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Plant Breeding and Propagation
Outline
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Crop Plant Evolution
Plant Breeding
 Sexually Compatible Germplasm
 Sexually Incompatible Germplasm
- Bacterial Gene Cloning
 Transgenic Plants
 Seed Propagation
 Asexual Plant Propagation
Crop Plant Evolution
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Approximately 200,000 species of flowering
plants.
 Six species provide 80% of calories
consumed by humans worldwide.
- Wheat, Rice, Corn, Potato, Sweet Potato,
and Cassava.
 Eight additional plants complete the list of
major crops grown for human consumption.
- Sugar Cane, Sugar Beet, Bean, Soybean,
Barley, Sorghum, Coconut, and Banana.
Origins of Agriculture
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First plants were domesticated in the Near
East around 10,000 years ago.
 Root crops and legumes were
domesticated 2,000-3,000 years ago.
- Plants for forage, decoration, and drugs
were first domesticated about 2,000
years ago.
Regions of Domestication
Plant Breeding
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Plant breeding is accelerated evolution
guided by humans rather than nature.
 Breeders replace natural selection with
human selection to modify plant genetics.
- Primary goal of plant-breeding programs
is commonly improved yield.
 Disease resistance, pest resistance,
and stress tolerance contribute to
yield.
Plant Breeding
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Strategies
 Self-Pollination - Plants are capable of
fertilizing themselves.
- Tend to be highly homologous.
 Significant inbreeding
 Wheat, Rice, Peas, Tomatoes
- Pure-Line Selection - Collecting seeds
from several plants, growing seeds from
an individual plant in a row, and then
selecting the most desirable row.
Plant Breeding
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Cross-Pollination - Plants must be fertilized
from other individuals.
- Tend to be highly heterozygous.
 Corn, Rye, Alfalfa and most Fruit.
- Mass Selection - Many plants from a
population are selected, and seeds from
these plants are then used to create the
next generation.
 Seeds from the best are used to
create the next generation.
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Plant Breeding Using
Sexually Compatible Germplasm
Outcrossing in cross-pollinated crops often
results in hybrid vigor (heterosis).
Cross-pollinated plants tend to exhibit
inbreeding depression.
 Modern breeders cross pollinate to delete
deleterious alleles.
- Heirloom Varieties are grown as openpollinated populations.
 Genetic variability allows crop
production under different
environmental conditions.
Germplasm Collection and Gene Banks
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A plant’s germplasm is the sum total of its
genes.
 Current agricultural varieties are often
genetically uniform, and thus may not be
good sources of genetic variability.
 Gene Banks have been established to
meet current and future demands of plant
genetic diversity.
- Seeds or other propagules are put into
long-term storage.
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Plant Breeding Using
Sexually Incompatible Germplasm
Protoplast Fusion
 Cells of each species are grown in a liquid
nutrient solution.
- Cell walls are chemically stripped to
produce protoplasts.
 Protoplasts of two species are mixed
together and stimulated with the aid of
an electric current or chemical
solution, to fuse with each other.
 Few successes
Plant Breeding Using
Sexually Incompatible Germplasm
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Gene Splicing and Transgenic Plants
 Transgenic Plants are produced by
inserting genes from one plant into
another.
- Recombinant DNA
 Restriction Enzymes cut DNA into
fragments with sticky ends.
Plant Breeding Using
Sexually Incompatible Germplasm
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Plasmids are commonly used as cloning
vectors.
 Small circular bacterial DNA.
After cloning, the gene is inserted into plant
cells via transformation.
 Agrobacterium tumefaciens
 Particle Guns
Bacterial Gene Cloning
Bacterial Gene Cloning
Making A Transgenic Plant
Transgenic Plants
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Pros
 Transgenic crops are often environmentally
friendly.
- Farmers can use fewer pesticides.
Cons
 Effect on non-target organisms, such as
insects, in the food web.
 Movement of herbicide resistance to weeds.
Seed Propagation
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Hybrid varieties are often grown from seed
produced by crosses between two inbred
parents.
 Inbred line varieties are typically grown
from seed and allowed to self-pollinate.
Mature seeds are harvested and stored in a
controlled environment.
 Viability is best when seeds are maintained
in cool, dry storage.
Seed Propagation
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In preparation for planting, seeds may be
dusted with a protectant, such as a fungicide.
Seeds must be planted in a suitable bed.
 Moist soil to allow seeds to imbibe water.
- Dry enough to maintain suitable oxygen
levels.
Asexual Plant Propagation
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Cuttings
 Produce adventitious roots.
- Cells near the wound must dedifferentiate
and create a new meristematic region.
Layering
 Tip Layering - Bend tips until they touch the
ground, and then cover them with soil.
 Air Layering - Wounding or Girdling to
produce roots.
Cuttings and Air Layering
Cuttings and Air Layering
Asexual Plant Propagation
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Grafting
 Segments of different plants are connected
and induced to grow together as one plant.
- Scion - Top section of a graft.
- Rootstock - Bottom section of a graft.
 Successful grafting depends on good
contact between the vascular cambium
of the scion and that of the rootstock.
Asexual Plant Propagation
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Micropropagation
 Grow and maintain plants in a disease-free
status in test tubes.
- Grown in-vitro in sterile medium.
- Relies on totipotency of plant cells.
 Capacity of a cell to give rise to any
structure of a mature organism.
Asexual Plant Propagation
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Micropropagation usually begins with the
establishment of an explant in tissue culture.
 Plant parts are disinfested.
 Induced to develop multiple shoots.
 Microshoots separated and placed in a new
medium by subculturing.
 Root Formation
 Transfer plants back to outdoor
environment.
Review
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Crop Plant Evolution
Plant Breeding
 Sexually Compatible Germplasm
 Sexually Incompatible Germplasm
- Bacterial Gene Cloning
 Transgenic Plants
 Seed Propagation
 Asexual Plant Propagation
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