1. AN ASSIGNMENT
ON
PHYSIOLOGICAL EFFECTS OF VIRUS INFECTED
PLANTS
SUMITTED BY:
ROUF AHMAD
L-2011-BS-249-M
SUBMITTED TO:
Prof. Dr. S.S. KANG
Deptt. of Plant pathology

2. INTRODUCTION:
Plant cells serve for an infecting virus as biochemical and molecular environment which can
by the viral genome be determined to sustain the replication of the virus. This is achieved by
the use of the host cells' protein synthesizing system for the production of non-structural
proteins (NSP), including nucleic acid replicating enzymes, and the coat protein (CP) of the
virus. The latter serves, together with viral nucleic acid (NA) molecules for the formation of
new infective viral particles. The processes incited by a virus may disturb the biochemical
balance of the host cells since host cell components including its energy conferring systems
have to be used for the synthesis of viral components.These processes are in many virus-host
systems not fully compatible with the host cells' physiological balance and, therefore, incite
various symptoms of cellular degeneration, in the extreme cellular death.
Various external symptoms indirectly illustrate the complicated interactions between
virus and host cells. They may occur in the whole host plant or may be localized on plant
parts near the original infection sites. Virus symptoms may be specific to certain plant organs
like the flowers (flower break symptoms) or to certain tissues like the phloem. Various
distinct regular or irregular patterns of chlorotic or necrotic tissues develop on leaves
indicating that the distribution of systemically spread virus is not really uniform throughout
the plant. The effects of systemically spread virus are obviously regulated by more or less
balanced interactions between the host metabolism, the virus and environmental factors.
Hypersensitivity phenomena occur which lead to sudden cell death immediately upon the
initial cell infection. These examples indicate highly complex interactions which have largely
not been elucidated. The external symptoms are produced by infected tissues relatively late,
at a time when viral propagation is already completed in the respective tissue. On the cellular
level degeneration symptoms caused by the infection are observed e.g. in form of chloroplast
senescence (lipid accumulation, vacuolization).
Various physiological changes in virus infected plants can be discussed as under:-
A. Nucleic Acids and Proteins
(i) DNA
It is widely assumed that the small RNA viruses have little effect on host-cell DNA
synthesis, but there are very few, if any, definitive experiments bearing on the question.

3. Virus infection may well have some effect on host-cell DNA synthesis, but such effects are
likely to be fairly small and difficult to establish because (i) DNA content per cell may
increase for some time in a normal expanding leaf; (ii) minor DNA fractions, which might be
affected by virus infection, may be difficult to isolate and identify; and (iii) any effect might
be very transitory and, therefore difficult to detect in asynchronous infections. Using a radio
autographic technique to assay for DNA synthesis in individual cells, Atchison (1973) found
that there was a drop in DNA synthesis in the terminal 1 mm of French bean roots about the
time they were invaded by tobacco ring spot virus. This was soon followed by a transient
drop in the mitotic index.
(ii) Ribosomes and ribosomal RNA
Effects of virus infection on ribosomal RNA synthesis and the concentration of
ribosomes may differ with the virus, strain of virus, time after infection, and the host and
tissue concerned. In addition, 70 S and 80 S ribosomes may be affected differently.
In TMV-infected leaves viral RNA may come to represent about 75% of the total
nucleic acids without having any marked effects on the main host RNA fractions except to
cause a reduction in 16 S and 23 S chloroplast ribosomal RNAs (Fraser 1987b). However,
under some conditions cytoplasmic ribosomal RNA synthesis is also inhibited. A reduction in
chloroplast ribosomes without a marked effect on cytoplasmic ribosomes is a fairly common
feature for mosaic diseases (e.g., BSMV in barley, Brakke et al 1987b; TYMV) .
In Chinese cabbage leaves chronically infected with TYMV the concentration of 70 S
ribosomes in the yellow-green islands in the mosaic is greatly reduced compared to that in
dark green islands in the same leaf (Reid and Matthews. 1966). here' is little effect on the
concentration of cytoplasmic ribosomes in such yellow-green islands of tissue. The extent of
this reduction depends very much on the strain of TYMV, and it also becomes more severe
with time after infection. Loss of 70 S ribosomes more or less parallels the loss of
chlorophyll, "white" strains causing the most severe loss.
A somewhat different result is obtained if the effect of TYMV infection with time in a
young systemically infected leaf is followed. Chloroplast ribosome concentration falls
markedly about the time virus concentration reaches a maximum. About the same time there
is a significant increase in cytoplasmic ribosome concentration, which is mainly due to the
stunting effect of infection. On the other hand, if the effects of virus infection on these

4. components for the plant as a whole are considered, a different picture emerges. Infection
reduces both cytoplasmic and chloroplast ribosomes.
These results emphasize the fact that infection of a growing plant with a virus
introduces an additional time-dependent variable into a system in which many normal
interacting components are changing with time. Analyses made on only one or two
components of the system, or at some particular time, arc unlikely to give much in sight into
virus replication and the nature of the disease process. Very little is known about any effects
of virus infection on host tRNAs, nuclear RNAs, or mitochondrial ribosomal RNAs.
(iii) Proteins
The coat protein of a virus such as TMV can come to represent about half the total
protein in the diseased leaf. This can occur without marked effects on the overall content of
host proteins. Many other viruses multiply to a much more limited extent. Effects on host
protein synthesis are not necessarily correlated with amounts of virus produced. A reduction
in the amount of the most abundant host protein-ribulose bisphosphate carboxylase-
oxygenase (rbcs)-is one of the commonest effects of viruses that cause mosaic and yellowing
diseases (e.g, TYMY, Reid and Matthews, 1966; wheat streak mosaic' Potyvirus, White and
Brakke 1983).
Fraser (1987b) estimated that TMV infection reduced host protein synthesis by up to
75% during the period of virus replication. Infection did not alter the concentration of host
polyadenylated RNA, nor its size distribution. This suggested that infection may alter host
protein synthesis at the translation stage rather than interfering with transcription. Many
viruses infecting vertebrates inhibit host-cell translation by a variety of mechanisms, bringing
about conditions that favor translation of viral mRNAs (Schneider and Shenk, 1987). The
mechanisms used by plant viruses are beginning to be studied. For example, Stratford and
Covey (1988) found that there were changes in the levels of specific translatable mRNAs in
response to infection of turnip leaves with CaMV. More such changes were found with a
severe strain. In particular the mRNA encoding the precursor to the small subunit of the was
markedly decreased following infection with the severe strain.
It is known that the coat protein of TMV, and some other viruses, can encapsulate
some host RNAs in vivo. Sleat et al (1988b) transformed tobacco seedlings to express
chloramphenicol acetyl transferase (CAI) mRNA. Transformed plants that also contained the

5. TMV origin of assembles quench 3' to the CAT gene showed a threefold suppression of CAY
activity compared with plants without the origin of assembly sequence. Thus it is possible
that the coating of host mRNAs in viral coat protein may be a mechanism for the shutting off
of specific host mRNAs during virus replication.
Saunders et al. (1989) used another approach to the same problem. They generated a
library of cDNA clones corresponding to the host RNAs isolated from turnip leaves infected
with CaMV during the early vein-clearing stage. Hybridization was used to selected clones
that represented RNAs whose levels had been raised or lowered by infection. For example,
one RNA that was greatly reduced in amount was Identified as the mRNA for the ribulose
1,5-bisphosphate carboxylase small subunit polypeptide. Overall, the findings of Stratford
and Covey (1988) and Saunders et al (1989) suggest that there are few major changes in host
gene expression during infection with CaMV .
2. Lipids
The sites of virus synthesis within the cell almost always contain membrane
structures. TYMV infection alters the ultrastructure of chloroplast membranes, and
rhabdovirus particles obtain their outer membrane by budding through some host-cell
membrane. There have been a few studies of the effects of virus infection on lipid
metabolism (e.g., Trevathan et al., 1982) but none of these has illuminated the mechanism by
which viruses change and use' plant membrane systems.
3. Carbohydrates
Some viruses appear to have little effect on carbohydrates in the leaves, while others
may alter both their rate of synthesis and rate of translocation. These changes may be
illustrated in a simple manner.
Leaves that have been inoculated several days previously with a virus that does not
cause necrotic local lesions are harvested in the morning or after some hours in darkness,
decolorized, and treated with iodine. The local lesions may show up as dark-staining areas
against a pale background, indicating a block in carbohydrate translocation. On the other
hand, if the inoculated leaves are harvested in the afternoon on after a period of
photosynthesis, decolorized and stained with iodine, the local 1esions may show up as pale

6. spots against the dark-staining background of uninfected tissue .Thus, virus infection can
decrease the rate of accumulation of starch when leaves are exposed to light.
From the few diseases that have been examined in any detail, it is not possible to
make very firmly based generalizations about other carbohydrate changes, but the following
may be fairly common effects: (i) a rise in glucose, fructose, and sucrose in virus - infected
leaves; (ii) a greater rise in these sugars caused by mild strains of a given virus compared
with severe strain; and (iii) effect of infection on mesophyll cells, not yet understood, may
reduce translocation of carbohydrates out of the leaves.
4. Cell Wall Compounds
Although cytological studies have demonstrated ultrastructural changes in the cell
walls in many virus infections, the biochemical basis of such changes would be difficult to
study. Future work may show that virus infection has effects on various activities in the cell
wall compartment, which is not metabolically inert. Eighty-five percent of detectable
peroxidase activity and 22% of the acid phosphatase are located in the cell wall of healthy
tobacco leaves (Yung and Northcote, 1975), Elevated peroxidase activity has been reported
as a response of tobacco and many other hosts to virus infection (Matthews, 1981).
5. Respiration
Many studies have been made of the effects of virus infection on rate, and pathways
of respiration, but it is not possible to relate the results to the processes involved virus
replication. In summary for man many host – virus combinations where necrosis does not
occur, there is a rise in respiration rate, which may begin before symptoms appear and
continue for a time as disease develops. In chronically infected plants, respiration is often
lower than normal. In the one systemic disease so for examined in detail, there is no
detectable change in the pathway of respiration. In host-virus combinations where necrotic
local lesions develop, there is an increase in respiration as necrosis develop . This increase is
accounted for, at least in part, by activation of the hexose monophosphate shunt pathway
(Matthews 1981; Fraser, 1987b).
6. Photosynthesis
In a tobacco mutant in which some islands of leaf tissue had no chlorophyll, TMV
replication occurred in white leaf areas in the intact plant. However, replication did not occur

7. if the white tissue was detached and floated on water immediately after inoculation (R, E. F.
Matthews, unpublished). Detached white tissue supplied with glucose supported TMV
replication, indicating that the process of photosynthesis itself is not necessary for replication
of this virus, Nevertheless, virus infection usually affects the process of photosynthesis.
Reduction in carbon fixation is the most commonly reported effect in leaves showing mosaic
or yellows diseases. This reduction usually becomes detectable some days after infection.
Photosynthetic activity can be reduced by changes in chloroplast structure, by reduced
content of photosynthetic pigments or ribulose bisphosphate carboxylase, or by reduction in
specific protein associated with the parti les of photosystem II (Naidu et al .. 1986). However,
such changes appear to be secondary, occurring some time after infection when much virus
synthesis had already taken place. In tobacco plants infected with various strains of TMV,
electron transport rates were reduced when loss of chlorophyll occurred. In inoculated laves,
photosystem II appeared to be irreversibly damaged in inoculated leaves even when no
macroscopic symptoms were apparent (van Kooten et al 1990). A variety of effects of
localized and systemic TMV infection in tobacco were observed in experiments with isolated
chloroplasts. However, some enzyme activities were little affected (Montalbini and Lupattelli
1989).
Some effects on photosynthesis are known that appear to be closely linked in time to
the early period of maximum virus production. In chloroplasts isolated from Chinese cabbage
leaves infected with TYMV, the Hill reaction and cyclic and noncyclic photophosphorylation
were all increased compared with healthy leaved during the phase of active virus
multiplication (on an equal chlorophyll basis (Goffeau and Bove, 1965). At a late stage of
infection, photosynthetic activity way lower than in controls measured on chloroplasts
isolated from whole plants in young Chinese cabbage leaves infected with TYMV there was a
substantial diversion of a the products of photosynthetic carbon fixation away from sugars
and into organic acids and amino acids. This change was most marked during the period of
virus increase and returned to the normal pattern when virus replication was near completion
(Bedbrook and Matthews, 1973). An increase in the activity of the enzymes
phosphoenolpyruvate carboxylase and aspartate aminotransferase followed a similar time
course.
Magyarosy (et al (1973) found a similar shift from the production of sugars to amino
acids and organic acids in squash plants systemically infected with squash mosaic

8. Comovirus. They isolated chloroplasts from healthy and diseased leaves and snowed that
both produced a similar pattern of carbon fixation products and that the total carbon fixed
was about the same. They concluded that the virus-induced production of amino acids was
taking place in the cytoplasm.
In summary, during the period of rapid replication, virus infection may cause on
diversion of the early products of carbon fixation away from sugars and. into pathways ways
that lead more directly to the production of building blocks for the synthesis of nucleic acids
and proteins. The most general result of virus infection is a reduction in photo photosynthetic
activity. This reduction arises from a variety of biochemical and physical changes. The
relative importance of different factors varies with the disease.
7. Transpiration
In chronically virus-infected leaves transpiration rate and water content have been
found to be generally lower than in corresponding healthy tissues. The reported effects over
the first 1-2 weeks after inoculation vary. Results are difficult to compare and interpret
because different viruses and host species have been used together with different conditions
of growth and different tissue sampling procedures.
Bedbrook (1972) used the cobalt chloride paper method (Stahl, 1894) to estimate
relative transpiration rate and to give a measure of stomatal opening. He compared, in intact
Chinese cabbage plants, dark green islands in leaves showing mosaic patterns due to TYMV
infection and various islands of tissue fully invaded by the virus. In darkness or low light
intensity, stomata in darker green and pale green islands were closed, while those in islands
of more severely affected lamina were open. In plants that had been held in full daylight the
dark and pale green islands were transpiring rapidly. Transpiration from severely affected
islands was much less. These and other experiments showed that TYMV infection lowers the
responsiveness of the stomata to changes in light intensity, the lowered response being most
marked with strains causing the greatest reduction in chlorophyll. Because of diminished
transpiration, the temperature of sugar beet leaves in susceptible plants infected with
BNYVV was 2-3°C higher than that of a tolerant variety (Xeller et al 1989).
8. Activities of Specific Enzymes
Much of the work dealing with the effect of virus infection on specific enzymes is
difficult to interpret for the following reasons: (i) where differences have been found, it has

9. usually been assumed that virus infection alters the amount of enzyme present and little
consideration has been given to the possibility that infection may affect enzyme activities
through changes in the amount of enzyme inhibitors or activators released when cells are
disrupted; (ii) the difficulty of deciding on an appropriate basis for expressing enzyme
activity has often been ignored; and (iii) much of the work was done before the widespread
existence of isoenzymes was recognized.
There have been many studies involving the use of polyacrylamide gel electro-
phoresis to fractionate and assay isoenzymes and to study the consequences of virus infection
on these patterns. It is relatively easy to generate data by this means. It is much less easy to
provide meaningful interpretations of any observed changes. There are several reasons for
these difficulties:
1. In the healthy plant there may be a continually changing developmental sequence of
isoenzymes
2. Electrophoretically· distinct isoenzymes may be determined genetically or may be
different conformational forms derived by posttranslational modification from the
same primary structure.
3. The pattern of isoenzymes in the normal host may differ in closely related genotypes,
and the effects. of virus infection may differ with these.
4. Isoenzymes may be distributed in several subcellular sites. For example, a different
set of peroxidase isoenzymes was associated with the cell wall and with the soluble
fraction in extracts of normal maize root tips. Virus infection may affect various sites
in different ways.
5. Virus-induced cell death may lead to changes in isoenzyme patterns that do not differ
significantly from those induced by entirely unrelated causes of necrosis.
6. The observed effect of virus infection may depend on the substrate used for
isoenzyme assay.
7. Aggregation states (e.g., monomer = dimer) may affect the kinetic properties of an
enzyme, for example, aspartate aminotransferase.
Studies have been reported that involve representatives of all the major groups of

10. enzymes (oxidoreductases and so on), but none of these has taken the preceding variables
adequately into account.
9. Hormones
There is little doubt that virus infection influences hormone activities in infected plant
and that hormones play some part in the induction of disease. Quantitative effects of
infection on concentration have been shown for all the major groups of plant hormones
(reviewed by Fraser, 1987b). Virus infections tend to decrease auxin and gibberellin is
concentrations and increase that of abscisic acid. Stimulation of ethylene production is
associated with necrotic or chlorotic local responses.
10. Low-Molecular-Weight Compounds
There are numerous reports on the effects of virus infection on concentration of low-
molecular-weight compounds in various parts of virus-infected plants. The analyses give rise
to large amounts of data, which vary with different hosts and viruses, and which are
impossible to interpret in relation to virus replication. Some of these effects can be briefly
discussed as:
a. Amino Acids and Related Compounds
The most consistent change observed has been an increase in one or both of the
amides, glutamine and asparagine. The imino acid pipecolic acid has been reported to occur
in relatively high concentrations in several virus-infected tissue. A general deficiency in
soluble nitrogen compounds compared with healthy leaves may occur during periods of rapid
virus synthesis.
b. Compounds Containing Phosphorus
Phosphorus is a vital component of all viruses and as such may come to represent
about one-fifth of the total phosphorus in the leaf. In spite of this we still have no clear
picture for any virus of the source of virus phosphorus, or the effects of infection on host
phosphorus metabolism.
In Chinese cabbage leaves infected with TYMV, sampled 12-20 days after
inoculation, a rise in virus phosphorus was accompanied by a corresponding fall in nonvirus-
insoluble phosphorus, suggesting that this virus uses phosphorus at the expense of (but not

11. necessarily directly from) some insoluble source of phosphate in the leaf (Ma thews et al
1963).
c. Leaf Pigments
Virus infection frequently involves yellow mosaic mottling, or a generalized
yellowing of the leaves. Such changes are obviously due to a reduction in leaf pigments.
Many workers have measured the effects of virus infection on the amounts of pigments in
leaves. Frequently it appears to involve a loss of the chlorophylls, giving the 'Yellowish
coloration due to carotene and xanthophyll, but the latter pigments are also decreased in some
diseases. Changes in chloroplast pigments are probably often secondary changes, since many
viruses appear to multiply and accumulate in other parts of the cell, and since closely related
strains of the same virus may have markedly different effects on chloroplast pigments even
though they multiply to the same extent.
The reduction in amount of leaf pigments can be due either to an inhibition of
chloroplast development or due to the destruction of pigments in mature chloroplasts. The
first effect probably predominates in young leaves that are developing as virus infection
proceeds. The rapidly developing chlorosis frequently observed in local lesions when mature
green leave are inoculated with a virus must be due to destruction of pigments already
present. In systemically infected leaves, TYMV reduced the concentration of all six
photosynthetic pigments to a similar extent. This was due to a cessation of net synthesis, and
subsequent dilution by leaf expansion.
Dark green islands of tissue in Chinese cabbage leaves showing mosaic symptoms
had essentially normal concentrations of pigments. Small leaves near the apex of large
Chinese cabbage plants are shielded from light and contain little chlorophyll. When such
cream-colored leaves, about 2 cm long, were excised and exposed to light, those from healthy
plants became uniformly dark green, while those from TYMV-infected plants developed a
prominent mosaic pattern of dark green islands and yellow areas within 24 hours. Thus, in
young expanding leaves chlorophyll synthesis is inhibited in those islands of tissue in which
TVMV is replicating.

12. d. Flower Pigments
In view of the work that has been done on the genetics and biochemistry of normal
flower coloration, surprisingly little is known about the biochemistry of the flower-breaking
process, which is such a conspicuous feature of many virus diseases. In tobacco plants
infected with TMV, the normal pink color of the petals may be broken by white stripes or
sectors. We have found the virus present only in the white areas. However, presence or
absence of Virus may not be the only cause for color breaks. In sweet peas (Lathrus
odoratus) infected with what was presumably a single virus-bean yellow mosaic Potyvirus-a
pale pink flower sometimes became flecked with both darker pink and white areas.
Virus infection usually appears to affect only the vacuolar anthocyanin pigments. The
pigments residing in chromoplasts may not be affected. For example, the brown wallflower
(Cheiranthus cheirii), which contains an anthocyanin, cyanin, and a yellow plastid pigment
(Gairdncr 1936), breaks to a yellow color when infected by turnip mosaic Potyvirus (TuMV).
A preliminary chromatographic examination of broken and normal parts of petals infected
with several viruses showed that the absence of color was due to the absence of particular
pigments rather than to other factors, such as change in pH within the vacuole (R. E. F.
Matthews, unpublished).
Kruckelmann and Seyffert (1970) examined the effect of TuMV infection on several
genotypes of Matthiola incana R. Br. Infection brought about both white stripes and pigment
intensification. Observations on a set of known host genotypes have shown that virus
infection affected only the activities of genes controlling the quantities of pigments produced.
It appeared to have no effect on the activities of genes modifying anthocyanin structure.

13. REFRENCES:
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