Cell Growth: Stimulus and Response

How do plants respond to stimuli?

A. Perception of a Stimulus

1. There is an environmental signal or cue (eg: unilateral light, gravity vector, development)
2. Cells in Perception Cells Transduce Signal
   • Statoliths Fall on Sensitive Wall (gravitropism)
   • Blue-light Photoreceptor Absorbs Light (phototropism)
3. Cell responds to Transduced Signal through Hormone Concentration
   1. Hormone Synthesis Increase or Decrease
   2. Hormone Degradation Increase or Decrease
   3. Hormone Transport Increase or Decrease
   4. Hormone Conjugation: Bound vs Free form--storage or release

B. Response of Responding Cells

1. The Hormone Binds with Receptor
   • Membrane Protein of Cell Membrane (need secondary messenger?)
   • Cytoplasmic Protein or Membrane Protein (of Internal Membranes?)
   • Most Plant Hormones are small enough to penetrate
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2. The Receptor Causes Change Ultimately Making a Response
   1. Fast Growth Response
      1. Cell Membrane Receptor is ATP-driven Proton Pump
      2. Protons Pumped from Cytosol into Cell Wall Space
      3. pH Drop in Cell Wall Space
      4. Optimizes/Activates Xyloglucanase and Rhamnogalacturonase already in wall
      5. Xyloglucan and Rhamnogalacturonan cross-linking cleaved
      6. Wall Elasticity is reduced...Plasticity is increased
      7. Cellulose Microfibrils slip past one another due to turgor pressure
      8. Cell Expands/Elongates depending on microfibril orientation.
   2. Sustained Growth Response
      1. Receptor is in Nucleus
      2. DNA is activated to become template
      3. RNA is transcribed
      4. Protein is translated
      5. Transcriptional/translational/post-translational controls invoked/repealed
      6. Active enzymes catalyze responses (Seed Germination in Barley/Lettuce)
      7. Needed Components:
         • Energy for all steps (accelerated respiration)
         • Wall Polysaccharide Subunit (sugar) Synthesis: Krebs/Calvin Cycles
         • Wall Polymer Synthesis Enzymes: Cellulose, Pectin, Hemicellulose Synthases
         • Polymerization Enzyme Transport across Cell Membrane--exocytosis
         • Organization of new microfibrils and cross-linking.

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