plant reproduction chapter 42. 2 reproductive development angiosperms represent an evolutionary...
TRANSCRIPT
Plant Reproduction
Chapter 42
2
Reproductive Development
Angiosperms represent an evolutionary innovation with their production of flowers and fruits
Plants go through developmental changes leading to reproductive maturity by adding structures to existing ones with meristems-A germinating seed becomes a vegetative plant through morphogenesis
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Gameteproduction and
pollination
Fertilization
Embryodevelopment
Fruitand seed
maturation
Developmentof plant body
Maturationand flowering
Dispersaland
germination
2n
2n
2n
2n
2n
2n
n
n
Zygote
CHAPTER 42
CHAPTER 37
CHAPTER 36
2n
2n
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Reproductive Development
Flowering is the default state
In Arabidopsis, the gene embryonic flower (EMF) prevents early flowering
-emf mutants lacking a functional EMF protein flower immediately
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Reproductive Development
The juvenile-to-adult transition can be induced by overexpressing a flowering gene
-LEAFY (LFY) was cloned in Arabidopsis
-Overexpression of LFY in aspen, causes flowering to occur in weeks instead of years
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Flower Production
Four genetically regulated pathways to flowering have been identified
1. The light-dependent pathway
2. The temperature-dependent pathway
3. The gibberellin-dependent pathway
4. The autonomous pathway
Plants can rely primarily on one pathway, but all four pathways can be present
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Let’s Take a Look at theAutonomous Pathway in Some
Detail First
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Autonomous Pathway
The autonomous pathway does not depend on external cues except for basic nutrition
It allows day-neutral plants to “count” nodes and “remember” node location
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Upper Axillary Bud Released from Apical Dominance Lower Axillary Bud Released from Apical Dominance
Intact plant Shoot removed Replacement shoot
Shootremovedhere
5 nodes*removed
5 nodes*replaced
Intact plant Shoot removed Replacement shoot
Shootremovedhere
13 nodes*removed
13 nodes*replaced
*nodes = leaf bearing node
Autonomous Pathway--Plants Can Count
-Tobacco plants produce a uniform number of nodes before flowering-Upper axillary buds of flowering tobacco remember their position if rooted or grafted
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Shootremovedhere
Shootremovedhere
Shoot Florally Determined Shoot Not Florally Determined
a. b.
Intact plant
Shootremoved
Rooted shoot Floweringrooted shoot
Intact plant
Shootremoved
Rooted shoot Floweringrooted shoot
Autonomous Pathway--Plants Can Remember
Not-Florally Determined Plants are said not to remember...Florally Determined plants are said to remember
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Control plant:no treatment
Experimental plant:pot-on-pot treatment
Experimental plant:Lower leaves were
continually removed
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Roots Inhibit Flowering
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A Model of All the Flowering Pathways
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inhibitionactivation
Repression of Floral Inhibitors
Activation of Floral Meristem Identity GenesAdult meristem Floral meristem
Temperature-dependentpathway
Autonomouspathway
Flower-repressing
genes
Flower-promoting
genes
Vernalization
Autonomousgene expression
Cold
Gibberellin-dependentpathway
Light-dependentpathway
Gibberellin
CO
Light
LFY
AP1
ABCDEfloral organidentity genes
Floral organdevelopment
Gibberellin binds to the promoter of LFY
CO is a transcription factor that turns on other genes, resulting in the expression of LFY
-Phytochromes regulate CO transcription
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Flower Structure
Floral organs are thought to have evolved from leaves
A complete flower has four whorls-Calyx, corolla, androecium, and gynoecium
An incomplete flower lacks one or more of these whorls
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Stamen Anther Filament
CarpelStigmaStyleOvaryOvule
Petal
Receptacle
Sepal
all stamens = androeciumall carpels = gynoeciumall petals = corollaall sepals = calyx
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Male structure
Female structure
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Gamete Production
Plant sexual life cycles are characterized by an alternation of generations-Diploid sporophyte haploid gametophyte
In angiosperms, the gametophyte generation is very small and is completely enclosed within the tissues of the parent sporophyte-Male gametophyte = Pollen grains-Female gametophyte = Embryo sac
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Gamete Production
Gametes are produced in separate, specialized structures of the flower
Reproductive organs of angiosperms differ from those of animals in two ways:
1. Both male and female structures usually occur together in the same individual
2. Reproductive structures are not permanent parts of the adult individual
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Anther Microsporemother cell
Pollen sac
Megaspores
Pollen grains(microgametophytes)
Tube cellnucleus
Generative cell
Ovule
Megasporemother cell
SurvivingmegasporeAntipodals
Polarnuclei
Degeneratedmegaspores
Eight-nucleate embryo sac(megagametophyte)
Synergids
Eggcell
diploid (2n)
haploid (n)
MEIOSIS
MEIOSIS
MITOSIS
MITOSIS
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Microspores
Generative Cellsgo on to make 2 spermCells
Megaspore enlarges and
undergoes
repeated mitotic divisions
to produce eight haploid
nuclei...Egg cell is enclosed
within a seven-celled
embryo sac
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Pollination
Pollination is the process by which pollen is placed on the stigma
-Self-pollination = Pollen from a flower’s anther pollinates stigma of the same flower
-Cross-pollination = Pollen from anther of one flower pollinates another flower’s stigma
-Also termed outcrossing
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Pollination
Successful pollination in many angiosperms depends on regular attraction of pollinators
Flowers & animal pollinators have coevolved resulting in specialized relationships
-Bees are the most common insect pollinators
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Pollination
Flowers that are visited regularly by birds often have a red color
-Usually inconspicuous to insects
Hummingbirds obtain nectar from flowers that match the length and shape of their beaks
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Pollination
Self-pollinating plants usually have small, relatively inconspicuous flowers that shed pollen directly into the stigma
Self-pollination is favored in stable environments
1. Plants do not need to be visited by animals to produce seed
2. Offspring are more uniform and probably better adapted to their environment
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Pollination
Several evolutionary strategies promote outcrossing
1. Separation of male and female structures in space
-Dioecious plants produce only ovule or only pollen
-Monoecious plants produce male and female flowers on the same plant
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Pollination
2. Separation of male and female structures in time
-Even if functional stamens and pistils are both found in the same flower,
they may reach maturity at different times
-Plants in which this occurs are called dichogamous
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Pollination
3. Self-incompatibility
-Pollen and stigma recognize each other as self and so the pollen tube is blocked
-Controlled by alleles at the S locus
-Gametophytic self-incompatibility
-Block is after pollen tube germination
-Sporophytic self-incompatibility
-The pollen tube fails to germinate
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a. b.
Gametophytic Self-Incompatibility Sporophytic Self-Incompatibility
S1S2
pollen parent
S2
S1S2
S2
S1
S2
X
S1 S2
X X
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S2S3
stigma ofpollen recipient
S1S2
pollen parent
S2S3
stigma ofpollen recipient
Pollination
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Fertilization
Angiosperms undergo a unique process called double fertilization
-A pollen grain that lands on a stigma forms a pollen tube that pierces the style
-While the pollen tube is growing, the generative cell divides to form 2 sperm cells
-When pollen tube reaches the ovule, it enters one of the synergids and
releases the two sperm cells
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Fertilization
-Then double-fertilization occurs
-One sperm cell nucleus fuses with the egg cell to form the diploid (2n) zygote
-Other sperm cell nucleus fuses with the two polar nuclei to form the triploid (3n) endosperm nucleus
-Eventually develops into the endosperm that nourishes
embryo
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Generative cellTube cell
Stigma
Style
Ovary
Ovule
Carpel
Pollination
Pollen grain
Embryosac
Tube cell
Sperm cells
Tube cellnucleus
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Growth of pollen tube
Pollen tube
Double fertilizationRelease of sperm cells
Zygote (2n)
Antipodals
Polar nuclei
Egg cell
Synergids
Endosperm nucleus (3n)
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Plant Life Spans
Once established, plants live for variable periods of time, depending on the species
Woody plants, which have extensive secondary growth, typically live longer than herbaceous plants, which don’t
-Bristlecone pine, for example, can live upward of 4,000 years
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Plant Life Spans
Perennial plants are able to flower and produce seeds and fruit for an indefinite number of growing seasons
-May be herbaceous or woody
-In deciduous plants all the leaves fall, and the tree is bare, at a particular time of year
-In evergreen plants, the leaves drop throughout the year, and so the plant is never completely bare
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Plant Life Spans
Annual plants grow, flower, and form fruits and seeds, and typically die within one growing season
-Are usually herbaceous
-The process that leads to the death of the plant is called senescence
Biennial plants have two-year life cycles
-They store energy the first year and flower the second year