Photomorphogenesis: plant responses to light

Plant Phys and BiotechBiology 3470

Lecture 6, Tues. 24 Jan 2006 Text Chapter 17

Rost et al., “Plant biology”, 2nd edn

Photomorphogenesis is the plant’s response to light

• An obviously integral element of normal development in autotrophic organisms like plants

• We will be looking at the role of phytochrome in perceiving and translating light signals into changes in plant shape and function

• Light dictates a plant’s metabolism– Quality (spectrum)– Amount (flux) – more light, more photosynthesis, higher

growth rate– Timing (diurnal patterns) important in development

• Plants are sessile: must deal with environmental limitations to survive

• Plants use photoreceptors to detect environmental light changes

• These act as initiators of signal transduction cascades that ultimately direct plant’s response to light level

• Most light responses are controlled by chromoproteins– They contain a chromophore – absorbs light– They are an apoprotein – undergoes conformational

change (initiates signal transduction cascade)

A light perception system is critical for the meaningful regulation of plant metabolism

Phytochrome is the primary plant chromoprotein

• Phytochrome exists in 2 stable states that absorb light at different wavelengths

• There are multiple phytochromes in plants different proteins expressed at different times during development

• These collectively form the photochromic receptor system

(665 nm)

(730 nm)

Pr Pfr

Phytochrome protein synthesis

Phytochrome’s conformation is photoreversible: light changes the protein’s shape!Fig. 17.3

Only the Pfr form of phytochrome is involved in signal transduction

• Pr is the physiologically inactive form• Exposed to red light becomes Pfr active!

– You should see this in the lettuce seed experiment• How light interconverts Pr ↔ Pfr not clear

– Likely affects protein folding and dimerizationApoprotein

Chromophore (phytochromobilin)

Fig. 17.4: the structure of phytochrome

Conversion between Pr and Pfr involves rotation between rings of the chromophore

Measuring phytochrome effects• For this, physiologists use etiolated (not green) seedlings• Recall that red light converts Pr to Pfr

– Thus have a high Pfr/Pr ratio

– but Pfr is 100X more unstable than Pr and is degraded in vivo

• Therefore, under red light, total phytochrome levels drop over time

17.5

Pfr is the biochemically and physiologically active form of phytochrome

• In the presence of this physiologically active form, the transcription rate of phytochrome genes decreases– Enough protein is present to do its

job, no more is required– Pfr does this directly!

• Existing Pfr protein is gradually broken down (proteolyzed)

• This feedback loop maintains a relatively constant amount of phytochrome in autotrophic cells

Fig 17.7

There are 3 categories of plant responses to phytochrome

The categories are light-level dependent 1. VLFRs (very low fluence responses)

• < 10-3 mol photons/m2 (converts 0.01% of phytochrome)

2. HIR (high irradiance responses)• >1000 mol photons/m2, continuous irradiation,

dependent on actual fluence

3. LFRs (low fluence responses) • 1-1000 mol photons/m2, FR light reversible

• We will concentrate on LFRs only

Low fluence responses are the most studied changes induced by

phytochrome• These regulate important plant growth and

development responses including– De-etiolation

• the greening of etiolated seedlings• Important as seedlings emerge from the soil and

begin to become autotrophic

– Seed germination • breaking of seed coat, start of active metabolism in

new plant• Light can promote or inhibit germination, depending

on the species

De-etiolation is regulated by phytochrome• Etiolated – typical seedling response

when dark-grown– Long hypocotyl (stem below the cotyledons)– Not green – no chlorophyll, no chloroplasts– Little leaf development or unfolding

• Expose to light (de-etiolated):– Hypocotyl stops growing– Chlorophyll and chloroplasts develop– Leaves unfold

• Therefore plants need light to developmentally progress from heterotrophic autotrophic organism

Seeds can be one of two types• Positively photoblastic: ↑ germination in light (need high Pfr/Pr)• Negatively photoblastic: ↓ germination in light (need low Pfr/Pr)

Fig 17.9

Grown under normal light

De-etiolated

Etiolated

The time scales for phytochrome-mediated effects vary

• Usually long (h d) growth effects– Germination– De-etiolation– Circadian clock (daily light effects)

• Some short (s min)– Transmembrane potential

• Red light depolarizes membranes!• FR repolarizes• Due to ion movements via specific membrane transporters

(electrically gated channels)

• The main form of phytochrome (A) has characteristics suggesting that it is the primary plant light sensor – Degrades in light– Other, more sensitive forms of phytochrome may monitor

light quality (e.g., phy B)

Light quality has direct effects on plant growth via phytochrome

• Consider bean plants given an end-of-day R or FR light treatment

• Plant growth is inhibited by red light (R) high Pfr• FR light has no effect in etiolated seedlings high

Pr• The effect of phytochrome is thus developmentally

dependent• FR actually stimulates stem growth at high fluence

rates in green plants, R inhibits it• Contrast this observation with the germination of

lettuce seedlings

high Pfr (inhibitory)

high Pr (simulatory)

Fig. 17.12

FR Rcontrol

= growth rate

End of day light treatment

R FR

For stem elongation in green plants,

Real-world conditions explain the role of phytochrome

• All of the previous studies were performed in the lab

What happens in the real world?• Under a forest canopy, there is

– Little available red or blue light (absorbed!)

– Lots of FR (chlorophyll transparent)

– Low R/FR ratio

– This converts Pfr Pr

– This suppresses lengthening of internodes

• Thus the relative amount of R and FR modulates the production of the active form of phytochrome (Pfr) and thus the metabolic activity in the plant– Transcription of genes needed for growth

– Enzyme activity

Lots of PfrLittle Pfr

Lots of Pfr

Little Pfr

Fig. 17.14

The ratio of R/FR affects the amount of Pfr and its ability to control transcription

• The R/FR ratio changes frequently in the natural environment (along X-axis) – second-to-second: sunflecks, short

shading periods, etc.)• At bottom of canopy: little R

light• Therefore small changes in R/FR

cause large changes in Pfr

• Recall that Pfr is the active form of phytochrome – it activates or represses

transcription of certain genes

• Therefore, Pfr is a very good photoreceptor on the forest floor– it can respond quickly to the light

environment and adjust gene expression accordingly

Small change in R/FR causes large change in amount of Pfr (Recall that

high Pfr inhibits stem elongation!)

Fig. 17.13

Little R Lots of R

Amount of Pfr

How does phytochrome work in the natural environment?

• There are 5 phytochrome genes – why?• PhyA is the major phytochrome protein (≡ total

phytochrome) and accumulates– In seeds requiring R to germinate (surface germinators)– In germinated seedlings as they prepare to break

through soil

• But why accumulate Phy in etiolated plants and degrade it upon R exposure when the seedling breaks through the soil surface?

• Phytochrome mediates R, FR responses– Plants also respond to higher energy light (blue, UV-A)

using other photosensor molecules

Photosensors modulate plant responses to lightOther photosensors respond to light other than R and FR• Blue and UV light is also important2 photoreceptors recently isolated in plants• Cryptochrome: UV-A responsive

– Responds to UV-A– Flavoprotein with 2 chromophores (flavins) – mediates blue light suppression

of elongation of hypocotyls and cotyledon expansion

• Phototropin: blue light responsive– Like most receptors, found in _____ ____ of cells in actively growing regions

of etiolated seedlings– It is a kinase – autophosphorylates in blue light

• Key components of signal transduction chains (phosphorylation cascade)• Induced Ca2+ uptake regulation of cytoplasmic [Ca2+]• Ca conc. redistributions important in other tropisms gravitropism

– This is evidence for crosstalk between tropisms !

Protein Protein–Pkinase

phosphatase

How does phytochrome regulate gene expression? Fig. 17.17

• Pfr increases transcription rates of these genes• How?

– Pfr activates regulatory protein (RP)– Activated RP binds to light-responsive element (LRE)

upstream of light-responsive gene– This activates transcription of that gene

• Pfr is directly involved in mediated gene expression (mRNA synthesis)

• Light-grown plants express more of enzymes involved in autotrophism (rubisco, light-harvesting)

Gene regulatory proteins alter gene expression in partnership with Pfr

• Regulatory proteins can enter nucleus to affect gene expression – so can Pfr!

• The subcellular location of Pfr is affected by light

How? One example…• A regulatory protein (PIF3) binds

to the promoter of a light-sensitive gene

• PIF3 alone does not activate transcription of this gene

• Pfr enters the nucleus and binds to PIF3

• This binding changes the shape of PIF3, letting it turn on transcription

• Amount of R/FR regulates binding to PIF3 and thus transcription of gene

Fig. 17.18