Virus Symptom Development Research Articles

Dynamic interactions between plant viruses and their hosts for symptom development

As obligate intracellular parasites, plant viruses catalyze drastic alterations in the cellular physiology of host cells in order to support their own replication. This disruption often, but not always, manifests macroscopically as disease symptoms. The search for what distinguishes symptom-inducing virus strains from their asymptomatic counterparts has long been a central component of plant virology research. A consistent through line has been the conclusion that symptoms arise from specific interactions between viral and host components. The identification of viral components responsible for symptom development (i.e. viral symptom determinants) followed by the identification and characterization of interactions with host components has led to concrete mechanistic linkages between the viral and host interactants for some symptoms. The rise of systems biology approaches (e.g. transcriptomics and proteomics) has allowed host responses to be described in greater detail, providing a broad view of the molecular events of plant virus infections. Here, we review the most recent literature describing plant virus symptom determinants. This includes studies detailing specific virus-host interactions which lead to symptom development, as well as those which utilize systems biology approaches such as transcriptomics to probe the molecular changes underlying the development of virus symptoms. Emerging trends, and how they might inform the future of plant virus symptomatology research, are discussed.

Differential gene expression and physiological changes during acute or persistent plant virus interactions may contribute to viral symptom differences.

Viruses have different strategies for infecting their hosts. Fast and acute infections result in the development of severe symptoms and may cause the death of the plant. By contrast, in a persistent interaction, the virus can survive within its host for a long time, inducing only mild symptoms. In this study, we investigated the gene expression changes induced in CymRSV-, crTMV-, and TCV-infected Nicotiana benthamiana and in PVX- and TMV-U1-infected Solanum lycopersicum plants after the systemic spread of the virus by two different high-throughput methods: microarray hybridization or RNA sequencing. Using these techniques, we were able to clearly differentiate between acute and persistent infections. We validated the gene expression changes of selected genes by Northern blot hybridization or by qRT-PCR. We show that, in contrast to persistent infections, the drastic shut-off of housekeeping genes, downregulation of photosynthesis-related transcripts and induction of stress genes are specific outcomes with acute infections. We also show that these changes are not a consequence of host necrosis or the presence of a viral silencing suppressor. Thermal imaging data and chlorophyll fluorescence measurements correlated very well with the molecular changes. We believe that the molecular and physiological changes detected during acute infections mostly contribute to virus symptom development. The observed characteristic physiological changes associated with economically more dangerous acute infections could serve as a basis for the elaboration of remote monitoring systems suitable for detecting developing virus infections in crops. Moreover, as molecular and physiological changes are characteristics of different types of virus lifestyles, this knowledge can support risk assessments of recently described novel viruses.

Variation in virus symptom development and root architecture attributes at the onset of storage root initiation in 'beauregard' sweetpotato plants grown with or without nitrogen.

It has been shown that virus infections, often symptomless, significantly limit sweetpotato productivity, especially in regions characterized by low input agricultural systems. In sweetpotatoes, the successful emergence and development of lateral roots (LRs), the main determinant of root architecture, determines the competency of adventitious roots to undergo storage root initiation. This study aimed to investigate the effect of some plant viruses on root architecture attributes during the onset of storage root initiation in ‘Beauregard’ sweetpotatoes that were grown with or without the presence of nitrogen. In two replicate experiments, virus-tested plants consistently failed to show visible symptoms at 20 days regardless of nitrogen treatment. In both experiments, the severity of symptom development among infected plants ranged from 25 to 118% when compared to the controls (virus tested plants grown in the presence of nitrogen). The presence of a complex of viruses (Sweet potato feathery mottle virus, Sweet potato virus G, Sweet potato virus C, and Sweet potato virus 2) was associated with 51% reduction in adventitious root number among plants grown without nitrogen. The effect of virus treatments on first order LR development depended on the presence or absence of nitrogen. In the presence of nitrogen, only plants infected with Sweet potato chlorotic stunt virus showed reductions in first order LR length, number, and density, which were decreased by 33%, 12%, and 11%, respectively, when compared to the controls. In the absence of nitrogen, virus tested and infected plants manifested significant reductions for all first order LR attributes. These results provide evidence that virus infection directly influences sweetpotato yield potential by reducing both the number of adventitious roots and LR development. These findings provide a framework for understanding how virus infection reduces sweetpotato yield and could lead to the development of novel strategies to mitigate virus effects on sweetpotato productivity.

Why mosaic? Gene expression profiling of African cassava mosaic virus‐infected cassava reveals the effect of chlorophyll degradation on symptom development

Cassava mosaic disease, caused by cassava begomoviruses, is the most serious disease for cassava in Africa. However, the pathogenesis of this disease is poorly understood. We employed high throughput digital gene expression profiling based on the Illumina Solexa sequencing technology to investigate the global transcriptional response of cassava to African cassava mosaic virus infection. We found that 3,210 genes were differentially expressed in virus-infected cassava leaves. Gene ontology term and Kyoto Encyclopedia of Genes and Genomes pathway analysis indicated that genes implicated in photosynthesis were most affected, consistent with the chlorotic symptoms observed in infected leaves. The upregulation of chlorophyll degradation genes, including the genes encoding chlorophyllase, pheophytinase, and pheophorbide a oxygenase, and downregulation of genes encoding the major apoproteins in light-harvesting complex II were confirmed by qRT-PCR. These findings, together with the reduction of chlorophyll b content and fewer grana stacks in the infected leaf cells, reveal that the degradation of chlorophyll plays an important role in African cassava mosaic virus symptom development. This study will provide a road map for future investigations into viral pathogenesis.

Management of potato virus Y (PVY) in potato by some biocontrol agents under field conditions

The study was conducted to test the activity of Pseudomonas fluorescens, Rhodotorula sp and fermented neem extract to protect potato plants against potato virusY disease development under field conditions. Infected potato tubers were soaked in P. fluorescens, Rhodotorula sp suspensions and in fermented neem extracts separately and sown in the field in completely randomized block design. The development of virus symptoms and the accumulation of virus in the plant based on Enzyme Linked Immunosorbent Assay (ELISA) were followed. The results obtained showed that the treatment of potato tubers with the three agents have significantly accelerated plant emergence, 5-6 days early than non treated ones, and improved plant growth, the plant dry weights ranged from 120-177 g/plant compared to 42 g/plant in non treated plants. The enhancement of plant growth was found associated with reduction in disease severity based on symptoms development and restriction of virus concentration as proved by ELISA absorbance of 405 nm, 0.14-0.23 compared with 2.50 in non treated plants. The results indicated that the use of bioagent to induce systemic resistance provide an efficient tool, as insecticide alternative to manage potato virus Y in potato.

FIRST REPORT OF STRAWBERRY MILD YELLOW EDGE VIRUS IN ARGENTINA

Numerous viruses have been mentioned infecting strawberry in a single or mixed infection. Many of them do not induce distinct symptoms in commercial cultivars, and often the only signs of infection are loss of vigor and reduced yield. Strawberry mild yellow edge virus (SMYEV) is one of the most widespread virus diseases of cultivated strawberry. Most of the grown cultivars are tolerant to infection with this virus and they do not show symptoms. However, infections by severe strains and mixed infection with other viruses lead to decline and to the development of virus symptoms. In strawberry production areas in Tucuman province, Argentina, strawberry plants were detected with dwarfism, stocky plants, mosaic, mottle, and leaf distortion. Plants were serologically tested for SMYEV by ELISA (Agdia kit). Of 21 symptomatic plants analyzed, 8 were positive (38%). Simultaneously, 454 symptomless plants were tested and 12 of them were found positive (2.6%). ELISA positive plants were further analyzed by immunocapture reverse transcription seminested polymerase chain reaction (IC-RT-seminested PCR) with specific primers for SMYEV. IC was performed with inmunoglobuline from Agdia kits. RT-seminested PCR reaction was done using three specific primers A, B, and C. Using this technique a fragment of the expected size was amplified. After graft inoculation of Fragaria vesca and F. virginiana indicator clones, necrosis, mosaic, epinasty, chlorosis, and distorted leaves were observed, along with other symptoms. Further research is being conducted to identify other possible viruses infecting this crop.

RNA silencing can explain chlorotic infection patterns on plant leaves

BackgroundRNA silencing has been implicated in virus symptom development in plants. One common infection symptom in plants is the formation of chlorotic tissue in leaves. Chlorotic and healthy tissue co-occur on a single leaf and form patterns. It has been shown that virus levels in chlorotic tissue are high, while they are low in healthy tissue. Additionally, the presence of siRNAs is confined to the chlorotic spots and the boundaries between healthy and infected tissue. These results strongly indicate that the interaction between virus growth and RNA silencing plays a role in the formation of infection patterns on leaves. However, how RNA silencing leads to the intricate patterns is not known.ResultsHere we elucidate the mechanisms leading to infection patterns and the conditions which lead to the various patterns observed. We present a modeling approach in which we combine intra- and inter-cellular dynamics of RNA silencing and viral growth. We observe that, due to the spread of viruses and the RNA silencing response, parts of the tissue become infected while other parts remain healthy. As is observed in experiments high virus levels coincide with high levels of siRNAs, and siRNAs are also present in the boundaries between infected and healthy tissue. We study how single- and double-stranded cleavage by Dicer and amplification by RNA-dependent RNA polymerase can affect the patterns formed.ConclusionThis work shows that RNA silencing and virus growth within a cell, and the local spread of virions and siRNAs between cells can explain the heterogeneous spread of virus in leaf tissue, and therewith the observed infection patterns in plants.

Color Mulches Influence Yield and Insect Pest Populations in Tomatoes

Field studies were conducted for three seasons, Fall 1988 and Spring and Fall 1989, on the effect of six mulch colors: blue, orange, red, aluminum, yellow, and white (fall) or black (spring), on fruit yields and on insect vectors of Sunny' tomato (Lycopersicon esculentum Mill.). Plant growth and yields were inconsistent with mulch colors during the three seasons. In Fall 1988, in a once-over harvest, extra-large (≥ 70 mm diameter) and marketable fruit yields were higher (P ≤ 0.05) on blue than on the conventional white mulch. In Spring 1989, early marketable yields on red mulch were higher than on black mulch, and in Fall 1989, under high stress from tomato mottle virus (TMoV) transmitted by silverleaf whitefly [Bemisia argentifolii (Bellows and Perring)], seasonal yield of extra-large fruit was better on orange than white mulch. In Fall 1988 and 1989, fruit size and marketable yields were reduced on yellow mulch. Aphids (Aphididae), thrips (Thripidae), and whiteflies were counted monthly in traps placed on the mulched beds. Aphids were least numerous on the aluminum and yellow and most numerous on the blue mulch. Where differences occurred, the fewest thrips were captured on aluminum and the fewest whiteflies were captured on the yellow, aluminum and orange mulches. Although differences were not always significant, the fewest adult whiteflies also were observed on foliage of tomato plants grown on these latter three mulches. Later in the seasons, as plant foliage covered the mulch, differences in the number of insects captured were similar for all mulch colors. Low numbers of whiteflies on the orange and aluminum mulches early in Fall 1989 delayed virus symptom development and increased yields. Virus symptom development was not delayed and yields were low on the yellow mulch, in spite of the low number of whiteflies. When averaged over all mulch colors, extra-large and marketable fruit yields increased linearly with delayed symptom development. It is proposed that, under high insect stress, mulches should be selected for their effects on insects in addition to their effects on soil temperature and plant morphology.

Acquisition of potato leafroll virus byMyzus persicae from secondarily-infected potato plants of different genotypes

The acquisition of potato leafroll virus (PLRV) byMyzus persicae nymphs from the top leaves of potato plants was studied throughout a growing season in relation to the antigen titre in those leaves and the feeding behaviour of the aphid. Secondarily-infected plants of eight potato genotypes with different levels of field resistance served as virus sources. Early in the growing season, plants were efficient sources for virus acquisition. The amount of viral antigen detected inM. persicae nymphs fed on the top leaves was strongly correlated with the titres of viral antigen in these leaves. Virus acquisition from the top leaves of older potato plants was markedly impaired and could not be correlated with their virus titre. With increasing age of the potato plants and the development of virus symptoms, the virus titre in the leaves declined and the initial weak correlation between the virus titre and field resistance ratings disappeared. Thus, screening secondarily-infected potato plants for field resistance to PLRV based on the concentration of viral antigen in leaves or in aphids fed on them should be avoided later in the growing season. The feeding rate ofM. persicae, measured by the number of honeydew droplets excreted, did not account for the reduced uptake of virus from older plants since it was not influenced by the age of the plant. Throughout the growing season, the feeding rate ofM. persicae nymphs on PLRV-infected plants was higher on genotypes with low levels of field resistance to PLRV than on genotypes with high ones.

Determinants of tomato golden mosaic virus symptom development located on DNA B

Infectious clones have been constructed from two strains of the bipartite geminivirus tomato golden mosaic virus. The common strain and the yellow vein strain show marked phenotypic differences in Nicotiana benthamiana which are reproduced following infection with the cloned viral genomes. Pseudorecombinants between the two strains, produced by exchange of genome components (DNAs A and B), established that the difference in symptoms in several species of the Solanaceae is determined by DNA B. Recombinants produced in vitro between the DNA B components showed that determinants of symptom development map to the common region and gene BL1. DNA B is known to carry functions necessary for spread of viral DNA through the host plant. Our results emphasize the link between symptom type and virus spread.

Factors Influencing Cucumber Mosaic Virus Symptom Development in Pepper

Abstract Beginning one week after the onset of flower bud formation, greenhousegrown pepper (Capsicum annuum L.) plants were innoculated each week with cucumber mosaic virus (CMV) for a period of 12 weeks. In the initial experiment, begun 30 Dec. 1980, almost all fruit with CMV symptoms were harvesed from plants which were inoculated 5-8 weeks after flower initiation. When the experiment was repeated, beginning 31 May 1981, symptoms were found only on fruit harvested from plants which were inoculated 3-6 weeks after flower initiation. Increasing the temperature from 21° to 32°C decreased the number of days between inoculation and the initial appearance of virus crystals. Increasing the day length from 5 to 20 hr also decreased the incubation period. The results suggest the best stage of plant development for inoculation with CMV if fruit symptoms are desired for a disease screening program, and the best temperature (32°), and day length (20 hr) for a reduced incubation period.

Zinc nutrition of Nicotiana tabacum L. in relation to multiplication of tobacco mosaic virus

Multiplication of tobacco mosaic virus was studied in Nicotiana tabacum L. grown in water culture vessels containing a complete nutrient solution except for zinc which was supplied at various concentrations. Development of virus symptoms was more pronounced in plants grown at zinc levels optimal for growth than in plants grown at zinc levels that were deficient. Optical density and local lesion assays for virus concentration of sap extracts from whole plants and from tip leaves showed that the concentration of virus nucleoprotein and of infective virus increased with increasing levels of zinc up to the level which was optimal for growth, and then decreased at levels which were toxic. Symptoms of zinc deficiency of plants increased after inoculation with tobacco mosaic virus, indicating that virus multiplication was associated directly or indirectly with utilization of zinc. When young inoculated plants were grown under zinc-deficient conditions for approximately 5 days and were then provided with zinc so that the total amount of zinc supplied was equivalent to that required for optimal host growth, the virus concentration remained less (during a period of 14 days) than that in plants grown continuously under optimal conditions. This suggested that virus multiplication was indirectly rather than directly associated with utilization of zinc. Virus concentration in floating leaf discs obtained from plants grown at different concentrations of zinc was found to be similar to that in whole plants. When virus-infected leaf discs were incubated on solutions of indole-3-acetic acid at concentrations of 5 × 10 −5, 1 × 10 −3, and 1 mg/l, there were no significant effects on virus concentration. A decrease in virus concentration, virus symptom expression, and plant growth resulted when indole-3-acetic acid at concentrations of 5 × 10 −5, 1 × 10 −3, 1, and 10 mg/l was added to the nutrient solution in which plants were growing at different concentrations of zinc. The decrease was greater at the higher concentrations of indole acetic acid than at the lower.