Photomorphogenesis: plant responses to light Plant Phys and Biotech Biology 3470 Lecture 6, Tues. 24 Jan 2006 Text Chapter 17 Rost et al., “Plant biology”, 2 nd 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) PrPr Pf r Phytochrome protein synthesis Phytochrome’s conformation is photoreversible: light changes the protein’s shape! Fig. 17.3

Only the P fr form of phytochrome is involved in signal transduction P r is the physiologically inactive form Exposed to red light  becomes P fr  active! –You should see this in the lettuce seed experiment How light interconverts P r ↔ P fr not clear –Likely affects protein folding and dimerization Apoprotein 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 P fr /P r ratio –but P fr is 100X more unstable than P r and is degraded in vivo Therefore, under red light, total phytochrome levels drop over time 17.5

P fr 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 P fr 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) <  mol photons/m 2 (converts 0.01% of phytochrome) 2.HIR (high irradiance responses) >1000  mol photons/m 2, continuous irradiation, dependent on actual fluence 3.LFRs (low fluence responses)  mol photons/m 2, 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 FRRcontrol = 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 P fr  P r –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

The ratio of R/FR affects the amount of P fr 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 P fr Recall that P fr is the active form of phytochrome –it activates or represses transcription of certain genes Therefore, P fr 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 P fr (Recall that high P fr inhibits stem elongation!) Fig 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 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 Ca 2+ uptake  regulation of cytoplasmic [Ca 2+ ] Ca conc. redistributions important in other tropisms  gravitropism –This is evidence for crosstalk between tropisms ! Protein Protein–P kinase phosphatase

How does phytochrome regulate gene expression? Fig P fr increases transcription rates of these genes How? –P fr activates regulatory protein (RP) –Activated RP binds to light-responsive element (LRE) upstream of light-responsive gene –This activates transcription of that gene P fr 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 P fr ! 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