11: Plant Response to Light

• Plants receive + respond to environmental signals:

  • Light
  • Gravity
  • Pressure
  • Wounds
• Sensory cells perceives external stimulus and transduces the information to an internal signal
• A cell-cell signal released by the sensory cell travels throughout the body
• Target cells receive the cell-cell signal and change activity in a way that produces an appropriate response
• Steps in information processing:

  1. Sensory cell
  2. Cell-cell signal
  3. Target cell
• Receptor proteins detect environmental signals and change shape, transferring to an internal signal, usually a hormone
• Hormones may be transmitted from cell-to-cell by:

  • Specialized transport proteins in membranes
  • Through xylem or phloem
  • Simple diffusion
• Signal molecules (ligands) elicit a response in cells with a matching receptor located:

  • Inside the cell (if signal molecule can diffuse through the membrane)
  • In the membrane (ligand can't diffuse through the membrane)
• Signal Transduction via:

  • Phosphorylation
  • Secondary messengers
• Phosphorylation cascades:

  • Trigger: when signal binds receptor protein
  • Transfer of P from ATP to receptor or associated protein
• Secondary messengers:

  • Triggers production or release of intracellular signals
  • Interact to modify cellular response, amplifying the signal
• Response to signal binding a receptor:

  • Activation of membrane transport proteins
  • Changes membrane electrical potential or the cell wall pH
  • Changes gene expression (for specific genes)
• Plants sense + respond only to specific wavelengths of light
• Phototropism: directed movement in response to light
• Plants bend toward light. Direction of growth is directed through elongation of cells on the shaded side of the plant
• Chlorophyll a and b strongly absorb light of blue wavelength
• PHOT1 gene codes for PHOT1 protein, a blue-light receptor
• PHOT1 protein becomes phosphorylated after exposure to blue light, leading to phototropic response
• Phototropins: photoreceptors that detect blue light and initiate phototropic responses

  • Chloroplasts (move to locations in cells to optimize light absorption)
  • Stomata (open to allow CO2 into the cell)
  • Other blue-light receptors control stem elongation + flower production
• Coleoptile: protective sheath covering the emerging shoot
• Darwins' further experiments:

  • Coleoptile was removed --> no longer bent towards the light
  • Covered tips of coleoptiles with opaque material --> stopped seedlings from bending towards the light
  • Opaque collars below the tip area --> seedlings continued to bend

  Final proposal: substance at the coleoptile tip acts as a signal that is transported to the area of bending
• Frits Went experiment:

  • Agar blocks exposed to coleoptile tips to collect the phototropic hormone
  • Placed off-centre on decapitated coleoptiles
  • Grew coleoptiles in the dark --> bend away from agar blocks
  • Named this hormone "auxin"
• Auxin (aka indole acetic acid, IAA) causes plant cells to elongate
• Auxin-binding proteins are found in the stem and leaf:

  • ABP1 auxin-binding protein receptor in the membrane
  • T1R1 intracellular auxin receptor
• When auxin binds these receptors, the resulting signaling pathway:

  • Increases plasma membrane proton pumps, pumping H+ out of cell, lowering pH of cell wall
  • Result is cell elongation: acid-growth hypothesis
• Acid-Growth Hypothesis:

  
  

  2 things must happen for a plant cell to get larger:

  1. Cell wall must loosen up
  2. Water must enter, creating turgor pressur eon cell wall + increase cell size
• Acid-Growth Hypothesis for Cell Elongation:

  • Lowering pH to 4.5 activates expansins, cell-wall proteins that "unzip" H--bonds between cellulose + other cell-wall polymers loosening the cell wall
  • Electrochemical gradient created brings other positively charged ions into cell
  • Water follows by osmosis, increasing turgor pressure to expand the loosened cell wall
• Red light (660 nm) drives photosynthesis, like blue light. Far-red (720 nm) not absorbed strongly by photosynthetic pigments (much of the far-red light passes through leaves)
• Red and far-red light act like on-off switches for seed germination

  • Red light promotes germination
  • Far-red light inhibits germination
  • Last wavelength sensed by the seed impacts % germination
• Phytochrome: absorbs both red and far-red light, existing in 2 shapes
• Photoreversibility: switching behavior

  • Pr (phytochrome "red") absorbs red light
  • Pfr (phytochrome "far-red") absorbs far-red light
• Phytochrome controls etiolation
• Seedling grows with etiolated morphology:

  • Fast-growing stems
  • Thin, spindly, pale yellow colour
  • Maximizes chance of reaching the surface
• Once the plant breaks to the surface and sense light, activated phytochromes promote de-etiolation:

  • Stem growth slows down
  • Cotyledons expand
  • Chloroplasts develop
• Photoperiodism:

  • Response based on photoperiod (relative length of day and night)
  • Allows plants to respond to seasonal change
• Plants fall into 3 categories:

  1. Long-day plants (bloom in midsummer when days are longer than a certain length)
  2. Short-day plants (bloom in the spring or fall when days are shorter than a certain length)
  3. Day-neutral plants (flower without regard to photoperiod)
• Photoperiodism:

  • Plants have a "clock" that resets each morning
  • Clock protein levels rise during the day + trigger expression of costans (CO) gene
• CO proteins:

  • A transcription factor that affects production of a flowering hormone
  • Long-day plants (high CO protein level stimulates flowering)
  • Short-day plants (high CO protein level inhibits flowering)
• Photoperiodism (effect of red and far-red light):

  • Red light acts as daylight
  • Far-red acts as night/shade
• Florigen: flowering hormone