molecular control of flowering

Question 1

flowering

Answer 1

• most cell division occurs at the meristem
• cell differentiation in the shoot apical meristem (SAM) changes from vegetative (plant growth) to reproductive (flowering)
• once cells move away from meristem they then differentiate
• most of our and many other species’ calorie intake comes from fruits/seeds that develop from flowers

Question 2

Chailaknyan, flowering stimulus in Crysanthenum leaves

Answer 2

• 1937 USSR
• short day flowering plant
• stripped off leaves from top part of plant
• box placed over certain parts of plant to induce short day conditions
• leaves in long days, apex in short days = no flowering
• leaves in short days, apex in long days = flowering
• flowering stimulus must be mobile
• originally though to be a hormone, names ‘florigen’
• identification of ‘florigen’ took 70 years

Question 3

Hamner, flowering experiments on ‘cocklebur’

Answer 3

• 1942 USA
• short day plant
• selected plants with two shoots
• placed different part of plant under short and long day conditions
• 1/8 of a leaf exposed to short days is sufficient to trigger flowering in the other stem (exposed to long days)

Question 4

Zeefaart, grafting P.frutescens leaves

Answer 4

• 1965 USA
• short day plant
• grafting a single leaf from a plant exposed to short day conditions on a plant exposed to long day conditions induced flowering
• can repeat experiment with the same leaf on multiple plants in succession
• signal can be transferred to other plants but does not transfer to other leaves
• signal moves from leaf to apex, likely to be transported by phloem
• ‘photoassimilates’ (organic products of CO2) move from leaf to apex in phloem

Question 5

Takeba and Takimoto, rate of ‘florigen’ movement in Japanese morning glory

Answer 5

• 1966, Japan
• long day plant, tall climber
• cut off donor leaves, receptor bud still flowers
• timed time it took for flower to develop
• moved 102cm in 2hr
• rate of flow ~50cm/h
• same rate as phloem movement, 50-100cm/h

Question 6

Zeefaart, is ‘florigen’ the same substance in short day and long day plants?

Answer 6

• USA 1982
• Crassulacean family
• Kalanchoe (SDP), Sedum (LDP), Echeveria (SLDP), Bryophyllum (LSDP)
• grafting of one plant’s scion (top) on to another plant’s vascular ring
• place root stock under inductive conditions and scion under non-inductive conditions
• ‘florigen’ moved from stock to scion and triggered flowering in all combinations of plant
• same signal in LDP and SDP

Question 7

LSDP and SLDP

Answer 7

• SLDP = plant needs short then long days to flower
• LSDP = plant needs long then short days to flower

Question 8

flowering mutants in Arabidopsis thaliana

Answer 8

• ‘facultative’, long days promote flowering, are not essential
• ft-1 mutant never flowers
• single location in genome induces flowering

Question 9

discovery of ‘florigen’ protein, not hormone

Answer 9

• 4 papers published simultaneously in 2007
• FT = flowering locus T
• encodes small mobile FT protein in leaves
• integrates environmental and endogenous cues regulating flowering time
• day length sensory in leaves related to circadian rhythm main inducer, produces FT protein through cascade
• moves to buds through phloem and induces floral regulator genes in meristem

Question 10

constans (CO)

Answer 10

• main regulator of FT
• regulated by circadian rhythm
• response to long days activates transcription of FT
• CO protein degraded in dark, an increase in day length causes levels to reach peak, able to interact with Ft promoter

Question 11

circadian clock

Answer 11

• all organisms perceive the day
  and night cycles
• to adapt and prepare for these
  cycles, a mechanism based on
  genetic feedback loops has been
  developed
• the circadian clock in plants
  affects many processes on a
  genetic level
• the oscillations in output genes are influenced by the direct binding of rhythmically expressed clock proteins to promoters or transcription factors

Question 12

vernalisation

Answer 12

= the flowering response of some plants to a transition from cold temperatures (winter) to warm (spring)
- effects on flowering are quantative, longer cold exposure promotes flowering more than shorter ones
- after vernalization plants do not necessarily initiate flowering but acquire the competence to do so
- gibberellins activate proteins to switch off FLC production, allowing FT to be expressed (if light conditions are right)

Question 13

climate change and flowering

Answer 13

• warmer winters can shorten the period of cold needed for vernalization, potentially delaying or preventing flowering
• changes in vernalization can lead to shifts in plant phenology including flowering time and leaf growth which can disrupt ecosystems and plant-pollinator interactions
• changes in vernalization can have a significant impact on yields of crops that rely on this process for flowering and seed production e.g. winter wheat and oilseed rape

Question 14

virus induced flowering

Answer 14

• ‘disarmed’ virus engineered to express FT
• crop infected with virus, virus and FT in phloem, flowering

Question 15

floral regulator genes discovered as mutants

Answer 15

• LEAFY (gene symbol LFY) = master regulator of floral genes
  mutant ‘lfy’ flowers have leaf-like organs instead of petals, stamens or carpals
• LFY in Arabidopsis, homologous in other species, same protein same phenotype
• p35S:LFY, wild type LFY gene expressed from p35S constitutive promoter (always on), Arabidopsis flowers at ‘rosette’ stage without inflorescence stem

Question 16

ABC model of flowering

Answer 16

• different zones within floral structure develop into different floral parts
• once the meristem finishes the transition from vegetative to floral, different genes are expressed around the meristem to influence different developmental patterns generating different floral organs
• depends on different transcription factors
• homeotic genes expressed, overlapping gene expression zones