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- Today’s Lecture -Trichome development

And Switching into reproductive

phase

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For more info-- Szymanski DB, Lloyd AM, Marks MD. Progress in the

molecular genetic analysis of trichome initiation and morphogenesis in Arabidopsis. Trends Plant Sci. 2000 May;5(5):214-9. Review.

Michaels, S. D. and R. M. Amasino. 2000. Memories of winter: vernalization and the competence to flower. Plant Cell & Environment 23: 1145 -1154. See-- http://www.biochem.wisc.edu/amasino/publications.html

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Plant Trichomes-- insight into cellular differentiation in plants Trichome = hair like structure that

extend from the epidermis of aerial tissues Function of trichomes-- boundary layer

between epidermis and environment Reduce heat and water loss Protection against herbivory and pathogens Secretion of chemicals or physically limit

access

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Trichome on Lemon Basil

Capitate trichome from Ocimum ssp. (Lemon Basil)http://fisher.bio.umb.edu/pages/JFTrich/trichome.htm2001

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Plumbago-- trichomes protecting nectories

http://www.botany.hawaii.edu/faculty/webb/BOT201/Angiosperm/plumbago.htm

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Example-- Arabidopsis trichomes

Fully formed & Developing Developing

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Arabidopsis Trichome

An Arabidopsis trichome is a single cell

Trichomes can be branched

The basal portion is called the stalk

There are three spikes in the apical

portion.

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Trichome Number and Distribution Normal plants have leaves with evenly

spaced solitary trichomes The Try mutation causes formation of

small clusters of 2-5 trichomes; therefore the normal TRY gene regulates the number of trichomes at any specific position

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Trichome development-- Steps in development

Cell proliferation-- number of cells per trichome Differentiation Inter-cellular communication-- lateral inhibition to

get regular spacing Morphogenesis control-- branching

Control in development Integration with leaf development (cell division

and expansion) Hormone levels Vegetative phase development

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Restricted Cell Division in Trichome development

A normal trichome has endoreduplication (DNA replication) but no mitosis and cytokinesis, THEREFORE-- cell division is inhibited

The SIAMESE (SIM) gene is required for this inhibition

In sim mutants, trichome cell continue to divide, resulting in multi-celled trichomes

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Morphology versus Cell Division Because plant cells don’t move, cell

division is thought to be critical for plant morphogenesis

In the sim mutant, trichomes are multi-cellular, but they have normal morphology

This is indicates that alteration of cell

division does not automatically change

morphology

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Genetic Regulation of Trichome Initiation

Series of checks and balances

Initiation Glabrous1 (GL1) Transparent Testa

Glabra1 (TTG1) Glabrous3 (GL3)

• Checks Glabrous1 (GL1) Transparent Testa Glabra1 (TTG1) Triptychon (TRY) Caprice (CPC)

TTG1 GL3GL1

CPC

TTG1TRY

GL1

TRICHOMEINITIATION

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GL1 and TTG proteins GL1 encodes a MYB-domain protein;

MYB domain proteins are probably transcription factors

TTG encodes a WD-40 domain protein; WD-40 domain can interact with other proteins

GL1 and TTG proteins regulate the expression of genes needed for trichome initiation and endoreduplication

Evidence suggests these form a protein complex that controls trichome initiation

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Genetic Regulation of Trichome Formation

Endoreduplication-- synthesis of DNA w/o cell division

Glabrous2 (GL2) Inhibit by Triptychon

(TRY)

Morphogenesis (aerial expansion of cell and spacing)

Glabrous3 (GL3) Glabrous2 (GL2)

GL3GL2TRY

TRICHOME INITIATION

Morphogenesis

DNA synthesis in absence of cell division (polyploid)

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Genes Regulating Early Cell Growth

GL2 is needed for the local outgrowth, and encodes a homeodomain protein

Many genes regulate the extension growth and morphogenesis.

Mutants in these genes have abnormally-shaped trichomes

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Cytoskeleton and Morphology

Actin filament is a major component of cytoskeleton Actin filament form thick cables during

trichome growth and disruption of actin filament causes distorted growth

Actin filament organization is abnormal in

trichome growth mutants: alien, crooked,

dis1, gnarled, klunker, and wurm

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Genes involved in branching Several genes control the branching

to trichomes Evidence indicates that they interact

with cytoskeleton and microtubule formation

Zwichel -- kinesin-like motor protein Furca gene family, Angustifolia and

stachel

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Pathways in trichome development

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- Meristems - What controls the type of

structures meristems generate? Vegetative growth verses Reproductive growth

(inflorescence and flowers) Shift in plant development is a change in fate

of meristem

Vegetative Inflorescence flowers

Timing of switch is regulated by environmental factors (light, temperature, gravity, moisture and nutrients)

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Shoot meristem formation and maintenance [Review article] Michael Lenhard, Thomas Laux Current Opinion in Plant Biology 1999, 2:44-50.

Meristem identity

Vegetative

versusInflorescence?

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Regulation of vegetative to reproductive transition Internal vs.

external factors

Signals may vary among plants

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When to flower? That is the question.

Variation among plants in triggers to flower (annual / biennial / perennial)

Three independent pathways:

Long-day pathway (photoperiod)

Autonomous Pathway Vernalization (cold-treatment or

extended exposure to cold)

Gibberrellin (GA)

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Photoperiod or daylength controls switch from vegetative growth to reproductive growth is

From floral induction to floral shape [Review article] Detlef Weigel. Current Opinion in Plant Biology 1998, 1:55-59.

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Light Perception-- Red and Far Red Light

Plants have several types of light receptors

One type is called phytochrome, which can absorb red and far red light

In Arabidopsis, there are five isoforms of phytochromes: PhyA, PhyB, PhyC, PhyD, and PhyE

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Phytochromes Phytochromes have two domains:

A light-sensing domain A protein kinase domain

Light (photon) probably causes a change of conformation in the light sensing domain, which activates the protein kinase domain

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Photoperiod-- Controls Constans (Co) gene Affects downstream genes

controlling the meristem. Meristem identity genes -- Master

regulatory genes of floral initiation Induction of inflorescence

Leafy (LFY) Terminal flower (termination of meristem)

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Autonomous Pathway Flowering is inhibited by Flowering locus C

(FLC) Autonomous-pathway genes (FCA, FLD, LD

and FVE) promote flowering by down-regulating FLC.

In biennial plants, FRI (Frigida) up-regulates FLC leading to delayed flowering (i.e. vegetative growth is prolonged).

FRI is dominant over the effects of the autonomous pathway genes.

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Autonomous Pathway Balance between vegetative and

flowering phases, depends upon FLC expression in annual plants

Richard M. Amasino, 2001http://www.biochem.wisc.edu/amasino/

Low FLC, Flower on! Mutant LD, Flower off!

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-- Vernalization -- Cold treatment triggers flowering by

downregulating FLC expression

Flower off! Flower on!Richard M. Amasino, 2001http://www.biochem.wisc.edu/amasino/

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Interaction of pathways to flowering-- as determined by genetic experiments

Reeves PH, Coupland G. Analysis of flowering time control in Arabidopsis by comparison of double and triple mutants. Plant Physiol. 2001 Jul;126(3):1085-91

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Floral initiation -- genetic regulation Meristem identity genes -- Master

regulatory genes of floral initiation Induction of inflorescence

Leafy (LFY) Unusual floral organs (UFO)

Apetala1 (AP1) Cauliflower (CAL)

Apetala2 (AP2) Maintenance of flowering -

Terminal flower Agamous

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Master regulatory genes of floral initiation Induction of inflorescence-- switch from

vegetative to inflorescence Primary genes-- initial role in floral fate Leafy and Apetala1 -- major roles,

LOF -- no floral structures GOF -- precocious flowers form

Gene regulation -- transcriptional activators Leafy expressed first, turns on Apetala1 and

Cauliflower-- Feedback on each other

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Floral meristem Identity Genes -- induction of inflorescence

Genetic control of shoot and flower meristem behavior [Review article] Sarah J Liljegren, Martin F Yanofsky.

Current Opinion in Cell Biology 1996, 8:865-869.

lof gof

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Master regulatory genes of floral initiation

Maintenance of flowering - How many flowers to form?

How to stop making additional

flowers within a flower?

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Maintenance of flowering Two genes of opposite actions

Terminal flower -- keeps meristem active forming additional floral meristems (stalk of flowers)--

LOF, one flower Agamous -- terminate cell division and

additional generation of floral program within a floral meristem

LOF-- flower within a flower, etc.

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The making of a flower: control of floral meristem identity in Arabidopsis [Review] Pidkowich, Klenz and Haughn. Trends in Plant Science, 1999, 4:2:64-70

Floral Meristem ID genes --

LOF phenotypes

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Next lecture--

How to make a flower?

(Please read Chapter 20 before this lecture)