SOME PROPERTIES OF THREE VIRUSES ISOLATED FROM A DISEASED PLANT OF SOLANUM JASMINOIDES PAXT. FROM INDIA

Three mechanically transmissible viruses were isolated from a diseased Solanum jasminoides plant obtained from India. One is a strain of potato virus Y , which in some potato varieties produces symptoms resembling those caused by potato virus C , but unlike potato virus C it is readily transmitted by Myzus persicae. The second, named tobacco wilt virus, is also transmitted by M. persicae but much less readily, whereas the third, named datura necrosis virus, is not. All three have a wide host range, but neither tobacco wilt nor datura necrosis viruses infects potato plants. All three have long flexuous particles and similar general properties.
Simultaneous infection with datura necrosis virus usually decreases the concentration reached by potato virus Y in tobacco plants but not in Nicotiana glutinosa.     

The relationships of some viruses causing necrotic diseases of the potato

Potato virus B , and some other viruses with reactions in potato varieties different from any previously described, are strains of virus X . All produce intracellular inclusions which vary with different hosts and virus strains. Except with virus B, the inclusions are larger and more frequent in potato than in tobacco or tomato. All give systemic infection when inoculated to tobacco, tomato and potato varieties in which they are carried or cause mosaic symptoms; some give systemic infection when inoculated to varieties in which they cause top-necrosis, whereas others give only local lesions.
Potato virus C is a strain of virus Y: in tobacco and a few potato varieties both produce similar symptoms, but in those varieties in which Y causes leaf-drop streak, C causes top-necrosis. C causes systemic infection when inoculated to tobacco and to potato varieties in which it causes mosaic symptoms, but not when inoculated to potato varieties in which it causes top-necrosis. Virus C was not transmitted by M . persicae. Viruses C and Y produce a few small intracellular inclusions in potato and tobacco.
Virus A is not related to Y or X : no inclusions were found in plants infected with A alone.     

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.     

The behaviour of some naturally occurring strains of potato virus Y

Potato virus Y was obtained from field crops of potatoes in many strains which differed widely in virulence and caused diseases in the variety Majestic ranging from severe leaf-drop streak to mild mosaic. The symptoms caused by these strains in seven potato varieties and tobacco are described and compared with those caused by the serologically related potato virus C. No changes were noted in the behaviour of any of the strains over three years, during which they were transmitted to many different plants.
Potato virus C was not transmitted by Myzus persicae , the most efficient vector of other strains of virus Y. Nor was virus C transmitted by eleven other species of aphides, eight of which transmitted virus Y. The efficiency with which different species acted as vectors of virus Y varied greatly, and it is suggested that in some species only occasional individuals can transmit.
Possible mechanisms for the evolution of viruses C and Y are indicated, and the effects of changes in virus, vector, and host on the prevalence of insect-transmitted viruses are discussed.     

Infection of potato mesophyll protoplasts with five plant viruses

Methods are described for preparing potato mesophyll protoplasts that are suitable for infection with inocula of virus nucleoprotein or RNA. The protoplasts could be infected with four sap-transmissible viruses (tobacco mosaic, tobacco rattle, tobacco ringspot and tomato black ring viruses) and with potato leafroll virus, which is not saptransmissible. No differences were observed in ability to infect protoplasts with potato leafroll virus strains differing either in virulence in intact plants or in aphid transmissibility.     

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.     

THE EFFECT OF DIFFERENT HOST PLANTS OF POTATO VIRUS C IN DETERMINING ITS TRANSMISSION BY APHIDS

A stock of potato virus C derived from Edgecote Purple potatoes in 1945 was not then transmitted by aphids, although more than 2000 aphids were used in conditions optimal for transmitting the serologically related potato virus Y. This stock of virus C has been propagated continuously since, by manual inoculation in a series of Nicotiana glutinosa and N. tabacum , and in 1955 it was transmitted by the aphid Myzus persicae (Sulz.): about one in twenty of the aphids transmitted it compared with one in two for potato virus Y.
Virus C derived from the Edgecote Purple potatoes in 1955 was not transmitted by aphids; both stocks of virus C produced only local lesions in Majestic potato leaves, and gave similar symptoms in tobacco.
When inoculated to Majestic potatoes and then returned to tobacco plants, potato virus C usually ceased to be aphid transmitted and did not recover this property in any of the subsequent subcultures.
Transmission from stock by aphids did not isolate a strain of virus C which was any more readily transmitted by aphids, indeed, for the first two or three subcultures, aphids usually transmitted more readily from plants inoculated manually. But the few isolates which remained aphid transmissible, after a second passage through potato, were rather readily transmitted.
These results suggest that the ability of a virus to be aphid transmitted is, at least in part, determined by the host plant in which it is multiplying, but the nature of the changes which determine this ability are unknown.     

Use of protein A to improve sensitisation of latex particles with antibodies to plant viruses

Protein A-coated latex (PAL) was compared with uncoated latex (L) for sensitisation with antibodies to five plant viruses: apple mosaic virus (ApMV), arabis mosaic virus (AMV), plum pox virus (PPV), potato virus Y ordinary strain (PVY°) and prunus necrotic ringspot virus (NRS V). A range of globulin concentrations was used with each antiserum and detection end points determined in serial dilutions of infective sap. When sensitised with antibodies to ApMV, PAL detected ApMV readily, whereas L did not. When sensitized with antibodies to PVY° and AMV, PAL gave higher detection end points than L. However, PAL gave little increase in sensitivity with the antisera to PPV and NRSV. Non-specific aggregation of latex, which sometimes occurred in very dilute sap with PAL, could be dispersed by adding 0.02% Tween-20 to the extraction buffer. Globulins of PVY° and AMV could be used at higher dilutions with PAL than with L, giving a saving in antiserum. Both types of latex sensitised with PVY° antibody globulins readily detected the tobacco veinal necrosis and C strains of this virus.     

A new lineage sheds light on the evolutionary history of Potato virus Y

Potato virus Y (PVY) is one of the rare plant viruses for which some biological traits (host range and symptomatology) are highly correlated with phylogeny, allowing the reconstruction of the evolutionary history of these traits. In this article, a new lineage of PVY isolates from Chile is described, showing unique genomic and biological properties. This lineage was found to be the sister group of all other PVY isolates and helped in the reconstruction of the ancestral traits and evolutionary history of PVY, suggesting that veinal necrosis in tobacco is an ancestral state and that adaptation to pepper ( Capsicum spp.) and potato ( Solanum tuberosum ) has been modified several times during PVY history.     

Expression of genes for resistance to potato virus Y in potato plants and protoplasts

Plants of several potato clones with major gene resistance to potato virus Y (PVY) developed necrotic local lesions and systemic necrosis after manual inoculation with common (PVY_o) or veinal necrosis (PVY_N) strains of the virus. The clones reacted similarly, although their resistance genes are thought to be derived from four different wild species of Solarium. Mesophyll protoplasts from each clone became infected when inoculated with RNA of PVY_o by the polyethylene glycol method. The proportion of protoplasts infected, assessed by staining with fluorescent antibody to virus particles, was similar to that of protoplasts of susceptible potato cultivars. In contrast, plants of potato cultivars Corine and Pirola, which possess gene Ry from S. stoloniferum, developed few or no symptoms when manually inoculated or grafted with PVY_o. Moreover, only very few protoplasts of these cultivars produced virus particle antigen after inoculation with PVY_o RNA. The extreme resistance to PVY of cvs Corine and Pirola was therefore expressed by inoculated protoplasts whereas the resistance of the necrotic-reacting potato clones was not.     

STUDIES ON THE HOST RANGE, PROPERTIES AND MODE OF TRANSMISSION OF BEET RINGSPOT VIRUS

An apparently undescribed mechanically transmissible virus has been named beet ringspot virus (BRV). It occurs naturally in Scotland in sugar-beet, turnip, swede, potato and many kinds of weed plants. BRV is readily distinguished from raspberry ringspot virus by the symptoms produced in Chenopodium amaranticolor , French bean, tobacco and Petunia hybrida plants. BRV lost infectivity when heated for 10 min. at 63°C. but not at 60°C.: at 20°C. its longevity in vitro was about 3 weeks. BRV was precipitated by ammonium sulphate, ethanol and acetone.
Protection experiments with tobacco plants, and serological tests, gave no evidence that BRV was related to tobacco ringspot, raspberry ringspot, potato bouquet or tobacco rattle viruses, but showed that viruses isolated from different host plants and from different localities were strains of BRV.
BRV is soil-borne: in glasshouse experiments sugar-beet, beetroot, potato, turnip, swede, French bean, Fragaria vesca , oat and wheat plants often became systemically infected when grown in soil from the site of a disease outbreak, but the virus was restricted to the roots of many infected plants. When sugar-beet seedlings were grown in virus-containing soil, BRV was first detected in their roots, where its concentration increased, before progressively increasing amounts of virus were found in the shoots.
Soils from five localities were found to contain BRV. BRV has been found only where the soil is light in texture, and often in fields where raspberry ringspot virus occurs.     

Tests for transmission of four potato viruses through potato true seed

The Andean potato calico strain of tobacco ringspot virus (TRSV-Ca) was detected in 2–9% of potato seedlings grown from true seed from plants of cv. Cara and clone G5998(6) infected with TRSV-Ca. Similarly, a potato isolate of the oca strain of arracacha virus B (AVB-O) was detected in 4–12% of progeny seedlings of cv. Cara and clone D42/8 infected with AVB-O. Potato virus T (PVT) passed through 33–59% of seed from PVT-infected cv. Cara, but only 0–2% infection was detected in seedlings from seed of PVT-infected clone D42/8. By contrast, no infection was detected in seedlings grown from seed from plants of G5998(6), D42/8 or cv. Cara infected with Andean potato latent virus strains Hu (APLV-Hu) or Caj (APLV-Caj), although both strains passed through seed of Nicotiana clevelandii. AVB-O, PVT and TRSV-Ca were detected in all tests of pollen from flowers of infected potato plants, but APLV-Hu and APLV-Caj were detected less frequently. AVB-O and PVT were transmitted through 2% and 8% respectively, of seed from healthy potato plants pollinated with pollen from infected plants. However, no transmission through seed was detected when pollen from TRSV-Ca infected plants was used. None of the four viruses were transmitted to healthy potato plants pollinated with pollen from infected plants. APLV-Hu caused exceptionally severe symptoms in the cv. Cara plants used for seed production, but the Bolivian strain of PVT induced only mild symptoms rather than the severe systemic necrosis previously reported for the type of strain of PVT in this cultivar. No symptoms developed in potato seedlings infected with TRSV-Ca, AVB-O or PVT through the seed.     

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.     

Accumulation of potato virus Y is enhanced in Solatium brevidens also infected with tobacco mosaic virus or potato spindle tuber viroid

The accumulation of potato virus Y?(PVY?) and potato leaf roll virus (PLRV) was studied in plants of Solanum brevidens co-infected with each of six viruses or a viroid. Virus could not be detected by ELISA in plants of S. brevidens infected solely with PVY. However, accumulation of PVY was increased c. 1000-fold in plants doubly infected with tobacco mosaic virus or potato spindle tuber viroid (PSTVd). PVY titres in doubly infected plants of S. brevidens were between 1% and 0.1% of those found in the PVY-susceptible interspecific Solanum hybrid DTO-33. Double infections of 5. brevidens by PVY and alfalfa mosaic virus or potato viruses M, S, T or X did not significantly enhance PVY accumulation. Accumulation of PLRV was not enhanced in plants co-infected with any of the six viruses or PSTVd.     

The suppression of one plant virus by another

Severe etch virus prevents die multiplication of potato virus Y and Hyoscyamus virus 3 and replaces them even in plants in which they are established. Mild etch virus reduces the concentration of potato virus Y but does not suppress it completely. Cucumber virus 1 multiplies normally in mixed infections with any of the three other insect-transmitted viruses. Possible implications of these results on the mechanism of virus multiplication are discussed; it is suggested that these viruses inactivate in cell sap at approximately the same rate as they denature in vitro.
No differences were found between the stability of antibodies to viruses with different properties.     

PRIMULA OBCONICA, A CARRIER OF TOBACCO NECROSIS VIRUSES

A tobacco necrosis virus has been isolated from the leaves and flowers of naturally infected Primula obconica plants. Although the virus produces no necrotic symptoms, it is not distributed uniformly through primulas, but occurs only in isolated regions, most of the tissues being apparently virus-free.
When inoculated to healthy primulas, three tobacco necrosis viruses were found to behave similarly. They all enter and multiply locally, but produce no symptoms; movement from the inoculated areas occurs only rarely and then does not cause a full systemic infection but only further localized infections. Multiplication of the viruses in primula is slower than in tobacco or French bean, which react necrotically.
The uncertainties in interpreting results of tests for tobacco necrosis viruses are described and possible explanations of natural infections are discussed.     

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.     

ANEMONE MOSAIC–A VIRUS DISEASE

The name anemone mosaic is proposed for a previously unrecorded virus disease of Anemone coronaria L.; infected plants have mottled leaves, and broken and distorted flowers. This virus can cause winter browning, and can contribute to crinkle in anemones.
The virus infected forty-seven out of ninety plant species tested; it was transmitted by mechanical inoculation, and by four of the six aphid species tested. Most aphids ceased to be infective within 30 min. when continuing to feed after leaving an infected plant.
Properties in vitro varied according to conditions of the tests; the thermal inactivation point was always below 62°C., the dilution end-point did not exceed 1/2500, and the virus inactivated at 18°C., the fewer than 72 hr.
Intracellular inclusion bodies were produced in all hosts examined.
Anemone mosaic virus is very similar to viruses placed in the turnip virus 1 group of Hoggan & Johnson, and is serologically related to cabbage black ringspot virus, although AMV infection did not protect plants against infection with cabbage black ring-spot virus.
Weeds naturally infected with AMV were found in anemone plantations, and this virus was detected, together with cucumber mosaic and tobacco necrosis viruses, in corms imported into this country.     

Sorghum chlorotic spot virus binds to potyvirus cylindrical inclusions in tobacco leaf cells

Nicotiana benthamiana can be doubly infected with either potato virus Y or tobacco etch virus and sorghum chlorotic spot virus (SCSV). Immunogold labeling showed that cylindrical inclusions of either potyvirus bind virions of the unrelated rod-shaped furovirus SCSV. Not all cells in doubly infected N. benthamiana plants contained both viruses. In cells infected by the potyviruses but not by SCSV, cylindrical inclusions did not label with the antiserum to SCSV. Numbers of cells infected with SCSV did not increase in doubly infected plants compared to those in plants infected with SCSV alone. Systemic infection of N. benthamiana by either potyvirus was not prevented by SCSV infections. This provides further evidence that unrelated rod-shaped viruses can bind to potyvirus cylindrical inclusion bodies, and that this phenomenon is not limited to graminaceous hosts.     

Studies on the spread of certain plant viruses in the field

Studies on the spread of potato viruses X and Y and cucumber mosaic virus in the field are described: tobacco was used as the experimental plant. The plants were set out in the form of a cross, one series with the leaves in contact and one with the leaves not touching. No spread of potato virus X was observed, but there was extremely rapid permeation of virus Y throughout the plots. The spread of cucumber mosaic virus was much slower than that of virus Y.