Phytochrome

1
Phytochrome

• Light sensing pigment molecule
• Found primarily in root cap leaf cytoplasm
• Convertible forms allow reversible effects
• Several genes code for different forms of P
• Activated P sets off signal transduction pathways

2

• Phytochrome affects
• Etiolation vs. green growth program
• Innate response is etiolation program
• Light signals turns on green growth program

3

• Affects (cont)
• Seed germination program
• Can affect reference angle of gravitropism
• Governs diurnal events
• Flowering time
• Shade-avoidance response
• Neighbor detection system

4
Photoconversion

• New phytochrome molecules made as Pr
• No developmental effects
• Storage form
• Absorbs red light best
• Red light absorption converts Pr to Pfr
• Causes characteristic developmental effects
• Absorbs far-red light best
• Converts Pfr to Pr

5
Color vision

• Concentration of PfrPr varies with the color of
  light
• But not with intensity of light
• Thus, photostationary state varies w/ light
  quality
• Relative concentrations of PrPfr affect
  development

6
Shade avoidance response

• Remember innate response is the etiolation
  program unless light converts Pr to Pfr (response
  green growth program)
• Shade differs in color from open sunlight
• less R light in shade relative to open sunlight
  thus more Pr than Pfr (response similar to
  etiolation program)
• Sun growth

7
Sun grown signal transduction-what happens with
more shade?

Pfr

8

• Pfr may inhibit elongation by interfering w/ GA
  or sensitivity to GA
• Two types of Phytochrome
• Pr I ? ? Pfr I
• abundant in etiolated plants
• degrades quickly in light
• Pr II ? ? Pfr II
• always a small amt.
• governs green growth
• At least 5 genes for P proteins (Phy A ? Phy E)

9
Vegetative ? floral programs

• Factors that affect flower induction
• Maturity
• Temperature
• Chilling ( vernalization) _at_ 5-10o C
• Stress
• Dehydration
• Mechanical damage
• Hormones
• C2H4
• GA
• Photoperiod

10
Do plants measure days or nights?

• Nights!!

Inductive photoperiod a photoperiod that
stimulates flowering
11
Photoperiod response classes

• Day neutral plants flower when mature
• Crop plants, annuals
• Long day plants (LDP)
• an inductive signal is generated when nights fall
  below a critical night length (CNL)
• Spring ? early summer
• Many biennials perennials
• Short day plants (SDP)
• an inductive signal is generated when nights
  become longer than a critical night length
• Late summer ? fall ? early winter
• Poinsettia, Chrysanthemum, X-mas cactus

16
CNL
CNL
Night length
SDP
Genes specify CNL
8
LDP
6/21
12/21
12/21
12
How to determine a response class?

• Variables that influence photoperiod
• of inductive photoperiods
• Absolute
• Relative
• Temperature

13
Leaf
-
S.A.M.
14
Dawn-dusk sensor

• Phytochrome!!
• Pfr may inhibit the timer
• Normally, timer stays at zero during day
• At night Pfr declines timer starts running
• Biological clock (aka circadian rhythms)

15
Timer

• Timing system poorly understood a clock
• Slow metabolic pathway
• Genes involved
• System is in young expanded leaves
• Demonstrated with grafting experiments

16
Induction signal

• Mobile signal emitted from leaf?shoot tip
• In living cells the symplast likely phloem
• Same signal in LD/SD plants
• Not identified, but named florigen
• Inhibitory signal antiflorigen

17
Pr
Pfr
decay
-
SDP

Florigen generator
Alarm substance
-
LDP


CNL

Biological clock
Induction signal florigen
Vegetative program

Floral program