1. Plant Breeding and Propagation
Chapter 14
Lecture Outline
Plant Breeding and Propagation
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2. Outline Crop Plant Evolution Plant Breeding Plant Propagation
Breeding Methods Using Sexually Compatible Germplasm
Breeding Methods Using Sexually Incompatible Germplasm
Plant Propagation
Seed Propagation
Asexual Plant Propagation

3. 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

4. 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.

5. Crop Plant Evolution Origins of Agriculture
Regions of domestication

6. Potato tuber diversity
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 plant- breeding 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

7. 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.
Significant inbreeding
Wheat, rice, oats, barley, peas, tomatoes, peppers, some fruit trees: apricots, nectarines, citrus
Pure-line selection:
Seeds collected from several plants.
Seeds from individual plant grown in same row.
Most desirable row selected.

8. Plant Breeding Breeding Methods Using Sexually Compatible Germplasm
Strategies
Cross-pollinating plant - Must be fertilized from other individuals
Tend to be highly heterozygous
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.
Seeds from the best plants chosen and propagated, for many generations.

9. 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.
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.

10. 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.

11. 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

12. 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.
Sticky ends base- pair with tail of other fragments cut with same enzyme.

13. 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.

14. Plant Breeding Breeding Methods Using Sexually Incompatible Germplasm
To clone gene:

15. 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:
Agrobacterium tumefaciens used to insert transfer DNA (T-DNA) that contains gene of interest from its plasmids into plant’s chromosomes.
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.

16. Plant Breeding Breeding Methods Using Sexually Incompatible Germplasm
Transformation of plant by Agrobacterium tumefaciens:

17. Plant Breeding Breeding Methods Using Sexually Incompatible Germplasm
Steps in making a transgenic plant:

18. 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?
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.

19. 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

20. 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

21. 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

22. 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.

23. 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.
Tip layering
Blackberries, boysenberries
Air layering - Branch or main stem wounded or girdled to produce roots.
Tropical trees and shrubs

24. 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.

25. 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

26. 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.

27. Review Crop Plant Evolution Plant Breeding Plant Propagation
Breeding Methods Using Sexually Compatible Germplasm
Breeding Methods Using Sexually Incompatible Germplasm
Plant Propagation
Seed Propagation
Asexual Plant Propagation