Selection of perennial and Italian ryegrass plants resistant to ryegrass mosaic virus

Perennial ryegrass plants collected from fields and Italian ryegrass plants grown from seed were selected for resistance to infection by ryegrass mosaic virus (RMV) by repeated manual inoculation. Two of 108 perennial ryegrass plants and one of 150 Italian ryegrass plants were symptomless after seven and nine inoculations respectively. These three plants were propagated vegetatively. Plants of the two perennial ryegrass clones showed no symptoms after further manual inoculations with the initial isolate of RMV, or with an inoculum from infected plants collected from several fields, or after inoculation by viruliferous mites. Electron microscopy and back tests indicated that the plants were virus free. Some plants of the selected Italian ryegrass clone became infected after a further inoculation with mites or sap, but fewer than similarly inoculated unselected plants.     

Serological detection of ryegrass mosaic virus and ryegrass seed-borne virus

Ryegrass mosaic virus (RMV) was reliably detected in both perennial (S24) and Italian (S22) ryegrass, by enzyme-linked immunosorbent assay (ELISA) when plants had been infected for 8 wk. ELISA detected more infections in field-grown perennial ryegrass cv. Premo than either visual assessment or electron microscopy. However, with plants of Italian ryegrass cultivars only recently infected with RMV, positive reactions were more difficult to separate from the reactions of RMV-free plants, which varied considerably with cultivar, some giving high absorbance values. Immunosorbent electron microscopy showed that the RMV antiserum also contained antibodies to ryegrass seed-borne virus (RGSV), suggesting that these high values were caused by RGSV infection in the material tested.     

Effect of ryegrass mosaic virus on the quality of perennial ryegrass

In 1976 fields of perennial ryegrass located on two farms in the west of Scotland were found to be far more severely and extensively infected by ryegrass mosaic virus (RMV) than had been seen in earlier surveys of ryegrass fields in 1972 and 1973. The chemical analysis of samples of ryegrass from 10 of the fields examined in 1976 showed that RMV significantly reduced the organic matter content, organic matter digestibility, water soluble carbohydrate content and D value of the grass. Furthermore, the losses suffered by grass with the severe necrotic symptom of RMV were significantly greater than those incurred by the grass with the mild mottle only. A correlation analysis between water soluble carbohydrate level and D value showed that whilst the loss in water soluble carbohydrates in mottled grass accounted for much of the loss in digestibility this was not the case in the necrotic grass where loss of green tissue appeared to be a more important contributory factor.     

Cauliflower mosaic virus as a gene expression vector for plants

It is possible to replace the CaMV (cauliflower mosaic virus) ORF (open reading frame) II with foreign sequences without interfering with virus viability. Such recom-binants can induce the synthesis of substantial amounts of a foreign protein in infected plants and confer new properties to these plants. However, so far only three genes have been successfully cloned and expressed in this way. The expression mechanism of CaMV demands precise replacement of ORF II and probably certain structural features of the viral 35S RNA, which should not be disturbed by inserted sequences. Since these features are largely unknown, it cannot at present be pre-dicted whether an insert will be tolerated. It is more likely that larger inserts will disturb the viral gene expression mechanism than smaller ones.     

The effect of ryegrass mosaic virus on the yield of perennial ryegrass swards

The influence of ryegrass mosaic virus (RMV) on yield of perennial ryegrass swards was examined under two harvesting frequencies and at two levels of nitrogen fertilizer, in a field trial planted with a clone of ryegrass, cv. S 321, using small plots established with proportions of healthy and infected plants. RMV reduced the height and the number of tillers of plants grown in pots and reduced the height also of plants in the field. The swards were planted in August and at a harvest 8 wk later RMV reduced the yield from 2–12 to 1–52 t dry matter (d.m.)/ha. In the first full harvest year, RMV caused only a small reduction in yield where no N fertilizer was applied and the application of a very small amount of fertilizer would have recouped the loss. On the other hand RMV severely restricted the increase in yield resulting from heavy dressings of the fertilizer. Thus where 400 kg N/ha was applied, RMV reduced the total annual yield from 18-6 to 13-8 t d.m./ha. The effect of the virus was especially great both in the Spring, the period of maximum herbage production, and when the level of fertilizer was high.     

Decreased severity of ryegrass mosaic virus after passage through resistant perennial ryegrass

After exposure to infection in the field, the proportion of plants showing distinct symptoms of ryegrass mosaic virus (RMV) was less in perennial than in Italian ryegrass. The perennial ryegrass cv. Mascot had a smaller proportion of plants with symptoms than cv. S.23. Far milder symptoms were induced in test plants by RMV from naturally infected perennial ryegrass plots than from Italian ryegrass plots. Within perennial ryegrass, RMV from cv. Mascot caused milder symptoms than that from cv. S.23. Severe RMV isolated from Italian ryegrass cv. Trident (RMVT) became milder after one passage through cv. Mascot, although not as mild as RMV obtained from field plots of cv. Mascot (RMVM). Families from two highly resistant perennial ryegrass clones and two randomly selected clones of cv. S.23 crossed in all possible combinations varied in symptom severity when inoculated with RMVT but not when inoculated with RMVM. Families inoculated with RMVT also yielded virus which varied in the severity of symptoms induced in test plants, families with severe symptoms yielding severer virus. Thus, much of the variation in the resistance of these clones could be attributed to infection with RMV of differing severity. Resistance was controlled by several genes which were mainly additive in their effect.     

The use of African cassava mosaic virus as a vector system for plants

This paper describes the development of a gene-displacement vector based on DNA1, one of two single stranded circular genomic components of a bipartite geminivirus, African cassava mosaic virus (ACMV). The DNA1 molecules of ACMV were cloned as dimers into a plant transformation vector and the constructs have been integrated into tobacco protoplasts by PEG-mediated DNA transfer. In transgenic plants extrachromosomal copies of DNA1 monomers could be detected. Deletion of the coat protein-encoding gene in chimeric constructs resulted in free DNA1 copies of reduced size, and extrachromosomal recombinant molecules were detected after displacement of the coat protein-encoding region by foreign DNA fragments of comparable size. Due to the absence of the second component of ACMV, DNA2, the transgenic plants are free from viral infection symptoms which allows the establishment of healthy transformants that carry a recombinant construct in an extrachromosomal form.     

Electron microscopy of cocksfoot streak virus and its differentiation from ryegrass mosaic virus in naturally infected Dactylis glomerata plants

The length and rigidity of some cocksfoot streak virus (CSV) particles were greater (1) in ammonium molybdate negative stain at pH 5 than at pH 8 and (2) when magnesium was added either directly to extracted plant sap or indirectly via increased plant uptake. Most CSV particles were flexuous and 750–760 run long in low concentrations of magnesium but many were rigid and 800–810 nm long in higher concentrations. In sodium phosphotungstate and ammonium molybdate at pH 5, frequency distributions of particle lengths from cocksfoot plants infected with both CSV and ryegrass mosaic virus (RMV) produced two modal peaks, one at less than 700 nm (RMV) and the other at more than 750 nm (CSV); in ammonium molybdate at pH 8, however, only one peak (at 725 nm) was resolved. In ultrathin sections of CSV-infected cocksfoot leaves, virus-like particles were scattered or in bundles within the cytoplasm, or aligned in aggregates near the tonoplast. Some cells contained pinwheel inclusions, often with attached or associated laminar plates and occasionally with scroll-like or circular tubes. These CSV-induced inclusions were easily distinguished from those induced by RMV, even when both types occurred in the same cell.     

Wheat curl mite resistance: interactions of mite feeding with wheat streak mosaic virus infection

The majority of plant viruses are dependent on arthropod vectors for spread between plants. Wheat streak mosaic virus (family Potyviridae, genus Tritimovirus, WSMV) is transmitted by the wheat curl mite, Aceria tosichella Keifer, and this virus and vector cause extensive yield losses in most major wheat (Triticum aestivum L.)-growing regions of the world. Many cultivars in use are susceptible to this vector-virus complex, and yield losses of 10-99% have been documented. wheat curl mite resistance genes have been identified in goat grass, Aegilops tauschii (Coss) Schmal., and transferred to hexaploid wheat, but very few varieties contain effectively wheat curl mite resistance, due to virulent wheat curl mite populations. However, wheat curl mite resistance remains an effective strategy to reduce losses due to WSMV. The goal of our project was to identify the most effective, reproducible, and rapid method for assessing wheat curl mite resistance. We also wanted to determine whether mite resistance is affected by WSMV infection, because the pathogen and pest commonly occur together. Single and group wheat curl mite infestations produced similar amounts of leaf rolling and folding on wheat curl mite-susceptible wheat varieties that were independent of initial wheat curl mite infestation. This finding will allow accurate, efficient, large-scale screening of wheat germplasm for wheat curl mite resistance by infesting plants with sections of wheat leaf tissue containing mixed stages of wheat curl mite. The wheat curl mite-resistant breeding line 'OK05312' displayed antibiosis (reduced wheat curl mite population development). The effect of WSMV infection on wheat curl mite reproduction was genotype-dependent. Mite populations increased on infected wheat curl mite- and WSMV-susceptible plants compared with uninfected plants, but WSMV infection had no significant effect on wheat curl mite populations on resistant plants. OK05312 is a strong source of wheat curl mite resistance for wheat breeding programs.     

Ultrastructural pathology of leaf cells of ryegrass (Lolium multiflorum) infected with ryegrass mosaic virus.

Ryegrass mosaic virus particles and virus induced lamellar inclusions were found in mesophyll and epidermal cells of virus infected ryegrass leaves. The lamellar inclusions were occasionally found in phloem cells also. Virus particles occurred in cytoplasm, inside plasmodesmata and often in membrane bound sacs embedded in a matrix between plasmalemma and cell wall at or near plasmodesmata. Electron dense plugs protruding from plasmodesmata, finger-like cell wall outgrowths and cell wall deposits usually at plasmodesmata were also observed. Cytopathological changes in organelles in infected cells included dense deposits in the cisternae of endosplasmic reticulum and Golgi apparatus, mitochondria with electron-dense or opaque matrix, proliferating cristae and deteriorating unit membrane; and disintegrating chloroplasts.     

Transfer of polygenic resistance to ryegrass mosaic virus from perennial to Italian ryegrass by backcrossing

Polygenic resistance to ryegrass mosaic virus (RMV) was transferred from perennial to Italian ryegrass cv. Tribune by repeated cycles of backcrossing, polycrossing and selection. The resulting synthetic variety (Bb 2113) was highly resistant to infection by RMV whereas the Italian ryegrass cultivars Tribune, RvP, Bartolini, Augusta, Wilo and Dalita all were highly susceptible. Leaf width and length of Bb 2113 was similar to that of cv. Tribune and, like Tribune, Bb 2113 had awned florets. In a field trial, Bb 2113 had a similar persistency rating and number of inflorescences in the regrowth to cv. Tribune but produced 11% less dry matter. The results emphasise the need for a more efficient means of transferring small numbers of genes from one species of Lolium to another than backcrossing provides.     

Agrobacterium-mediated inoculation of plants with tomato golden mosaic virus DNAs

We have adapted the agroinfection procedure of Grimsley and co-workers [4,5] to develop a simple, efficient, reproducible infectivity assay for the insect-transmitted, split-genome geminivirus, tomato golden mosaic virus (TGMV). Agrobacterium T-DNA vectors provide efficient delivery of both components of TGMV when used in mixed inoculation of wild-type host plants. A greater increase in infection efficiency can be obtained by Agrobacterium delivery of the TGMV A component to permissive transgenic plants. These permissive plants contain multiple tandem copies of the B component integrated into the host genome. An inoculum containing as few as 2000 Agrobacterium cells can produce 100% infection under these conditions. Further, our results show that there is a marked effect of the configuration of the TGMV A components within the T-DNA vector on time of symptom development. We have also found that transgenic plants carrying tandem copies of the A component do not complement the B component. Possible mechanisms to explain these results and the potential use of this system to further study the functions of the geminivirus components in infection are discussed.     

Weed plants as sources of cucumber mosaic virus

Severe yellowing and stunting was widespread in lettuce crops in Britain in 1968. Cucumber mosaic virus was consistently obtained from affected plants and from weeds which were prevalent in lettuce fields. Infection was most frequent in Stellaria media but also occurred, more or less commonly, in Senecio vulgaris, Urtica urens, Tripleurospermum maritimum ssp. inodorum, Capsella bursa-pastoris, Lamium purpureum and Sonchus oleraceus. Most infected weeds were symptomless. Infected weeds were still common in January-April 1969 and are considered to be a major overwintering source of the virus for aphid transmission to lettuce.     

Tolerance to ryegrass mosaic virus, its assessment and effect on yield

Symptom severity of eighteen populations of Italian ryegrass infected artificially in the glasshouse and naturally in the field with ryegrass mosaic virus (RMV) was strongly correlated. A smaller proportion of plants of the more tolerant populations showed symptoms in the field, but this was probably due to an association of tolerance with increasing incidence of symptomless infection rather than with resistance to infection. Under sward conditions, the yield of a sensitive genotype was reduced by 27% and that of two more tolerant ones by 15 and 13 %. The percentage dry matter yield loss of the most sensitive genotype was similar in all cuts, despite the appearance of extensive necrosis at the time of one cut. With the more tolerant genotype the yield loss varied from 7 to 25 % according to cut. Over the period of the experiment RMV infection did not increase plant mortality.     

The effect of ryegrass mosaic virus on carbon assimilation and growth of ryegrasses

The growth of simulated swards of Italian and perennial ryegrass, artificially infected with ryegrass mosaic virus (RMV) or RMV-free, was investigated. During the build up of RMV infection, dry-matter yield was usually decreased in infected swards, and the effect was confirmed when single vegetative regrowths of swards were investigated in detail. The primary cause of lower yields in RMV-infected swards appears to be a decrease in net canopy photosynthesis (maximum decrease about 50%) and an associated increase in dark respiration (maximum increase about 50%). The decrease in net canopy photosynthesis is due primarily to a lowered rate of net photosynthesis of constituent leaves within the canopy. A secondary effect of RMV infection is to decrease tillering (maximum decrease about 30%) which results in a change in canopy structure and in particular a lower leaf area index. In the present experiments, lower light utilization by the swards is less important than the decrease in leaf photo-synthetic efficiency in lowering yield.