Plant Breeding and Propagation Outline Crop Plant Evolution
Plant Breeding and Propagation
Outline • • Crop Plant Evolution Plant Breeding v Sexually Compatible Germplasm v Sexually Incompatible Germplasm - Bacterial Gene Cloning v Transgenic Plants v Seed Propagation v Asexual Plant Propagation
Crop Plant Evolution • Approximately 200, 000 species of flowering plants. v Six species provide 80% of calories consumed by humans worldwide. - Wheat, Rice, Corn, Potato, Sweet Potato, and Cassava. v 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 • First plants were domesticated in the Near East around 10, 000 years ago. v Root crops and legumes were domesticated 2, 000 -3, 000 years ago. - Plants forage, decoration, and drugs were first domesticated about 2, 000 years ago.
Regions of Domestication
Plant Breeding • Plant breeding is accelerated evolution guided by humans rather than nature. v Breeders replace natural selection with human selection to modify plant genetics. - Primary goal of plant-breeding programs is commonly improved yield. Ø Disease resistance, pest resistance, and stress tolerance contribute to yield.
Plant Breeding • Strategies v Self-Pollination - Plants are capable of fertilizing themselves. - Tend to be highly homologous. Ø 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 v Cross-Pollination - Plants must be fertilized from other individuals. - Tend to be highly heterozygous. Ø 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. Ø Seeds from the best are used to create the next generation.
• • 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. v Modern breeders cross pollinate to deleterious alleles. - Heirloom Varieties are grown as openpollinated populations. Ø Genetic variability allows crop production under different environmental conditions.
Germplasm Collection and Gene Banks • A plant’s germplasm is the sum total of its genes. v Current agricultural varieties are often genetically uniform, and thus may not be good sources of genetic variability. v 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 v Cells of each species are grown in a liquid nutrient solution. - Cell walls are chemically stripped to produce protoplasts. Ø 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 • Gene Splicing and Transgenic Plants v Transgenic Plants are produced by inserting genes from one plant into another. - Recombinant DNA Ø Restriction Enzymes cut DNA into fragments with sticky ends.
Plant Breeding Using Sexually Incompatible Germplasm • • Plasmids are commonly used as cloning vectors. v Small circular bacterial DNA. After cloning, the gene is inserted into plant cells via transformation. v Agrobacterium tumefaciens v Particle Guns
Bacterial Gene Cloning
Bacterial Gene Cloning
Making A Transgenic Plant
Transgenic Plants • • Pros v Transgenic crops are often environmentally friendly. - Farmers can use fewer pesticides. Cons v Effect on non-target organisms, such as insects, in the food web. v Movement of herbicide resistance to weeds.
Seed Propagation • • Hybrid varieties are often grown from seed produced by crosses between two inbred parents. v Inbred line varieties are typically grown from seed and allowed to self-pollinate. Mature seeds are harvested and stored in a controlled environment. v Viability is best when seeds are maintained in cool, dry storage.
Seed Propagation • • In preparation for planting, seeds may be dusted with a protectant, such as a fungicide. Seeds must be planted in a suitable bed. v Moist soil to allow seeds to imbibe water. - Dry enough to maintain suitable oxygen levels.
Asexual Plant Propagation • • Cuttings v Produce adventitious roots. - Cells near the wound must dedifferentiate and create a new meristematic region. Layering v Tip Layering - Bend tips until they touch the ground, and then cover them with soil. v Air Layering - Wounding or Girdling to produce roots.
Cuttings and Air Layering
Cuttings and Air Layering
Asexual Plant Propagation • Grafting v 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. Ø Successful grafting depends on good contact between the vascular cambium of the scion and that of the rootstock.
Asexual Plant Propagation • Micropropagation v Grow and maintain plants in a disease-free status in test tubes. - Grown in-vitro in sterile medium. - Relies on totipotency of plant cells. Ø Capacity of a cell to give rise to any structure of a mature organism.
Asexual Plant Propagation • Micropropagation usually begins with the establishment of an explant in tissue culture. v Plant parts are disinfested. v Induced to develop multiple shoots. v Microshoots separated and placed in a new medium by subculturing. v Root Formation v Transfer plants back to outdoor environment.
Review • • Crop Plant Evolution Plant Breeding v Sexually Compatible Germplasm v Sexually Incompatible Germplasm - Bacterial Gene Cloning v Transgenic Plants v Seed Propagation v Asexual Plant Propagation
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