Aphid gene expression following polerovirus acquisition is host species dependent

Upon acquisition of persistent circulative viruses such as poleroviruses, the virus particles transcytose through membrane barriers of aphids at the midgut and salivary glands via hemolymph. Such intricate interactions can influence aphid behavior and fitness and induce associated gene expression in viruliferous aphids. Differential gene expression can be evaluated by omics approaches such as transcriptomics. Previously conducted aphid transcriptome studies used only one host species as the source of virus inoculum. Viruses typically have alternate hosts. Hence, it is not clear how alternate hosts infected with the same virus isolate alter gene expression in viruliferous vectors. To address the question, this study conducted a transcriptome analysis of viruliferous aphids that acquired the virus from different host species. A polerovirus, cotton leafroll dwarf virus (CLRDV), which induced gene expression in the cotton aphid, Aphis gossypii Glover, was assessed using four alternate hosts, viz., cotton, hibiscus, okra, and prickly sida. Among a total of 2,942 differentially expressed genes (DEGs), 750, 310, 1,193, and 689 genes were identified in A. gossypii that acquired CLRDV from infected cotton, hibiscus, okra, and prickly sida, respectively, compared with non-viruliferous aphids that developed on non-infected hosts. A higher proportion of aphid genes were overexpressed than underexpressed following CLRDV acquisition from cotton, hibiscus, and prickly sida. In contrast, more aphid genes were underexpressed than overexpressed following CLRDV acquisition from okra plants. Only four common DEGs (heat shock protein, juvenile hormone acid O-methyltransferase, and two unannotated genes) were identified among viruliferous aphids from four alternate hosts. Gene ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations indicated that the acquisition of CLRDV induced DEGs in aphids associated with virus infection, signal transduction, immune systems, and fitness. However, these induced changes were not consistent across four alternate hosts. These data indicate that alternate hosts could differentially influence gene expression in aphids and presumably aphid behavior and fitness despite being infected with the same virus isolate

Investigating the effects of planting date and Aphis gossypii management on reducing the final incidence of cotton leafroll dwarf virus

This is the first study to research management strategies for cotton leafroll dwarf virus (CLRDV) in the southeastern U.S. The efficacy of aphid vector management to reduce final CLRDV incidence was investigated concurrent with efforts to monitor aphid population dynamics and timing of CLRDV spread. Adjusting the planting date and insecticide applications did not reduce the final incidence of CLRDV, which was confirmed in 60–100% of plants per plot using RT-PCR. Aphid population density was reduced, but not eliminated with foliar insecticide applications. Aphis gossypii was the only species observed on cotton and was the dominant species collected in pan traps. Three distinct periods of virus spread were detected with sentinel plants including early, mid-and late-season. Most virus spread occurred during large aphid dispersal events

BIOLOGICAL, EPIDEMIOLOGICAL AND MOLECULAR INSIGHTS INTO THRIPS-IRIS YELLOW SPOT TOSPOVIRUS PEST COMPLEX

Iris yellow spot tospovirus (IYSV) (genus Tospovirus, family Bunyaviridae), transmitted by Thrips tabaci L. causes an economically important disease in both onion bulb and seed crop in the USA and other onion-growing regions of the world. Onion thrips as a pest alone can cause up to >60 % crop loss. Besides Allium spp, several weeds were found to be hosts of IYSV.IYSV isolates collected from different states in the USA were evaluated to determine the existence of biologically distinct strains. On the basis of the ability to cause systemic infection, disease severity, senescence and death of the inoculated plants, isolates were delineated as mild or severe isolates.Since the genome structure of only the small (S) RNA of IYSV was known, the large (L) and medium (M) RNAs of the virus were sequenced. The L RNA was 8,880 nucleotides in length, coding the 331.17 kDa RNA-dependent RNA polymerase in the viral complementary (vc) strand. The M RNA was 4,817 nucleotides long coding the movement protein (34.7kDa) in the viral sense and the glycoprotein precursor (128.4 kDa) in the vc strand.An ELISA protocol was developed for detecting IYSV in single adult thrips using a polyclonal antiserum produced against the nonstructural protein (NSs) coded by the small (S) RNA. The approach enabled estimating the proportion of viruliferous thrips among the field-collected thrips. This will help better understand the epidemiology of IYSV.To understand the molecular basis of the emergence of new tospoviruses, a system was developed to study virus-virus interactions. It was found that two distinct and economically important tospoviruses, IYSV and Tomato spotted wilt virus (TSWV) complement each other to overcome host defense. The small RNA expression profiles of IYSV and TSWV in single-and dually-infected datura plants showed that systemic leaves of dually-infected plants had reduced levels of TSWV N gene-specific small interfering RNAs (siRNAs). This identifies a new role for the viral gene silencing suppressor in potentially modulating the biology and host range of viruses and underscores the important role of virally-coded suppressors of gene silencing in virus infection of plants

Induction of Plant Resistance in Tobacco (Nicotiana tabacum) against Tomato Spotted Wilt Orthotospovirus through Foliar Application of dsRNA

Thrips-transmitted tomato spotted wilt orthotospovirus (TSWV) continues to be a constraint to peanut, pepper, tobacco, and tomato production in Georgia and elsewhere. TSWV is being managed by an integrated disease management strategy that includes a combination of cultural practices, vector management, and growing virus-resistant varieties where available. We used a non-transgenic strategy to induce RNA interference (RNAi)-mediated resistance in tobacco (Nicotiana tabacum) plants against TSWV. Double-stranded RNA (dsRNA) molecules for the NSs (silencing suppressor) and N (nucleoprotein) genes were produced by a two-step PCR approach followed by in vitro transcription. When topically applied to tobacco leaves, both molecules elicited a resistance response. Host response to the treatments was measured by determining the time to symptom expression, and the level of resistance by absolute quantification of the virus. We also show the systemic movement of dsRNA_N from the inoculated leaves to younger, non-inoculated leaves. Post-application, viral siRNAs were detected for up to nine days in inoculated leaves and up to six days in non-inoculated leaves. The topical application of dsRNAs to induce RNAi represents an environmentally safe and efficient way to manage TSWV in tobacco crops and could be applicable to other TSWV-susceptible crops

Complementation between two tospoviruses facilitates the systemic movement of a plant virus silencing suppressor in an otherwise restrictive host

New viruses pathogenic to plants continue to emerge due to mutation, recombination, or reassortment among genomic segments among individual viruses. Tospoviruses cause significant economic damage to a wide range of crops in many parts of the world. The genetic or molecular basis of the continued emergence of new tospoviruses and new hosts is not well understood though it is generally accepted that reassortment and/or genetic complementation among the three genomic segments of individual viruses could be contributing to this variability since plants infected with more than one tospovirus are not uncommon in nature. Two distinct and economically important tospoviruses, Iris yellow spot virus (IYSV) and Tomato spotted wilt virus (TSWV), were investigated for inter-virus interactions at the molecular level in dually-infected plants. Datura (Datura stramonium) is a permissive host for TSWV, while it restricts the movement of IYSV to inoculated leaves. In plants infected with both viruses, however, TSWV facilitated the selective movement of the viral gene silencing suppressor (NSs) gene of IYSV to the younger, uninoculated leaves. The small RNA expression profiles of IYSV and TSWV in single- and dually-infected datura plants showed that systemic leaves of dually-infected plants had reduced levels of TSWV N gene-specific small interfering RNAs (siRNAs). No TSWV NSs-specific siRNAs were detected either in the inoculated or systemic leaves of dually-infected datura plants indicating a more efficient suppression of host silencing machinery in the presence of NSs from both viruses as compared to the presence of only TSWV NSs. Our study identifies a new role for the viral gene silencing suppressor in potentially modulating the biology and host range of viruses and underscores the importance of virally-coded suppressors of gene silencing in virus infection of plants. This is the first experimental evidence of functional complementation between two distinct tospoviruses in the Bunyaviridae family

Whitefly-Transmitted Viruses of Cucurbits in the Southern United States

Cucurbits are economically important crops that are widely cultivated in many parts of the world, including the southern US. In recent years, higher temperatures have favored the rapid build-up of whiteflies in the fall-grown cucurbits in this region. As a result, whitefly-transmitted viruses (WTVs) have severely impacted the marketable yield of cucurbits. In this review, we discuss three major groups of WTVs negatively impacting cucurbit cultivation in the southern US, including begomoviruses, criniviruses, and ipomoviruses. Here, we discuss the available information on the biology, epidemiology and advances made toward detecting and managing these viruses, including sources of resistance and cultural practices

Alternative Methods to Manage Silverleaf Whitefly [Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae)] in Zucchini (Cucurbita pepo L.)

Whiteflies are the most detrimental insect pest for cucurbit crops in Georgia, USA. Plant growth and yield are severely reduced due to whitefly feeding and, more importantly, many plant viruses they vector. Chemical management of whiteflies has been ineffective. An integrated pest management (IPM) strategy is needed to reduce whitefly numbers to an acceptable level in cucurbit crops in Georgia. This study evaluated the impact of plastic mulch color, exclusionary row covers, particle clay, and organic insecticidal solutions (soap and neem oil) on plant growth, fruit yield, and whitefly index (WFindex) in zucchini squash (Cucurbita pepo L). The study was conducted during the fall of 2019 and 2020. Particle clay applications reduced zucchini foliar temperatures and WFindex and increased zucchini fruit yields. Black plastic mulch reduced fruit yield due to increased root zone temperature and did not impact whitefly numbers. Neem oil and soap decreased the WFindex but inconsistently affected fruit yield and quality. Our findings indicate that neem oil, soap, and particle clay may be part of an IPM program for whiteflies. However, further evaluations are needed in large plots and commercial fields to confirm the preliminary results of this study

Mutational analysis of two highly conserved motifs in the silencing suppressor encoded by tomato spotted wilt virus (genus Tospovirus, family Bunyaviridae)

Tospoviruses cause serious economic losses to a wide range of field and horticultural crops on a global scale. The NSs gene encoded by tospoviruses acts as a suppressor of host plant defense. We identified amino acid motifs that are conserved in all of the NSs proteins of tospoviruses for which the sequence is known. Using tomato spotted wilt virus (TSWV) as a model, the role of these motifs in suppressor activity of NSs was investigated. Using site-directed point mutations in two conserved motifs, glycine, lysine and valine/threonine (GKV/T) at positions 181-183 and tyrosine and leucine (YL) at positions 412-413, and an assay to measure the reversal of gene silencing in Nicotiana benthamiana line 16c, we show that substitutions (K182 to A, and L413 to A) in these motifs abolished suppressor activity of the NSs protein, indicating that these two motifs are essential for the RNAi suppressor function of tospoviruses

Engineering artificial microRNA-based resistance against tospoviruses

Custom-designed, artificial micro-RNAs are shown to provide targeted suppression of virus replication in infected plants thereby making plants resistant to virus infection

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Not Availablehrips-transmitted Iris yellow spot virus is an economically important viral pathogen of Allium crops worldwide. A global analysis of known IYSV nucleocapsid gene (N gene) sequences was carried out to determine the comparative population structure, spatial and temporal dynamics with reference to its genetic diversity and evolution. A total of 98 complete N gene sequences (including 8 sequences reported in this study) available in GenBank and reported from 23 countries were characterized by in-silico RFLP analysis. Based on RFLP, 94% of the isolates could be grouped into NL or BR types while the rest belonged to neither group. The relative proportion of NL and BR types was 46% and 48%, respectively. A temporal shift in the IYSV genotypes with a greater incremental incidence of IYSV BR was found over IYSV NL before 2005 compared to after 2005.Not Availabl