Lecture 13

• Tropisms

  Growth response that results in plant organs turning toward or away from stimuli
• Developmental Transitions


  Environmentally-sensitive (often) decisions to move onto a new stage of the life cycle (e.g. germination, flowering, fruit ripening, seed dispersal, senescence)
• Components of Developmental Plasticity

  1. Perception

  2. Transduction

  3. Response
• Sensor Molecules


  • Phototropins
• Integrator


  • PINs? + Hormonal Signal: Auxin
• Transduction


  • Asymmetrical Auxin Movement
• Local Actors


  • Proton Pumps
• Response


  • Cell wall loosening by expansins
• Gravitropism

  Directional growth in response to the force of gravity
• Roots


  • Positively gravitropic, bend toward gravity vector
• Shoots

  • Negatively gravitropic, bend away from gravity vector
• Gravity sensing


  • Starch granules called statoliths (or amyloplasts) that settle to the lowest side of cells
• Location of statoliths


  • Root cap and endodermis surrounding shoot vasculature
• Statolith sinking

  Triggers signals that lead to lateral transport of auxin to the lower side of the root / shoot
• Auxin inhibits growth in root but promotes growth in shoot
• Shoot movement is more quickly powered by local photosynthate
• Statoliths fall more slowly in root caps
• Auxin inhibits root elongation by making apoplast more basic
• Auxin promotes shoot elongation by making apoplast more acidic
• Thigmotropism

  • Response to touch
• Hydrotropism


  • Response to water
• Heliotropism


  • Solar tracking
• Dormancy


  State of slow metabolism and suspended growth and development
• Germination

  Growth of an organism from a seed or spore
• Seeds can be non-dormant but not germinate
• Seeds in the soil that are dormant or not germinating are called the seed bank
• Plasticity in the timing or form of development

  To be adaptive, current environmental cues must be predictive of future environmental conditions
• Plasticity in the timing or form of development

  1. Cues experienced at an earlier developmental stage

  2. Trigger developmental changes that take time to occur

  3. Next stage experiences a new environment with different conditions
• Dormancy

  State of slow metabolism and suspended of growth and development
• Germination

  The growth of an organism from a seed or spore
• Seeds can be non-dormant but and yet not germinate
• Seed bank


  Seeds in the soil that are dormant or not germinating
• Understanding the regulation of germination is important for agriculture and conservation
• Precocious germination is bad for crops
• Knowing how to maintain and break dormancy key for long-term seed storage efforts
• Abscisic Acid (ABA)

  Hormone that generally slows growth, levels in seeds may increase 100x as they mature, must be inactivated by cold or removed (washed out)
• Gibberellins (GAs)


  Hormone that promotes both cell division and elongation, may loosen cell walls and allow expansin activity, in monocots, embryo releases GAs to stimulate conversion of endosperm into sugars
• ABA:GA ratio


  Sets the balance between dormancy vs. germination
• Light Quality

  For sun-loving species, light quality (the color spectrum of incoming light) is a good predictor of shade vs. full sun, A lower red:far-red ratio indicates shade because leaves above have absorbed photosynthetically useful red wavelengths, Measured by phytochromes, sensors of red and far-red light
• Phytochrome activation


  Reversible nature allows dormancy to be reversed