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Phytochrome and Flowering

Today�s topics: •  More about Hormones

•  Apical dominance •  Ripening •  Dormancy

•  Phytochrome •  Red / Far red •  Control of flowering •  Shade avoidance

12 November 2021

Auxin and Cell Expansion

(a) Apical bud intact (not shown in photo)

(b) Apical bud removed

(c) Auxin added to decapitated stem

Axillary buds

Lateral branches

�Stump� after removal of apical bud

Auxin promotes apical dominance Control of Apical Dominance

Control of Apical Dominance Ethylene is a ripening hormone

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One bad apple CAN spoil the whole bunch . . .

Figure 39.14

(a) ein mutant (b) ctr mutant

ctr mutant ein mutant

Leaf abscision is controlled by the ratio of ethylene to auxin

0.5 mm

Protective layer

Stem Petiole

Abscission layer

ABA is a dormancy hormone

Seed Dormancy

Cold Tolerance

ABA Drought stress in roots causes stomata to

close

(a) Before exposure to light (b) After a week’s exposure to natural daylight

De-etiolation (greening response)

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Phytochrome pigment is activated by Red light Reception

CYTOPLASM

Plasma membrane

Phytochrome activated by light

Cell wall

Transduction Response Transcription factor 1 NUCLEUS

cGMP

Second messenger produced

Protein kinase 1

Transcription factor 2

P

Protein kinase 2

P

Transcription

Light Translation

Ca2+ channel opened

Ca2+

De-etiolation (greening) response proteins

1 2 3

You have seen other signaling pathways like this

Figure 39.16

Results

Red Dark Red Dark

Dark (control)

Red Dark

Data from H. Borthwick et al., A reversible photoreaction controlling seed germination, Proceedings of the National Academy of Sciences USA 38:662–666 (1952).

Far-red

Far-red Red Red Far-red Red Far-red

Phytochrome is a molecular switch that detects Red light

Far Red is just another color

Phytochrome

Chlorophyll

Red Far Red

Many plants us day length as a cue for flowering

Poinsettias are “short day” plants

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Fig. 39-21 24 hours

Light

Critical dark period

Flash of light

Darkness

(a) Short-day (long-night) plant

Flash of light

(b) Long-day (short-night) plant

Phytochrome measures

photoperiod

Plants actually measure the length

of night, not day

Fig. 39-22

24 hours

R

RFR

RFRR

RFRRFR

Critical dark period Short-day

(long-night) plant

Long-day (short-night)

plant

Phytochrome is a molecular switch that detects Red light

Leaves produce the flowering signal

Condition Number of plants that flowered

Short nights, intact plant

0/6

Long nights, intact plant

6/6

Long nights, all leaves removed

0/6

Long nights, all but one leaf removed

6/6

Leaves produce the flowering signal

Flowers No Flowers

Masked leaf has a long “night”

R/Fr ratio also used to detect neighbors

More Red

More Far Red