Plant Responses

Organisms respond to the environment and receptors observe changes in the environment

  • Plants can look very different based on their environment

Signal Transduction Pathway:

  1. Reception:

    • Receptors- proteins that undergo changes in shape in response to specific stimuli→chemical, mechanical, light

      • lock and key mechanism

    • Etiolation = morphological adaptations for growing in darkness

  2. Transduction:

    • conversion of an external signal from receptor into a signal that organisms can process in a response center

    • Secondary messengers- small molecules in the cell than can amplify the signal and transfer it from the receptor to other proteins that carry out the response

      • like a relay race

  3. Response

    1. Transcriptional Regulation: turning genes on or off

      • increasing/decreasing the synthesis of of mRNA coding a specific enzyme

    2. Post translational Modification: changing enzymes already there

      • activates pre-existing enzymes

    3. De-etiolation: the process of “greening”, w/ morphological and biochemical changes of photosynthesis

Chemical Communication:

  • Plants tune responses w/ hormones

    • signaling molecules produced by one part of an organism’s body and transported to other parts where it binds to specific

  • Tropism- growth response toward/away from a stimulus

    • Phototropism- growth toward/away from light

      • differential growth of cells on light+dark sides

      • cells elongate faster on dark side as bending occurs below the growing tip of plant (opp side), which goes toward the light

    • Ratio of auxin and cytokinin controls cell differentiation

    • Thigmotropism- response to touch

    • Gravitropism- sensing of gravity

Moving Hormones

  • move through circulatory system (plasmodesmata, xylem, phloem)

  1. Auxin: promotes elongation

    1. produced mainly in apical meristems

    2. moves only from tip to base in phloem→Polar transport (unidirectional)

    3. inhibits growth of lateral axiliary buds when apicad bud present→removal stimulates cytokinin→lateral axilliary bud growth

    4. Used as a herbicide (preserves monocots but overdoses eudicot weeds)

    5. promote fruit development (esp in greenhouses)

    6. in agent orange

  2. Cytokinin: stimulates cytokinesis (cell direction)

    1. produced mainly in plant roots

    2. transported in xylem sap- opposite direction of auxin

    3. bushier plants

    4. anti-aging effects on plants

  3. Gibberellins

    1. expressed in apical buds, young leaves, and developing seeds

    2. promotes stem elongation and leaf growth

      1. bolting= rapid growth of the floral stalk

    3. Promotes pollen development, fruit growth w/ auxin, and seed germination

      1. used to make bigger seedless grapes

  4. Abscisic Acid (ABA)

    1. synthesized in most plant cells

    2. inhibits growth and maintains seed dormancy

    3. antagonizes growth hormones

    4. promotes leaf senescence (aging→death)

    5. drought tolerance→ closes stomata, used in agriculture, including synthetics

  5. Ethylene:

    1. gas produced by nearly all parts of plant in response to stressful conditions

    2. promotes fruit ripening→fruits look great on outside but don’t taste good

    3. speeds rate of senescence in deciduous trees→leaf abscission (leaves drop prematurely (esp close to gas leaks))

      1. Prevents desiccation

      2. leaf nutrient reuptake from leaves b/4 abscission

      3. abscission layer at base of petiole→change in auxin/ethylene

    4. Triple response to mechanical stress- enables growing shoot to avoid an obstacle

      1. slowing of stem elongation

      2. thickening of stem

      3. curvature of stem

Time

  • Circadian Rhythms

    • 24 hr cycle in eukaryotes

    • Internal timekeeper controlling biological rhythms

    • persist w/o external cues (ex: opening/closing stomata, fold/unfold leaves of flowers)

Light

  • Photomorphogenesis- light triggered events in plant growth/ development

  • diff wavelengths of light drive diff processes in plants

  • blue light photoreceptors- pigments that absorb blue light

    • initiate response such as phototropism (light induced opening of stomata and growth towards light)

    • slowing of hypocotyl elongation when seedling breaks ground

  • phytocrome receptors- absorb red light

    • regulate seed germination and shade avoidance

Gravity

  • Gravitropism- sensing of gravity

    • shoots go up and roots go down in response to gravity

    • positive= down, negative= up

    • occurs as soon as seed germinates

    • statoliths- dense cytoplasmic components that settle under influence of gravity to lower portions of the cell

Touch

  • Thigmotropism- directional growth in response to touch

    • Thigmomorphogenesis- changes in form that result from mechanical perturbation

    • Rapid leaf movements- mimosa pudica, carried out through action potentials

Attacks

  • Defenses against pathogens

  • Epidermis/periderm = 1st line of defense, helps to have a good, thick cuticle

  • Plant immune system- have systems analogous to humans

    • innate immunity- pathogen associated molecular patterns (PAMP)

    • adaptive immunity- effector triggered immunity

  • Herbivory

    • physical defenses = thorns, spines/modified leaves

    • chemical defenses = toxins/ unpalatable compounds