lecture 12

• Water potential (Ψ)


  The physical property that predicts which direction water will move, typically measured in megapascals (MPa)

  
• Free water moves from regions of high Ψ to low Ψ
• Ψ


  Ψ = Ψs + Ψp, where Ψs = solute potential and Ψp = pressure potential
• Solute potential of pure water is defined as 0, and thus Ψs is always negative when solutes are present
• Transpiration-cohesion-tension mechanism


  Water transpiring from the leaf pulls water through the xylem due to tension (a negative pressure potential Ψp) along an overall water potential gradient

  
• Xylem


  • Tracheids and vessel elements are dead when functional, with no solutes of their own and lignified cell walls that can withstand the tension pressure
• Stomata opening and closing is important for limiting water loss
• Plants are able to continue to extract water from the soil


  When the water potential (Ψ) of the root is more negative than the water potential (Ψ) of the soil
• Phloem


  Sieve-tube elements and companion cells that move sugars and other organic molecules throughout the plant

  
• Movement of Phloem Sap


  Sugars are actively loaded into sieve-tubes

  1. Osmosis drives water into phloem from xylem, creating positive pressure (Ψp) and causing sap to flow
  2. Sugars are unloaded by diffusion into sinks
  3. Water is recycled back to xylem by osmosis
• Sugar allocation to sinks is a critical aspect of crop breeding and management
• The phloem is also a molecular information superhighway, distributing hormones and other molecules that modulate plant growth, development, and defense

  
• Movement of Phloem Sap


  1. Sugars are actively loaded into sieve-tubes from apoplast or companion cells (making Ψs more negative)

  2. Osmosis drives water into phloem from xylem, creating positive pressure (Ψp) and causing sap to flow

  3. Sugars are unloaded by diffusion into sinks (making Ψs less negative)

  4. Water is recycled back to xylem by osmosis
• Many competing demands for finite resources
• Sink strength (i.e., how metabolically demanding a sink is) is not the only determinant of where flux passes through the plant
• The phloem is also a molecular information superhighway
• Some hormones and other molecules that modulate plant growth, development, and defense are distributed by specifically the phloem (others may move by xylem too)
• Phloem movement may mediate whole-plant, environmentally-sensitive regulation of source-sink dynamics

  
• Phloem movement is of core importance
• Phenotypic Plasticity


  Phenotypic variation produced due to individuals of the same genotype experiencing different environments
• Developmental Plasticity


  Environment-dependent changes in growth, developmental patterning, or developmental timing
• Often the case that an environmental cue experienced by one tissue or life stage determines the developmental response by another tissue or life stage
• Components of Developmental Plasticity

  1. Perception

  2. Transduction

  3. Response
• Cues and Sensors


  • Light - Phytochromes, Cryptochromes, Phototropins
  • Salinity, Metals, Nutrients - Ion and import channels, Nutrients themselves
  • Density, Touch - Mechanoreceptors, Phytochromes
  • Herbivores, Parasites, Mutualists - Chemoreceptors, Immune System, Direct Induction
  • Temperature - Phytochromes too!, Protein aggregates
  • Gravity - Starch granules
• Sensing is not Interpreting
• How are multiple cues integrated?
• What makes a cell respond to cues only at particular developmental stages?
• How is the response executed in the correct time frame?
• How is the experience of the cue(s) remembered after the cue is gone?
• Information Transfer and Encoding in Developmental Plasticity


  1. Perception - Sensing and interpreting complex environmental cues

  2. Transduction - Communicating that information to nucleus OR to another tissue entirely

  3. Response - Local changes to cells and tissues appropriate to the stimuli perceived
• Hormone


  Mobile signal that acts to alter development or function of a target tissue
• Developmental Plasticity


  • 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
• Phototropism: growth in response to a directional light source
• In angiosperms, phototropic reaction is specifically induced by blue light
• How does a stem bend?

  1. Movement is mediated by growth, i.e., irreversible cell elongation

  2. Growth is asymmetric. Shaded side grows faster.
• Where is the light perceived?


  1. Light is sensed in the shoot tip

  2. Initial experiments examined grass coleoptiles

  3. Always get bending as long as tip is present and lit
• If tip is site of perception but cell elongation happens below the tip, what connects detection to response?
• Answer


  A mobile growth regulator (hormone)
• Additional experiments


  1. Asymmetrical placement in dark

  2. Symmetric placement in dark

  3. Symmetric placement with unilateral white or blue light
• Why does the plant still respond if hormone is not asymmetrically applied?