BS1003: The transition to flowering. Pat Heslop-Harrison

Cell and Developmental BiologyModule BS1003

Lectures on Slideshare and blackboard (under Schwarzacher)

Plant Cell and Developmental BiologyTinyurl.com/phhLight

Tinyurl.com/phhFlowering

Pat Heslop-Harrison

phh4@le.ac.uk

Sugar beet seed genetics

• Change from multigerm with several seeds joined together

• To monogerm with single seeds –giving the 40% yield increase of the Metro article

• Genetic character (cf lettuce and seed germination)

Cell and Developmental BiologyModule BS1003

Lectures on Slideshare and blackboard (under Schwarzacher)

Plant Cell and Developmental BiologyTinyurl.com/phhLight

Tinyurl.com/phhFlowering

Pat Heslop-Harrison

phh4@le.ac.uk

British Sugar. Display at Cereals 2012.

Aim: To develop your knowledge & understanding of the cell and developmental biology of plants

Objectives: You should be able to describe….• Last lecture: The role of light in regulating

growth - photomorphogenesis

• The transition to flowering

• Next up: Dr Trude Schwarzacher on genomics and biotechnology

By the end of this lecture you will:

1. How do plants „decide‟ when to flower?

Flowering is seasonal (in temperate regions)

Why?

Much about flowering is new since 2009 and you need to look in recent textbooks; Wikipedia is (unusually) poor on this topic!

By the end of this lecture you will:

1. How do plants „decide‟ when to flower

2. How is temperature sensed?

3. How is daylength sensed?

Much about flowering is new since 2009 and you need to look in recent textbooks; Wikipedia is (unusually) poor on this topic!

http://tinyurl.com/FlowerCuesAndrés F, Coupland G. 2012. The genetic basis of flowering responses to seasonal cues. Nature Reviews Genetics 13, 627-639. (Sept 2012). doi:10.1038/nrg3291 – much of this lecture!

Rafflesia arnoldii

THE TRANSITION TO FLOWERING

1 meter11 kg

Photoperiodism

• Phytochromes play a critical role

• Influences the timing of dormancy and flowering.

• Flowering plants can be classified as long-day, short-day, or day-neutral plants according to the way their flowering responds to night length

• Plants measure night length

• Long-day plants – flower in spring or early summer, when the night period is shorter (and thus the day length is longer) than a defined period

• Short-day plants – flower only when the night length is longer than a defined period such as in late summer, autumn or winter, when days are short

• Day-neutral plants – flower regardless of the night length, as long as day length meets the minimal requirements for plant growth

VEGETATIVE VERSUS REPRODUCTIVE GROWTH

Flower development involves a dramatic change in the STRUCTURE and ACTIVITY of the SHOOT APEX

Vegetativemeristem

Leafprimordia

Inflorescencemeristem

Flowerprimordia

Floralmeristem

Floral organprimordia

Apical meristem transformations

• Shoot apical meristem Inflorescence & floral meristems

• VEGETATIVE SHOOT APEX - simple structure

• 1. LEAF PRIMORDIA EMERGE IN A SPIRAL ARRANGEMENT (PHYLLOTAXY)

• 2. REPETITIVE

• 3. INDETERMINATE

SUMMARY

FLORAL APEX - more complex1. SHOOT STOPS ELONGATION GROWTH2. INITIATES MULTIPLE FLORAL ORGANS3. NON-REPETITIVE4. DETERMINATE

Development of a single flower bud of Arabidopsis

Coordinated growth of different organs

P

P

C

Plant age Light InformationTemperature

FACTORS THAT INFLUENCE FLOWERING

Flowering Signals

• 1. PLANT AGE - JUVENILE TO ADULT FORM

• “RIPENESS-TO-FLOWER”

• eg. Tobacco will only flower after 15-20 nodes

• eg. Many tree species flower only after >10 years

Development of competence to flower• ENDOGENOUS TIMING MECHANISM?

• DIFFUSIBLE FACTORS?

• TEST IN GRAFTING EXPERIMENTS

• MANGO

juvenile mature

If the juvenile shoots, which normally fail to flower, are grafted on to a mature plant, they will flower

• Two GENERAL CHARACTERISTICS that could be required for the ability to flower:

• THE CHRONOLOGICAL AGE OF THE PLANT

• THE LARGER SIZE OF THE PLANT

Century plant(Agave americana)

Botanic GardensUniversity of Leicester

LATE FLOWERING MUTANTS of Arabidopsis

Both OLD and LARGEBut still flower late

Genetic Control

Primulaprimrose/cowslip LDP

Poinsettia Christmas Cactus SDP

SHORT DAY PLANTCoffea arabica SoybeanStrawberryChrysanthemum Christmas cactusDahliasLate summer/autumn

LONG DAY PLANTSWheat/SpinachLettuce/RadishGladiolus/IrisArabidopsisPrimroseLate spring/summer

LIGHT: Duration or photoperiod

• SECOND MAJOR FACTOR• INFLUENCING THE • 'DECISION' TO FLOWER • IS LIGHT (DAYLENGTH)

• 1. LONG DAY PLANTS

• 2. SHORT DAY PLANTS

• 3. DAY-NEUTRAL PLANTS• eg. tobacco, tomato, sunflower• dandelions, cucumbers, roses,

snapdragons, carnations, cotton

LDP

SDP

Day Neutral

FloweringResponse

Photoperiod(h)

CDL = Critical Daylength

WHY USE DAYLENGTH OR OTHER

ENVIRONMENTAL SIGNAL?

• Predicts future season/weather• Allows synchronization of growth and

reproduction• - WITH EACH OTHER PLANT• - WITH THE ENVIRONMENT

• BUT DAYLENGTH CANNOT BE USED TO DISTINGUISH BETWEEN AUTUMN & SPRING

Both have short nights, but very different outcomes!

Harry Allard photoperiod experiments

Tournois (1914) first noted

LIGHT TREATM ENT FLOW ERING RESPONSE

SDP LDP

Relationship between photoperiod and flowering response

Length of the DARK PERIODdetermines the flowering responseIn both SDP & LDP

VegetativeFlower

Vegetative Flower

Vegetative Flower

Vegetative Flower

Promotes flowering in LDP

Day break no effect

Night break inhibits flowering in SDP

HOW DO PLANTS DETECT THE LENGTH OF DARKNESS?

• RED/FAR RED REVERSIBILITY OF THE PHOTOPERIODIC RESPONSE

• MODELS:• SD PLANTS - REQUIRE LONG NIGHTS• - PFR IS DEGRADED TO PR• - PFR INHIBITS FLOWERING • - LOW PFR SIGNALS FLOWERING

• RED LIGHT NIGHT BREAK PREVENTS FLOWERING BY CONVERTING PR TO PFR - inhibitor

REDPR PFR >>>> BIOLOGICAL ACTIVITY

INHIBIT FLOWERING

FAR-RED/DARK

LONG DAY PLANTS > REQUIRE SHORT NIGHTS

• PFR PROMOTES FLOWERING

• INSUFFICIENT DEGRADATION OF PFR TO PR

• RED LIGHT BREAK IN A LONG DARK PERIOD INDUCES FLOWERING BY PREVENTING DEGRADATION OF PFR TO PR

REDPR PFR >>>> BIOLOGICAL ACTIVITY

PROMOTE FLOWERING

FAR-RED/DARK

TEMPERATURE

• SOME PLANTS FLOWER MORE RAPIDLY WHEN SEEDLINGS ARE GIVEN A COLD TREATMENT:

• The promotion of flowering by cold is known as

• VERNALIZATION

• EFFECTIVE TEMPERATURE -2 to +120C• Eg. Autumn sown, Winter wheat/Winter rye• Long term Winter „memory‟ winter > summer

(~200 days)• Many biennials > rosette form over winter >

flower spring/early summer

Sugar beet: biennial crop flowering in „second‟ yearPlant early for maximum establishment and growth before winter… but not so early that it flowers the following summer

Vernalization• Cabbage (biennial)

• Requires exposure to the environmental cue of prolonged winter cold to flower the second spring after planting.

Cabbage grown in the greenhouse for 5 years without vernalization.

Where are the signals detected and how?

• APPROPRIATE light or temperature IS DETECTED, AND THE SIGNAL TRANSDUCED INTO A RESPONSE AT THE SHOOT APEX – perhaps months later

• LEAF (not the apical meristem) IS THE SITE OF DETECTION OF PHOTOPERIOD

EVIDENCE?

• „BAGGING‟

• GRAFTING

•

Cocklebur (Xanthium) –Bonner & Hamner

SDP

BAGGING EXPERIMENTS (Cocklebur= SDP)

Bagging of apical leaf on plant grown in LONG DAYS (un-induced) leads to flowering

Signal

LD LD

GRAFTING EXPERIMENTS (Cocklebur = SDP)

• graft SD-induced leaf onto LD-uninducedstock

induces flowering

repeat cell memory Signal moves

leaf to apex

LD

SD

LD LD

LD LD

• FLOWERING SIGNAL TRAVELS FROM LEAF TO THE SHOOT APEX

• MICHAEL CHAILAKHYAN (1930) POSTULATED A CHEMICAL SIGNAL OR FLOWERING HORMONE

FLORIGEN• 2007: FT-protein is + „florigen‟ (George Coupland)• mRNA and protein made in leaf phloem companion

cells in response to light perception• Protein (small – 20kD) travels in phloem to SAM• In SAM, FT protein combines with another protein

and acts as transcription factor for flower induction genes

• Arabidopsis molecular genetics enabled progress• (NB: Big change: need post-2009 textbook; Wikipedia is poor!)

Tinyurl.com/FlowerCuesFernando Andrés & George Coupland

“Expression of key transcription factors in A. thaliana is regulated by day length or by temperature, creating plasticity in flowering behaviour so that it occurs only under certain environmental conditions. These environmentally responsive transcription factors converge on a small number of floral integrator genes that initiate the early stages of flowering, and this convergence creates a coordinated response to seasonal cues.”

In Arabidopsis, CO (=constans gene) codes for a transcription factor protein that activates FT gene

FT mRNA then transcribed to FT protein and transported to shoot apical meristem(Andres & Coupland)

From Sadava et al. Life: the science of biology. 10th Edition

Complex regulation of transcription factor genes, TF proteins, dependent mRNA transcription and translation!

Overview – 8 LecturesProf Dave Twell

8. Pattern Formation in Plants (Embryogenesis)

9. Meristems & Organogenesis

10. Chemical Communication Systems in Plants

Prof Pat Heslop-Harrison

11. The Role of Light in Plant Development

12. The Transition to FloweringDr Trude Schwarzacher

13. The Biology of Crown Gall

14. Genetic Engineering of Plant Development

15. Genes, Genomes & Genomics in Plants

Module Booklet

Lectures on SlideshareTinyurl.com/phhfloweringTinyurl.com/phhlight

Information in light:Quantity/IntensityQuality/WavelengthDirectionPeriodicity/Daylength

Phytochrome regulates growth & development through gene activation

REDPR PFR >>>> BIOLOGICAL ACTIVITY

FAR-RED

or Dark (slow)

MODEL

Gene activation(includes +ve and –ve responses)

See YouTube – BS1003 Phytochrome practical

LIGHT TREATM ENT FLOW ERING RESPONSE

SDP LDP

Relationship between photoperiod and flowering response

Length of the DARK PERIODdetermines the flowering responseIn both SDP & LDP

VegetativeFlower

Vegetative Flower

Vegetative Flower

Vegetative Flower

Promotes flowering in LDP

Day break no effect

Night break inhibits flowering in SDP

• FLOWERING SIGNAL TRAVELS FROM LEAF TO THE SHOOT APEX

• MICHAEL CHAILAKHYAN (1930) POSTULATED A CHEMICAL SIGNAL OR FLOWERING HORMONE

FLORIGEN

• 2007: FT-protein is + „florigen‟• mRNA and protein detects light; made in leaf• Protein travels in phloem to SAM• In SAM, FT protein is transcription factor

for flower induction genes

From Sadava et al. Life: the science of biology. 10th Edition

Cell and Developmental BiologyModule BS1003

Lectures on Slideshare and blackboard (under Schwarzacher)

Plant Cell and Developmental BiologyTinyurl.com/phhLight

Tinyurl.com/phhFloweringTinyurl.com/FlowerCues

Pat Heslop-Harrison

phh4@le.ac.uk