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Plant Reproduction

BIOL 304

11/21/2008-11/26/2008

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• Transition to reproduction

• Flower organ development

• Gametogenesis and fertilization

Plant Reproduction

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Transition to reproduction

Vegetative phaseReproductive phase

Inflorescence

Flower

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Production of flowers involves two transitions in Arabidopsis

SC: stem cellP: organ primordiaSe: sepal

1. Convert SAM to inflorescence meristem (infinite, making lateral organs)

2. Convert inflorescence meristem to floral meristem (terminal, flowers)

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Factors regulating the transitions

Vegetative meristem

Inflorescence meristem

Floral meriste

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• Genes (flowering-time genes and floral identity genes)

• Light (photoperiod)• The biological clock• Temperature • Hormones

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affecting the transition of vegetative growth to reproductive growth

WT emf2

Flowering-time genes

emf1

embryonic flower

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affecting formation of inflorescence and floral meristems

Floral identity genes

Inflorescence (from Inflorescence meristem)

Flower (from Floral meristem)

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terminal flower 1 (tfl1): Convert the inflorescence meristem to the flower meristem.

leafy (lfy): produce more inflorescences, delayed flowering

Mutations in floral identity genes

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Factors regulating the transition to reproduction

Vegetative meristem

Inflorescence meristem

Floral meriste

m

EMF

TFLLFY

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Garner and Allard (1920’s)

The discovery of photoperiodism

Soybeans (Glycine max) planted over a three-month period all flowered about the same time

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Many more experiments were followed: • Eliminate a variety of environmental

conditions: Nutrition, temperature, and light intensity

• Relative length of day and night decides the flowering time

Photoperiodism: ability of an organism to measure the proportion of daylight during a 24-hour period

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Varies according to the latitude and seasonal changes.

Photoperiod

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Critical daylength

CriticalDaylength

(CD)

Xanthium: a short day plant, flowers when CD is LESS than 15.5 hours.

Hyoscyamus: a long day plant, flowers when CD is MORE than 11 hours.

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Plants are induced to flower by different photoperiods

• short day (SD) : plants are stimulated to flower when the length of day falls below a threshold

• long day (LD): plants are stimulated to flower when the length of day exceeds a threshold

• Day neutral (DN): plants flower indifference to the changes of day length.

• Long-short-day: flowering requires certain number of short days are preceded by a certain number of long days.

• Short-long-day: flowering requires certain number of long days are preceded by a certain number of short days.

• Intermediate-daylength: not flowering if the daylength is too short or too long.

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Do plants really measure the length of the daylength?

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• Xanthium flowers when the dark period exceeds 8.5 hours.

Hamner and Bonner (1938): Xanthium strumarium, a SD plant with CD = 15.5 hours

• Short interruption of dark period, even by a pulse of light as short as 1 minute delays flowering.

• The relative length of dark is not the determining factor.

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Similar results were obtained with other SD plants.

For LD plantsA longer dark period inhibits flowering.Light break induces flowering.

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What tissues/organs perceive photoperiod?

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Exp. 1: The leaf or apex of Perilla (a short day plant) was exposed to different daylength.

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Exp. 2: Grafting experiment with Perilla

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• the flowering signal is generated in the leaf

• the signal goes one way: from the leaf to the apex

• Grafting transmittable

The flowering signal: florigen

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vegetative or reproductive growth?

SAM

Florigen

Florigen

Florigen

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• Present in plants, animals, fungi, and some photosynthetic bacteria

• An internal time measuring system (“clock”) that runs on its own with a periodicity of nearly 24 hours. It can be “reset” by external signals.

The biological clock

Temperature

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The Arabidopsis biological clock

The central oscillator: CCA1, LHY, and TOC1 (these are transcription factors) and other proteins

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The Arabidopsis biological clock

• CCA1 and LHY are expressed during the day and together repress expression of TOC1 during the day

• TOC1 is expressed at night and is required for activation of CCA1 and LHY1, beginning just before morning

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• Lack of the nyctinastic movement: diurnal rise and fall of leaves

• Altered flowering time in some mutants

• cca1: early flowering• lhy: early flowering• toc1: early flowering• Some other clock mutants can

be late flowering

Mutations in the clock genes

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Temperature: Vernalization

Vernalization: low temperature treatment can promote flowering in some plants.

• The vernalization-effective temperature and duration of low temperature treatment may vary.

• Vernalization is perceived by the shoot apex.

• The vernalization state is grafting transmissible.

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• Can be induced quickly• Increases plant resistance to freezing

stress• Does not affect flowering time.

Cold acclimation

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Hormone GA regulates flowering time

GA1: an enzyme involved in GA biosynthesis

ga1: In addition to the dwarf phenotype, the mutant flowers late under LD conditions and does not flower under SD conditions.

GA treatment promotes flowering time.

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Flower development in ArabidopsisVegetative meristem

Inflorescence meristem

Floral meristem

Flower: sepals, petals, stamens,

and carpels

Transition to reproduction: Genes & other factors

Flower organ development:Organ identity genes

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Flower organs

• petal• stamen• carpel• sepal

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The flower is generated from the floral meristem

the floral meristem

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Produced in 4 concentric whorls with the same ordersepal (whorl 1) stamen (whorl 3)

petal (whorl 2) carpel (whorl 4)

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Flower organs

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(the ap1 mutant is similar)

stamen-carpel-stamen-carpelsepal-petal-stamen-carpel

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wt pistillata (pi)

sepal-sepal-carpel-carpelsepal-petal-stamen-carpelapetala3 (ap3)

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sepal-petal-stamen-carpel sepal-petal-petal-sepal

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The “ABC” model for flower development

• The ABC genes function in the distinct fields. • The A and C genes are mutually exclusive in their

expression.

A

B

C

AP1, AP2

AP3, PI

AG

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The A genes: ap1 or ap2 mutants should (and do) make carpel-stamen-stamen-carpel

ap1 or ap2

WT

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The B genes: ap3 or pi mutants should (and do) make sepal-sepal-carpel-carpel

WT

ap3 or pi

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The C genes: ag mutants should (and do) make sepal-petal-petal-sepal

WT

ag

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Gametogenesis and Fertilization

Flower organ function:

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generative cell

Diploid pollen mother cells undergo meiosis to produce a tetrad of haploid microspores. Each microspore develops into a pollen grain containing two haploid cells (mitosis I):• the generative cell (small)

•The vegetative cell (large)

Male gametogenesis

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• the vegetative cell grows to produce the pollen tube

• the generative cell produce 2 sperm cells (mitosis II)

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an ovule primordium emerges as a bump on the inner wall (placenta) of the ovary

the megasporocyte undergoes meiosis to produce 4 haploid cells, only one of which (the megaspore) survives.

Female gametogenesis

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placental wall

Female gametogenesis

the megaspore undergoes 3 mitotic divisions to produce 8 cells:

•3 antipodal cells •2 synergid cells •2 central cell nuclei •1 egg cell (EC)

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Female gametogenesis

placental wall

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Double fertilizationPollens land on the stigma, hydrate, and begin to germinate the pollen tube

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Pollen tubes grow, by tip growth, down through the stigma and style and into the ovary, toward the ovules.

The pollen tube navigates to the micropyle and discharges the two sperm cells.

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• One sperm fertilizes the egg cell to develop into the embryo.

• the other sperm fertilizes the diploid central cell nucleus to develop into the endosperm.

Micropyle

Egg

Synergids

Antipodal cells

Central cell nuclei

Sperms

Pollen tube

Ovule

Double fertilization

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Plant reproduction

Ovule (1 to many)Ovary

Silique

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Fruit development

• The ovary and other tissue together produce a fruit.

• Fruit is important for seed dispersal in many species

• Many foods are also called “vegetables”: tomatoes, pea pods, squash

• Fruit size, texture, and sugar content are determined by genes.

• Ethylene stimulates fruit ripening.

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Life cycle of a flowering plant

2n

2n

2n

2n 1n

2n