Photomorphogenesis: plant responses to light

1. Photomorphogenesis: plant responses to light Rost et al., “Plant biology”, 2nd edn Plant Phys and Biotech Biology 3470 Lecture 6, Tues. 24 Jan 2006 Text Chapter 17

2. 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

3. A light perception system is critical for the meaningful regulation of plant metabolism • 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)

4. 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 Phytochrome’s conformation is photoreversible: light changes the protein’s shape! (665 nm) Phytochrome protein synthesis Pr Pfr Fig. 17.3 (730 nm)

5. 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 dimerization Apoprotein Conversion between Pr and Pfr involves rotation between rings of the chromophore Chromophore (phytochromobilin) Fig. 17.4: the structure of phytochrome

6. 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

7. Pfr is the biochemically and physiologically active form of phytochrome Fig 17.7 • 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

8. There are 3 categories of plant responses to phytochrome The categories are light-level dependent • VLFRs (very low fluence responses) • < 10-3 mmol photons/m2 (converts 0.01% of phytochrome) • HIR (high irradiance responses) • >1000 mmol photons/m2, continuous irradiation, dependent on actual fluence • LFRs (low fluence responses) • 1-1000 mmol photons/m2, FR light reversible • We will concentrate on LFRs only

9. 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

10. De-etiolation is regulated by phytochrome De-etiolated Etiolated • 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 Grown under normal light Fig 17.9 • 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)

11. 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)

12. Light quality has direct effects on plant growth via phytochrome Fig. 17.12 • 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 = growth rate R FR control FR R End of day light treatment high Pfr (inhibitory) high Pr (simulatory) For stem elongation in green plants,

13. Real-world conditions explain the role of phytochrome Fig. 17.14 • 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 Little Pfr Lots of Pfr Little Pfr Lots of Pfr

14. The ratio of R/FR affects the amount of Pfr and its ability to control transcription Small change in R/FR causes large change in amount of Pfr (Recall that high Pfr inhibits stem elongation!) • 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 Amount of Pfr Lots of R Little R Fig. 17.13

15. 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

16. kinase phosphatase Protein Protein–P Photosensors modulate plant responses to light Other photosensors respond to light other than R and FR • Blue and UV light is also important 2 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 !

17. How does phytochrome regulate gene expression? Fig. 17.17 • Pfr is directly involved in mediated gene expression (mRNA synthesis) • Light-grown plants express more of enzymes involved in autotrophism (rubisco, light-harvesting) • 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

18. 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