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Lecture 2 from Pat Heslop-Harrison for BS1003 - Cell and Developmental Biology. The transition to flowering. How do plants decide to flower? How do they respond to daylength (photoperiod) and temperature? For information from light, phytochrome is the photoreceptor, but not the clock/time measuring process. Pfr (phytochrome far red) is always the active form of phytochrome, but the function is different in long day plants and short day plants. Pfr promotes flowering in LDPs but inhibits flowering in SDPs.
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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
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
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
