THE INFLUENCE OF LIGHT INTENSITY ON THE SUSCEPTIBILITY OF PLANTS TO CERTAIN VIRUSES

Reducing the light intensity under which plants were grown in summer to one-third increased their susceptibility to infection with tobacco necrosis, tomato bushy stunt, tobacco mosaic and tomato aucuba mosaic viruses. With the first two viruses shading increased the average number of local lesions per leaf by more than ten times and by more than five times with the second two.
Reducing the light intensity increased the virus content of sap from leaves inoculated with Rothamsted tobacco necrosis virus by as much as twenty times. As it also reduced the total solid content of sap by about one-half, purification was greatly facilitated; crystalline preparations of the virus were readily made from shaded plants but not from unshaded controls.
Reducing the light intensity also increased the virus content of systemically infected leaves; the greatest effect was with tomato bushy stunt virus with which increases of up to ten times were obtained, but with tobacco mosaic and aucuba mosaic viruses there were also significant increases.
The importance of controlled illumination in raising plants for virus work and the possible mechanisms responsible for the variations in susceptibility are discussed.     

SOME EFFECTS OF HIGH TEMPERATURE ON THE SUSCEPTIBILITY OF PLANTS TO INFECTION WITH VIRUSES

When plants were kept at 36°C. for some time before inoculation, their susceptibility to infection by five mechanically transmissible viruses was greatly increased. When kept at 36° after inoculation, fewer local lesions were produced than at lower temperatures, but the effects of the post-inoculation treatment differed with different viruses. Tomato spotted wilt and tobacco mosaic viruses multiply in plants at 36°, and the post-inoculation treatment reduced the local lesions they caused to numbers that varied between 10 and 90% of the control; these two viruses also have large thermal coefficients of heat inactivation. By contrast, tobacco necrosis, tomato bushy stunt and cucumber mosaic viruses, were much affected by post-inoculation treatment, lesion formation being completely prevented by exposure to 36° for a day or more. These three viruses appear not to multiply in plants at 36°, and although they have high thermal inactivation points, they have small temperature coefficients of thermal inactivation.
The extent to which lesion formation was affected by pre- or post-inoculation exposure of plants to 36° depended not only on the length of the treatment, but also on the physiological condition of the plants.
The symptoms of infected plants changed considerably if kept at 36°. At 36° Nicotiana glutinosa , inoculated with tobacco mosaic virus, gave chlorotic local lesions instead of necrotic ones, and became systemically infected. When systemically infected plants were brought to ordinary glasshouse temperature, the infected tissues all collapsed and died in a day.     

EFFECTS OF DARKNESS ON THE CONSTITUTION OF TOBACCO LEAVES AND SUSCEPTIBILITY TO VIRUS INFECTION

An attempt was made to find the causes of increased susceptibility to virus infection when tobacco plants are kept in the dark before inoculation. The changes in certain nitrogen fractions, viz. insoluble-N, amino-N, amide-N, ammonia-N and nitrate-N, and in dry matter and water content were followed in tobacco plants subjected to a period of darkness before inoculation with tobacco aucuba mosaic virus. Only nitrate-N was strongly correlated with the susceptibility to infection, but the evidence suggests that the correlation is indirect and not causal.
Dry matter and water content, determined either as dry matter percentage of fresh weight or measured separately on a leaf-disk basis Ivere found to vary directly with variation in susceptibility.     

FACTORS AFFECTING THE PRODUCTION OF LOCAL LESIONS BY PLANT VIRUSES

Beans inoculated with tobacco necrosis virus were kept in the dark at different temperatures for 1 hr. before and 1 hr. after inoculation; in this experiment the number of lesions increased with temperature over the range 55–82° F.
The effect of 30 min. periods of darkness before or after inoculation depended on the time of day, the number of local lesions usually being decreased. Prolonging the night period before inoculation sometimes increased the number of lesions.
Light appeared to be more important than temperature in controlling the daily variation in susceptibility. However, in a test over a 30 hr. period this variation continued even when plants were placed under constant conditions before and after inoculation.
When plants that had been kept in the dark were exposed to light of about 800 f.c. intensity for 1 min. immediately before inoculation the number of local lesions was doubled.     

SOME EFFECTS OF VIRUS INFECTION ON LEAF WATER CONTENTS OF NICOTIANA SPECIES

Infection with tobacco mosaic virus decreases the water content which detached tobacco leaves attain when kept for 20 hr. in conditions of minimum water stress, and does so more when the plants are kept in light before inoculation than when they are kept in darkness. No such effects of infection during the first day after inoculation were obtained with tobacco leaves infected with either tobacco etch virus or potato virus X , or with Nicotiana glutinosa leaves infected with tobacco mosaic virus. These results, like those showing early effects of TMV on respiration and photosynthesis of tobacco leaves, suggest that inoculation with TMV affects deeper leaf tissues than the epidermis earlier in tobacco leaves than in other leaves, and earlier than other viruses in tobacco leaves.     

RASPBERRY YELLOW DWARF, A SOIL-BORNE VIRUS

An apparently undescribed virus, provisionally named raspberry yellow dwarf virus (RYDV), was isolated from naturally infected raspberry, strawberry, blackberry and several weed species by mechanical inoculation of sap to Chenopodium amaranticolor. The severe disease it caused in Malling Exploit raspberry usually occurred patchily in otherwise normal plantations: these patches increased in size from year to year. RYDV was differentiated from raspberry ringspot and tomato black ring viruses by the symptoms produced in C. amaranticolor , tobacco and Petunia hybrida. RYDV lost infectivity when sap was heated for 10 min. at 61° C., diluted 10^-5or kept for 15 days at 18° C. RYDV was precipitated without inactivation by acetone and by ammonium sulphate.
Isolates of RYDV from different plants and localities, and of different virulence, were identified by plant-protection and serological tests. Such tests gave no evidence that RYDV was related to raspberry ringspot, tobacco ringspot, tomato black ring or cucumber mosaic viruses.
Raspberry and sugar-beet plants became systemically infected with RYDV when grown under glass in soil from a field where the disease had occurred in raspberry plants, and where the virus persisted in the soil for 3 years after the raspberry plants were removed. RYDV seems to be widely disseminated in England but recently introduced and rare in eastern Scotland.
Like raspberry ringspot and tomato black ring viruses, RYDV causes symptoms of the ringspot type in tobacco, has a wide natural and experimental host range, is soil-borne and of local importance. Such features seem characteristic of ringspot viruses as a group.     

Transgenic tobacco plants expressing a coat protein gene of tobacco mosaic virus are resistant to some other tobamoviruses

Transgenic tobacco plants expressing the coat protein (CP) gene of tobacco mosaic virus were tested for resistance against infection by five other tobamoviruses sharing 45-82% homology in CP amino acid sequence with the CP of tobacco mosaic virus. The transgenic plants (CP+) showed significant delays in systemic disease development after inoculation with tomato mosaic virus or tobacco mild green mosaic virus compared to the control (CP-) plants, but showed no resistance against infection by ribgrass mosaic virus. On a transgenic local lesion host, the CP+ plants showed greatly reduced numbers of necrotic lesions compared to the CP- plants after inoculation with tomato mosaic virus, pepper mild mottle virus, tobacco mild green mosaic virus, and Odontoglossum ringspot virus but not ribgrass mosaic virus. The implications of these results are discussed in relation to the possible mechanism(s) of CP-mediated protection.     

SOME EFFECTS OF HOST NUTRITION ON THE SUSCEPTIBILITY OF PLANTS TO INFECTION BY CERTAIN VIRUSES

Fertilizer treatments that greatly influenced the growth of tobacco and potato plants in pots had little effect on the number that became infected with potato virus Y when the plants were colonized by equal numbers of infective aphids, though the number was slightly decreased by nitrogen and increased by phosphorus.
The number of local lesions produced on leaves of tobacco and Nicotiana glutinosa by tomato aucuba mosaic and tobacco mosaic viruses was increased by additions of both nitrogen and phosphorus, provided that these also increased growth. The predominant effect of both nutrients in increasing susceptibility was indirect by increasing plant size, but over certain critical ranges both elements also increased the numbers of lesions produced per unit leaf area. Conditions of maximum susceptibility approximated closely to those producing optimal growth, and susceptibility, whether measured by lesions per half-leaf or per unit area, was decreased by a deficiency or excess of either element. Sometimes the addition of nitrogen reduced susceptibility when still increasing plant growth.     

RED CLOVER MOTTLE VIRUS

A virus, provisionally named red clover mottle virus (RCMV), isolated from red clover plants in England, seems distinct from any previously described. It was transmitted by mechanical inoculation of sap to many legumes and to Gomphrena globosa L., but it was not transmitted by six aphid species, or through soil or through seeds.
RCMV is inactivated in 10 min. between 60 and 63°C., and in 8 days at 18°C., but survives for long periods at -20; sap was not infective when diluted more than 1/1000. The virus is soluble in the pH range (4–7) in which it is stable. It was precipitated without inactivation by 50% saturated ammonium sulphate solution, but it was inactivated by ethanol or acetone. Partially purified preparations contained polygonal particles about 28 mμ in diameter. Serological tests showed no antigens in common with broad bean mottle, true broad bean mosaic or lucerne mosaic viruses.     

THE EFFECT OF TEMPERATURE ON INFECTION WITH STRAINS OF CUCUMBER MOSAIC VIRUS

Cucumber mosaic virus strains differed in their ability to multiply in plants at 37° C. Some strains multiplied in inoculated leaves and produced systemic symptoms in plants at this temperature; plants systemically infected with one such strain remained infected after prolonged treatment at 37° C. Other strains did not appear to multiply in inoculated leaves at 37° C. and heat treatment was successful in freeing plants from infection with these. Tests with one strain of each type showed both to be rapidly inactivated in expressed sap at 37° C.
Strains of cucumber mosaic virus forming small necrotic local lesions in leaves of french bean var. Canadian Wonder, produced many fewer lesions in plants kept after inoculation at 25° C. for 24 hr. and then at 15° C. than in plants kept continuously at the lower temperature.     

HEAT-THERAPY OF VIRUS-INFECTED PLANTS

Virus-free plants were produced from parents systemically infected with the following five viruses: tomato bushy stunt, carnation ring spot, cucumber mosaic, tomato aspermy and Abutilon variegation. The leaves formed while the infected plants were kept at 36°C. were free from symptoms, and test plants inoculated from these remained uninfected. When cuttings were taken from the infected plants at the end of the treatment most grew into healthy plants. The treated plants themselves usually developed symptoms after varying lengths of time at 20°C, but some that before treatment were infected with tomato aspermy, cucumber mosaic or Abutilon variegation viruses, remained permanently healthy.
The same method failed to cure plants infected with tomato spotted wilt, potato virus X and tobacco mosaic virus, although it decreased their virus content. Heat-therapy seems not to be correlated with the thermal inactivation end point of the virus in vitro.     

Identification of soybean mosaic, southern bean mosaic and tobacco ringspot viruses from soybean in the People's Republic of China

Samples of soybean plants with virus-like symptoms were collected from several locations in the People's Republic of China in 1981. These samples were used to prepare inocula for mechanical inoculation to soybean. Twenty-one virus cultures were obtained, the identities of which were determined by serology, symptomatology and host range. Sixteen cultures contained only soybean mosaic virus, four of which were more pathogenic than any previously studied; one culture contained only tobacco ringspot virus, another only southern bean mosaic virus, and three other cultures mixed infections of soybean mosaic and southern bean mosaic viruses. This is the first report of the occurrence of tobacco ringspot virus and southern bean mosaic virus in soybean in the People's Republic of China.     

Host-Pathogen Interactions : XXXII. A Fungal Glucan Preparation Protects Nicotianae against Infection by Viruses

A glucan preparation obtained from the mycelial walls of the fungus Phytophthora megasperma f.sp. glycinea and known as an elicitor of phytoalexins in soybean was shown to be a very efficient inducer of resistance against viruses in tobacco. The glucan preparation protected against mechanically transmitted viral infections on the upper and lower leaf surfaces. Whether the glucan preparation was applied by injection, inoculation, or spraying, it protected the plants if applied before, at the same time as, or not later than 8 hours after virus inoculation. At concentrations ranging from 0.1 to 10 micrograms per milliliter, the glucan preparation induced protection ranging from 50 to 100% against both symptom production (necrotic local lesions, necrotic rings, or systemic mosaic) and virus accumulation in all Nicotiana-virus combinations examined. However, no significant protection against some of the same viruses was observed in bean or turnip. The host plants successfully protected included N. tabacum (9 different cultivars), N. sylvestris, N. glutinosa, and N. clevelandii. The viruses belonged to several taxonomic groups including tobacco mosaic virus, alfalfa mosaic virus, and tomato black ring virus. The glucan preparation did not act directly on the virus and did not interfere with virus disassembly; rather, it appeared to induce changes in the host plant that prevented infections from being initiated or recently established infections from enlarging. The induced resistance does not depend on induction of pathogenesis-related proteins, the phenylpropanoid pathway, lignin-like substances, or callose-like materials. We believe the induced resistance results from a mechanism that has yet to be described.     

The effect of light wavelength on the susceptibility of plants to virus infection

Plants exposed for 24–72 h to light of different wavelengths differed in their subsequent susceptibility to virus infection. French bean leaves were less susceptible to infection by tobacco necrosis virus and Nicotiana glutinosa leaves were less susceptible to infection by tobacco mosaic virus when previously exposed to blue or red light than when exposed to green light. These differences were most pronounced at low energy levels. Leaves exposed to each kind of light were less susceptible than those kept in darkness.     

THE DISTRIBUTION OF VIRUSES IN DIFFERENT LEAF TISSUES AND ITS INFLUENCE ON VIRUS TRANSMISSION BY APHIDS

Exposing both surfaces of leaves systemically infected with cabbage black ring spot virus (CBRSV) or henbane mosaic virus to ultra-violet radiation decreases the infectivity of expressed sap to about one-fifth. As irradiation probably inactivates virus mainly in the epidermis, which occupies about one-quarter the volume of the leaves, these viruses seem to occur at much higher concentrations in sap from the epidermis than in sap from other cells. By contrast, tobacco mosaic virus seems not to occur predominantly in the epidermis.
CBRSV and henbane mosaic virus are normally transmitted most frequently by previously fasted aphids that feed for only short periods on infected leaves, but aphids treated like this transmit rarely from leaves that have been exposed to ultraviolet radiation. Irradiation has relatively little effect on the proportion of aphids that transmit after long infection feedings. Fasting seems to increase transmission by increasing the probability that aphids will imbibe sap from the epidermis of leaves they newly colonize. With longer periods on infected leaves, the ability of fasted aphids to transmit probably decreases because they then feed from deeper cells and their stylets contain sap with less virus. Only virus contained in the stylets seems to be transmitted, not virus taken into the stomach. About half the transmissions of henbane mosaic virus by aphids that have colonized tobacco leaves for hours may be caused by insects that temporarily cease feeding on the phloem and newly penetrate the epidermis.
Irradiating infected leaves affected the transmission of sugar-beet mosaic virus in the same way as that of henbane mosaic virus, but had little effect on the transmission of beet yellows virus, whose vectors become more likely to transmit the longer they feed on infected plants.     

VIRUSES OF COWPEA, VIGNA UNGUICULATA L. (WALP.), IN NIGERIA

The cowpea strain of tobacco mosaic virus was isolated from a range of leguminous hosts at Ibadan, but was rare in cultivated crops. Systemic symptoms in species infected experimentally are described.
A new virus of cowpea was also found in Nigeria. The physical properties (thermal inactivation point 56° C., dilution end-point 1/50,000 and longevity in vitro 4 days at 25° C.) differ from those for cowpea viruses reported elsewhere and the name cowpea yellow mosaic virus is proposed. This virus produces local lesions in French bean ( Phaseolus vulgaris L.) and local and systemic lesions in Bengal bean ( Mucuna aterrima Holland), but does not infect other leguminous hosts. The virus was purified and an antiserum prepared against it.
Both viruses are transmitted by a beetle ( Ootheca mutabilis Sahlb.) which loses infectivity within 48 hr. of leaving plants infected with either or both viruses.     

SOME PROPERTIES OF FOUR STRAINS OF CUCUMBER MOSAIC VIRUS

Different strains of cucumber mosaic virus differ in their host range, symptoms caused, virulence towards different plants, transmissibility by aphids, dilution end-point and thermal inactivation point.
There are seasonal variations in the susceptibility of some host species; French bean is apparently immune during the summer but during the winter produces countable local lesions suitable for quantitative assays.
Different host species differ in the ease with which cucumber mosaic virus is transmitted to and from them; systemic infection in beet rarely occurs unless the virus is introduced into young tissues. Inhibitors of infectivity in sap of sugar beet and Phytolacca sp. make mechanical transmission from these to other hosts difficult; the inhibitors interfere less with the infection of hosts in which they occur than with the infection of tobacco.
Cucumber mosaic virus has a low temperature coefficient of thermal inactivation and much infectivity is destroyed by heating at temperatures below the thermal inactivation point.
Myzus persicae (Sulz.) is a more efficient vector than M. ornatus Laing which is more efficient than Macrosiphum euphorbiae (Thomas); although individual aphids can cause more than one infection, most cease to be infective in feeding periods of from one to five minutes.     

THE EFFECT OF DARKENING ON THE SUSCEPTIBILITY OF PLANTS TO INFECTION WITH VIRUSES

The susceptibility of French bean plants to infection by the Rothamsted strain of tobacco necrosis virus as measured by the local-lesion method is increased by a rise in temperature and usually by darkening the plant before inoculation. If part only of a leaf is darkened, that part becomes more susceptible. Plants in full light also become more susceptible if carbon dioxide is removed from the air, whereas the susceptibility of plants in the dark is not altered.
Darkening leaves decreases their content of malic, fumaric, succinic and glycolic acids and increases the content of citric acid; the content of oxalic and malonic acids remains constant. These changes occurred in winter and summer and whether or not darkening increased susceptibility.
The effect on susceptibility of individual acids infiltrated into the leaf was measured in leaves kept in the light or in the dark before inoculation. None of the acids used produced any large change in susceptibility.     

Involvement of Volatile Substances in Systemic Resistance of Barley Against Erysiphe graminis f. sp. hordei Induced by Pruning of Leaves

Horsegram yellow mosaic disease was shown to be caused by a geminivirus; horsegram yellow mosaic virus (HYMV). The virus could not be transmitted by mechanical sap inoculation. Leaf dip and purified virus preparations showed geminate virus particles, measuring 15-18 * 30 nm. An antiserum for HYMV was produced and in enzyme-linked immunosorbent assay (ELISA) and immunosorbent electron microscopy (ISEM) tests HYMV was detected in leaf extracts of fieldinfected bambara groundnut, french bean, groundnut, limabean, mungbean, pigeonpea and soybean showing yellow mosaic symptoms. Bemisia tabaci fed on purified HYMV through a parafilm membrane transmitted the virus to all the hosts listed above but not to Ageratum conyzoides, okra, cassava, cowpea, Croton bonplandianus, Lab-lab purpureus, Malvastrum coromandalianum and tomato. No reaction was obtained in ELISA and ISEM tests between HYMV antibodies and extracts of plants diseased by whitefly-transmitted agents in India such as A. conyzoides yellow mosaic, okra yellow vein mosaic, C. bonplandianus, yellow vein mosaic, M. coromandalianum yellow vein mosaic, tomato leaf curl and cassava mosaic. HYMV was also not found to be related serologically to bean golden mosaic, virus.     

STUDIES ON THE APHID TRANSMISSION OF A STRAIN OF HENBANE MOSAIC VIRUS

A virus causing a wilt of Datura stramonium was identified as a strain of henbane mosaic virus. It causes necrotic local lesions in Nicotiana rustica , and local lesions are demonstrable in tobacco by staining with iodine. Some of the factors affecting its transmission by Myzus persicae (Sulz.) were studied quantitatively using these lesions.
Infective aphids differed little in their ability to cause infection, and usually produced two or three lesions. The duration of the feeding puncture did not affect the number of infections and had little effect on the percentage of aphids becoming infective. Transmissible virus did not seem to be continually imbibed while aphids fed on infected plants, and there were indications that it was acquired immediately before aphids withdrew their stylets from the leaf. Aphids became infective when allowed to make feeding punctures into epidermis stripped from infected leaves.
M. persicae transmitted during feeding punctures as brief as 5–10 sec; the probability of single feeding punctures resulting in infection reached a maximum with those lasting from 20 to 30 sec, during which the stylets did not penetrate as far as the centre of the epidermal cell and little or no saliva appeared to be ejected. M. persicae did not transmit the virus when its stylets were artificially wetted with infective sap.
Periods of darkness before inoculation with datura wilt virus increased the susceptibility of Nicotiana rustica to infection by rubbing, but not to infection by aphids.