Plant Perception of Light
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.