Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 39.1 Signal Transduction Pathways Link Signal Reception to Response
Plants constantly sense and respond to their environment through signal transduction pathways. These allow them to convert external signals like light into physiological responses—such as turning a dark-grown sprout into a green, functional shoot.
Etiolation and De-Etiolation: A Model System
- Etiolation: Growth in darkness — long, pale stems, underdeveloped roots, unexpanded leaves.
- De-etiolation: Triggered by light — stems thicken, leaves expand and turn green, roots elongate.
This process illustrates how environmental cues trigger internal responses through signal transduction.
Reception: Detecting the Signal
- Light is detected by phytochrome, a cytoplasmic photoreceptor.
- Activated phytochrome changes shape, initiating the pathway.
- Evidence: aurea mutant tomatoes lacking phytochrome show reduced greening, which is restored when phytochrome is added.
Transduction: Passing the Signal
- Involves second messengers:
- Ca²⁺ (calcium ions): light opens channels, raising cytosolic Ca²⁺.
- cGMP (cyclic GMP): activated by phytochrome to trigger protein kinases.
These messengers activate different protein kinases, initiating gene expression via multiple pathways.
Response: Activating the Machinery
Two outcomes:
- Post-translational modification: Protein kinases phosphorylate target proteins to activate/deactivate them.
- Transcriptional regulation: Transcription factors activate genes involved in:
- Photosynthetic enzyme production
- Chlorophyll biosynthesis
- Hormone regulation (e.g., auxin, brassinosteroids)
The result: stem elongation slows, chlorophyll production begins, and the plant becomes green and light-adapted.
Turning the Signal Off
- Protein phosphatases deactivate signaling proteins by removing phosphate groups.
- This prevents overreaction and resets the system when light disappears.
Experimental Insights
- Applying only cGMP produces partial de-etiolation.
- A full response requires both cGMP and Ca²⁺ — showing that multiple pathways must act together.
In a Nutshell
Plants detect signals like light using receptors such as phytochrome. These activate second messengers (Ca²⁺ and cGMP), which in turn activate kinases and gene expression, transforming the plant's physiology. De-etiolation is a prime example of how external signals can initiate internal changes, revealing the sophistication of plant sensory systems.