13: Plant Maturation + Hormones

• Plants control growth in response to environment + age
• Hormones act as growth regulators in plants:

  • Auxins
  • Cytokinins
  • Gibberellins
  • ABA
  • Brassinosteroids
  • Ethylene
• Apical Dominance: the phenomenon in plants where a main shoot dominates and inhibits the outgrowth of other shoots
• Auxin travels to tissues below preventing lateral growth
• Auxin transport is unidirectional

  • From shoots to roots via parenchyma cells in ground and vascular tissues
• Removal of apical meristem causes lateral buds to grow

  • Lateral branches sprout + take over for the main shoot
• Auxin produced by seeds promotes fruit development

  • Falling auxin levels involved in leaf and fruit abscission
• Auxin in growing roots + shoots is needed for differentiation of xylem and phloem cells + development of vascular tissues
• Auxin stimulates growth of roots in tissue cultures and cuttings
• Cytokinins:

  • Most are made in root apical meristem + travel through the xylem
  • Regulate growth by binding receptors in target cells
  • Receptors activate genes regulating cell division: cyclin gene (CycD3)
  • Cell cycle stops in absence of cytokinins
• Cdc25: cell division control

  • Phosphatase controls cell cycle by dephosphorylating Cdk (cyclin-dependent kinase) --> driving mitosis forward --> causing plant growth
• CycD3:

  • Cyclin binds Cdk protein --> stimulation of elongation factor E2F --> drives transition from G1 to S cell cycle phase
• Gibberellins/ Gibberellic Acid (GA): stimulates plant growth and seed germination
• Plants grow when temperature + soil moisture are suitable and become dormant under conditions of cold/ drought
• Abscisic acid (ABA): inhibits growth and seed germination
• Seeds contain nutrients (endosperm) to fuel growth as many germinate underground away from light
• Seed Dormancy:

  • Seeds remain viable until conditions improve
  • Feature of plants inhabiting seasonal environments (dormancy is rare in tropical wet forests, where conditions are always suitable for germination)
  • ABA triggers accumulation of storage compounds + prevents germination
• To germinate:

  • Most seeds need: H2O, O2, warm temperatures
  • Seed coat must be disrupted or scarified
  • Habitats where wildfires are frequent:
  • Seeds must be exposed to fire to break the seed coat or melt away a resin coating
  • Chemicals in smoke infused soil, dissolve in soil water --> germination
  • Small-seeded species germinate near soil surface
  • Need exposure to red light before they can break dormancy and germinate
• Alpha-amylase enzyme digests starch in endosperm, releasing sugars for embryo
• GA increases alpha-amylase levels
• Process: GAs activate production of alpha-amylase

  1. Seed absorbs water and germination begins
  2. GAs diffuse from the embryo to the aleurone layer
  3. Cells in the aleurone layer respond by releasing digestive enzymes, such as alpha-amylase
  4. The enzymes digest starch, releasing sugars and other molecules to the growing plant
• ABA causes stomata to close.

  • Dry roots released ABA, signaling drought
  • When guard cells are turgid, stomata open
  • When guard cells are flaccid, stomata close
• Activation of phototropins by blue light leads to water entry and stomatal opening
• Activation of ABA receptors leads to water exit and stomatal closing
• Process: Stomata open in response to blue light:

  1. Pumping by H+ ATPases increases. Protons leave guard cells
  2. K+ and Cl- enter cells along electrochemical gradients via inward directed K+ channels and H+/Cl- cotransporters
  3. H2O follows by osmosis. Cell opens and stoma opens
• Process: Stomata close in response to ABA:

  1. Pumping by H+ ATPases stops. Outward-directed Cl- channels open. Cl- exits along electrochemical gradient
  2. Change in membrane potential opens outward-directed K+ channels. K+ exits along electrochemical gradient
  3. H2O follows by osmosis. Cells shrink and stoma close
• Brassinoseroids (Br): promote growth + regulate body size in plants
• Brassinosteroids (BR):

  • Bind receptors on plasma membrane + activate signal transduction (phosphorylation cascades) altering gene expression
  • Interact with other hormones for:
  • Cell elongation
  • Cell division
  • Root growth
  • Photo-morphogenesis
  • Stomatal + vascular differentiation
  • Seed germination
• BR hormones are perceived by:

  • BRI1 (Brassinosteroid insensitive 1) receptor (kinase)
  • BRL (Brassinosteroid localized) receptors (kinase)
• Ethylene:

  • Fruit ripening
  • Flower fading
  • Leaf abscission
• During fruit ripening:

  • Starch is converted to sugar
  • Cell walls degrade (softening)
  • Chlorophyll is broken down
  • Produce pigments + aromas that signal ripeness to attract animals for seed dispersal
  • Fruit growers manipulate ethylene levels to control fruit ripening
• Process: Leaf senescene + abscission:

  1. Less auxin produced as leaves age or in response to colder temp. + shorter day length
  2. As auxin levels drop, abscission zone in the petiole becomes more sensitive to the ethylene present, activating enzymes that weaken cells in the abscission zone
  3. Eventually cell walls degrade enough that leaf falls