Plant Responses and Hormones

Plant Responses

• Plants are responsive to touch, damage (e.g., from herbivores), and light.
• Chapter 39 focuses on:
  • Signal transduction.
  • Plant hormones and their roles.
  • Plant responses to light.

Complexity of Plant Processes

• Internal processes in plants are as complex as those in animal cells.
• Plants utilize proteins and photoreceptors, similar to rods and cones in animal eyes, to respond to light.
• Plasma desmata facilitate the movement of molecules and proteins between plant cells, analogous to gap junctions in animal cells but larger, enabling more significant molecule transfer.

Signal Transduction

• Hormones or stimuli interact with the cell membrane or outside the cell, leading to a response.
• Two main types of responses:
  • Changes in gene expression (genomic pathway).
    • Signal transduction alters which genes are expressed in the nucleus.
    • Transcription factors are activated by kinases in the cytoplasm and nucleus.
  • Modification of existing proteins (biochemical activation).
    • Signal transduction can amplify signals; one receptor activates multiple molecules.
    • Example: One activated receptor activates multiple guanaline cyclase molecules, which activate kinases, then transcription factors.
• Kinases are activated, leading to either upregulation or downregulation of enzymatic activity.
• Protein phosphatases remove phosphate groups added by kinases, generally downregulating the signal.
  • Protein phosphatases are less specific than kinases; one phosphatase can deactivate multiple kinases.

De-etiolation

• Process where plants are exposed to sunlight, triggering:
  • Photosynthesis.
  • Enzyme production.
  • Hormone release to stimulate root and shoot growth.
• Signals lead to different responses based on the cell type activated.
• Signals spread through plasma desmata.

Plant Hormones

• Hormones have effects at low concentrations, with different cells responding differently to the same hormone.

• Major classes of plant hormones:

  • Auxins:

    • Growth-promoting hormones that form gradients in plants.
    • Often generated at growing ends.
    • Involved in phototropism, where plants bend toward light.
    • Promote elongation of grass tips.
    • Indole Acetic Acid (IAA) is a common auxin.
    • Transport is polar, synthesized in apical tips and transported down, loosening cell walls and allowing cell expansion.
    • Changes gene expression, stimulating sustained growth.
    • Auxin concentration affects plant response; higher at the top, lower further down.
    • Influences phyllotaxy (leaf arrangement) and vein patterns.
    • Important in fruit development; seeds produce auxin.

  • Cytokinins:

    • Zeatin (from maize) is an example.
    • Affect differentiation lower in the plant and have anti-aging effects by preventing breakdown, stimulating protein production, and mobilizing nutrients

  • Gibberellins:

    • Cause rapid elongation or stretching in stalks.
    • Impact dormancy; break seed dormancy.
    • Ratio of abscisic acid to gibberellins determines seed growth.

  • Abscisic Acid (ABA):

    • Maintains seed dormancy during harsh conditions (e.g., cold, dry).
    • Triggers stomata closure in response to water stress, conserving water.

  • Ethylene:

    • Enhances fruit ripening.
    • Induced by stress.
    • Causes leaf abscission (falling off).
    • Produced when a plant encounters an obstacle, redirecting growth
    • Associated with programmed cell death (apoptosis).
    • Concentration relative to auxin influences leaf abscission.

  • Brassinosteroids:

    • Involved in various processes, including gene expression.

Light Responses

• Plants optimize gathering light, distributing resources, positioning leaves, and organizing roots.

• Plants detect the presence, direction, and intensity of light.

• Two major classes of light receptors:

  • Blue light photoreceptors.
  • Phytochromes:
    • Respond to red and far-red light.
    • Red light (PR) increases germination, while far-red light (PFR) inhibits it.

• Phytochromes and Circadian Rhythms:

  • Regulate daily movements (e.g., raising or lowering leaves).
  • Studied by altering day/night cycles and evaluating protein states.

Flowering Regulation

• Spider chromes control flowering based on the ratio of PR to PFR.
• Some plants require specific day lengths or flashes of light to flower.
• Floragin, regulated by the Feet gene, induces flowering.

Gravitropism

• Plants sense gravity using statoliths (dense starch granules) in cells.
• Statolith position triggers signals that affect growth, helping plants determine which way is up.

Thigmotropism

• Response to touch.

Stress Responses

• Plants respond to stresses like flooding, salt, and heat.

  • Flooding: Causes oxygen deprivation; plants break down the cortex to create air tubes.

  • Salt: Decreases water potential; plants increase solutes in roots.

  • Heat: Denatures proteins; plants use transpiration for cooling and produce heat shock proteins to help proteins fold correctly.

  • Cold: Decreases membrane fluidity; plants increase unsaturated fatty acids in membranes and produce antifreeze proteins.

Pathogen and Herbivore Defense

• Plants defend against pathogens and herbivores.

  • Pathogen Defense:

    • PAMPs (pathogen-associated molecular patterns) trigger antimicrobial chemicals and cell wall toughening.
    • R genes (resistance genes) are activated by specific effector molecules.
    • Systemic Acquired Resistance (SAR) provides systemic protection after initial infection.

  • Herbivore Defense:

    • Chemicals deter animals (e.g., irritants, toxins).
    • Physical defenses: Tough or chewy textures.
    • Mimicry: Some plants mimic insect eggs to deter other insects from laying eggs.

Leaf Excision (Abscission)

• Process where leaves fall off trees, especially in autumn.

  • Excision layer forms, cutting off water and sugar transport.
    • Chlorophyll breaks down, revealing carotenoids (yellow and orange pigments).
    • Sugars may be trapped in vacuoles, leading to red hues.