Chapter 3- Human Manipulation of Plants
Sporic life cycles involve 2 types of multicellular bodies:
-a diploid, spore-producing sporophyte
-a haploid, gamete-producing gametophyte
Sexual Reproduction in Plants
Each anther generally has four pollen sacs.
Each pollen sac produces a number of microsporocytes.
Sexual Reproduction in Plants
Each microsporocyte undergoes meiosis to produce four haploid microspores.
Review-Meiosis
The nucleus of each microspore then divides without cytokinesis to form the pollen grain with 2 nuclei.
Review-Mitosis
Sexual Reproduction in Plants
The ovule is the site of meiosis and ultimately the formation of the seed.
The megasporocyte divides by meiosis to produce the haploid megaspores.
Of the four megaspores, generally only one remains.
The single megaspore undergoes successive nuclear divisions to produce an embryo sac.
A unique part of the life cycle of angiosperms is double-fertilization.
In double-fertilization, the two sperm nuclei in the pollen tube each participate in a fertilization event.
One sperm nucleus fuses with the egg to form the zygote.
The other sperm nucleus with the polar nuclei to form a triploid primary endosperm nucleus.
Sexual Reproduction in Plants
Although sexual reproduction involving separate plants (outcrossing) is
is normal, flowering plants show considerable variation in the
production of seeds.
Some plants result from the fusion of gametes within a single parent,
(self-fertilization) or (selfing).
Some plants result without fusion of gametes, (asexual reproduction).
Self-fertilization/Self-pollination
The presence of both male and female gametophytes within the same
flower on the same plant raises the possibility of self-pollination.
The term inbreeding is used for:
-the fusion of gametes within a flower (or within an individual)
-the fusion of gametes from separate, but genetically very similar plants
Self-fertilization can occur only if a plant is self-compatible.
-if the pollen of that plant is capable of germinating on the stigmas and fertilizing the eggs of its own flowers
Cross-fertilization/Outcrossing)
In nature, most species will outcross even if they are self-compatible.
Cross-fertilization/Outcrossing -
Reproductive strategies
Many plants have mechanisms to increase the chance of cross-pollination even though they may be capable of self-pollination.
The most obvious mechanism to insure cross-pollination is the separation of the sexes into different individuals (dioecious
plants).
The willows are dioecious, with separate male and female plants.
Self-pollination can be prevented (or greatly reduced) in
monoecious plants by various factors including:
Self-pollination can be prevented (or greatly reduced) in monoecious plants by various factors including:
2) A genetic system that prevents the elongation of the pollen tube on a stigma of the same plant (self-incompatibility)
3) A genetic system that prevents genes expressed in cells at the stigma surface allow genetically distinct pollen from flowers
of the same species, but not genetically identical pollen.
Cross-fertilization/Outcrossing)
– Outcrossing, even at low levels, ensures variability and genetic diversity in the offspring.
- Organisms in nature that produce variable offspring (outcrossing) will ultimately leave more survivors in
comparison to offspring produced through selfing.
Self-fertilization/Self-pollination
Despite the advantages of outcrossing, annual herbs and many weeds characteristically self-pollinate.
These self-fertilizing plants are mostly independent of pollinators, flowering time and other external conditions:
Agriculturalists favor homozygous or inbred lines of crops because these plants typically produce:
Self-fertilization/Self-pollination
Inbred lines are also used as parental stocks for production of hybrid seed.
Plants that are the product of two inbred lines are often larger and produce larger seed crops than either homozygous parent.
This phenomenon is known as hybrid vigor or heterosis.
Inbred lines are used to produce hybrid corn.
Humans have also selected mutations to facilitate self-pollination for fruit production.
This figure shows the postulated changes in the flowers of cultivated tomatoes through human selection from the natural,
outcrossing system to the self-pollinating system.
Spatial separation of the anthers and the stigma fostered outcrossing in wild tomatoes, because the pollen from the anthers
could not reach the longer style of the same flower.
Humans selected for a shorter style where the stigma is below the level of the anthers and pollen will naturally fall on the stigma.
This self-pollination system assures a high level of fruit set.
Asexual reproduction/Vegetative reproduction
Asexual reproduction is another propagation method that has often been used by humans to manipulate plants.
When plants reproduce asexually, the “daughter” plants are genetically identical to the “parent” because no meiosis and
subsequent recombination has occurred.
Apomixis involves the production of seeds directly from diploid maternal cells.
No male-female gamete fusion is involved, so the seeds are genetically identical to the mother.
Vegetative propagation
Because vegetative propagation employs pieces of mature tissue to start plants:
Asexual reproduction/Vegetative reproduction
Grafting is an artificial method of asexual propagation that ensures:
-perpetuation of the genotype of the plant from which the graft
was taken
-production a “plant” as mature as a sapling several years old
In the process of grafting, a branch or bud of a desirable woody tree or shrub is joined to a rootstock or stem of another
individual.
The living cambium layers of the scion (branch or bud) of the desired plant and the stock (rootstock or stem) are aligned.
In this way, the actively dividing tissues come into contact and grow together.
If the graft is successful, then the crop-bearing part of the plant will express the genes of the plant from the scion.
Polyploidy
Large-scale changes in chromosome number, such as whole genome doubling have played a major role in plant evolution and the
history of domesticated crops.
Polyploidy is common in plants and represents a way in which hybrid individuals with dissimilar chromosome sets can become
reproductively successful.
Polyploidy
Chromosome number increases can occur if homologous chromosomes fail to separate during Meiosis I.
If these diploid cells fuse with gametes cells that have the normal haploid number, zygotes will have three times the haploid
number of chromosomes.
gamete
2n= 4
normal
gamete
n= 2
zygote
3n= 6
If a gamete cell with a double number of chromosomes fuses with another unreduced diploid gamete, the zygote will have four
times the number chromosomes of the normal haploid cells.
gamete
2n= 4
gamete
2n= 4
zygote
4n= 8
Polyploidy- Autopolyploid
Polyploidy-Allopolyploid
Polyploidy
Polyploids can also be produced
by a doubling of the chromosomes
after a zygote is formed.
Polyploidy
Polyploid plants are important in agriculture because they are usually bigger and usually have larger fruits or seed crops than
diploid parents.
https://botanistinthekitchen.wordpress.com/2015/09/18/triple-threat-watermelon/
The earliest written report of successful chemical induction of autopolyploidy was made in 1937 by Blakeslee and Avery,
who were working at the Carnegie Institution at Cold Spring Harbor in New York. Following up on a tip from a colleague,
they found that they could induce chromosome doubling by applying a plant-derived alkaloid called colchicine to either
seeds or seedlings. Some of their experiments charmingly involved an atomizer “purchased in Woolworths for twenty
cents.”
Colchicine, an alkaloid compound from Colchicum autumnale, the autumn crocus, is used to artificially induce chromosome
doubling in plants.
Colchicine works as a mitotic inhibitor by binding to tubulin and disrupting the formation of the mitotic spindle.
Seedless watermelons are triploids (3n)
Tissue Culture
Plant Biotechnology
A plasmid is a tiny, circular piece of double-stranded DNA present in bacterial cells, as carriers of genes from one species to
another.
The most common bacterium used in genetic engineering is Agrobacterium tumifaciens.
Top seven genetically modified crops grown in the United States
Chapter 3 Human Manipulation of Plants-study outline questions
REVIEW- general structure of a sporic life cycle; sexual reproduction in angiosperms (Fig. 3.3); meiosis (Fig. 3.4); mitosis (Fig 3.5);
definition of double fertilization
Understand self-fertilization/self-pollination (Fig. 3.6)
-Inbreeding
-Characteristics of self-pollinating plants
-Explain how inbred lines, selected mutations and hybrid vigor are used for agriculture? Examples? (Fig 3.10; Fig. 3.11;
Fig. 3.9)
Understand cross-fertilization/outcrossing
-Characteristics of outcrossing plants
-Reproductive strategies to decrease selfing or increase chances of cross-pollination
--protogyny/protoandry (Fig. 3.7)
--self-incompatibility
--pollen grain inhibition
-How does outcrossing ensure variability and genetic diversity in nature?
Understand asexual reproduction
-apomixis; vegetative propagation (examples-Fig. 3.12); grafting (Fig. 3.13)
-What are some agricultural advantages of vegetative propagation?
Understand polyploidy (Fig. 3.14); autopolyploidy; allopolyploidy
-Why are polyploid plants important in agriculture?; What are some major crops of presumed polyploid origins? (Table 3.2)
-How is colchicine used to artificially induce chromosome doubling in plants? Example: seedless watermelons
Understand plant biotechnology
-How is tissue culture important in biotechnology? What is a plasmid? What is the most common bacterium used in
genetic engineering (Fig. 3.15)
What are the top 7 genetically modified crops that are grown in the U.S.?