Chapter 14
Lecture Outline
Plant Breeding and
Propagation
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Outline

Crop Plant Evolution

Plant Breeding
• Breeding Methods Using Sexually Compatible
Germplasm
• Breeding Methods Using Sexually Incompatible
Germplasm

Plant Propagation
• Seed Propagation
• Asexual Plant Propagation
Crop Plant Evolution

Approximately 250,000 living 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 list of major crops
grown for human consumption.
–

Sugar cane, sugar beet, bean, soybean, barley, sorghum,
coconut, and banana
Plants domesticated by altering them genetically.
• Domesticated plant - Reproductive success depends
on human intervention.
• Ongoing evolutionary process
Crop Plant Evolution


Origins of Agriculture
Agricultural practices arose independently in
many parts of world.
People began to domesticate plants in Near East
(Iran) around 10,000 years ago.
• Domestication in Asia and New World 1,000 to 3,000
years later

First crops were cereal grains.
• Root crops and legumes domesticated 1,000 to 2,000
years later.
• Followed by vegetables, then oil, fiber and fruit crops
• Plants for forage, decoration, and drugs first
domesticated about 2,000 years ago.
Crop Plant Evolution
Origins of Agriculture
Regions of domestication
Plant Breeding

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 plantbreeding programs is
improved yield, with disease
resistance, pest resistance,
and stress tolerance
contributing to yield.
Genetic variation provides
foundation for improving
plants through breeding.
Potato tuber diversity
Plant Breeding
Breeding Methods Using Sexually Compatible Germplasm

Strategies
• Self-pollinating plant - Capable of fertilizing itself
–
Tend to be highly homogeneous - Genes come from
same parent.
o Significant inbreeding
« Wheat, rice, oats, barley, peas, tomatoes,
peppers, some fruit trees: apricots, nectarines,
citrus
–
Pure-line selection:
o Seeds collected from several plants.
o Seeds from individual plant grown in same row.
o Most desirable row selected.
Plant Breeding
Breeding Methods Using Sexually Compatible Germplasm

Strategies
• Cross-pollinating plant - Must be fertilized from
other individuals
–
Tend to be highly heterozygous
o Corn, rye, alfalfa, clover and most fruit, nuts and
vegetables
–
Mass selection - Many plants from a population
selected, and seeds from these plants used to create
next generation.
o Seeds from the best plants chosen and propagated,
for many generations.
Plant Breeding
Breeding Methods Using Sexually Compatible Germplasm

Strategies
• Outcrossing in cross-pollinated crops often results
in hybrid vigor (heterosis).
• Self pollination of cross-pollinating plants results in
inbreeding depression.
–
–
–
Due to expression of deleterious recessive alleles
Modern breeders force self-pollination in cross-pollinated
species to create inbred lines in which deleterious alleles
eliminated.
o Selected inbred lines crossed to produce hybrid seed.
« Successful in corn
Heirloom varieties grown as open-pollinated populations.
« Genetic variability allows crop production under
different environmental conditions.
Plant Breeding
Breeding Methods Using Sexually Compatible Germplasm


Impossible to improve population if there is
no genetic variability for trait.
Germplasm - Sum total of a plant’s genes
• Current agricultural varieties are often genetically
uniform, and thus may not be good sources of
new genetic variability.
–
Homogeneity makes them vulnerable to pest outbreaks.
• Gene banks established to meet current and
future demands of plant genetic diversity.
–
Seeds or other propagules put into long-term storage.
Plant Breeding
Breeding Methods Using Sexually Incompatible Germplasm


Using sexually incompatible germplasm Across species boundaries
Protoplast fusion
• Cells of each species grown in liquid nutrient
solution.
• Cell walls chemically stripped to produce protoplasts.
• Protoplasts of two species mixed together and
stimulated, with aid of an electric current or chemical
solution, to fuse with each other.
• Grow hybrid fusions by tissue culture.
–
–
Form somatic hybrids - New plants that carry genes from
two distantly related species
Few successes
Plant Breeding
Breeding Methods Using Sexually Incompatible Germplasm

Transgenic plants - Produced by inserting
genes from virtually any organism into plants
• Recombinant DNA
technology used.
–
Restriction enzymes
from bacteria cut DNA
into fragments with
one DNA strand
longer than other,
creating sticky ends.
o
Sticky ends basepair with tail of other
fragments cut with
same enzyme.
Plant Breeding
Breeding Methods Using Sexually Incompatible Germplasm

Transgenic plants
• Plasmids commonly used as cloning vectors.
–
Plasmid - Small circular bacterial DNA capable of
independent replication
• To clone gene:
–
A bacteria plasmid and a gene of interest from foreign
DNA cut by restriction enzymes.
–
Gene becomes inserted into plasmid.
–
Transformation - Bacteria, Escherichia coli, stimulated to
take up plasmid.
–
Bacteria multiply.
Plant Breeding
Breeding Methods Using Sexually Incompatible Germplasm
• To clone gene:
Plant Breeding
Breeding Methods Using Sexually Incompatible Germplasm

After cloning, gene inserted into plant cells via
transformation:
• Plasmids removed from E. coli.
• Same restriction enzymes used to cut out gene.
• Plant is transformed with new gene.
–
Two techniques used:
o Agrobacterium tumefaciens used to insert transfer DNA
(T-DNA) that contains gene of interest from its plasmids
into plant’s chromosomes.
o
Particle guns - Shoot DNA into plant tissue
« Tungsten or gold pellets coated with cloned gene shot
into plant cells.
« Process of how this works is a mystery.
Plant Breeding
Breeding Methods Using Sexually Incompatible Germplasm

Transformation of plant by Agrobacterium
tumefaciens:
Plant Breeding
Breeding Methods Using Sexually Incompatible Germplasm

Steps in making a transgenic plant:
Plant Breeding
Breeding Methods Using Sexually Incompatible Germplasm

Pros of transgenic plants
• Transgenic crops environmentally friendly.
–

Farmers can use fewer and less noxious chemicals for
crop production.
Cons of transgenic plants
• Unanswered questions need to be answered:
– Effects on non-target organisms, such as beneficial insects,
worms or birds?
– Effects on humans eating transgenic crops?
o Allergic reaction to transgene protein product?
– Movement of herbicide-resistance genes into weeds?
– Evolution of insects to be able to eat transgenic plants?
– Encouragement of farmers to head farther away from
sustainable agricultural farming.
Plant Propagation
Seed Propagation

Hybrid varieties often grown from seed
produced by crosses between two inbred
parents.
• Produce highly variable population
• Corn

Inbred line varieties typically grown from
seed and allowed to self-pollinate.
• Produce plants nearly identical to parents
Plant Propagation
Seed Propagation

Mature seeds harvested and stored in a
controlled environment.
• Viability best when seeds maintained in cool, dry
storage.

In preparation for planting, seeds dusted
with a protectant, such as fungicide.

Seeds planted in a suitable bed.
• Moist soil to allow seeds to imbibe water
• Dry enough to maintain suitable oxygen levels
Plant Propagation
Asexual Plant Propagation

Asexual propagation - Uses vegetative parts

Crown division - Plant separated into several
pieces, each with crown portion and roots.
Crown division
of daylily
Plant Propagation
Asexual Plant Propagation

Cuttings - Propagation from parts of plants
• If stem used, produces adventitious roots.
–
Cells near the wound must dedifferentiate and create a
new meristematic region.
–
Sometimes rooting is stimulated by auxin.
• Identical copies of
valuable plants
can be made.
• Disadvantage -
Diseases carried
by mother plant
propagated.
Plant Propagation
Asexual Plant Propagation

Layering
• Works well for some
plants that are not easy
to propagate by cuttings.
• Tip layering - Tips bent
until touch ground, and
then covered with soil.
–
–
Roots form on buried stem.
Blackberries, boysenberries
Tip layering
• Air layering - Branch or main stem wounded or
girdled to produce roots.
–
Tropical trees and shrubs
Plant Propagation
Asexual Plant Propagation

Grafting - Segments of different
plants connected and induced to
grow together as one plant.
• Fruit and nut trees
• Scion - Top part of graft
• Rootstock - Bottom portion
–
Selected for winter hardiness,
dwarfing and disease resistance
• Success depends on good contact
between vascular cambium of scion
and that of rootstock.
Plant Propagation
Asexual Plant Propagation

Micropropagation - Grow and maintain plants
in a disease-free status in test tubes
• Advantages:
–
Can grow large numbers of plants in small area.
–
Minimal maintenance required.
–
Rapid multiplication
• Grown in-vitro in sterile medium and maintained in
controlled environments
• Relies on totipotency of plant cells
–
Totipotency - Capacity of a cell to give rise to any
structure of a mature organism
Plant Propagation
Asexual Plant Propagation

Micropropagation begins with establishment of
explants in tissue culture.
• Explant - Excised piece of stem or leaf tissue
• Plant parts disinfested and inserted into growth
medium in test tubes.
• Induced to develop multiple shoots = microshoots
• Microshoots separated and placed in new medium by
subculturing.
• Roots induced by transferring to rooting medium.
• Plants transferred back to outdoor environment.
Review

Crop Plant Evolution

Plant Breeding
• Breeding Methods Using Sexually Compatible
Germplasm
• Breeding Methods Using Sexually Incompatible
Germplasm

Plant Propagation
• Seed Propagation
• Asexual Plant Propagation