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Plant-mediated interactions between plant viruses and their vectors are important determinants of the population dynamics of both types of organisms in the field. The whitefly Bemisia tabaci can establish mutualism with begomoviruses via their shared host plants. This mutualism is achieved by the interaction between virulence factors and their host proteins. While the virulence factor βC1 encoded by tomato yellow leaf curl China betasatellite (TYLCCNB), a subviral agent associated to the begomovirus tomato yellow leaf curl China virus (TYLCCNV), may interact with plant protein MYC2, thereby establishing the indirect mutualism between TYLCCNV and whitefly, whether other mechanisms are involved remains unknown. Here, we found the in vitro and in vivo interactions between βC1 and tobacco protein S-phase kinase associated protein 1 (NtSKP1). Silencing the expression of NtSKP1 enhanced the survival rate and fecundity of whiteflies on tobacco plants. NtSKP1 could activate the transcription of genes in jasmonic acid (JA) pathways by impairing the stabilization of JAZ1 protein. Moreover, βC1-NtSKP1 interaction could interfere JAZ1 degradation and attenuate the plant JA defense responses. These results revealed a novel mechanism underlying the better performance of whiteflies on TYLCCNV/TYLCCNB-infected plants.

Actinomycetes comprise a large group of biocontrol bacteria, yet no studies have reported on their effects against plant virus diseases. Here, we evaluated the control effects and the possible mechanisms of Streptomyces pactum Act12 against tomato yellow leaf curl virus disease (TYLCD) through field and pot experiments. We also analyzed changes in plant growth and rhizosphere microbial community composition following the application of Act12. The pre-inoculation of soil with the Act12 agent enhanced the activities of peroxidase and chitinase while upregulating the expression of genes related to plant systemic resistance (PR-1 and SIPI-II) and specific TYLCD resistance (SIPer1 and SIVRSLip) in tomato leaves under field conditions. The effects of Act12 at inducing a decline in TYLCD severity and promoting plant growth were deemed satisfactory in all three field experiments. In the pot experiments, Act12 induced a decline in the viral DNA level and an enhancing of peroxidase, polyphenoloxidase, and phenylalanine ammonia lyase activities in tomato leaves. Additionally, Act12 treatment reduced the accumulation of H2O2 and O·−2, increased the levels of salicylic and jasmonic acids, and upregulated the expression of genes related to plant resistance and RNA interference in tomato leaves. Applying Act12 also increased the diversity of bacteria and the abundance of potential beneficial microbes (e.g., Trichoderma and Bacillus) in the root-zone and root-surface soils, resulting in modular differentiation of co-occurrence networks. In conclusion, applying the S. pactum Act12 agent reduced TYLCD severity and promoted plant growth in tomato. Whether the changes of resistance-related gene expression and rhizosphere microbial community composition contribute to TYLCV resistance needs to be further investigated.

The Panel on Plant Health performed for the European Union (EU) territory a pest categorisation of Tomato yellow leaf curl virus (TYLCV) and three related viruses, Tomato yellow leaf curl Sardinia virus (TYLCSV), Tomato yellow leaf curl Axarquia virus (TYLCAxV) and Tomato yellow leaf curl Malaga virus (TYLCMaV), which collectively cause the tomato yellow leaf curl disease (TYLCD) in Europe. The viruses are well‐defined species of the genus Begomovirus, are exclusively transmitted by members of the Bemisia tabaci species complex and have tomato, as well as a few other crops or weeds, as their hosts. TYLCV is listed on tomato plants for planting, other than seeds, in Annex IIAII of Directive 2000/29/EC. While establishment and local spread rely on the Bemisia vector, the viruses can also be disseminated over long distances by movement of infected plants for planting or by consignments of non‐host plants carrying viruliferous whiteflies. Establishment outdoors and spread are limited to regions with ecoclimatic conditions suitable for the establishment of vector populations in the open. Outbreaks can nevertheless occur in other regions under protected cultivation conditions. Because of the very high potential impact of TYLCD, tomato production in affected regions requires intensive crop management efforts to reduce impact. TYLCV appears to be present in almost all EU regions with suitable ecoclimatic conditions for its establishment in open fields, while the other three viruses do not appear to have reached their full establishment potential. All four viruses are absent from other regions of the EU but have the potential to cause temporary outbreaks there.

While the management of biological invasions is often characterized by a series of single-species decisions, invasive species exist within larger food webs. These biotic interactions can alter the impact of control/eradication programs and may cause suppression efforts to inadvertently facilitate invasion spread and impact. We document the rapid replacement of the invasive Bemisia Middle East-Asia Minor I (MEAM1) cryptic biotype by the cryptic Mediterranean (MED) biotype throughout China and demonstrate that MED is more tolerant of insecticides and a better vector of tomato yellow leaf curl virus (TYLCV) than MEAM1. While MEAM1 usually excludes MED under natural conditions, insecticide application reverses the MEAM1-MED competitive hierarchy and allows MED to exclude MEAM1. The insecticide-mediated success of MED has led to TYLCV outbreaks throughout China. Our work strongly supports the hypothesis that insecticide use in China reverses the MEAM1-MED competitive hierarchy and allows MED to displace MEAM1 in managed landscapes. By promoting the dominance of a Bemisia species that is a competent viral vector, insecticides thus increase the spread and impact of TYLCV in heterogeneous agroecosystems.

Deltasatellites are small noncoding DNA satellites associated with begomoviruses. The study presented here has investigated the biology of two deltasatellites found in wild malvaceous plants in the New World (NW). Infectious clones of two NW deltasatellites (from Malvastrum coromandelianum and Sidastrum micranthum) and associated begomoviruses were constructed. Infectivity in Nicotiana benthamiana and their natural malvaceous hosts was assessed. The NW deltasatellites were not able to spread autonomously in planta, whereas they were maintained by the associated bipartite begomovirus. Furthermore, NW deltasatellites were transreplicated by a monopartite NW begomovirus, tomato leaf deformation virus. However, they were not maintained by begomoviruses from the Old World (tomato yellow leaf curl virus, tomato yellow leaf curl Sardinia virus and African cassava mosaic virus) or a curtovirus (beet curly top virus). NW deltasatellites did not affect the symptoms induced by the helper viruses but in some cases reduced their accumulation. Moreover, one NW deltasatellite was shown to be transmitted by the whitefly Bemisia tabaci, the vector of its helper begomoviruses. These results confirm that these molecules are true satellites. The availability of infectious clones and the observation that NW deltasatellites reduced virus accumulation paves the way for further studies of the effect on their helper begomoviruses.

Sweet pepper (Capsicum annuum) is a popular crop worldwide and an asymptomatic host of the begomovirus (Geminiviridae) Tomato yellow leaf curl virus (TYLCV). A previous study showed that TYLCV could be transmitted by the seeds of tomato plants, but this phenomenon has not been confirmed in other plants. In 2015, four different cultivars of sweet pepper (‘Super Yellow,’ ‘Super Red,’ ‘Sunnyez’ and ‘Cupra’) known to be susceptible to TYLCV were agro-inoculated with a TYLCV infectious clone. Three months after inoculation, the leaves of the ‘Super Yellow’ cultivar showed 80% (8/10) susceptibility and the other three sweet pepper cultivars showed 30 to 50% susceptibilities. All of the ‘Super Yellow’ seed bunches (five seeds per bunch) from plants whose leaves were confirmed to be TYLCV-infected were also TYLCV-infected (8/8). The seeds of other cultivars showed 20 to 40% susceptibilities. Virus transmission rates were also verified with 10 bunches of seedlings for each cultivar (five seedlings per pool). Eight bunches of ‘Super Yellow’ seedlings (8/10) were confirmed to be TYLCV-infected and one to three bunches of each of the other cultivar seedlings were also infected. Viral replication in TYLCV-infected seeds and seedlings was confirmed via strand-specific amplification using virion-sense- and complementary-sense-specific primer sets. This is the first report of TYLCV seed transmission in sweet pepper plants and among non-tomato plants. Because sweet pepper is an asymptomatic host of TYLCV, seeds infected with TYLCV could act as a silent invader of tomatoes and other crops.

The selection of resistant crops is an effective method for controlling geminivirus diseases. ty-5 encodes a messenger RNA surveillance factor Pelota with a single amino acid mutation (PelotaV16G), which confers effective resistance to tomato yellow leaf curl virus (TYLCV). No studies have investigated whether ty-5 confers resistance to other geminiviruses. Here, we demonstrate that the tomato ty-5 line exhibits effective resistance to various geminiviruses. It confers resistance to two representative begomoviruses, tomato yellow leaf curl China virus/tomato yellow leaf curl China betasatellite complex and tomato leaf curl Yunnan virus. The ty-5 line also exhibits partial resistance to a curtovirus beet curly top virus. Importantly, ty-5 confers resistance to TYLCV with a betasatellite. Southern blotting and quantitative polymerase chain reaction analyses showed that significantly less DNA of these geminiviruses accumulated in the ty-5 line than in the susceptible line. Moreover, knockdown of Pelota expression converted a Nicotiana benthamiana plant from a geminivirus-susceptible host to a geminivirus-resistant host. Overall, our findings suggest that ty-5 is an important resistance gene resource for crop breeding to control geminiviruses.

Viruses are strict intracellular parasites that rely on the proteins encoded in their genomes for the effective manipulation of the infected cell that ultimately enables a successful infection. Viral proteins have to be produced during the cell invasion and takeover in sufficient amounts and in a timely manner. Silencing suppressor proteins evolved by plant viruses can boost the production of viral proteins; although, additional mechanisms for the regulation of viral protein production likely exist. The strongest silencing suppressor encoded by the geminivirus tomato yellow leaf curl virus (TYLCV) is V2: V2 suppresses both post-transcriptional and transcriptional gene silencing (PTGS and TGS), activities that are associated with its localization in punctate cytoplasmic structures and in the nucleus, respectively. However, V2 has been previously described to largely localize in the endoplasmic reticulum (ER), although the biological relevance of this distribution remains mysterious. Here, we confirm the association of V2 to the ER in Nicotiana benthamiana and assess the silencing suppression activity-independent impact of V2 on protein accumulation. Our results indicate that V2 has no obvious influence on the localization of ER-synthesized receptor-like kinases (RLKs) or ER quality control (ERQC)/ER-associated degradation (ERAD), but dramatically enhances the accumulation of the viral C4 protein, which is co-translationally myristoylated, possibly in proximity to the ER. By using the previously described V2C84S/86S mutant, in which the silencing suppression activity is abolished, we uncouple RNA silencing from the observed effect. Therefore, this work uncovers a novel function of V2, independent of its capacity to suppress silencing, in the promotion of the accumulation of another crucial viral protein.

In nature, a plant can be infected by multiple viruses simultaneously. However, the effects of coinfection on plant–vector interactions are less well studied. Two begomoviruses of the family Geminiviridae, Tomato yellow leaf curl virus (TYLCV) and Tomato yellow leaf curl China virus (TYLCCNV), occur sympatrically in China. Each of them is reported to increase the performance of whitefly vector via manipulation of plant traits. In this study, we examined the effects of coinfection by the two viruses TYLCV and TYLCCNV on plant–whitefly interactions, compared to that infected by a single virus. We found that plants infected by two viruses showed aggravated symptoms but the performance and preference of whiteflies were not altered significantly compared to singly-infected plants. Coinfection suppressed the transcription of genes involved in jasmonic acid (JA) signaling pathway in plants, but showed no significant difference to single-virus infected plants. These findings suggest that although TYLCV and TYLCCNV may synergistically induce plant symptoms, they did not manipulate synergistically plant-mediated responses to the insect vector.

In July 2016, an aggressive syndrome of tomato yellow leaf curl disease was reported in Sicily in tomato plants carrying the Ty-1 resistance gene. A total of 34 samples were collected and analyzed. Twenty-seven out of the 34 samples analyzed appeared to contain only recombinant molecules. One full sequence was obtained after cloning. Alignments and plot similarity analysis showed that the genome of the recombinant, named TYLCV-IL[IT:Sic23:16], was mostly derived from tomato yellow leaf curl virus (TYLCV), with a small region of 132 nucleotides in the non-coding region between the stem-loop and the start of the V2 ORF replaced by 124 nucleotides derived from a virus of a different species, tomato yellow leaf curl Sardinia virus. All plants in which the new recombinant was detected belonged to resistant tomato cultivars.