chapter 14 lecture outline plant breeding and propagation copyright © the mcgraw-hill companies,...

Chapter 14

Lecture Outline

Plant Breeding and Propagation

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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


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 EvolutionOrigins 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 EvolutionOrigins 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 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

Plant BreedingBreeding 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.


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.


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.


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 BreedingBreeding 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

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 base-

pair with tail of other fragments cut with same enzyme.

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.


• To clone gene:


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.


Transformation of plant by Agrobacterium tumefaciens:


Steps in making a transgenic plant:


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 PropagationSeed 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 PropagationSeed 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 PropagationAsexual Plant Propagation

Asexual propagation - Uses vegetative parts

Crown division - Plant separated into several pieces, each with crown portion and roots.

Crown division of daylily


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.


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

• Air layering - Branch or main stem wounded or girdled to produce roots.– Tropical trees and shrubs

Tip layering


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.


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


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


Plant Breeding

• Breeding Methods Using Sexually Compatible Germplasm

• Breeding Methods Using Sexually Incompatible Germplasm

Plant Propagation

• Seed Propagation

• Asexual Plant Propagation

chapter 14 lecture outline plant breeding and propagation copyright © the mcgraw-hill companies,...

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