Plant Responses to Light

Light and Developmental Responses in Plants

Introduction

  • Light is essential for plants, serving as both an energy source for photosynthesis and a carrier of environmental information for developmental processes.
  • This information influences flowering, growth, and coordination with seasons and environments.

Plant Perception of Light

  • Plants perceive light and different wavelengths to translate them into developmental responses.
  • Examples include:
    • Phototropism: Growth towards light mediated by auxin.
    • Shading responses: Seen when plants are covered or shaded.
    • Potato shoot formation: Shoots form in light but remain white in the dark.

Types of Developmental Responses

  • Phototropism: Growth towards a light source.
  • Shade Avoidance: Plants elongate in shaded conditions to seek more light including:
    • Hypocotyl elongation.
    • Altered leaf development with less green leaves.
    • Changes in germination and growth timing.
  • Responses to Light Period Timing: Coordination of flowering with day length, crucial in temperate regions.

The Light Spectrum

Solar Spectrum
  • The atmosphere filters out large amounts of light.
  • Visible light ranges from approximately 350 nm (blue) to 700 nm (red).
  • Plants are also exposed to far-red light, UVA, and UVB.
Light Receptors
  • Plants use light receptors to perceive different wavelengths, similar to how eyes function in animals.
  • These receptors are membrane proteins with photoreceptors that exist in different types of cells.
General Structure of Light Receptors
  • Consist of:
    • A protein (often a dimer) that may sit in the membrane.
    • An intracellular signaling domain (e.g., a kinase domain that phosphorylates other proteins).
    • A chromophore: A functional group covalently bonded to the protein.
Types of Photoreceptors
  • Phytochromes: perceive red and far-red light, mediating developmental responses.
  • Cryptochromes: perceive UVA and blue light.
  • Phototropin: absorbs in UVA and blue light.
  • Zeitlupe: another blue light receptor.

Identifying Photoreceptors

  • Researchers use mutants to study light responses. They observe wild-type and mutant plants under different light conditions.
  • Mutants defective in responding to specific wavelengths are identified.
  • Responses include:
    • Germination in light.
    • Seedling development (de-etiolation).
    • Flowering transitions.
    • Neighbor detection and shade avoidance.
    • Phototropism.
Example: Cryptochrome Mutants
  • Double mutants (e.g., cryptochrome 1 and 2 knockouts) are used to identify photoreceptors by looking at the wavelengths that cause developmental response in wild type but not in mutant.
  • In blue light, the double mutant doesn't respond, indicating the mutated photoreceptor perceives blue light.

How Photoreceptors Work: Phytochromes

  • Phytochromes are photoreversible, switching between activated and deactivated states based on absorbed wavelengths.
Absorbance Spectra
  • Phytochromes have two forms:
    • PR: Absorbs red light.
    • PFR: Absorbs far-red light.
  • The forms convert into each other depending on the light perceived.
  • PR+red light→PFRPR + red\ light \rightarrow PFR
  • PFR+far red light→PRPFR + far\ red\ light \rightarrow PR
  • PFR→darkPRPFR \xrightarrow{dark} PR
  • PFR is usually the active form that triggers physiological changes.
Chromophore Conformation
  • The chromophore's conformation changes upon light absorption, causing a switch between absorbance after red or far-red light exposure.
  • Example: Phytochrome Chromophore
    • A cis isomer switches to trans configuration upon red light absorption, activating the photoreceptor.
    • The reverse occurs with far-red light.
  • Example: Cryptochrome
    • Blue light causes the formation of FADH-.
Protein Conformational Changes
  • In phytochromes, red light irradiation causes conformational changes that allow the protein to move into the nucleus and activate gene expression.
  • The chromophore is derived from a heme group, forming phytobilin and then phytochrome.
Multiple Phytochromes
  • Plants such as Arabidopsis have multiple phytochromes (e.g., A and B) with different kinetics and stability.
  • Phytochrome A is more highly expressed in the dark, while phytochrome B is more stable across light and dark.
  • This variation leads to diurnal responses, where the ratio of different phytochromes changes between day and night.
  • Other light-stable phytochromes include B, C, D, and E.

Summary of Photoreceptor Function

  • Light of different wavelengths is perceived by distinct photoreceptors.
  • Photoreceptors consist of a protein coupled to a chromophore.
  • Interaction with light changes the chromophore's conformation, activating the photoreceptor.
  • The activated photoreceptor alters the transcription of genes, mediating developmental responses.

Shade Avoidance Responses

Light Spectrum in Forests
  • Plants in the shade receive a different light spectrum than those in full sunlight.
  • Sunlight at the top of the canopy includes wavelengths from 400 to 800 nm.
  • At the bottom of the canopy, blue, green, and red wavelengths are reduced significantly, while far-red light penetrates more.
Typical Shade Avoidance Responses
  • Seed germination: Some seeds germinate only in the light, while others wait for light exposure.
  • Hypocotyl elongation: Plants elongate to find more light.
  • Changes in flowering time: Plants may flower earlier under stress.
Agricultural Implications
  • Farmers exploit shade avoidance responses by controlling planting density.
  • Optimal density balances competition and shading to maximize yield.

Molecular Mechanisms of Light Signaling

Phytochrome Interacting Factors (PIFs)
  • PIFs are transcription factors that bind to promoter regions (e.g., G-box) of light-regulated genes.
  • Similar to ARF transcription factors in auxin signaling.
  • In the dark or far-red light, PIFs are free to activate or inhibit gene expression.
  • Red Light Activation:
    • Active PFR moves into the nucleus and interacts with PIFs.
    • This interaction leads to PIF degradation via proteolysis.
  • PFR recruits PIFs for degradation and this is similar to how auxin binds to its receptor recruitment of auxiliary proteins to repressors, and shredding them up.
Light-Inhibited vs. Dark-Inhibited Changes
  • Some PIFs repress genes in the dark, while others activate them.
  • The active PFR form degrades PIFs, leading to either light-inhibited or dark-inhibited gene expression changes.
Interaction with Auxin Pathway
  • Light signaling interacts with the auxin pathway.
  • Downstream genes activated by PIFs can be auxin response genes.
  • Cryptochrome and phytochrome mediate activation of different PIFs, which then alter the expression of auxin genes.
Phototropism
  • Induced by blue light and mediated by cryptochrome.
  • Leads to changes in auxin transport, causing bending towards the light.
Gibberellin and Germination
  • Far-red light activates phytochrome(PFR) formation, which can activate the transcription of genes that synthesize and activate gibberellin and this is needed for seed germination.
  • Gibberellin, a plant hormone, is needed for seed germination and shoot elongation.
  • Phytochromes can activate multiple downstream pathways, including hormonal pathways.
Chloroplast Movement
  • Chloroplasts change their localization depending on light intensity.
  • In low light, they maximize light interception.
  • In high light, they move to avoid damage (avoidance response).
  • In the dark, they sink to the bottom of the cell.

Mechanism of Chloroplast Movement

  • Photoreceptors in the plasma membrane interact with proteins that alter the polymerization of actin filaments.
  • Actin filaments move organelles around the cell.
  • Chloroplast movement is directed by polymerizing and depolymerizing the actin cytoskeleton.

Flowering Responses

Short-Day Plants
  • Flower when the day is short (actually require a minimum continuous dark period).
  • Interruption of the dark period with a flash of light inhibits flowering.
  • The PFR form inhibits flowering in this case.
Long-Day Plants
  • Flower when the day is long (require a maximum continuous dark period).
  • A light pulse during the dark period induces flowering.
  • The PFR form induces flowering.

Evolution of Light Receptors

  • Evolution of light receptors is linked to the evolution of land plants.
  • Early plants in the sea primarily had blue light receptors.
  • Red and far-red light photoreceptors evolved as plants colonized land.
  • Gene duplication and diversification led to different conversion rates and expression patterns of photoreceptors.

Summary of Photomorphogenesis

  • Photomorphogenesis is a change in plant development in response to light.
  • Activated photoreceptors interact with proteins (e.g., PIFs) that are degraded upon interaction.
  • This alters the expression of downstream genes, including auxin and other hormone-related genes.
  • Light responses are ancient, starting with blue light responses in marine algae and diversifying to include red light responses in land plants.