotted Wilt Orthotospovirus in Peanut Cultivars on Virus Population Genetics and Thrips Fitness

Thrips-transmitted tomato spotted wilt orthotospovirus (TSWV) is a major constraint to peanut production in the southeastern United States. Peanut cultivars with resistance to TSWV have been widely used for over twenty years. Intensive usage of resistant cultivars has raised concerns about possible selection pressure against TSWV and a likelihood of resistance breakdown. Population genetics of TSWV isolates collected from cultivars with varying levels of TSWV resistance was investigated using five TSWV genes. Phylogenetic trees of genes did not indicate host resistance-based clustering of TSWV isolates. Genetic variation in TSWV isolates and neutrality tests suggested recent population expansion. Mutation and purifying selection seem to be the major forces driving TSWV evolution. Positive selection was found in N and RdRp genes but was not influenced by TSWV resistance. Population differentiation occurred between isolates collected from 1998 and 2010 and from 2016 to 2019 but not between isolates from susceptible and resistant cultivars. Evaluated TSWV-resistant cultivars differed, albeit not substantially, in their susceptibility to thrips. Thrips oviposition was reduced, and development was delayed in some cultivars. Overall, no evidence was found to support exertion of selection pressure on TSWV by host resistance in peanut cultivars, and some cultivars differentially affected thrips fitness than others.

Fine mapping of a thrips resistance QTL in Capsicum and the role of diterpene glycosides in the underlying mechanism

A major thrips resistance QTL in Capsicum was fine-mapped to a region of 0.4 Mbp, and a multidisciplinary approach has been used to study putative underlying mechanisms. Resistance to thrips is an important trait for pepper growers. These insects can cause extensive damage to fruits, flowers and leaves on field and greenhouse grown plants worldwide. Two independent studies in Capsicum identified diterpene glycosides as metabolites that are correlated with thrips resistance. In this study, we fine-mapped a previously defined thrips resistance QTL on chromosome 6, to a region of 0.4 Mbp harbouring 15 genes. Two of these 15 candidate genes showed differences in gene expression upon thrips induction, when comparing plants carrying the resistance allele in homozygous state to plants with the susceptibility allele in homozygous state for the QTL region. Three genes, including the two genes that showed difference in gene expression, contained a SNP that was predicted to lead to changes in protein structure. Therefore, these three genes, i.e. an acid phosphatase 1 (APS1), an organic cation/carnitine transporter 7 (OCT7) and an uncharacterized locus LOC107874801, are the most likely candidates for playing a role in thrips resistance and are a first step in elucidating the genetic basis of thrips resistance in Capsicum. In addition, we show that the diterpene glycoside profiles did not differ between plants with the resistance and susceptibility allele for the chromosome 6 QTL, suggesting that these compounds do not play a role in the resistance conferred by the genes located in the major thrips resistance QTL studied.

Current Status and Potential of RNA Interference for the Management of Tomato Spotted Wilt Virus and Thrips Vectors

Tomato spotted wilt virus (TSWV) is the type member of the genus Orthotospovirus in the family Tospoviridae and order Bunyavirales. TSWV, transmitted by several species of thrips, causes significant disease losses to agronomic and horticultural crops worldwide, impacting both the yield and quality of the produce. Management strategies include growing virus-resistant cultivars, cultural practices, and managing thrips vectors through pesticide application. However, numerous studies have reported that TSWV isolates can overcome host-plant resistance, while thrips are developing resistance to pesticides that were once effective. RNA interference (RNAi) offers a means of host defence by using double-stranded (ds) RNA to initiate gene silencing against invading viruses. However, adoption of this approach requires production and use of transgenic plants and thus limits the practical application of RNAi against TSWV and other viruses. To fully utilize the potential of RNAi for virus management at the field level, new and novel approaches are needed. In this review, we summarize RNAi and highlight the potential of topical or exogenous application of RNAi triggers for managing TSWV and thrips vectors.

Overview of Biotic Stresses in Pepper (Capsicum spp.): Sources of Genetic Resistance, Molecular Breeding and Genomics

Pepper (Capsicum spp.) is one of the major vegetable crops grown worldwide largely appreciated for its economic importance and nutritional value. This crop belongs to the large Solanaceae family, which, among more than 90 genera and 2500 species of flowering plants, includes commercially important vegetables such as tomato and eggplant. The genus includes over 30 species, five of which (C. annuum, C. frutescens, C. chinense, C. baccatum, and C. pubescens) are domesticated and mainly grown for consumption as food and for non-food purposes (e.g., cosmetics). The main challenges for vegetable crop improvement are linked to the sustainable development of agriculture, food security, the growing consumers' demand for food. Furthermore, demographic trends and changes to climate require more efficient use of plant genetic resources in breeding programs. Increases in pepper consumption have been observed in the past 20 years, and for maintaining this trend, the development of new resistant and high yielding varieties is demanded. The range of pathogens afflicting peppers is very broad and includes fungi, viruses, bacteria, and insects. In this context, the large number of accessions of domesticated and wild species stored in the world seed banks represents a valuable resource for breeding in order to transfer traits related to resistance mechanisms to various biotic stresses. In the present review, we report comprehensive information on sources of resistance to a broad range of pathogens in pepper, revisiting the classical genetic studies and showing the contribution of genomics for the understanding of the molecular basis of resistance.

Tomato Plant Flavonoids Increase Whitefly Resistance and Reduce Spread of Tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV), a begomovirus (genus Begomovirus) is the causal agent of tomato yellow leaf curl disease (TYLCD), which causes severe damage to tomato (Solanum lycopersicum) crops throughout tropical and subtropical regions of the world. TYLCV is transmitted by the whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) in a circulative and persistent manner. Our previous studies showed that tomato flavonoids deter B. tabaci oviposition, but the effects of tomato flavonoids on the settling and feeding behavior of B. tabaci and on its transmission of TYLCV are unknown. Using two near-isogenic tomato lines that differ greatly in flavonoid levels, we found that high flavonoid production in tomato deterred the landing and settling of B. tabaci. Moreover, electrical penetration graph studies indicated that high flavonoid levels in tomato reduced B. tabaci probing and phloem-feeding efficiency. As a consequence, high flavonoid levels in tomato reduced the primary and secondary spread of TYLCV. The results indicate that tomato flavonoids provide antixenosis resistance against B. tabaci and that the breeding of such resistance in new varieties could enhance TYLCD management.

Thrips Resistance Screening Is Coming of Age: Leaf Position and Ontogeny Are Important Determinants of Leaf-Based Resistance in Pepper

Capsicum is a genus containing important crop species, many of which severely suffer from thrips infestation. Thrips feeding damages leaves and fruits, and often results in virus infections. Only a few insecticides are still effective against thrips, underlining the importance of finding natural resistance in crops. Capsicum is a perennial plant which is usually cultivated for several months, during which time the fruits are harvested. From the young vegetative stage to the mature fruit bearing stage, the plants are at risk to thrips infestation. Constitutive resistance to thrips over the entire ontogenetic development is therefore a key trait for a more sustainable and successful cultivation of the hot and sweet pepper. In addition to ontogeny, leaf position can affect the level of thrips resistance. Pest resistance levels are known to differ between young and old leaves. To our knowledge, no studies have explicitly considered ontogeny and leaf position when screening for constitutive resistance to thrips in Capsicum. In this study we analyze whether ontogeny and leaf position affect leaf-based resistance to Frankliniella occidentalis and Thrips tabaci, in 40 Capsicum accessions, comprising five different species. Our results show that resistance to both thrips species in Capsicum varies with ontogenetic stage. This variation in resistance among ontogenetic stages was not consistent among the accessions. However, accessions with constitutive resistance in both the flowering and fruit ripening stage could be identified. In addition, we found that thrips resistance is overall similar at different leaf positions within the ontogenetic stage. This implies that resistance mechanisms, such as defense compounds, are constitutively present at sufficient levels on all leaf positions. Finally, we found that resistance to F. occidentalis and resistance to T. tabaci were not correlated. This indicates that leaf-based resistance in Capsicum is thrips species-specific. Because of the variation in resistance over ontogeny, identifying Capsicum accessions with resistance over their entire lifespan is challenging. For resistance screening, accounting for leaf position may be less of a concern. To identify the defense mechanisms responsible for thrips resistance, it is important to further analyze and compare resistant and susceptible accessions.

Resistance to Thrips in Peanut and Implications for Management of Thrips and Thrips-Transmitted Orthotospoviruses in Peanut

Thrips are major pests of peanut (Arachis hypogaea L.) worldwide, and they serve as vectors of devastating orthotospoviruses such as Tomato spotted wilt virus (TSWV) and Groundnut bud necrosis virus (GBNV). A tremendous effort has been devoted to developing peanut cultivars with resistance to orthotospoviruses. Consequently, cultivars with moderate field resistance to viruses exist, but not much is known about host resistance to thrips. Integrating host plant resistance to thrips in peanut could suppress thrips feeding damage and reduce virus transmission, will decrease insecticide usage, and enhance sustainability in the production system. This review focuses on details of thrips resistance in peanut and identifies future directions for incorporating thrips resistance in peanut cultivars. Research on thrips-host interactions in peanut is predominantly limited to field evaluations of feeding damage, though, laboratory studies have revealed that peanut cultivars could differentially affect thrips feeding and thrips biology. Many runner type cultivars, field resistant to TSWV, representing diverse pedigrees evaluated against thrips in the greenhouse revealed that thrips preferred some cultivars over others, suggesting that antixenosis "non-preference" could contribute to thrips resistance in peanut. In other crops, morphological traits such as leaf architecture and waxiness and spectral reflectance have been associated with thrips non-preference. It is not clear if foliar morphological traits in peanut are associated with reduced preference or non-preference of thrips and need to be evaluated. Besides thrips non-preference, thrips larval survival to adulthood and median developmental time were negatively affected in some peanut cultivars and in a diploid peanut species Arachis diogoi (Hoehne) and its hybrids with a Virginia type cultivar, indicating that antibiosis (negative effects on biology) could also be a factor influencing thrips resistance in peanut. Available field resistance to orthotospoviruses in peanut is not complete, and cultivars can suffer substantial yield loss under high thrips and virus pressure. Integrating thrips resistance with available virus resistance would be ideal to limit losses. A discussion of modern technologies such as transgenic resistance, marker assisted selection and RNA interference, and future directions that could be undertaken to integrate resistance to thrips and to orthotospoviruses in peanut cultivars is included in this article.

QTL mapping of thrips resistance in pepper

A QTL for thrips resistance on pepper chromosome 6 was identified and validated. This QTL affects thrips larval development and explains 50% of the variation. Thrips is one of the most damaging pests in pepper (Capsicum). Resistance to thrips was identified in Capsicum annuum. This study was aimed at the elucidation of the genetic background of thrips resistance in Capsicum through QTL mapping. The QTL analysis was carried out for Frankliniella occidentalis resistance in an F2 population consisting of 196 plants derived from an interspecific cross between the highly resistant C. annuum AC 1979 as female parent and the highly susceptible C. chinense 4661 as male parent. Fifty-seven SSR, 109 AFLP, and 5 SNP markers were used to construct a genetic map with a total length of 1636 cM. Damage caused by larvae and the survival of first and second instar larval stages observed in a no-choice test were used as parameters of resistance. Interval mapping detected one QTL for each of these parameters, all co-localizing near the same marker on chromosome 6. Use of this marker as co-factor in a multiple-QTL mapping analysis failed to uncover any additional QTLs. This QTL explained about 50% of the genetic variation, and the resistance allele of this QTL was inherited from the resistant parent. Thrips resistance was not linked to trichome density.

Host plant resistance against tomato spotted wilt virus in peanut (Arachis hypogaea) and its impact on susceptibility to the virus, virus population genetics, and vector feeding behavior and survival

Tomato spotted wilt virus (TSWV) severely affects peanut production in the southeastern United States. Breeding efforts over the last three decades resulted in the release of numerous peanut genotypes with field resistance to TSWV. The degree of field resistance in these genotypes has steadily increased over time, with recently released genotypes exhibiting a higher degree of field resistance than older genotypes. However, most new genotypes have never been evaluated in the greenhouse or laboratory against TSWV or thrips, and the mechanism of resistance is unknown. In this study, TSWV-resistant and -susceptible genotypes were subjected to TSWV mechanical inoculation. The incidence of TSWV infection was 71.7 to 87.2%. Estimation of TSWV nucleocapsid (N) gene copies did not reveal significant differences between resistant and susceptible genotypes. Parsimony and principal component analyses of N gene nucleotide sequences revealed inconsistent differences between virus isolates collected from resistant and susceptible genotypes and between old (collected in 1998) and new (2010) isolates. Amino acid sequence analyses indicated consistent differences between old and new isolates. In addition, we found evidence for overabundance of nonsynonymous substitutions. However, there was no evidence for positive selection. Purifying selection, population expansion, and differentiation seem to have influenced the TSWV populations temporally rather than positive selection induced by host resistance. Choice and no-choice tests indicated that resistant and susceptible genotypes differentially affected thrips feeding and survival. Thrips feeding and survival were suppressed on some resistant genotypes compared with susceptible genotypes. These findings reveal how TSWV resistance in peanut could influence evolution, epidemiology, and management of TSWV.

Virus diseases of peppers (Capsicum spp.) and their control

The number of virus species infecting pepper (Capsicum spp.) crops and their incidences has increased considerably over the past 30 years, particularly in tropical and subtropical pepper production systems. This is probably due to a combination of factors, including the expansion and intensification of pepper cultivation in these regions, the increased volume and speed of global trade of fresh produce (including peppers) carrying viruses and vectors to new locations, and perhaps climate change expanding the geographic range suitable for the viruses and vectors. With the increased incidences of diverse virus species comes increased incidences of coinfection with two or more virus species in the same plant. There is then greater chance of synergistic interactions between virus species, increasing symptom severity and weakening host resistance, as well as the opportunity for genetic recombination and component exchange and a possible increase in aggressiveness, virulence, and transmissibility. The main virus groups infecting peppers are transmitted by aphids, whiteflies, or thrips, and a feature of many populations of these vector groups is that they can develop resistance to some of the commonly used insecticides relatively quickly. This, coupled with the increasing concern over the impact of over- or misuse of insecticides on the environment, growers, and consumers, means that there should be less reliance on insecticides to control the vectors of viruses infecting pepper crops. To improve the durability of pepper crop protection measures, there should be a shift away from the broadscale use of insecticides and the use of single, major gene resistance to viruses. Instead, integrated and pragmatic virus control measures should be sought that combine (1) cultural practices that reduce sources of virus inoculum and decrease the rate of spread of viruliferous vectors into the pepper crop, (2) synthetic insecticides, which should be used judiciously and only when the plants are young and most susceptible to infection, (3) appropriate natural products and biocontrol agents to induce resistance in the plants, affect the behavior of the vector insects, or augment the local populations of parasites or predators of the virus vectors, and (4) polygenic resistances against viruses and vector insects with pyramided single-gene virus resistances to improve resistance durability.

Tospoviruses in the Mediterranean area

Tospoviruses are among the most serious threats to vegetable crops in the Mediterranean basin. Tospovirus introduction, spread, and the diseases these viruses cause have been traced by epidemiological case studies. Recent research has centered on the close relationship between tospoviruses and their arthropod vectors (species of the Thripidae family). Here, we review several specific features of tospovirus-thrips associations in the Mediterranean. Since the introduction of Frankliniella occidentalis in Europe, Tomato spotted wilt virus (TSWV) has become one of the limiting factors for vegetable crops such as tomato, pepper, and lettuce. An increasing problem is the emergence of TSWV resistance-breaking strains that overcome the resistance genes in pepper and tomato. F. occidentalis is also a vector of Impatiens necrotic spot virus, which was first observed in the Mediterranean basin in the 1980s. Its importance as a cause of vegetable crop diseases is limited to occasional incidence in pepper and tomato fields. A recent introduction is Iris yellow spot virus, transmitted by the onion thrips Thrips tabaci, in onion and leek crops. Control measures in vegetable crops specific to Mediterranean conditions were examined in the context of their epidemiological features and tospovirus species which could pose a future potential risk for vegetable crops in the Mediterranean were discussed.

Whitefly resistance traits derived from the wild tomato Solanum pimpinellifolium affect the preference and feeding behavior of Bemisia tabaci and reduce the spread of Tomato yellow leaf curl virus

Breeding of tomato genotypes that limit whitefly (Bemisia tabaci) access and feeding might reduce the spread of Tomato yellow leaf curl virus (TYLCV), a begomovirus (genus Begomovirus, family Geminiviridae) that is the causal agent of tomato yellow leaf curl disease. TYLCV is restricted to the phloem and is transmitted in a persistent manner by B. tabaci. The tomato breeding line ABL 14-8 was developed by introgressing type IV leaf glandular trichomes and secretion of acylsucroses from the wild tomato Solanum pimpinellifolium accession TO-937 into the genetic background of the whitefly- and virus-susceptible tomato cultivar Moneymaker. Results of preference bioassays with ABL 14-8 versus Moneymaker indicated that presence of type IV glandular trichomes and the production of acylsucrose deterred the landing and settling of B. tabaci on ABL 14-8. Moreover, electrical penetration graph studies indicated that B. tabaci adults spent more time in nonprobing activities and showed a reduced ability to start probing. Such behavior resulted in a reduced ability to reach the phloem. The superficial type of resistance observed in ABL 14-8 against B. tabaci probing significantly reduced primary and secondary spread of TYLCV.

Occurrence and Relative Incidence of Viruses Infecting Soybeans in Iran

A survey was conducted to determine the incidence of Alfalfa mosaic virus (AlMV), Bean common mosaic virus (BCMV), Bean yellow mosaic virus (BYMV), Blackeye cowpea mosaic virus (BlCMV), Cucumber mosaic virus (CMV), Pea enation mosaic virus (PEMV), Peanut mottle virus (PeMoV), Soybean mosaic virus (SMV), Tobacco mosaic virus (TMV), Tobacco ringspot virus (TRSV), Tobacco streak virus (TSV), Tomato ringspot virus (ToRSV), and Tomato spotted wilt virus (TSWV) on soybean (Glycine max) in Iran. Totals of 3,110 random and 1,225 symptomatic leaf samples were collected during the summers of 1999 and 2000 in five provinces of Iran, where commercial soybean is grown, and tested by enzyme-linked immunosorbent assay (ELISA) using specific polyclonal antibodies. Serological diagnoses were confirmed by electron microscopy and host range studies. The highest virus incidence among the surveyed provinces was recorded in Mazandaran (18.6%), followed by Golestan (15.7%), Khuzestan (14.2%), Ardabil (13.9%), and Lorestan (13.5%). Incidence of viruses in decreasing order was SMV (13.3%), TSWV (5.4%), TRSV (4.2%), TSV (4.1%), PEMV (2.9%), BYMV (2.2%), ToRSV (2.1%), AlMV (1.3%), BCMV (0.8%), and CMV (0.6%). Additionally, 1.5% of collected leaf samples had positive reactions in ELISA with antiserum to TMV, indicating the possible infection of soybeans in Iran with a Tobamovirus that is related serologically to TMV. Of 195 leaves from plants showing soybean pod set failure syndrome (PSF) in Mazandaran and Lorestan, only 14 (7.2%) samples had viral infection. No correlation was observed between PSF and presence of the 13 viruses tested, suggesting the involvement of other viruses or factors in this syndrome. To investigate the presence of seed-borne viruses, including SMV, TRSV, ToRSV, and TSV, 7,830 soybean seeds were collected randomly at harvesting time from the major sites of soybean seed production located in Mazandaran and Golestan provinces. According to ELISA analyses of germinated seedlings, 7.1 and 8.9% of the seed samples from Golestan and Mazandaran provinces, respectively, transmitted either SMV, TRSV, ToRSV, or TSV through seed. We also showed that SMV and other seed transmissible viruses, as well as TSWV, usually are the most prevalent viruses in soybean fields in Iran. In this survey, natural occurrence of AlMV, BCMV, BlCMV, BYMV, CMV, PEMV, PeMoV, and TSWV was reported for the first time on soybeans in Iran.

Tomato spotted wilt virus Infection Improves Host Suitability for Its Vector Frankliniella occidentalis

ABSTRACT The effect of Tomato spotted wilt virus (TSWV) infection on plant attractiveness for the western flower thrips (Frankliniella occidentalis) was studied. Significantly more thrips were recovered on infected than were recovered on noninfected pepper (Capsicum annuum) plants in different preference tests. In addition, more offspring were produced on the virus-infected pepper plants, and this effect also was found for TSWV-infected Datura stramonium. Thrips behavior was minimally influenced by TSWV-infection of host plants with only a slight preference for feeding on infected plants. Offspring development was positively affected since larvae hatched earlier from eggs and subsequently pupated faster on TSWV-infected plants. These results show a mutualistic relationship between F. occidentalis and TSWV.