Transcript chapt37_lecture

CHAPTER 37
LECTURE
SLIDES
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Vegetative Plant Development
Chapter 37
Embryo Development
• Begins once the egg cell is fertilized
• The growing pollen tube enters
angiosperm embryo sac and releases two
sperm cells
– One sperm fertilizes central cell and initiates
endosperm development
– Other sperm fertilizes the egg to produce a
zygote
• Cell division soon follows, creating the embryo
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• First zygote division is asymmetrical,
resulting in cells with 2 different fates
– Small cell divides repeatedly forming a ball of
cells, which will form the embryo
– Large cell divides repeatedly forming an
elongated structure called a suspensor
• Transports nutrients to embryo
• The root–shoot axis also forms at this time
– Cells near suspensor become root
– Cells at the other end become shoot
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
3n endosperm
Polar nuclei
Egg
Sperm
Micropyle
Pollen tube
2n zygote
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• Asymmetrical cell division is also observed
in the zygote of the brown alga Fucus
– Bulge develops on one side of embryo
– Cell division occurs there, resulting in
• A smaller cell that develops into a rhizoid that
anchors the alga
• A larger cell that develops into the thallus, or main
algal body
– Axis is first established by the point of sperm
entry, but it can be changed by environmental
signals
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• Arabidopsis mutants have revealed the
normal developmental mechanisms
– Suspensor mutants undergo aberrant
development in the embryo followed by
embryo-like development of the suspensor
– Thus, the embryo normally prevents embryo
development in suspensor
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Body Plan
• In plants, three-dimensional shape and
form arise by regulating amount and
pattern of cell divisions
– Vertical axis (root–shoot axis) becomes
established at a very early stage
– Same is true for establishment of a radial axis
(inner–outer axis)
• First cells divide producing a solid ball of
cells
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• Apical meristems establish the root–shoot
axis in the globular stage, from which the
three basic tissue systems arise
– Dermal
– Ground
– Vascular tissue
• These tissues are organized radially
around the root–shoot axis
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
embryo
suspensor
a.
Kindly provided by Prof. Chun-ming Liu, Institute of Botany, Chinese Academy of Sciences
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
embryo
suspensor
a.
b.
a-d: Kindly provided by Prof. Chun-ming Liu, Institute of Botany, Chinese Academy of Sciences
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
embryo
suspensor
a.
b.
c. a-d: Kindly provided by Prof. Chun-ming Liu, Institute of Botany, Chinese Academy of Sciences
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
embryo
suspensor
a.
b.
Cotyledon
Ground
meristem
Protoderm
Procambium
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c.
d.
a-d: Kindly provided by Prof. Chun-ming Liu, Institute of Botany, Chinese Academy of Sciences
• Both shoot and root meristems are apical
meristems, but are independently
controlled
• SHOOTMERISTEMLESS (STM)
– Necessary for shoot formation
– Plants that do not make STM protein fail to
produce viable shoots, but produce roots
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• HOBBIT gene is required for root formation
– Hobbit mutants form shoot meristems, but no root
meristems form
– HOBBIT protein allows auxin to induce the expression
of a gene or genes needed for correct cell division to
make a root meristem
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• One way that auxin induces gene
expression is by activating a transcription
factor
– MONOPTEROS (MP) is a gene that codes for
an auxin-induced transcription factor
• Necessary for root formation, but not shoot
– Once activated, MP protein binds to the
promoter of another gene, leading to
transcription of a gene or genes needed for
root meristem formation
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• c. A wild-type seedling depends on auxin-induced genes
for normal root initiation during embryogenesis
• d. The hobbit seedling has a stub rather than a root
because abnormal cell divisions prevent root meristem
formation
• e. The monopteros seedling also fails to develop a root
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Formation of Tissue Systems
• Primary meristems differentiate while the
plant embryo is still at the globular stage
– No cell movements are involved
• Outer protoderm develops into dermal
tissue that protects the plant
• Ground meristem develops into ground
tissue that stores food and water
• Inner procambium develops into vascular
tissue that transports water and nutrients
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Morphogenesis
• The globular stage gives rise to heartshaped embryo with bulges called
cotyledons
– Two in eudicots and one in monocots
• These bulges are produced by embryonic
cells, and not by the shoot apical meristem
– This process is called morphogenesis
– Results from changes in planes and rates of
cell division
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• Form of a plant body is largely determined
by the plane in which its cells divide
– Also controlled by changes in cell shape as
cells expand osmotically after they form
• Based on the position of the cell plate
– Determined by microtubules and actin
– Microtubules also guide cellulose deposition
as the cell wall forms around the new cell
• Cells expand in the directions of the two sides with
the least cellulose reinforcement
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• Early in embryonic development, most
cells can give rise to a wide range of cell
and organ types, including leaves
– As development proceeds, the cells with
multiple potentials are restricted to the
meristem regions
– Many meristems have been established by
the time embryogenesis ends and the seed
becomes dormant
• After germination, apical meristems
continue adding cells to the growing root
and shoot tips
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• During embryogenesis, angiosperms
undergo three other critical events
1. Development of a food supply
2. Development of seed coat
3. Development of fruit surrounding seed
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• Endosperm varies between plants
– In coconuts it includes the liquid “milk”
– In corn it is solid
– In peas and beans it is used up during
embryogenesis
• Nutrients are stored in thick, fleshy cotyledons
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Seeds
• In many angiosperms, development of the
embryo is arrested soon after meristems
and cotyledons differentiate
• Integuments develop into a relatively
impermeable seed coat
• Encloses the seed with its dormant
embryo and stored food
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• Seeds are an important adaptation
1. They maintain dormancy under unfavorable
conditions
2. They protect the young plant when it is most
vulnerable
3. They provide food for the embryo until it can
produce its own food
4. They facilitate dispersal of the embryo
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• Once a seed coat forms, most of the
embryo’s metabolic activities cease
• Germination cannot take place until water
and oxygen reach the embryo
• Seeds of some plants have been known to
remain viable for thousands of years
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• Specific adaptations ensure that seeds will
germinate only under appropriate
conditions
– Some seeds lie within tough cones that do not
open until exposed to fire
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Fruits
• Most simply defined as mature ovaries
(carpels)
• During seed formation, the flower ovary
begins to develop into fruit
• It is possible for fruits to develop without
seed development
– Bananas are propagated asexually
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• The ovary wall is termed the pericarp
– 3 layers: exocarp, mesocarp and endocarp
– Their fate determines the fruit type
• Fruits contain 3 genotypes in 1 package
– Fruits and seed coat from prior sporophyte
generation
– Remnants of gametophyte generation
produced egg
– Embryo represents next sporophyte
generation
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Fruit Dispersal
• Occurs through a wide array of methods
– Ingestion and transportation by birds or other
vertebrates
– Hitching a ride with hooked spines on birds
and mammals
– Burial in caches by herbivores
– Blowing in the wind
– Floating and drifting on water
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Germination
• Defined as the emergence of the radicle
(first root) from the seed coat
• Germination begins when a seed absorbs
water and metabolism resumes
– Oxygen must be available
– May requires additional environmental signals
such as specific wavelength of light,
appropriate temperature, or stratification
(period of low temperature exposure)
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• Germination can occur over a wide
temperature range (5o–30oC)
• Some seeds will not germinate even under
the best conditions
– Presence of ungerminated seeds in the soil of
an area is termed the seed bank
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• Germination requires energy sources
– Starch stored in amyloplasts, proteins, or fats and oils
– Food sources can readily be digested during
germination, producing glycerol and fatty acids, which
yield energy through cellular respiration
• In cereal grain kernels, the single cotyledon is
modified into a massive scutellum
– Its abundant food is used first during germination
– Later it serves as a conduit from the endosperm to the
rest of the embryo
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• Embryo produces gibberellic acid
– Hormone signals the aleurone (outer
endosperm layer) to produce a-amylase
– Breaks down the endosperm’s starch into
sugars that are passed to embryo
– Abscisic acid, another hormone, can inhibit
starch breakdown
• Establishes dormancy
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• As the sporophyte pushes through the
seed coat, it orients with the environment
such that the root grows down and shoot
grows up
– Shoot becomes photosynthetic
– Postembryonic phase is under way
• In monocots
– Coleoptile protect shoot
– Coleorhiza surrounds radicle
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• Emergence of the embryonic root and
shoot from the seed during germination
varies widely from species to species
– In most plants, the root emerges before the
shoot appears
– Cotyledons may be held below or above
ground
– Cotyledons may become green or may shrivel
quickly
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