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Lecture 7 - Introduction to Plant Development
•Reading for Lectures 7 and 8:
"Growth and Development of Flowering Plants" In Plants, Genes and
Agriculture, p.122-136 and p.151-161 on Reserve in the library
•Review
--------------------------------------------------------------------------------Lecture 7: Consequences of Immobility on Plant Development
1. How do genes instruct the building of large structures which make
up a cell and whole organism? Demo of blocks
-Genes encode enzymes.
-Enzymes organize chemistry.
-Enzymes build simple molecules (nucleotides, amino acids,
sugars, etc.).
-Other enzymes use these small molecules as subunits to build
largermolecules.
Once evolution could invent simple chemistry, it could then build
large structures by simply making long chains of simple molecules,
much like Lego blocks.
What are the subunits of:
DNA/RNA:
Proteins:
Cell walls (cellulose):
Hydrophobic Membranes:
Slide 7.1
Subunits Make Large Cellular Structures
Fig. 1 - nucleotides = DNA
Fig. 3Glucose, Fructose, Sucrose, Starch
cellulose
Fig. 2 - amino acids - proteins:
Fig. 4. Fatty acids- 2C Acetyl CoA
for compartment membranes:
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Figures 3 and 4 from :
Plants, Genes and Agriculture,pp.86 and 89
M. Chrispeels and D.Sadava
Jones and Bartlett Publishers, Boston, 1994
Fig 1 and 2 from: An Introduction to Genetic Analysis (6th ed)
A.J. Griffiths et al., page346
W.H. Freeman and Co., New York, 1996
Slide 7.2
Other Enzymes Assemble Large Macromolecules into
Organized Subcellular Structures
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From Biochemistry and Mol. Biology of Plants p.3
Different cell types arise as different genes switch on/off to encode
different subsets of enzymes to build and organize different subcellular
structures or different biochemistry.
Slide 7.03
2. Plant vs Animal Cell Growth
Plants do not move. Why do they not need to?
Unlike animals, plants make their own food via photosynthesis.
•Plants-rigid due to cell walls. Why is this compared to animals?
Animals have bones. Plants use cell walls for rigidity instead.
• walls consist of chains of sugars (5C and 6C) including cellulose,
a polymer of linked glucose (6C) molecules (15-30% of walls).
•Cellulose is the most abundant compound in the world and full of
potential calories.
Why can cows, but not humans access energy from cellulose??
images from Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.67 + p.81
ASPP, Rockville MD, 2000
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Slide 7.04
•Unlike some animal cells, plant cells cannot move during development.
•Plant growth occurs through a combination of cell expansion and cell
division.
•Plants growth like a building of "Lego building blocks", each locked
together after it is formed. demo Lego blocks
•Understanding what controls plant cell growth and the orientation of
the planes of cell division is crucial to understanding the shape of a plant
organ (eg. leaf)
Periclinal division - division plane is parallel to tissue surface
Anticlinal division -- division plane is perpendicular to tissue surface
With which divisions would you increase:
-length of a stem? ______________
-width (girth) of a stem? ___________
From Anatomy of Seed Plants (2nd Ed) p. 278
K. Esau
John Wiley and Sons, New York, 1977
3. Immobile - Consequences for organ and cell types and overall
architecture.
Much of plant development (architecture, organ and cell types) can be
explained by the fact that a plant is:
(i) immobile;
(ii) needs to compete for nutrients (carbon, Nitrogen, H20, etc.) to build
complex molecules (DNA, RNA, proteins, lipids) in order to....
(iii) spread its DNA to the next generation (reproduction
Slide 7.05
Why? Let's design a land plant organism from scratch as evolution did
noting that plants, unlike animals, produce their own energy via
photosynthesis, and cannot move to obtain water and nutrients (N, S..):
Need?
Features of Organ/Cell type?
Why this structure?
The Result: Architecture of a Plant:
How does the relative arrangement of leaves on the stem
(="phyllotaxy") affect a plant's survival and utility to field agriculture?
-whorled -- several leaves at the same node, but in a spiral
–spiral - one leaf at each node, but in a spiral
-decussate - two leaves at the same node, 180º apart
-distichous - one leaf at each node, but alternating 180º (eg. corn)
Hence, a plant is nothing but an efficient nutrient gathering machine
designed to obtain nutrients in order to increase its survival and spread
its DNA.
What about animals (including plant pests/herbivores and you)??
•All organisms have these same needs (nutrient acquisition and
reproduction). Animals, however, are mobile.
•Because they are mobile, animals must have brains to integrate the
environment with their needs -- this results in behaviour in order to
choose where to go and not to go.
•Plants do not have brains (see....you learned something in this course
after all).
•How do animals fulfill the above needs?
Slide 7,06
4. Immobile - Consequences for body plan flexibility ("plasticity")
A. A plant must remain flexible to changing environmental conditions:
i) The need for sun in response to changing canopy competition or
season/cloud cover.
ii) The need for soil nutrients, hence continuous root growth.
Demo - 2 twins vs 2 plants
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Oat root system
From Biology of Plants p.470
P. Raven, R. Evert and S.Eichhorn
Worth Publishers, New York, 1992
A plant achieves this by elaborating its body plan post-embryonically
via persistent meristems:. demo on board of plant vs animal body plan
meristem - a region of a plant containing a population of embryonic,
dividing cells that replenishes itself and gives rise to daughters that will
assume specialized cell fates.
From Anatomy of Seed Plants (2nd Ed) p.10
K. Esau
John Wiley and Sons, New York, 1977
Dicot Shoot Apical Meristem Root meristem
Slide 7.07
Primary meristem -- what is their purpose??
Secondary meristems: - What is their purpose??
R.A. Kerstetter and S. Hake (1997)
Shoot meristem formation in vegetative development.
Plant Cell 9, p.1002.
ASPP Press, Rockville, MD, 2000
B. Meristems may help plants to survive herbivory
eg. grasses - where is the shoot apical meristem located and why? Demo
of corn
C. In addition to meristems, plants produce"buds". The have the
potential to produce branches.
axillary bud - at leaf axil - gives rise to branch & may be dormant.
adventitious bud - from undifferentiated cells in root or stem
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Plants, Genes and Agriculture,p.124
M. Chrispeels and D.Sadava
Jones and Bartlett Publishers, Boston, 1994
From Anatomy of Seed Plants (2nd Ed) p.10
K. Esau
John Wiley and Sons, New York, 1977
Slide 7.08
D. The plant body plan can be very diverse and flexible by mixing and
matching numbers of developmental units, through reiteration of
organs. eg. the phytomer.
From Patterns in Plant Development (2nd ed), p.221
T.A. Steeves and I.M. Sussex
Cambridge University Press, Cambridge, UK, 1990
E. The body plan of some plants is fixed ("determinate growth")
whereas others are more flexible depending on the environmental
condition ("indeterminate growth").
In large-scale agriculture, do farmers want plants with determinate
or indeterminate growth and why??
5. Immobile - Consequences from the transition from water to
land
What adaptations did plants have to make to grow on land??
Slide 7.09
6. Immobile - Consequences of an unpredictable environment on
land for reproduction
A. Unlike animals, the germline (gamete-producing cells) are NOT setapart from the somatic cells early in development.
Why is this strategy useful for the plant??
B. Plants can regenerate new shoot apical meristems from somatic cells
(somatic regeneration). Why is this advantageous? demo
C. Some plants use vegetative propagation (new shoots without
meiosis or sex) as a means to produce seeds.
Strawberry
Potato
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Plants, Genes and Agriculture,p.153
M. Chrispeels and D.Sadava
Jones and Bartlett Publishers, Boston, 1994
Vegetative propagation allows the progeny to have the same genetic
make-up as the parent and this is useful in propagating berries, nuts, and
the grafting of fruit trees (apples, etc.).
Why/when is this useful??
Examples of vegetative propagation:
•strawberries send out horizontal stems (runners)
•potatoes - underground thickened stems (tubers) that sprout stems
•tuberous roots of sweet potatoes, cassava
Slide 7.10
7. Immobile - Consequences of having parents that cannot look
after a plant in its childhood
•Whereas many land animals protect, feed, and shelter their offspring
after birth, plants do not.
•Instead, higher plants allow young embryos with the first leaves and
primary root to develop inside a protected ovary and then, for after
germination, provide their offspring with nutrient storage reserves
made accessible to each embryo, and these are encapsulated and
protected within the hard wall of a seed.
From Biology of Plants p.450
P. Raven, R. Evert and S.Eichhorn
Worth Publishers, New York, 1992
Plants, Genes and Agriculture, p.135
M. Chrispeels and D.Sadava
Jones and Bartlett Publishers, Boston, 1994
What are the molecules that make up these seed reserves?
For parents to do this, they must remobilize their nutrient reserves from
leaves and roots to sinks (seeds and fruits). This is the basis of agriculture
How can this "grain-fill" be enhanced?
Slide 7.11
The plant invests differently in the storage reserves of the male pollen
versus the female egg. Why??
Citrus egg
inside ovary
inside flower
From Anatomy of Seed Plants (2nd Ed) p. 440
K. Esau
John Wiley and Sons, New York, 1977
Seeds have a low water content (~15%) and a very low metabolic rate
and may survive drought or wintering
The storage reserves are used during germination to allow fast early
growth of the seedling until it can photosynthesize and acquire nutrients
at high rates.
The weed lifestyle is to germinate and grow quickly. Why??
In general, fast seedling growth of annual crops in agriculture saves
water -- in a study, when the number of corn plants/hectare was
increased from 20,000 to 40,000, corn yield increased by 65%, but total
water use by only 20%. The increase in total transpiration was
compensated by a decrease in evaporation from the soil due to leaf cover
Slide 7.12
8. Immobile - Consequences on the need to spread DNA to the next
generation
What are the 3 main strategies have the flowering plants (Angiosperms)
use to spread their DNA to new environments and/or to mate with a new
population?
eg. cucumber fruit
From Anatomy of Seed Plants (2nd Ed) p. 441
K. Esau
John Wiley and Sons, New York, 1977
Fruits may also develop without embryo fertilization (eg. banana, fig,
melon, tomato, pumpkin) or have aborted (seedless) embryos
(eg. cherry, grape, peach).
9. Summary of key concepts
Slide 7.13