Sorghum Diseases in India

nonparticle proteins may also be determined.
The nonparticle proteins are named and sized,
and if possible, functionally characterized.
Other components of viruses, such as polyamines,
may be identified. Lipids are found in
enveloped viruses. At least two virus groups
contain envelopes of 10 to 30% lipids. Virus proteins
may have other molecules attached. For
example, some viruses are glycosylated and
others may be phosporylated (Matthews 1981).
Nucleic acid can be analyzed for type—
deoxyribonucleic acid (DNA) or ribonucleic acid
(RNA)--and strandedness. Both RNA and DNA
may be either single- or double-stranded (ssRNA,
ssDNA, dsRNA, dsDNA) (Dodds et al. 1984;
Harrison 1985; Matthews 1981). To date, unusual
or minor bases have not been reported in plant
virus nucleic acids. The number of nucleic add
species is another identifying criterion, since the
genome of a virus may be comprised of one or
more separate RNAs (Goldbach 1986; Matthews
1981). The molecular weight of the nucleic acid
and its base composition and ratios are distinguishing
characteristics. Another distinguishing
genomic characteristic is the presence or absence
of a covalently linked protein or methylated
cap at the five prime (50 end. At the three
prime (3') end, the virus may or may not have an
amino acid-accepting ability. Also, the 3' end
may have a polyadenine or tRNA-like structure
with a CCA (anti)condon. Infectivity may or
may not require a protein capsid (nucleocapsid).
Subgenomic RNA may or may not be present
(Goldbach 1986; Matthews 1981; Rowlands et al.
1987). Also, some viruses (e.g., rhabdoviruses
and reoviruses) contain polymerases within the
particle (Joklik 1981). The absorbance of its viral
nucleic acid per unit weight at 260 µm varies,
depending o base composition, and may therefore
serve as a distinguishing factor.
Antiserum production, testing
After purification, antiserum can be produced.
The reaction of virus with polyclonal or monoclonal
antibodies is useful in virus identification
and study of virus relationships. Many assay
techniques are available for studying antigen antibody
reactions (Hill et al. 1984; Van Regenmortel
1982b). The two most useful for virus
identification are gel-diffusion tests and enzyme-linked
immunosorbent assay (ELBA)
156
(Kerlan et al. 1982). The gel-diffusion test most
commonly used is the Ouchterlony double-diffusion
test. The position on the plate where antigen
and antibody meet in optimum proportions
produce precipitation zones characteristic of the
particular antigen-antibody system used. In the
gel-diffusion technique, crude or purified virus
preparations may be used with any type of antibody.
An additional advantage is that a number
of dilutions of antigen or antiserum samples can
be tested at the same time. The ELISA technique
employs several systems, but the most commonly
used is the double antibody sandwich
(Koenig and Paul 1982). In this procedure, the
inside walls of a polystyrene plate are coated
with gamma globulin from the antiserum and
the plate is then washed. The virus sample is
added and incubated. After washing, the antibody
linked to an enzyme with gluteraldehyde
is added, the plate washed, and an enzyme substrate
added. The colorless substrate, if positive
to the virus, converts to a colored product that
can be observed visually or measured spectrophotometrically.
This test is very sensitive and
can provide quantitative data for study of hostpathogen
interaction, epidemiological investigations,
and characterization studies (Clark 1981;
Giorda et al. 1986a). Virus samples may be crude
or purified preparations, and the technique is
particularly suitable for processing large numbers
of samples. It is highly specific and can be
used where mixed infections, or more than one
virus strain, is involved (Clark 1981; Koenig and
Paul 1982).
Host response may also be used as a diagnostic
tool. The symptom response of a host to an
unknown virus can be compared to that of a
known virus and the symptom homology or heterology
established (Alexander et al. 1985; Giorda
et al. 1986b; Persley et al. 1985; Toler 1985).
Genotypes resistant to specific viruses can be
inoculated with the unidentified virus, and observed
during the growing season. The observations
provide corroborative evidence based on
resistance (Alexander et al. 1983; Giorda et al.
1986b; Toler 1985).
Traditional approaches to detection and diagnosis
of plant viruses include: symptoms; transmissibility;
host range; vector relationships;
immunological, physical, and chemical properties
of the virus particle; morphology; its capsid
protein; and genome. With the development of
biotechnology, new methods based on nucleic