L33 2025 - How Plants Perceive Their Environment - Light - Tagged

Page 2: Growth of Seedlings

Characteristics of Seedlings in Light and Darkness

• Darkness:

  • Long hypocotyl

  • Unexpanded cotyledons

  • No chlorophyll

• Light:

  • Short hypocotyl

  • Expanded cotyledons

  • Presence of chlorophyll

• Conclusion: Light controls plant development

Page 3: Gene Expression Regulation

Gene Regulation

• Approximately one-third of genes in plant seeds are regulated by light.

Page 4: Monitoring Gene Expression

Transgenic Plants and Reporter Genes

• Use of promoter-reporter gene fusions in transgenic plants to analyze transcription regulation.

  • Example: CAB gene promoter driving b-glucuronidase expression

• Conclusion: CAB expression is light-stimulated.

Page 5: Visualising Transcription Changes

Challenges with Reporter Proteins

• β-glucuronidase reporter unsuitable due to its protein being too stable.

• Requirement of unstable reporter: Firefly Luciferase

  • luciferin + ATP + O2 → luminescence

Page 6: Photon Counting

Observing Transgenic Seedlings

• photon counting image of transgenic seedlings expressing firefly luciferase.

Page 7: Monitoring CAB Gene Transcription

Evaluation Methodology

• CAB promoter activates luciferase expression in transgenic plants.

• Process: Adding luciferin to monitor luciferase activity.

Page 8: CAB Expression Over Time

Diurnal Changes in Expression

• CAB promoter leads to fluctuations in luciferase expression over a 24-hour period.

Page 9: Circadian Regulation

Aspects of CAB Gene Transcription

• Circadian regulation of CAB transcription visualised via luciferase activity over various time intervals.

Page 10: Circadian Rhythms

Biological Clock and Light Response

• circadian rhythm is like a biological clock, controlled by proteins

• Plants have input pathways

  • e.g., light

• that influence output pathways

  • e.g., transcription\

Page 11: Light Detection in Plants

Key Aspects Detected by Plants

• Presence/Absence of light

• Quantity of light

• Spectral quality of light

• Direction of light

• Duration of light

Page 12: Light Spectrum

Visualization of Different Light Types

• Light wavelength ranges

  • UV-A: 400 - 500 nm

  • Blue: ~450 nm

  • Red: ~600 nm

  • Far-red: ~700 nm

  • UV-B: shorter wavelength ranges.

Page 13: Photoreceptors

Different Types

• Phytochromes:

  • Detect mainly red and far-red light.

• Cryptochromes and Phototropins:

  • Involved in UV-A and blue light detection.

  • Primarily detect UV-A and blue light.

• UVR8:

  • A UV-B photoreceptor for regulatory functions.

Page 14: Photoreceptor Structure

Components

• apoprotein is the photoreceptor protein

• chromophore is attached to the apoprotein and is a small organic molecule that absorbs light.

• Together, the chromophore and apoprotein form the complete photoreceptor.

Page 15: Phytochrome Structure

Ligand Binding

• Phytochrome apoprotein binds a linear tetrapyrrole chromophore to function effectively.

Page 16: Phytochrome Light Absorption

Interconvertible Forms

• The phytochrome molecule mostly absorbs red and far-red light, existing in two forms:

  • Pr: inactive form present in darkness

  • Pfr: active form produced by red light exposure.

• Pr → Pfr = red

• Pfr → Pr = far-red

• Pr is present in dark-grown plants and illumination with red light produces Pfr.

• The Pfr form of phytochrome initiates biological responses

Page 17: Light-Induced Responses

Transition of Phytochrome Forms

• Many phytochrome controlled responses show red light induction and far-red photo-reversibility

• Example: Seed germination mechanisms in species like lettuce.

Page 18: Phytochrome and CAB Regulation

Gene Transcription Dynamics

• Phytochrome regulates CAB gene transcription rates under varying light conditions (DR vs. FR).

Page 19: Shade Detection by Phytochrome

Environmental Awareness

• Plants detect shade using R:FR ratios to distinguish light conditions between healthy and shaded areas.

Page 21: Shade Avoidance Response

Mechanism

• Controlled by phytochrome through measurement of Red to Far-Red (R:FR) ratios.

  • High R:FR indicates direct sunlight, while low R:FR indicates shading.

• low red does better

Page 22: Neighbor Detection

Importance in Plant Growth

• Detection of increased far-red light reflection suggests nearby plants, leading to growth adaptations such as stem elongation.

Page 23: Cryptochrome Functionality

UV-A and Blue Light Response

• Cryptochromes bind flavin and pterin chromophores that absorb UV-A and blue light

• Regulating processes like stem extension, gene expression, and flowering time.

Page 24: Plant Light Perception Mutants

Genetic Variability in Light Growth

• Study comparing wild-type and mutant plants (cry and phy) grown in white light.

• white light grew more

Page 25: Phototropins

• Phototropins bind flavin chromophores that absorb mainly UV-A and blue light

• Controls

  • plant responses - phototopism

Page 26: UV-B Protection Mechanism

Plant Adaptations

• Plants possess mechanisms to protect themselves from damaging UV-B radiation exposures.

Page 27: UV-B Influence on Gene Expression

Gene Regulation

• UV-B regulates gene expression leading to processes like flavonoid biosynthesis, enhancing protective features in the epidermis.

• UV-B → UVR8 → genes → Flavonoid biosynthesis → Flavonoids in epidermis

Page 28: Summary of Light Regulation

Major Conclusions

• Light is a major factor regulating plant development

• Light is sensed by several different photoreceptors that regulate particular responses

Page 29: Importance of Plant Studies

Research Value

• Plants serve as excellent experimental systems, providing insights for crop improvement and bioresource conservation.