PreviousA corn -shaped fruit
Fruit showing a sharp difference in size between the top and
bottom portions thus resembling an acorn. In cross-section, the
albedo is thicker at the stem end and thinner at the stylar end.
Fruit on stubborn- or exocortis-infected trees will sometimes
show this acorn-shaped appearance (see Figure 39)
Acquisition feeding
The feeding period during which the insect ingests sap containing
the pathogen
Alkaline phosphatase
An enzyme which hydrolyses certain phosphate containing compounds
under alkaline conditions; commonly obtained from calf intestine
mucosa
Amorphous plugs
Microscopic inclusions found in the trunk xylem tissue of
blight-affected trees. These inclusions are uniformly dense and
without structure (see Figure 33).
Antibody
A protein formed in blood serum in response to stimulation by an
antigen. Antibodies are specific for their respective antigens,
and antigens and antibodies are mutually attracted
Antibody-antigen
The reaction or attraction formed when reaction antigens meet
their corresponding antibodies or vice versa. This strong
attraction is the basis of all immunodetection systems
Antigen
A substance, often a virus or bacteria, which stimulates
production of antibodies in an animal. Specifically, it is the
corresponding molecule to the antibody in a serological test
Autoradiography
The technique or process of making a picture revealing the
presence of radioactive material, the film being laid directly on
the object to be tested. This is frequently used for detection of
radioactivity following hybridization, by exposing kilter paper
to sensitive X-ray film
Bark graft
One of a number of types of grafts used in the transmission of
graft-transmissible pathogens. A piece of bark is cut from the
host scion or rootstock and grafted to an indicator plant (see
Figures 135-137)
Biotin
One of the B-vitamin complex. It is widely distributed in plant
and animal tissue, is water soluble and binds strongly to a
glycoprotein named avidin.
Biotin derivatives of deoxyribonucleotides are incorporated into
probe DNA by nick translation (see Part III). After hybridization
the biotin can then be detected using streptavidinfluorescein
complexes. The streptavidin binds to the biotin by one of the
strongest known biological interactions.The enzyme (usually
peroxidase or alkaline phosphatase) is then reacted with its
substrate which gives a coloured product; fluorescein is detected
by fluorescence under light of certain wavelength
Biotinylated
A compound to which a small vitamin, biotin, has been attached.
Antibodies, enzymes and nucleic acids can be labelled with biotin
Biotinylated enzyme
An enzyme coupled chemically to biotin
Biotinylated probe
A DNA probe in which certain bases were modified by chemical
coupling of biotin
Blind bud
Not a true bud, but a section cut from that portion of the stem
between buds containing no "eye" and used primarily for
graft inoculations (see Eye, and Figure 127)
Blister inoculum
Inoculum tissue for graft transmission of psorosis-B used
primarily in cross-protection studies. The tissue contains the
blisters associated with the severe "B" form of
psorosis (see Figure 79a)
Blot
As a verb, this means to transfer DNA, RNA or protein to an
immobilizing matrix such as DMB-paper, nitrocellulose or nylon
membranes. As a noun, it usually refers to the autoradiograph
produced during the Southern or Northern blotting procedure
BRL Hybri-dot
A commercially available kit for applying small volumes of
extracts to a membrane for testing the presence of viruses or
viroids by hybridization
Budgraft inoculation
A bud, blind bud or chip bud cut from a stem of the plant or tree
to be indexed and grafted to an indicator plant or tree
Bud-union crease
A line, ridge or fold, usually discoloured as brown,
yellowish-brown or reddish-brown, and formed at the bud-union. It
is readily observed when the outer bark is removed (see Figures
56 and 102-104). Some bud-union creases are caused by pathogens
and others by incompatibility of rootstocks and scions
Canopy
As in a tree canopy. The outline or total area of a tree,
indicating its size
cDNA
Complementary DNA. The DNA complement of an RNA sequence. It is
synthesized by the enzyme RNA-primed DNA polymerase or reverse
transcriptase. The single-stranded DNA product of this enzyme
(the reverse transcript) may be converted into the
double-stranded form by DNA-primed DNA polymerase, and inserted
into a suitable vector to make a cDNA clone. cDNA cloning is
commonly used to achieve the expression of mammalian genes in
bacteria or yeast
cDNA probe
A radioactive specific DNA sequence used to detect complementary
sequences of RNA or DNA (see cDNA and Probe)
Certification programme
A programme developed by a country, state, university or research
centre for ensuring that selected budwood distributed to the
growers is free of graft-transmissible pathogens and the fruit
true-to-type. These pathogen-free certified trees are usually
registered, and budwood issued from these mother or
foundation-block trees can be used to produce additional buds in
an increase block for the development of certified trees
CF-11
A fibrous, graded, cellulose powder sold by the Whatman Company
Cheesy bark
Enlarged, swollen and softened bark tissue which, when cut into
with a knife, resembles cheese. This type of cheesy bark is
usually associated with the presence of severe CTV
Chip bud
A piece of bark tissue used for graft inoculation. It is used
when the bark of receptor indicator plants does not slip or open
up to accept a bud or blind bud (see description in Part II under
Grafting techniques, and also Figure 127)
Chromatography
The separation of mixtures of chemicals, compounds, proteins,
macro-molecules etc. into their constituents or components by
preferential adsorption by a solid such as a column of cellulose,
or by filter paper or by gel
Clonal
A budline derived from a single parent source by propagation from
that source
Columella
The internal, central, soft white column-like axis of the citrus
fruit
Complementary
See cDNA. A nucleic acid sequence is said to be complementary to
another if it is able to form a perfect hydrogen-bonded duplex
with it, according to the Watson-Crick rules of base pairing. A
viral genomic ssRNA is complementary to "negative
sense" ssRNA from which it is transcribed
Conjugated molecule
The temporary or permanent union, fusion or binding of two
substances. When used in ELISA, it is the combining of antibody
and enzyme proteins to form an enzyme-labelled antibody
conjugate. The enzyme can then be detected colorimetrically, and
the colour produced will give a fairly precise picture of the
amount of virus present
DEAE cellulose column
A plastic or glass tube open at the top and fitted with a
stopcock on the bottom. It contains diethylaminoethyl cellulose
Denature
So to modify (a protein) by heat, acid, or alkali that it retains
its primary structure but no longer has all its original
properties
Deproteination
To remove and separate proteins from other macromolecules from
samples to be tested by hybridization. This is normally achieved
by phenol extraction or by treating with a protein digesting
enzyme
Dialysis
A procedure using a membrane to separate various components in
solution in accordance with their ability to pass through the
membrane
Dicing
Cutting of tissue into small segments using a sharp knife or
razor-blade. In the ELISA technique, leaf or bark segments are
diced or cut up prior to grinding
DNA
Deoxyribonucleic acid. Any of a class of nucleic acids that
contain deoxyribose, found chiefly in the nucleus of cells, and
that functions in the transference of genetic characteristics and
in the synthesis of protein
DNA probe
A probe for detection of specific nucleic acid segments (see
Probe)
Dot-blot
A procedure which is used to determine the presence and
concentration of a particular RNA or DNA species. Different
concentrations of the non-radioactive nucleic acids are denatured
and applied as a dot to nitrocellulose paper or other support
matrix. This is then hybridized with the radioactive
complementary probe under study. After autoradiography, the
intensities of the radioactive images formed are quantified and
compared to a series to determine the concentration of the
non-radioactive molecule
dPAGE
Denaturing polyacrylamide gel electrophoresis. Electrophoresis in
a gel formed from polyacrylamide in the presence of a chemical
agent such as urea (8M) or heat. It functions to minimize the
effects of secondary and tertiary structure of the molecule on
electrophoretic mobility
Electro-blotting
The electrophoretic transfer of macromolecules (DNA, RNA or
protein) from a gel in which they have been separated, to a
support matrix such as a nitrocellulose or a charged nylon sheet.
An alternative to the capillary transfer usually used in
techniques such as Southern and Northern blotting
Electro-blot membrane
A solid charged medium on to which a molecule is fixed as a
result of electrophoresis from a source medium. This can be
nitrocellulose or a charged nylon matrix
Electro-elution
Removal of adsorbed material from an adsorbent by use of an
electric field: or, recovery of a charged molecular species by
electrophoretic migration from a source medium such as a
polyacrylamide gel to a liquid medium in which concentration of
the species can be accomplished
Electrophoretic techniques
Techniques which separate components suspended in a fluid media
or gel by the influence of an electric field
Electro-transfer
The movement of a charged molecule from one medium to a second by
migration in an electric field
ELISA
Enzyme linked immunosorbent assay. Two antibody preparations are
commonly used in ELISA. The primary antibody binds the antigen
which is itself bound by the second antibody. The second antibody
is linked to any enzyme whose activity is easily monitored, i.e.
by colour change. The extent of enzymatic reaction is then a
quantitative indication of the amount of antigen trapped by the
primary antibody
Elution
Removal by dissolving, such as the removal of adsorbed material
from an adsorbent by solvents
Enation
A small protuberance or growth found on a leaf, induced by insect
bites or caused by the vein-enation virus. Very distinct enations
can be seen on the veins of leaves of Mexican lime infected with
vein-enation virus (see Figure 98)
Enzyme-labelled antibody
See Conjugated molecule and ELISA
Epinasty
An increase or decrease in growth of the upper or lower leaf
surface or vein which causes the leaf to bend downward. Mild leaf
epinasty is shown in an exocortis-infected citron leaf in Figure
42d, and severe epinasty in leaves of citron in Figure 41 a
Ethidium bromide
An intercalating agent which allows the ready detection of
double-stranded nucleic acid molecules in agarose gels. The
nucleic acid/ ethidium bromide complex fluoresces brightly when
exposed to ultraviolet (UV) light (ethidium bromide is highly
carcinogenic)
Eye (of a bud)
The protruding meristematic portion of a bud which later enlarges
and grows into a young shoot or flower
Flecking
As in leaf flecking. Usually a lighter translucent spot or small
patch on leaves. Flecking in young or mature leaves is
symptomatic for a number of graft-transmissible pathogens but can
also be induced by environmental conditions or by the inherent
genetic make-up of the plant
Flush
The new, young and fresh growth of shoots and leaves
Foundation tree
In a certification programme, the foundation tree is the primary
tree derived from budwood which has been specially selected,
shoot-tip grafted and/or heat-treated, and which has been indexed
and certified as virus-free and also true-to-type. It will become
the primary source tree for all future progeny trees. A
foundation tree can be synonymous with a mother tree or mother
block tree
Gametic
A gametic seedling is one which was formed by sexual union and
derived from seed containing the chromosomes of both parents.
This is in contrast to the nucellar seedling containing only the
chromosomes from the mother tree (see Nucellar)
Gel
The inert matrix used for electrophoretic separation of nucleic
acids or proteins. Agarose gels are used for separation of DNA;
agarose or polyacrylamide for RNA, and polyacrylamide for
proteins
Gracilicute-like
In reference to the greening organism, gracilicute is a division
in the order of bacteria comprising organisms having a
Gram-negative, membranous cell wall containing peptidoglycan
Graft-transmission
The transmission of a virus or other pathogen(s) by grafting
tissue from the suspect host to an indicator plant
Gravity infusion
Used for testing the water uptake within a tree for the diagnosis
of blight. It is the flow of water from a burette or other
container suspended from a branch. The water is permitted to flow
into the tree by gravity via a rubber or plastic tube (see Figure
31)
Hybridization
The formation of stable duplexes between complementary nucleotide
sequences via Watson-Crick base pairing. The efficiency of
hybridization is a test of sequence similarity. DNA-DNA, DNA-RNA,
RNA-RNA hybrids may be formed. An alternate use of the word comes
from classical genetics, and particularly plant breeding. Here
hybridization means the formation of a novel diploid organism
either by normal sexual processes or protoplast fusion
Immunoassay
An assay system which detects proteins using an antibody specific
to that protein. A positive result is seen as a precipitate of an
antibody protein complex. The antibody can be linked to a
radioactive atom or to an enzyme which catalyses an easily
monitored reaction (see ELISA)
Immunoblotting
A procedure whereby either the antigen or antibody molecules are
bound to a proteinbinding substrate, such as cellulose nitrate,
and then exposed to the complementary antigen or antibody. The
antigen-antibody complex which forms on the membrane is detected
by an appropriately labelled antibody
Immunodiffusion
A procedure where antibody and/or antigen molecules are allowed
to migrate through an inert medium. A visible precipitate forms
at the zone where related antigen and antibody molecules meet in
a suitable concentration and react
Immunofluorescence
The result of detection of antigens, often within tissues, by use
of an antibody to which a fluorescent material is attached
Immunoglobulin
A blood serum protein which functions as an antibody, commonly, a
gamma globulin
Immunosorbents
A material which can adsorb serologically active molecules
(antigens or antibodies). Cellulose nitrate and some plastics,
such as certain polystyrenes, are good immunosorbents. Adsorbed
molecules typically retain serological functions
Inclusion bodies
Cytopathic intracellular structures referred to as inclusion
bodies are found in virus-infected plants. They contain virus
particles, other proteins or structures specific to the virus
and/or formed as a result of virus infection
Indexing
Any test which will consistently confirm the presence (or
absence) of a transmissible pathogen, or identify a disease. The
index test should be specific for the pathogen or disease
Indicator plant
A plant used to test or index for the presence of a transmissible
pathogen. The inoculated indicator plant will usually show very
specific symptoms, thus permitting the diagnosis of a particular
disease
Infection feeding
In contrast to acquisition feeding where the pathogen is ingested
by the insect, infection feeding is secondary or the follow-up
feeding where the pathogen is injected into the host plant by the
insect
Inoculation
The process of infecting an indicator plant, usually by graft,
mechanical or vector transmission
Inoculum tissue
Tissue which contains the transmissible pathogen or pathogens
Insipid (taste)
A dull, flat, savourless taste when fruit of stubborn-infected
trees are tasted. This insipid taste is helpful in diagnosing
possible stubborn infection when associated with other diagnostic
symptoms
Intermediate antibody
An antibody used in the second step of an indirect ELISA assay.
The intermediate antibody reacts to the antigen bound to the
plate but is not labelled. The intermediate antibody is detected
by using another antibody which is labelled and is specific only
for the intermediate antibody
Interveinal clearing (or flecking)
The development of a clearing, flecking or lighter colour between
leaf veins is usually associated with the presence of the concave
gum or psorosis pathogen (see Flecking and Figure 83b)
Inverse stem-pitting
A type of pitting usually associated with citrus tristeza virus
reaction of sweet orange, mandarin or grapefruit scions on sour
orange rootstock. When a small segment of the sour orange bark is
removed just below the bud-union, it will show numerous small
pits on the inner surface of the bark, with corresponding
needle-like pegs on the opposite wood surface of the trunk
Invisible pathogen
This refers to graft-transmissible pathogens which are present in
citrus trees but which induce little or no symptoms in these
trees. However, when budwood from these symptomless carriers is
propagated or inoculated into a susceptible citrus species,
symptoms can occur on the susceptible host (see Symptomless
carrier)
Leaf-disc graft
A graft using round discs cut from leaves with a paper punch. The
discs are matched to holes punched into leaves of an indicator
plant and secured using adhesive tape (see Figures 1321 34)
Leaf-piece graft
A graft using as inoculum a small section cut from a young leaf
of the test plant to be indexed. The leaf piece is then inserted
into the stem of a receptor indicator plant in the same manner as
a bud-graft and secured with wrapping tape
Linearization
The conversion of a nucleic acid that is normally circular into a
linear form of the molecule. This is done by cutting the circular
form at a single site
Loading
As in loading ELISA plates. In ELISA, it is the process of adding
a given amount of sample, buffer or any substance to the wells of
an ELISA plate or gel apparatus
Mechanical transmission
Transfer or transmission of graft-transmissible pathogens by
means other than grafting and not involving vectors. This can be
done by knife cut, razor slash, hand or cotton rubbing of sap on
leaves using carborundum powder, or by any other non-grafting
method
Molecular hybridization
See Hybridization
Molecular probe
See Probe
Mollicutes
In reference to mycoplasma-like organisms. One of the four
divisions of the kingdom Procaryotae characterized by having no
cell wall or peptidoglycan
Monoclonal antibodies
An antibody preparation which contains only a single type of
antibody molecule. Monoclonal antibodies are produced naturally
by myeloma cells. A myeloma is a tumour of the immune system. A
clone of cells producing any single antibody may be prepared by
fusing normal lymphocyte cells with myeloma cells to produce a
hybridoma
Mother trees or mother-block trees
Similar or synonymous with foundation or foundation-block trees
(see Foundation tree)
Negative stain
An electron-dense solution used to provide contrast around virus
particles viewed on a transmission electron microscope
Nick translation
A procedure to insert radioactive or other tagged bases in a DNA
probe. It is a process whereby damaged dsDNA molecules are
repaired with nucleotides, some of which are radioactive. It is a
good way to repair a probe
Nitrocellulose membrane
(Cellulose nitrate) A nitrated derivative of cellulose which is
made into membrane filters of defined porosity, e.g. 0.45 mm 0.22 mm. These
filters have a variety of uses in molecular biology, particularly
in nucleic acid hybridization experiments. In the Southern and
Northern blotting procedures, DNA and RNA, respectively, are
transferred from an agarose gel to a nitrocellulose filter. Some
centrifuge tubes are made of nitrocellulose; they are readily
punctured with a hypodermic needle, and are frequently used for
sucrose gradient
Northern blot, Northern transfer
A procedure analagous to Southern transfer but, in this case, RNA
not DNA is transferred or "blotted" from a gel to a
suitable binding matrix such as a nitrocellulose sheet.
Single-stranded RNA is separated according to size by
electrophoresis through an agarose or polyacrylamide gel; the RNA
is then blotted directly on to the support matrix with no
denaturation. RNA fixed to the supporting matrix can then be
hybridized with a radioactive single-stranded DNA or RNA probe
Nucellar (seedling)
Referring to citrus, it is the seedling formed from the nucellus
tissue surrounding the embryo. Nucellar seedlings resemble the
parent plant and are quite uniform. However, a nucellar seedling
or tree derived from a nucellar seedling is not identical to the
parent tree, and may have many characteristics which are
different. The nucellar process was used primarily to bypass
citrus pathogens, and in particular exocortis
Nucleic acid
A DNA or RNA molecule which can be single-stranded or
double-stranded
Nucleotide sequence
(See Sequencing.) It is the order (or sequence) of
alignment of nucleotides of nucleic acid molecules
OLP
Oak-leaf pattern. A translucent pattern similar to the outline of
an oak leaf, and induced in leaves of indicator plants or trees
infected with the concave-gum family of viruses, which include
concave gum, impietratura and cristacortis (see Figures 83a, 89
and 95)
PAGE
Polyacrylamide gel electrophoresis (see Polyacrylamide gels). A
method for separating nucleic acid or protein molecules according
to their molecular size. The molecules migrate through the inert
gel matrix under the influence of an electric field. In the case
of protein PAGE, detergents such as sodium dodecyl sulfate are
often added to ensure that all molecules have uniform charge.
Secondary structure can often lead to the anomalous migration of
a molecule. It is common, therefore, to denature protein samples
by boiling them prior to PAGE. In the case of nucleic acids,
denaturing formamide, urea, or methyl mercuric hydroxide are
often incorporated into the gel itself, which may also be run at
a high temperature. PAGE is used to separate the products of
DNA-sequencing reactions and the gels employed are highly
denaturing since molecules differing in size by a single
nucleotide must be resolved
Pegs
The needle-like growths or pinpoint projections observed on the
wood or bark. These may be symptomatic for certain diseases of
citrus. Pegs usually have corresponding pits on the opposite bark
or wood surface
Peptidoglycan
Polysaccharide chains covalently cross l inked by peptide chains.
The presence or absence of peptidoglycan in the cell walls of
bacteria is used to distinguish gracilicute-like organisms (which
contain peptidoglycans) from mycoplasma-like organisms (which
have no peptidoglycans)
Plant laboratory
A sophisticated greenhouse designed and used primarily for
indexing
Plate-trapped antigen
Antigen which is adsorbed directly on the ELISA plate without use
of a trapping antibody. For example, virus particles can be
trapped to the surface of ELISA plates from extracts of infected
tissue added to the wells of the plate. Other proteins, however,
are also adsorbed
Polyacrylamide gels
Often referred to, incorrectly, as acrylamide gels. These gels
are made by cross-linking acrylamide with
N,N'-methylene-bis-acrylamide. Polyacrylamide gels are used for
the electrophoretic separation of proteins and also RNA
molecules. DNA molecules usually have too high a molecular weight
to migrate far in polyacrylamide. Polyacrylamide beads are also
used as molecular sieves in gel chromatography and are marketed
under the brand name "Nio-gel"
Polyclonal antiserum
Antiserum harvested from the blood of immunized animals.
Polyclonal antisera contain a mixture of antibodies to the
various antigenic molecules present in the material used to
immunize the animal. In contrast, monoclonal antisera contain
only a single antibody
Polyembryonic
The production of one or more embryos from one seed. Nucellar
seed may produce a number of seedlings from a single seed and
this is sometimes called polyembryonic. In a true sense, it
should be called multiple-sprouted nucellar seedlings
Potting mix
A mixture of ingredients used as an artificial soil medium for
container growth of plants
Primary leaves
The first emerging leaves from a germinating seed. These leaves
may be the cotyledons or may differ in shape from the secondary
leaves
Primer
A low-molecular-weight species which promotes a reaction, such as
an oligonucleotide which binds to a template permitting a copy of
the template to be further synthesized
Probe
As a noun, probe is a specific DNA or RNA sequence which has been
radioactively labelled to a high specific activity. Probes are
used to detect complementary sequences by hybridization
techniques such as Southern or Northern blotting or colony
hybridization. The verb to probe is to hybridize in order to
detect a specific gene or transcript, e.g. "We probed our
bank with labelled viral RNA to detect clones containing viral
DNA sequences."
Probe denaturization
Treating the probe under conditions that will separate its
nucleic acid strands and enable their subsequent hybridization
with the target molecules to be tested
Probe labelling
A variety of procedures that permit the labelling of probes with
detectable tags
Procaryote
Bacteria-like organisms in the kingdom Procaryotae that have no
organized nucleus and are surrounded by a nuclear membrane.
Recent taxonomic literature suggests four divisions in the
kingdom according to the presence of peptidoglycan and the type
of cell wall. Both the greening organism and Spiroplasma citri
are procaryote organisms; one a gracilicute-like organism and the
other a mollicute
Protein A
Aprotein with a high affinity for antibody gamma globulins
Protuberance
As protuberances found in the rind of an impietratura-infected
fruit. These are raised bumps or blister-like emergences on the
rind, and when cut they will usually show gum underneath. The
surface of protuberances of impietratura-infected fruit is often
a shade of colour different from the rest of the rind (see Figure
84)
Puckered
In reference to leaves infected with infectious variegation
virus. The surface of the leaves appears drawn together into
wrinkles and irregular folds, with puffed areas
Radioactive probe
A nucleic acid that has been made radioactive by one of several
techniques (e.g. nick translation) and is to be used to detect a
complementary nucleic acid sequence
Recombinant plasmid
A bacterial plasmid DNA containing an insert of DNA from a
non-related source, e.g. a plasmid containing an insert of viral
cDNA. It is created by recombinant DNA technology
Restricted endonuclease
Enzymes which recognize and cut double-stranded DNA at specific
sites determined by the sequence of bases at that site
Reverse transcription
The enzyme which accomplishes the enzyme synthesis of a copy DNA
from an RNA template in the presence of a primer and nucleotide
triphosphates under appropriate conditions
Rf (replicative form)
The intercellular form of viral nucleic acid which is active in
replication, e.g. M 13 phase particles contain a single-stranded
DNA circle while the Rf of the same molecule is double-stranded
Ringspot
Circular, ring-like translucent or yellow spots on leaves (Figure
78a) or round spots on fruit (Figure 73). Ringspot is also the
name of a psorosis-like disease which induces these symptoms on
citrus leaves and fruit
RM
Resuspension medium (see TKM buffer)
RNA
Ribonucleic acid. The alternative reservoir of genetic
information to DNA. Viruses have single-stranded or
double-stranded RNA genomes. In organisms, RNA is transcribed
from DNA and is essential for the expression of the genetic
information contained within the DNA. RNA differs from DNA in
having ribose instead of deoxyribose as the sugar moiety in its
nucleotides, and in having uracil instead of thymine as one of
its two pyrimidine bases. RNA, but not DNA, may be degraded by
alkaline hydrolysis
Rugose
As in leaf rugosity, meaning rough or wrinkled, with puffed areas
on the upper side of the leaf (see Puckered)
Scaly bark
The term used for a condition of the bark in psorosis-infected
sweet orange, grapefruit or mandarin scions or rootstocks. The
bark is scaled and exfoliated (see Figures 72a and 72b)
Scaly butt
A term used in Australia indicating exfoliation or scaling of the
bark of the trifoliate orange rootstock due to infection with the
citrus exocortis viroid
Secondary leaves
In contrast to the primary leaves of a germinating seed, the
secondary leaves are the permanent type of leaves with a fixed
morphology. Many seedlings produce both primary and secondary
leaves, but many have just one type
Seed abortion
A term used to describe a condition of seed found in
stubborn-infected fruit. The seed is not truly aborted, but
appears smaller and has a darker purple or brown colour (see
Figure 24b)
Seedling index
Indexing for transmissible pathogens in citrus done primarily
using seedlings of sensitive varieties
Seedling yellows
A yellowing and severe stunting of young inoculated seedlings of
sour orange, lemon or grapefruit associated with infection by
severe citrus tristeza virus isolates
Sequencing
The determination of the order of nucleotides in a DNA or RNA
molecule, or that of amino-acids in a polypeptide chain
Sequencing gel
A long, polyacrylamide slab gel which has sufficient resolving
power to separate single-stranded fragments of DNA or RNA which
differ in length by only a single nucleotide. Electrophoresis is
carried out at high voltage and with the gel in a vertical
position. Urea is usually included in the gel mixture as a
denaturing agent. This prevents internal base pairing within the
single-stranded molecules and ensures that their relative speed
of migration is solely dependent on their length
Shock
As in the shock symptoms of psorosis-A-infected seedlings. After
a seedling is inoculated and cut back, the first growth of a
young shoot may show withering and bending, with loss of leaves.
Ultimately the young shoot will dry up and turn dark brown. This
is the shock symptom of psorosis-A (see Figures 75a and 76)
Shoot tip grafting
A micro-grafting procedure. The meristematic growing tip (the
meristem plus one to three leaf primordia) is excised by cutting
the very young tip using the cutting edge of a razor-blade
mounted in a special handle. This decapitated tip is then grafted
to a very young seedling with the aid of a binocular microscope.
The grafted plant is then grown in test-tubes usually in vitro
and later transplanted or grafted to produce a shoot-tip grafted
plant or tree
Side grafting
A method of inoculation which uses a piece of stem 4-5 mm thick
and 3-4 cm long. A wedge cut is made at the end of this stem
piece; a cut is then made into the seedling to be indexed and the
wedge end is fitted into the cut portion of the stem. The side
graft is then wrapped. This type of graft is preferred when
indexing for stubborn and greening diseases (see Figure 138)
Side shoot
A lateral branch or shoot. The Mexican lime seedling normally
produces many side or lateral branches compared with the sweet or
sour orange seedling which produces few or none
Single leader
In reference to the training of the young growing stem of a
seedling or budded plant, to grow as one shoot. This one shoot is
usually secured or tied to a stake with plant ties (see Figures
47 and 48)
Sleeve
As in a polythene sleeve enclosing side grafts. A polythene bag
has the bottom removed or cut out to resemble the open-ended cut
sleeve of a shirt or coat. The sleeve is placed over a recently
grafted plant and tied at the top and bottom to ensure a moist
chamber for the side grafts
Slipping
As in bark slipping. A condition where the bark readily separates
from the wood of the plant to be grafted. The plant is said to be
"slipping" during the budding operation when a cut is
made into the bark and it readily separates or opens up from the
wood of the stem
Slot-blot
See Dot-blot
Southern blot, Southern transfer
A technique which combines the resolving power of agarose gel
electrophoresis with the sensitivity of nucleic acid
hybridization. DNA fragments separated in an agarose gel are
denatured in situ and then blotted or transferred, usually by
capillary action, from the gel to a nitrocellulose sheet, or
other binding matrix placed directly on top of the gel.
Single-stranded DNA binds to the nitrocellulose and is then
available for hybridization with labelled P32 or by colour
change, in the case of a biotinylated probe. A very sensitive and
powerful technique, it is often described as "blotting"
Stake
A thin stick made of bamboo, wood or metal which is forced into
the soil of a containergrown plant and used to support the
growing plant. The plant is secured to the stake by affixing
special plant ties (see Figure 48)
STE buffer
Sodium chloride, Tris and EDTA (see Part III). Used in
purification of nucleic acids including dsRNA and viroids
Streptavidin
A microbial protein which binds biotin. It is preferred to avidin
because of its more specific binding (see Biotin). In molecular
hybridization, streptavidin reacts specifically with the biotin
molecules fixed on the probe
Stub
As in a rootstock stub. The short projecting portion of the stem
which remains after the rootstock is severed from the growing
scion. The stub is that small portion above the scion which
should be trimmed flush with the scion at a later time
Stylar end
The bottom portion of a hanging fruit
Substrate
The substance acted upon by an enzyme. Usually this substance
contains a chemical which, when acted upon by an enzyme, will
change colour and the colour can be easily seen and measured
Symptomless carrier
Refers to a tree or plant which contains a grafttransmissible
pathogen but shows no symptoms. Examples of a symptomless carrier
would be a sweet orange scion on a sour orange rootstock
containing tatterleaf virus and/or the exocortis viroid (see
Invisible pathogen)
Synergism
The joint action of agents such as two viruses acting together
and inducing a more intense symptom in an indicator plant than by
either agent acting alone
TAE buffer
Tris, sodium acetate.3H2O and sodium EDTA
Tattered
Ripped, torn or shredded in a non-uniform pattern. The irregular
leaf pattern associated with tatterleaf virus suggests an
irregularly torn leaf
Template
The molecule which is acted upon or copied, as in the production
of cDNA probes by reverse transcriptase
Template primer
See Primer and Template
Thermotherapy
Treatment of budwood or plants by heat to effectively
eliminate internal pathogens
TKM buffer
A buffer with Tris, KCl and MgCl2
TME
As in TME Tris buffer. Tris, MgCl2 and EDTA
Trapping antibodies
Antibodies used to coat the wells of ELISA plates (the first
layer in sandwich assay procedures). Antibodies adsorbed to the
solid surface of the plate trap related antigens from the sample
extracts placed in the plate for testing
Triturating
Rubbing or grinding, as with tissue ground with pestle and mortar
Undulating depressions
As in symptoms induced by the cachexia viroid on the trunks of
susceptible varieties. The depressions are not deep and sharp but
rather have soft bends and undulations (see Figure 44)
Vein corking
A thick, cork-like growth on leaf veins induced by severe
isolates of CTV or by boron deficiency (see Figure 8)
Viroid
A small molecular RNA, transmissible in plants, without an
extracellular protein component or translation capacity and which
can be pathogenic. It is composed of naked, single-stranded
low-molecular RNA (MW 80 000-130 000) which utilizes only host
components for its replication. They exist in solution as
rod-like structures arranged in a series of short base-paired and
non-base-paired regions
Virus
Viruses are macro-molecular transmissible agents capable of
causing diseases in plants and animals. They are small enough to
pass through a millipore filter of 0.2 microns. They have been
considered to be either living organisms or simply a molecular
complex of nucleic acids and proteins capable of multiplication
in living cells. Viruses are characterized by a core of nucleic
acids with a genome less than 3 x 108 daltons in
weight surrounded by a protein coat which can induce formation of
antibodies.