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AGR2451 Lecture 8 Notes -- Raizada
Review of Last lecture
•Plants are immobile
•The plant is simply a nutrient gathering machine. The nutrients
are gathered to allow the organism's DNA to propagate.
•Vegetative body is entirely designed to get nutrients
•Plant architecture - phyllotaxy, branching, reiteration
•Organ shape - cell elongation and orientation of cell division
•Immobile, so needs to disperse DNA by other means
•Plant body plan is determined post-embryonically from meristems flexibility
•Many types of meristems
•Explain how a vegetative meristem gives rise to a leaf primordia
(picture)
•Flexibility in means of reproduction -- where and how - sexual and
asexual
Slide 8.1
A Overview of Plant Reproduction
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.989
ASPP, Rockville MD, 2000
Slide 8.2
Lecture 8 -- Plant Development II "The Mechanics of Plant Reproduction"
Reproduction is the key process for plant agriculture and the key target
for breeding.
1. Flowers
The flower is made up of 4 concentric rings, each ring containing a
different organ type.
QuickTime™ and a
PNG decompressor
are needed to see this picture.
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.997
ASPP, Rockville MD, 2000
Describe the functions of the following floral structures:
sepalspetals stamenscarpelsSlide 8.3
How is diversity created amongst the flowers of the 235,000 species of
Angiosperms?
Martin, C. and Gerats, T. (1993) Control of pigment biosynthesis genes during
Petal development. Plant Cell 5, 1254. ASPP Publishing, Rockville, MD, USA
From Biology of Plants p.382
P. Raven, R. Evert and S.Eichhorn
Worth Publishers, New York, 1992
Slide 8.4
-***The key decision made by a plant is when to flower = age +
environment
What are these environmental cues and why?
How does a plant sense daylength?
-pigment phytochrome = receptor, 2 forms, Pr and Pfr -- Pfr builds up
in leaves and is the active signal to vegetative meristem, converts it to an
inflorescence (flower-producing) meristem
-This is not chlorophyll
Define:
Short-day plant
Long-day plant
Effect on
Human
Civilization?
From Biology of Plants p.579 and p.948
P. Raven, R. Evert and S.Eichhorn
Worth Publishers, New York, 1992
Slide 8.5
2. Gametes
-The male gamete (sperm) is stored in a multicellular
(micro)gametophyte, the pollen grain.
-Each pollen grain actually possesses 3 haploid cells formed by two
rounds of mitosis following meiosis:
-a vegetative cell -its nucleus regulates the enzymes that regulate the
gametophyte including pollen tube growth after fertilization
-two sperm cells -- function?
From Biology of Plants p.266
P. Raven, R. Evert and S.Eichhorn
Worth Publishers, New York, 1992
Slide 8.6
Gametes - Female
•Meiosis produces 4 haploid spores.
•Three die.
•The last one (the "megaspore") undergoes 3 mitotic divisions to form 8
nuclei (contained in 7 cells).
•The central cell contains 2 nuclei, and both of these nuclei will be
fertilized by one sperm to form a triploid endosperm.
•Another cell is the egg cell which is the female gamete.
•All of the 8 nuclei are located inside a multicellular
(mega)gametophyte, the embryo sac.
•The embryo sac is located inside the carpel (ovary) of the flower
embryo sac
Reiser, L. and Fischer, R.L. (1993) The ovule and the embryo sac.
Plant Cell 5, 1294. ASPP Publishing, Rockville, MD, USA
Slide 8.7
Gametes (Continued)
Why do plant gametophytes consist of multiple haploid cells and
multiple nuclei?
Do the different haploid cells in an embryo sac send signals to each
other and if so why?
Why do plants have double fertilization?
A single corn plant produces 25 million pollen grains but only 300-600
female eggs (in embryo sacs).
What use is there for producing so much pollen?
Excess pollen permits competition between pollen grains for females, by
testing the fitness of the parental genes in single copy form -- 60-90% of
the genes expressed in the vegetative part of the plant are expressed in
haploid pollen.
Excess airborne pollen has implications for breeding and GMOs.
What are these?
Slide 8.8
3. Fertilization
•The embryo sac(eggs) is buried deep in the flower where it isprotected.
•Pollen, however, lands far away from the egg on the surface of the
flower (of the same species) on the stigma.
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1021
ASPP, Rockville MD, 2000
•The stigma has a sticky surface of protruding papillae to catch the pollen.
•The stigma is at the protruding tip of a long hollow cylinder
(the "style") elevated to catch pollen.
•At the base of the style is the embryo sac containing the egg.
How do the two sperm get to the embryo sac?
From Biology of Plants p.392
P. Raven, R. Evert and S.Eichhorn
Worth Publishers, New York, 1992
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1019
ASPP, Rockville MD, 2000
Slide 8.9
Fertilization (Continued)
A corn silk is a long style terminating in an embryo sac on an ear of
corn. (demo)
Obviously, pollen from one species can land on the floral stigma of a
different species.
Why doesn't cross-species fertilization happen?
Some crops are self-fertilizing whereas others are outcrossers.
What are the advantages and disadvantages of each strategy?
Because pollen and eggs are often produced in the same flower, how do
outcrossers manage to have their eggs fertilized by sperm (pollen)
originating from other plants?
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1021
ASPP, Rockville MD, 2000
*Self-incompability is found in many plants, including Solanaceous
species and Brassica.
*Corn and the major crop grasses do not reject their own pollen.
Slide 8.10
Fertilization (continued)
The embryo and the endosperm are formed from separate fertilization
events with two sperm.
Why do higher plants have double fertilization?
QuickTime™ and a
PNG decompressor
are needed to see this picture.
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1023
ASPP, Rockville MD, 2000
Some higher plants can produce embryos without meiosis or fertilization.
Instead, cells from the parent embryo sac divide and directly produce an
embryo inside the sac. This process is called apomixis.
Hybrid vigor occurs in the F1 (first generation) progeny of two
divergent parents, but diminishes in the next generation.
For corn, how is hybrid seed generated?
•Apomixis may help farmers in the Developing World.
•Dandelions reproduce via apomixis. They are clones of one another.
Slide 8.11
4. Seeds
•Fertilization -embryo, an endosperm surrounded by a seed coat.
•As the seed is being formed, the plant continues to senesce (a slowdown in carbon fixation +photosynthesis) at end of the growing season.
•However, carbon and nutrient reserves are needed by the seed.
•This occurs via a breakdown of starch, protein bodies and oils in the
leaves, then translocated to the growing seeds (= "grain fill" period).
Lopes, M.A. and Larkins, B.A. (1993) Endosperm origin, development and function.
Plant Cell 5, 1389. ASPP Publishing, Rockville, MD, USA
***Much of the success in increasing crop yields (as in corn) has come
from delaying the onset of senescence, allowing the plants to continue
to fix carbon during grain fill.
•After the seed has matured, it becomes dormant - low water content
•Annual plants make a huge investment in reproduction as a percentage
of their dry matter. Perennial plants do not. Why not?
•The seed has to be full of pesticides, and humans have selected for
mutations in the genes that encode the enzymes that produce these toxic
biochemicals, a process that is ongoing,incomplete and poorly researched.
Slide 8.12
5. Seed germination
What environmental signals does a seed use to determine when to
germinate or not germinate and why??
In general, how would a seed sense these signals at the molecular level?
These signals ultimately cause the turning on of genes via transcription,
that cause cell division and lead to seedling germination and the orderly
determination of cell types and organs. The storage reserves are broken
down and used by the developing seedling.
Please hand in questionnaire!!
Slide 8.13