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The Science of Plants in Agriculture
Plant Science 102
Study Notes for Test #3
Genes, Mendel and Meiosis
What is a gene? A gene is a DNA sequence coding for a single polypeptide, t-RNA or
r-RNA.
The central dogma of genetics:
DNA
RNA
Nucleic
acid
Nucleic
acid
protein
Amino
acid
What are the components of a gene? Components of a gene include the specific gene
sequence and a promoter region. Genetic variation is due to changes (mutations)
in either the specific gene sequence or the promoter region.
What are alleles? Different forms of a gene are called alleles.
What is the difference between homozygous and heterozygous alleles? A plant can be
either homozygous or heterozygous for a specific gene. It is common to use
letters to symbolize genes, and upper and lower letters to distinguish different
alleles (i.e. A or a). Homozygous refers to a situation where all possible copies
(usually two) are the same (i.e. AA or aa). Heterozygous refers to a situation
where there are different alleles of a gene (i.e. Aa).
What is the difference between a recessive allele and a dominant allele? If phenotypic
expression is determined by only one copy of an allele (i.e. Aa) the allele (A) is
said to be dominant over the resessive allele (a).
What are the two divisions of meiosis? Reduction division (Meiosis I): is the
separation of homologous chromosomes. This is the division is identical to
mitosis and produces diploid cells. Equational division (Meiosis II): is the
separation of sister chromosomes. This results in producing four cells, having
half the number of chromosomes as the original cell so results in haploid cells.
What are Mendel’s two laws of genetics? 1 The law of segregation describes where a
pair of different alleles (i.e. Aa) for a given gene segregates with equal frequency.
So there would be an equal frequency of A gametes (haploid cells resulting from
meiosis) as there would be a gametes. 2 The law of independent assortment
where allelic pairs of genes will segregate independently. So if a plant is
heterozygous for two trails (AaBb) will have equal frequency of the four possible
gametes after meiosis. What would the gametes be? AB, Ab, aB, and ab.
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Plant Breeding
What is the difference between a self-pollinating species and an out-pollinating
species: Self-pollinators are tolerant to inbreeding; have few deleterious recessive
alleles; show little heterosis (hybrid advantage); and tend to have closed flowers
to promote self fertilization. Out-pollinators are intolerant to inbreeding (and
show a decline in performance when self pollinated); have many recessive
deleterious alleles; show high heterosis; and have open flowers or other systems
to promote cross fertilization.
What types of cultivar can be developed? Pure line (or homozygous) cultivar; outpollinating cultivar; hybrid cultivar; hybrid cultivar.
What are the major characteristics of species from which the following cultivar types
are developed? And, provide an example crop where that cultivar type is
developed.
Cultivar type
Characteristics
Pure line
Self-pollinating mating system, annual
seed production, able to develop
homozygous lines.
Out-pollinating
Hybrid
Clone
Out-pollinating species, annual,
biennial or perennial, highly
heterozygous and heterogeneous.
Out-pollinating, annual or biennial,
must show high heterosis, usually
must have system of hybrid seed
production.
Out-pollinator, highly heterozygous
and homogeneous, must allow asexual
propagation.
Crops
wheat,
barley,
pea,
canola
alfalfa,
corn, red
clover
corn,
tomato,
onion,
sunflower
potato,
apple,
bulbs,
strawberry
What factors would you consider when designing a breeding scheme? People, politics,
economics, future trends in agriculture, diseases and other pests.
How can breeders develop crops with improved profitability? By (1) increasing
harvestable product over the same area; (2) increase the inherent quality of the
harvested product; or (2) reduce the costs of production (i.e. improved disease
resistance so avoid pesticide costs) while maintaining high yield and quality.
What are the two operations in cultivar development? (1) Produce (or identify) genetic
variability, and (2) select desirable recombinants (lines) from the genetically
variable population.
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How do breeders produce genetic variability? (1) Artificial hybridization between
chosen parents; (2) induced mutation; (3) Interspecific (wide) crosses; (4) genetic
engineering.
What are most commonly used mutagens in plant breeding? Chemical, most
commonly ethyl-methane-sulphonate (EMS), or radiation (X-rays or gamma
rays).
What types of mutation can occur? Genome mutation: which are changes in
chromosome number due to addition or loss of whole chromosomes. Structural
changes in chromosome: including translocations, inversions, deletions and
duplications. Gene mutation (point mutation): that results in a change in a single
gene. Extra nuclear mutation: which is a mutation in one of the cytoplasmic
organelles.
What would be the genotypic and phenotypic segregation in the F2 generation after a
cross between a tall two-row barley (TTss) and a short six-row barley (ttSS),
given that the tall gene (T) gene is dominant to the short gene (t) gene and the
siz-row gene (S) is dominant to the two-row gene (s)?
tall, 2-row (TTss) x dwarf, 6-row (ttSS)
When this cross is made, the F1 shows both dominant characteristics, tall and 6-row. The
F1 individuals produce equal frequencies of four kinds of gametes during meiosis
(Mendel’s second law – above) (TS, Ts, tS, and ts). An easy way to illustrate the possible
combination of F2 progeny is using a punnet square, where the four gamete types from
one parent are listed in a row along the top, and the four kinds from the other parent are
listed in a column down the left hand side. The 16 possible genotype combinations are
then obtained by filling in the square, i.e.:
Gametes from
female parent
TS
Ts
tS
ts
TS
TSTS
TsTS
tSTS
tsTS
Gametes from male parent
Ts
tS
TSTs
TStS
TsTs
TstS
tSTs
tStS
tsTs
tstS
ts
TSts
Tsts
tSts
tsts
This also can be written as:
Gametes from
female parent
TS
Ts
tS
ts
TS
TTSS
TTSs
TtSS
TtSs
Gametes from male parent
Ts
tS
TTSs
TtSS
TTss
TtSs
TtSs
ttSS
Ttss
ttSs
ts
TtSs
Ttss
ttSs
ttss
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Collecting the like genotypes we get the following frequency of genotypes:
TTSS ; 2TTSs ; TTss ; 2 TtSS; 4 TtSs; 2 Ttss; ttSS; 2 ttSs; ttss,
and with the frequency of phenotypes:
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3
3
1
T_S_
T_ss
ttS_
ttss
= tall and 6-row
= tall and 2-row
= short and 6-row
= short and 2-row.
If a plant breeder makes a cross between a tall homozygous barley parent (TT) and a
short homozygous (dwarf) barley parent (tt). Where the tall gene (T) is
completely dominant over the recessive dwarf gene (t). If the first generation
(F1) is self pollinated and 800 second generation (F2) plants were grown out in
the field. At harvest all the dwarf plants (short) are discarded and a single seed
from each of the tall selected plants were grown out in the F3 generation. How
many of the F3 plants would you expect to be short (dwarfs)?
Well, only 200 (¼) of the F2 plants will be recessive short (tt) and these are discarded.
The rest (600, made up from 200 TT and 400 Tt) are tall as so selected. As
already stated, from the ‘selected lines 400 would be heterozygous tall (Tt) and
200 would be homozygous tall (TT). All TT would remain TT, while one quarter
(¼ or 100) of the heterozygous (Tt) plants would be short in the F3 generation. So
the answer would be 100 plants
What are the terms in the response to selection? The response to selection is - iσh2 ,
where i is the intensity of selection, σ is the phenotypic standard deviation, and h2
is the heritability.
What is heritability? Heritability is the proportion of total variance that is genetic. It is
calculated by dividing the genetic variance by the phenotypic (or total) variance.
The phenotypic variance for seed weight in an F2 population is 200 kg2 and the genetic
variance in that population is 150 kg2. What is the heritability for seed weight?
Heritability is genetic variance/phenotypic variance = 150/200 = 0.75.
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Plant diseases and insects
Which pests affect crop plants? Air-borne fungi, soil-borne fungi, bacteria, viruses,
eelworm, insects, others include mammals. Of course weeds are also a pest to
crop plants so could be included here.
What are the major characteristics of the following plant pests?
Pest type
Air-borne fungi
Soil-borne fungi
Bacteria
Virus
Insects
Description
Spore forming, microscopic air-borne organisms that are usually
pathotype differentiated. Spread rapidly.
Spore forming, microscopic to large soil-borne organisms that
have log persistence in soils. Spread slowly and usually manassisted.
Microscopic prokaryotic rod-like organisms. Little pathotype
differentiation. Spread slowly and man assisted.
Single stranded RNA (in plants). Unable to multiply outside host
(obligate). Many pathotype differentiated. Spread rate is related
to vector (insects = quick; mechanical = slow).
Many insects have a wide host range but some are species
specific. Infestations tend to be seasonal. Can spread rapidly
(winged insects) or slowly (non-flyers).
Provide examples of the following pest types.
Pest type
Air-borne fungi
Soil-borne fungi
Bacteria
Virus
Insects
Examples
Potato early and late blight, wheat, barley & maize rusts, powdery
mildew (many crops), cereal smuts, rice blast.
Potato wart, Brassica club root, Many Vertillilium and Fusarium
wilts, cereal take-all.
Potato ring-rot; vascular wilts of many crops; ratoon in sugar
cane; cotton blackarm.
Potato virus LR, X, Y; tobacco mosaic, barley yellow dwarf,
many clonal fruit viruses.
Colorado potato beetle, wheat Hessian fly, many aphids, cabbage
seedpod weevil.
What effect do plant pests have on our crops? They reduce harvestable yield and reduce
end-use quality.
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In what sector of the ‘disease triangle’ might you expect to have disease?
Susceptible Host
Favorable
environment
Pathogen
Disease
Given that plants are resistant to a pathotype differentiated disease because they have
at least one copies of dominant resistance gene (locks), which can only be
overcome, and hence the plant become susceptible to the disease, if the disease
pathotype has two copies of the appropriate recessive virulence gene (keys).
Given that plant resistance genes are donated as capital letters (A, B and C)
with susceptible genes as lower case letters (a, b, or c), and that disease virulent
genes are donated as a’, b’ or c’ with disease avirulent genes donated as A’, B’,
or C’, would the following plants be resistant or susceptible to the associated
disease pathotype?
Plant phenotype
No resistance genes
A_ bb cc
A_ bb C_
A_ B_ cc
Disease genotype
No virulence genes
A’a’ b’b’ c’c’
a’a’ B’B’ c’c’
A’A’ b’b’ c’c’
Plant response
Susceptible
Resistant
Susceptible
Resistant
With respect to disease resistance what is meant by hypersensitivity? Hypersensitivity
is a plant reaction in response to being infected by a disease. On infection, plant
cells around the infection site die and hence the pest can’t spread through the
plant.
Explain the following plant resistance mechanisms: Antibiosis describes a form of
plant resistance that reduces, survival, growth, development, or reproduction of
pests feeding on the plant; Antixenosis describes a form of plant resistance that
reduces pest preference or acceptance of the plant; Escape describes a
mechanism such that a plant morphology avoid a disease.
What is the difference between vertical and horizontal plant resistance? Vertical plant
resistance is when the disease is controlled by a single gene, whereas horizontal
disease resistance is controlled by many different genes.
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Weed control
How do weeds impact crop plants? Weeds cause yield loss as they compete for:
interceptable light, water, and nutrients. Weeds ca also harbor pests, and reduce
crop quality through weed seed contamination.
How can weeds be controlled in crops? Mechanically by pre-plant cultivation, inter-row
cultivation, or hand weeding. Culturally mainly by inter-cropping. Biologically
usually by insects, and chemically using herbicides.
Describe the mode of action of the following herbicide groups.
Group Description
Acetyl CoA Carboxytase (ACCase) inhibitors, binds to ACCase and disrupts fatty
1
acid synthesis, which leads to membrane degeneration.
Acetolactate Synthase (ALS) inhibitors, binds to ALS and disrupts synthesis of
2
branched amino acids.
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Tubulin inhibitors, interferes with cell division.
Synthetic auxins, upsets plant growth regulator balance by mimicking an increase of
auxins.
Triazines & Ureas. Binds to a pigment in photosystem II and disrupts
5,6&7
photosynthesis.
5-enol Pyruvilshikimate-3-phosphate synthate (EPES) synthase inhibitor, binds to
9 EPES synthase and disrupts pathway, which is responsible for producing the
precursors of aromatic amino acids.
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Why is herbicide mode of action important when considering herbicides resistant
weeds? Herbicide resistant weeds can arise by repeated use of herbicides within
the same group and hence with the same mode of action.
What is meant by the term "plant back restriction"? Several herbicides have a long
residual in soils and can cause damage to certain crops if they are planted into
treated soils before complete herbicide breakdown.
What does IPM stand for? Integrated Pest Management.
What techniques are utilized in IPM systems? Biological control where natural pest
predators and parasites are encouraged on introduced. Biopesticides that are
naturally occurring pesticides. Cultural control including planting resistant
cultivars, planting trap crops, intercropping, having an appropriate and broad
crop rotation. Mechanical & Physical control using cultivation & tillage.
Reproductive & Genetic control where harmful pest genes are introduced (i.e.
mass release of sterile insects), and chemical control where pesticides are used in
an appropriate manner.
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Biotechnology – molecular markers and plant transformation
What are bacteriophages? Viruses that parasitizes bacteria and pass DNA into bacteria
cells.
What are restriction enzymes? Restriction enzymes are produced by bacteria as a
defense mechanism against phages. Enzymes act like scissors by cutting phage
DNA at specific sites.
What are molecular markers used for? Marker assisted selection, in many cases (in
plant breeding) it can be difficult to evaluate characters, and it can be easier (and
quicker) to select according to a marker located closely on the same chromosome
as a gene of interest. DNA finger printing can be used to identify genotypes (or
cultivars), and can be used to secure proprietary ownership. This can also be used
to select parents with known genetic distance. Cytological where information can
be obtained regarding chromosomes (mainly in interspecific hybrids), and
Saturated gene mapping.
What can be achieved by plant transformation techniques that are not possible with
traditional plant breeding methods? Using these techniques it is possible to
transfer single genes from other species and non-plants into plant and to have
these transgenes expressed and to function successfully. This effectively
bypasses natural barriers which limit sexual gene transfer and allows breeders to
utilize gene from completely unrelated species or to create new variability beyond
that currently available in germplasm.
What plant characters have been developed through plant transformation? Herbicide
resistance (Roundup and Liberty Link) , insect resistance (mainly B.t. resistance),
virus resistance, modifies quality (starch, amino acids, fatty acids, and fruit
ripening), salt tolerance, and pharmaceuticals (edible viruses, anti-coagulants).
What vectors can be used to insert genes into plants? Agrobacterium tumefacians, and
particle gun (could also include viruses and electroporation).
How does Agrobacterium tumefacians cause crown galls in plants? A. tumefacians
cells contain Ti (tuber inducing) plasmids that insert T-DNA into the DNA of the
host plant and contains genes that cause rapid cell division expressed as crown
galls.
Describe (briefly) the process of plant transformation.




Find a gene of interest, produce a suitable construct and clone it.
Develop a mechanism to insert construct into a plants DNA.
Regenerate a whole plant from a single transformed cell.
Check functionality of transformants.