Chapter 14:  Propagation

In this chapter you will learn how phenotypic changes occur under domestication. The concept and importance of germplasm banks is presented. The topic of transgenic plants is framed by a discussion of the pros and cons of transgenic plants. The chapter discusses an overview of the principal types of vegetative propagation such as cuttings and grafting. Finally, micropropagation is outlined as a method of asexual propagation that utilizes sterile culture techniques.

At the end of this chapter the successful student will be able to

• Recognize the breeding methods used for self pollinating crops
• Recognize the breeding methods used for cross pollinating crops
• Recognize the significance of germplasm banks in crop improvement programs
• Recognize the major steps involved in transgenic plants
• Recognize the steps involved in seed propogation
• Recognize different methods of asexual plant propagation
  • Plant cuttings
  • Layering (Tip & Air)
  • grafting (stem cuts (Scion) & root cuts)
  • tissue culture
• Assignments
• Glossary

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Self pollinating crops

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.

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.

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Cross pollinating crops

• 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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Germplasm banks

• Germplasm Collection and Gene Banks
  • 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.
• 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

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Steps involved in transgenic plants

• Plant Breeding Using Sexually Incompatible Germplasm
  • 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
  • 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
  •  
• Making A Transgenic Plant
  • 
  • 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.

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Seed propogation

• 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.
• 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.

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Asexual plant propagation

• 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 (see image at left)
  • 
• 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
• 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.
  • 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.

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Last modified: October 08, 2004 by Cynthia Herbrandson  © Copyright 1999, Kellogg Community College. All rights reserved.