PHYTOCHROME AND PHOTOMORPHOGENESISPHYTOCHROME AND PHOTOMORPHOGENESIS

1. A blue protein pigment with a molecular mass of about 125 kDa.

2. Mediates photomorphogenesis responses to red (660 nm) and far-red (730 nm) light.

PHYTOCHROME CAN INTERCONVERT BETWEEN Pr PHYTOCHROME CAN INTERCONVERT BETWEEN Pr AND Pfr FORMSAND Pfr FORMS

Pr Pfr Red light

Far-red light

PHOTOSTATIONARY STATE - Red LightPHOTOSTATIONARY STATE - Red Light

o When Pr molecules absorb red light, most are converted to Pfr.

o Since Pfr molecules can also absorb red light (see absorption spectrum), some of the Pfr molecules are converted back to Pr.

o Thus after saturating irradiation with red light, only about 85% conversion of Pr to Pfr occurs.

PHOTOSTATIONARY STATE - Far-red lightPHOTOSTATIONARY STATE - Far-red light

o When Pfr molecules are exposed to far-red light, nearly all the molecules are converted to the Pr form.

o Since Pr molecules absorb almost no far-red light, there is almost no back reaction from Pr to Pfr.

o Thus, after saturating far-red light there is about 97% conversion of Pfr to Pr.

Pfr is the Physiologically Active Form of PhytochromePfr is the Physiologically Active Form of Phytochrome(Is phytochrome response due to build-up of Pfr or tp the (Is phytochrome response due to build-up of Pfr or tp the

loss of Pr?)loss of Pr?)

1. In general, the magnitude of the physiological response to red light is proportional to the amount of Pfr produced.

2. In some cases the magnitude of the response is proportional to the ration of Pfr to pr, or of Pfr to Ptot.

3. Phytochrome deficient (hy) Arabidopsis mutants have long hypocotyls in both darkness and white light. If the red light response were due to a lack of Pr, we would expect the opposite to be true, i.e. the hypocotyls would be short in both darkness and white light.

PHYTOCHROME IS A DIMER COMPOSED OF TWO PHYTOCHROME IS A DIMER COMPOSED OF TWO POLYPEPTIDESPOLYPEPTIDES

Synthesized in plastids

Two Types of Phytochrome Have Been identifiedTwo Types of Phytochrome Have Been identified

Type I Type I

a) About 9X more abundant in dark-grown tissues.a) About 9X more abundant in dark-grown tissues.

b) The Pfr form is rapidly degraded.b) The Pfr form is rapidly degraded.

c) The Pfr form feed-back inhibits its own synthesis.c) The Pfr form feed-back inhibits its own synthesis.

Type IIType II

a) Present in in equal amounts with Type I a) Present in in equal amounts with Type I phytochrome in light-grown tissues.phytochrome in light-grown tissues.

b) The Pfr form is not degraded.b) The Pfr form is not degraded.

c) Synthesis of Type II phytochrome is not feed-back c) Synthesis of Type II phytochrome is not feed-back inhibited by Pfr.inhibited by Pfr.

Phytochrome is Encoded By a Multigene Phytochrome is Encoded By a Multigene FamilyFamily

1. Arabidopsis has five structurally related phytochrome genes: PHYA, PHYB, PHYC, PHYD, and PHYE.

2. PHYA is the only Type I phytochrome

3. PHYB - PHYE are all Type II phytochromes.

Types I and II Phytochromes are Regulated DifferentlyTypes I and II Phytochromes are Regulated Differently

1. PHYA mRNA Pr Pfr Response Red

Far-red

Degradation Degradation

Ubiquitin + ATPATP

2. PHYB-E mRNA Pr Pfr Response Red

Far-red

PHYTOCHROME RESPONSES VARY IN LAG TIME PHYTOCHROME RESPONSES VARY IN LAG TIME

1. LAG TIME - THE TIME BETWEEN THE STIMULUS AND THE OBSERVED RESPONSE

A. PHYTOCHROME RESPONSES MAY BE RAPID (A FEW MINUTES) OR LONG TERM (SEVERAL WEEKS).

PHYTOCHROME RESPONSES VARY IN THE AMOUNT OF PHYTOCHROME RESPONSES VARY IN THE AMOUNT OF LIGHT REQURIED LIGHT REQURIED

A. FLUENCE - TOTAL NUMBER OF PHOTONS IMPINGING ON A UNIT SURFACE AREA (micromoles/m2)

1. VLFR - VERY LOW FLUENCE RESPONSE

1. LFR - LOW FLUENCE RESPONSE

B. IRRADIANCE - FLUENCE RATE; NUMBER OF PHOTONS IMPINGING ON UNIT SURFACE AREA PER UNIT TIME (micromoles/m2/s)

1. HIR - HIGH IRRADIANCE RESPONSE

PHYTOCHROME RESPONSES VARY IN THEIR PHYTOCHROME RESPONSES VARY IN THEIR RECIPROCITY RECIPROCITY

A. LAW OF RECIPROCITY -

FLUENCE = FLUENCE RATE X TIME

(micromoles/m2) = (micromoles/m2/sec) X (sec)

B. According to the Law of Reciprocity, treating plants with a brief duration of high fluence rate (bright) light will five the same response as treating them with a long duration of low fluence rate (dim) light.

C. Reciprocity applies to most photochemical reactions, like the exposure of film.

ECOLOGICAAL FUNCTIONS OF PHYTOCHROME: SHADE ECOLOGICAAL FUNCTIONS OF PHYTOCHROME: SHADE AVOIDANCEAVOIDANCE

PHYTOCHROME HELPS PLANTS TO ADAPT TO PHYTOCHROME HELPS PLANTS TO ADAPT TO CHANGING LIGHT CONDITIONS BY SENSING THE RATIO CHANGING LIGHT CONDITIONS BY SENSING THE RATIO OF RED AND FAR-RED LIGHTOF RED AND FAR-RED LIGHT

ECOLOGICAAL FUNCTIONS OF PHYTOCHROME: ECOLOGICAAL FUNCTIONS OF PHYTOCHROME: SHADE AVOIDANCESHADE AVOIDANCE

PHYTOCHROME HELPS PLANTS TO AVOID SHADING BY PHYTOCHROME HELPS PLANTS TO AVOID SHADING BY OTHER PLANTS BY SENSING THE RATIO Pfr/PtotalOTHER PLANTS BY SENSING THE RATIO Pfr/Ptotal

A GREATER PROPORTION OF FAR-RED LIGHT A GREATER PROPORTION OF FAR-RED LIGHT CONVERTS MORE Pfr TO Pr, AND THE Pfr/Ptotal RATIO CONVERTS MORE Pfr TO Pr, AND THE Pfr/Ptotal RATIO DECREASES.DECREASES.

ONLY “SUN PLANTS’ EXHIBIT SHADE AVOIDANCE.ONLY “SUN PLANTS’ EXHIBIT SHADE AVOIDANCE.

““SHADE PLANTS” ARE ADAPTED TO SHADING, AND SHADE PLANTS” ARE ADAPTED TO SHADING, AND THEREFORE DO NOT EXHIBIT SHADE AVOIDANCE.THEREFORE DO NOT EXHIBIT SHADE AVOIDANCE.

PHYTOCHROME REGULATES CIRCADIAN RHYTHMSPHYTOCHROME REGULATES CIRCADIAN RHYTHMS

CIRCADIAN RHYTHM = APPROXIMATELY A DAYCIRCADIAN RHYTHM = APPROXIMATELY A DAY

PERIOD = TIME BETWEEN SUCCESSIVE PEAKS OR TROUGHS IN A PERIOD = TIME BETWEEN SUCCESSIVE PEAKS OR TROUGHS IN A CYCLECYCLE

ENDOGENOUS RHYTHM = UNAFFECTED BY EXTERNAL STIMULI; ENDOGENOUS RHYTHM = UNAFFECTED BY EXTERNAL STIMULI; REGULATED BY INTERNAL FACTORSREGULATED BY INTERNAL FACTORS

OSCILLATOR = BIOLOGICAL CLOCK MECHANISMOSCILLATOR = BIOLOGICAL CLOCK MECHANISM

TEMPERATURE COMPENSATION = RHYTHM UNAFFECTED BY TEMPERATURE COMPENSATION = RHYTHM UNAFFECTED BY TEMPERATURETEMPERATURE

ENTRAINMENT = ABILITY OF A STIMULUS (TYPICALLY LIGHT) TO ENTRAINMENT = ABILITY OF A STIMULUS (TYPICALLY LIGHT) TO RESET OR SYNCHRONIZE THE RHYTHM. RESET OR SYNCHRONIZE THE RHYTHM.

THE CIRCADIAN OSCILLATOR INVOLVES A TRANSCRIPTIONAL NEGATIVE FEEDBACK LOOP

DIFFERENT PHYTOCHROMES HAVE DIFFERENT DIFFERENT PHYTOCHROMES HAVE DIFFERENT ECOLOGICAL FUNCTIONSECOLOGICAL FUNCTIONS

PHYTOCHROME B MEDIATES THE RESPONSE TO CONTINUOUS PHYTOCHROME B MEDIATES THE RESPONSE TO CONTINUOUS RED OR WHITE LIGHTRED OR WHITE LIGHT

PHYTOCHROME A IS REQUIRED FOR THE RESPONSE TO PHYTOCHROME A IS REQUIRED FOR THE RESPONSE TO CONTINUOUS FAR-RED LIGHTCONTINUOUS FAR-RED LIGHT

PHYAPHYA AND AND PHYBPHYB CAN HAVE MUTUALLY ANTAGONISTIC EFFECTS CAN HAVE MUTUALLY ANTAGONISTIC EFFECTS

PHYTOCHROME REGULATES GENE EXPRESSIONPHYTOCHROME REGULATES GENE EXPRESSION

• THE STIMULATION AND REPRESSION OF GENE TRANSCRIPTION THE STIMULATION AND REPRESSION OF GENE TRANSCRIPTION BY LIGHT CAN BE VERY RAPID (~5 MINUTES).BY LIGHT CAN BE VERY RAPID (~5 MINUTES).

•THESE RAPID CHANGES INVOLVE DIRECT ACTIVATION OF THESE RAPID CHANGES INVOLVE DIRECT ACTIVATION OF TRANSCRIPTION FACTORS BY ONE OR MORE SIGNAL TRANSCRIPTION FACTORS BY ONE OR MORE SIGNAL TRANSDUCTION PATHWAYS.TRANSDUCTION PATHWAYS.

•SOME OF THE EARLY GENE PRODUCTS ARE TRANSCRIPTION SOME OF THE EARLY GENE PRODUCTS ARE TRANSCRIPTION FACTORS WHICH ACTIVATE THE EXPRESSION OF OTHER GENES.FACTORS WHICH ACTIVATE THE EXPRESSION OF OTHER GENES.

•EXPRESSION OF THE EARLY GENES, CALLED PRIMARY RESPONSE EXPRESSION OF THE EARLY GENES, CALLED PRIMARY RESPONSE GENES, IS INDEPENDENT OF PROTEIN SYNTHESIS AND INVOLVES GENES, IS INDEPENDENT OF PROTEIN SYNTHESIS AND INVOLVES ACTIVATION OF PRE-EXISTING TRANSCRIPTION FACTOR PROTEINSACTIVATION OF PRE-EXISTING TRANSCRIPTION FACTOR PROTEINS

Primary response gene

Secondary response gene

PHYTOCHROME MOVES INTO THE NUCLEUSPHYTOCHROME MOVES INTO THE NUCLEUS

phyA-GFP, continuous FR light phyB-GFP, continuous white light

Phytochrome is a Serine/Threonine Protein KinasePhytochrome is a Serine/Threonine Protein Kinase

1.1. Cyanobacterial phytochrome is part of a two-component Cyanobacterial phytochrome is part of a two-component system signal transduction pathwaysystem signal transduction pathway

2.2. Cyanobacterial phytochrome is a histidine kinaseCyanobacterial phytochrome is a histidine kinase

3.3. Eukaryotic phytochrome is a serine/threonine kinaseEukaryotic phytochrome is a serine/threonine kinase.