The Past, Present, and Future of Wheat Dwarf Virus Management-A Review

Wheat dwarf disease (WDD) is an important disease of monocotyledonous species, including economically important cereals. The causative pathogen, wheat dwarf virus (WDV), is persistently transmitted mainly by the leafhopper Psammotettix alienus and can lead to high yield losses. Due to climate change, the periods of vector activity increased, and the vectors have spread to new habitats, leading to an increased importance of WDV in large parts of Europe. In the light of integrated pest management, cultivation practices and the use of resistant/tolerant host plants are currently the only effective methods to control WDV. However, knowledge of the pathosystem and epidemiology of WDD is limited, and the few known sources of genetic tolerance indicate that further research is needed. Considering the economic importance of WDD and its likely increasing relevance in the coming decades, this study provides a comprehensive compilation of knowledge on the most important aspects with information on the causal virus, its vector, symptoms, host range, and control strategies. In addition, the current status of genetic and breeding efforts to control and manage this disease in wheat will be discussed, as this is crucial to effectively manage the disease under changing environmental conditions and minimize impending yield losses.

Construction of an Agroinfectious Clone of a Korean Isolate of Sweet Potato Symptomless Virus 1 and Comparison of Its Infectivity According to Agrobacterium tumefaciens Strains in Nicotiana benthamiana

Sweet potato symptomless virus 1 (SPSMV-1) is a single-stranded circular DNA virus, belonging to the genus Mastrevirus (family Geminiviridae) that was first identified on sweet potato plants in South Korea in 2012. Although SPSMV-1 does not induce distinct symptoms in sweet potato plants, its co-infection with different sweet potato viruses is highly prevalent, and thus threatens sweet potato production in South Korea. In this study, the complete genome sequence of a Korean isolate of SPSMV-1 was obtained by Sanger sequencing of polymerase chain reaction (PCR) amplicons from sweet potato plants collected in the field (Suwon). An infectious clone of SPSMV-1 (1.1-mer) was constructed, cloned into the plant expression vector pCAMBIA1303, and agro-inoculated into Nicotiana benthamiana using three Agrobacterium tumefaciens strains (GV3101, LBA4404, and EHA105). Although no visual differences were observed between the mock and infected groups, SPSMV-1 accumulation was detected in the roots, stems, and newly produced leaves through PCR. The A. tumefaciens strain LBA4404 was the most effective at transferring the SPSMV-1 genome to N. benthamiana. We confirmed the viral replication in N. benthamiana samples through strand-specific amplification using virion-sense- and complementary-sense-specific primer sets.

Viromes of Ten Alfalfa Plants in Australia Reveal Diverse Known Viruses and a Novel RNA Virus

Alfalfa plants in the field can display a range of virus-like symptoms, especially when grown over many years for seed production. Most known alfalfa viruses have RNA genomes, some of which can be detected using diagnostic assays, but many viruses of alfalfa are not well characterized. This study aims to identify the RNA and DNA virus complexes associated with alfalfa plants in Australia. To maximize the detection of RNA viruses, we purified double-stranded RNA (dsRNA) for high throughput sequencing and characterized the viromes of ten alfalfa samples that showed diverse virus-like symptoms. Using Illumina sequencing of tagged cDNA libraries from immune-captured dsRNA, we identified sequences of the single-stranded RNA viruses, alfalfa mosaic virus (AMV), bean leafroll virus, a new emaravirus tentatively named alfalfa ringspot-associated virus, and persistent dsRNA viruses belonging to the families Amalgaviridae and Partitiviridae. Furthermore, rolling circle amplification and restriction enzyme digestion revealed the complete genome of chickpea chlorosis Australia virus, a mastrevirus (family Geminiviridae) previously reported only from chickpea and French bean that was 97% identical to the chickpea isolate. The sequence data also enabled the assembly of the first complete genome (RNAs 1–3) of an Australian AMV isolate from alfalfa.

Life on the Edge: Geminiviruses at the Interface Between Crops and Wild Plant Hosts

Viruses constitute the largest group of emerging pathogens, and geminiviruses (plant viruses with circular, single-stranded DNA genomes) are the major group of emerging plant viruses. With their high potential for genetic variation due to mutation and recombination, their efficient spread by vectors, and their wide host range as a group, including both wild and cultivated hosts, geminiviruses are attractive models for the study of the evolutionary and ecological factors driving virus emergence. Studies on the epidemiological features of geminivirus diseases have traditionally focused primarily on crop plants. Nevertheless, knowledge of geminivirus infection in wild plants, and especially at the interface between wild and cultivated plants, is necessary to provide a complete view of their ecology, evolution, and emergence. In this review, we address the most relevant aspects of geminivirus variability and evolution in wild and crop plants and geminiviruses' potential to emerge in crops.

Fine-tuning expression of begomoviral movement and nuclear shuttle proteins confers cell-to-cell movement to mastreviral replicons in Nicotiana benthamiana leaves

Geminiviruses are a group of small plant viruses responsible for devastating crop damage worldwide. The emergence of agricultural diseases caused by geminiviruses is attributed in part to their high rates of recombination, leading to complementary function between viral components across species and genera. We have developed a mastreviral reporter system based on bean yellow dwarf virus (BeYDV) that replicates to high levels in the plant nucleus, expressing very high levels of GFP. To investigate the potential for complementation of movement function by other geminivirus genera, the movement protein (MP) and nuclear shuttle protein (NSP) from the bipartite begomovirus Bean dwarf mosaic virus (BDMV) were produced and characterized in Nicotiana benthamiana leaves. While overexpression of MP and NSP strongly inhibited GFP expression from the mastreviral reporter and caused adverse plant symptoms, optimizing the expression levels of MP and NSP allowed functional cell-to-cell movement. Hybrid virus vectors were created that express BDMV MP and NSP from mastreviral replicons, allowing efficient cell-to-cell movement comparable to native BDMV replicons. We find that the expression levels of MP and NSP must be fine-tuned to provide sufficient MP/NSP for movement without eliciting the plant hypersensitive response or adversely impacting gene expression from viral replicons. The ability to confer cell-to-cell movement to mastrevirus replicons depended strongly on replicon size: 2.1-2.7 kb replicons were efficiently moved, while 3 kb replicons were inhibited, and 3.9 kb replicons were very strongly inhibited. Optimized expression of MP/NSP from the normally phloem-limited Abutilon mosaic virus (AbMV) allows efficient movement in non-phloem cells.

Deep Sequencing Data and Infectivity Assays Indicate that Chickpea Chlorotic Dwarf Virus is the Etiological Agent of the "Hard Fruit Syndrome" of Watermelon

Chickpea chlorotic dwarf virus (CpCDV), a polyphagous mastrevirus, family Geminiviridae, has been recently linked to the onset of the "hard fruit syndrome" of watermelon, first described in Tunisia, that makes fruits unmarketable due to the presence of white hard portions in the flesh, chlorotic mottling on the rind, and an unpleasant taste. To investigate the etiological agent of this disease, total RNA extracted from symptomatic watermelon fruits was subjected to small RNA sequencing through next generation sequencing (NGS) techniques. Data obtained showed the presence of CpCDV and two other viral species. However, following validation through polymerase chain reaction (PCR), CpCDV was the only viral species consistently detected in all samples. Watermelon seedlings were then challenged by an agroinfectious CpCDV clone; several plants proved to be CpCDV-infected, and were able to produce fruits. CpCDV infected and replicated in watermelon fruits and leaves, leading to abnormality in fruits and in seed production, similar to those described in field. These results indicate that CpCDV is the etiological agent of the "hard fruit syndrome" of watermelon.

Identification and characterization of a maize-associated mastrevirus in China by deep sequencing small RNA populations

BACKGROUND

Maize streak Reunion virus (MSRV) is a member of the Mastrevirus genus in the family Geminiviridae. Of the diverse and increasing number of mastrevirus species found so far, only Wheat dwarf virus and Sweetpotato symptomless virus 1 have been discovered in China. Recently, a novel, unbiased approach based on deep sequencing of small interfering RNAs followed by de novo assembly of siRNA, has greatly offered opportunities for plant virus identification.

METHODS

Samples collected from maize leaves was deep sequencing for virus identification. Subsequently, the assay of PCR, rolling circle amplification and Southern blot were used to confirm the presence of a mastrevirus.

RESULTS

Maize streak Reunion virus Yunnan isolate (MSRV-[China:Yunnan 06:2014], abbreviated to MSRV-YN) was identified from maize collected from Yunnan Province, China, by small RNA deep sequencing. The complete genome of this virus was ascertained as 2,880 nucleotides long by conventional sequencing. A phylogenetic analysis showed it shared 96.3 % nucleotide sequence identity with the isolate of Maize streak Reunion virus from La Reunion Island. To our knowledge, this is the first identification of MSRV in China. Analyses of the viral derived small interfering RNAs (vsiRNAs) profile showed that the most abundant MSRV-YN vsiRNAs were 21, 22 and 24 nt long and biased for A and G at their 5' terminal residue. There was a slightly higher representation of MSRV-YN siRNAs derived from the virion-sense strand genome than the complementary-sense strand genome. Moreover, MSRV-YN vsiRNAs were not uniformly distributed along the genome, and hotspots were detected in the movement protein and coat protein-coding region.

CONCLUSIONS

A mastrevirus MSRV-YN collected in Yunnan Province, China, was identified by small RNA deep sequencing. This vsiRNAs profile derived from MSRV-YN was characterized, which might contribute to get an insight into the host RNA silencing defense induced by MSRV-YN, and provide guidelines on designing antiviral strategies using RNAi against MSRV-YN.

Improved Efficiency of Zea mays Agroinoculation with Maize streak virus

Agroinoculation is a technique permitting the transmission of geminivirus genomes cloned in Agrobacterium tumefaciens into a wide variety of mono- and dicotyledonous host plants. Most geminiviruses are obligately transmitted by insect vector species under natural conditions; therefore, agroinoculation has greatly simplified the study of this group of viruses. In many cases, agroinoculation has replaced insect transmission, and has been used to compare virulence characteristics among viruses. Here we report on the discovery that, in agroinfectious Maize streak virus constructs, the orientation of cloned viral genomes relative to the Cauliflower mosaic virus 35S (CaMV35S) promoter of the binary cloning vector pBI121 can significantly affect agroinfectivity of the constructs. Rates at which plants became symptomatic were significantly higher when agroinoculating maize seedlings with constructs containing the CaMV35S promoter upstream of the viral replication-associated protein (Rep) gene than when the same viruses were cloned either in the opposite orientation or into a vector without a strong eukaryotic promoter sequence. Plants infected using the construct with Rep cloned downstream of the CaMV35S promoter also displayed more stunting and, in the early stages of the infection, more severe chlorotic streak symptoms.