seeds

• Seeds & seed plants

  Advantages of reproducing by seed: dormancy, longevity, dispersal
• How is seed development and germination regulated?

  1. Plant hormones

  2. Environment
• What makes a seed a seed?

  • Desiccation tolerance
  • Storage reserves
  • Dormancy
• Seed plants are very good at reproducing by seed
• Challenges in conquering the land

  • Leaving the water behind
  • Water free reproduction
  • Modification of the life cycle
  • Protection against desiccation
  • Mechanical support (2D to 3D)
  • Cells for water & nutrient uptake
  • Anchorage
  • UV-B radiation
• Seed plants do not need water for reproduction
• Gametophyte (haploid, 1n)


  Sporophyte (diploid, 2n)
• Seed plants evolved early, but became dominant only in the Cretaceous with rapid diversification of angiosperms
• Current distribution of plant species

  • Angiosperms (seeds enclosed in ovary) ~ 352,000 species
  • Gymnosperms (seeds not enclosed in an ovary) ~ 1,000 species
  • Pteridophytes (ferns, reproduction needs water) ~ 13,000 species
  • Bryophytes (mosses and liverworts, reproduction needs water) ~ 20,000 species
• Seed size varies hugely between plant species
• Largest seed


  • Coco de Mer (Arecaceae) Lodoicea maldivica 18 kg
• Smallest seed


  • Anoectochilus imitans (orchid from New Caledonia) 0.05 mm
• Orchids, e.g. vanilla, have usually very small seeds. Tree seeds are usually quite large.
• Desiccation tolerance


  How dry are seeds? 15-25% RH recommended for long term seed storage
• Cellular responses to desiccation

  • Removing all the water from a cell and retaining its viability is tremendously challenging
  • Membranes and cell walls can break or adhere to each other
  • Proteins can aggregate or denature
  • Toxic reactive oxygen species can accumulate
• How do desiccation-tolerant cells prevent irreversible damage?

  Accumulate LEAs and sugars to protect membrane integrity
• Late embryogenesis abundant (LEA) proteins


  Small, very hydrophilic proteins that fold only when water content decreases. Functions: membrane stability, antioxidant, protein stability
• Sugars


  Oligosaccharides that replace water and function to maintain membrane stability
• Storage reserves


  Why does a seed need storage reserves? To fuel growth until the seedling can produce its own energy through photosynthesis
• Where are storage reserves stored?


  In the embryo or endosperm
• What is stored as storage reserves?

  • Protein, oil, carbohydrate
• Majority of our food is seed-based, either directly (cereals, oil, pulses) or indirectly (seed as feed for meat production)
• Dormancy


  A mechanism to ensure a spread of germination from seed populations over time
• Dormancy cycle


  1. Quiescent dry seed (dormant)

  2. Imbibition (water uptake)

  3. Imbibed dormant seed

  4. After-ripening (period of dry storage)

  5. Imbibed non-dormant seed

  6. Germinated seed
• Types of dormancy


  • External structures (exogenous dormancy): physical, chemical, mechanical
  • Embryo (endogenous dormancy): physiological, morphological
• Most dormant seeds use a combination of these dormancy mechanisms
• Abscisic Acid (ABA)


  The major regulator of developmental events in the later stages of seed development and following imbibition: storage reserve accumulation, desiccation tolerance, dormancy, after-ripening time, germination
• Gibberellic Acid (GA)


  GA is absolutely required for germination. Seeds that do not produce GA are unable to germinate.
• Hormonal control of seed germination

  ABA:GA ratio determines whether germination takes place, not absolute hormone contents.
• Germination


  Complete when the radicle (embryo root) has emerged from all covering layers (testa and endosperm)
• Phases of transition from seed to seedling

  1. Pre-Germination: Imbibition, physical uptake of water

  2. Phase I: Radicle emergence, cell division, protein/RNA synthesis, DNA repair

  3. Phase II: Mobilisation of storage reserves, DNA synthesis, respiration, mitochondrial repair and multiplication

  4. Phase III: Seedling establishment
• What triggers germination?


  • Parasitosis
  • Symbiosis
  • Light
  • Temperature
  • Oxygen
  • Smoke
• Light


  An extremely important factor regulating seed germination. Phytochrome is an important sensor of light.
• Uce GA


  Unable to germinate
• Hormonal control of seed germination

  1. ABA

  2. GA

  3. ABA:GA ratio determines whether germination takes place, not absolute hormone contents
• Germination is complete when the radicle (embryo root) has emerged from all covering layers (testa and endosperm)
• The transition from seed to seedling

  1. Pre-Germination

  2. Water Uptake (Imbibition, Physical uptake of water)

  3. Radicle Emergence (Germination, Cell division, Protein/RNA synthesis, DNA repair)

  4. Seedling Establishment (Mobilisation of Storage Reserves, DNA synthesis, Respiration, mitochondrial repair and multiplication)
• What triggers germination?


  • Parasitosis
  • Symbiosis
  • Light
  • Temperature
  • Oxygen
  • Smoke
• Phytochrome


  An important sensor of light quality that determines germination potential
• Red light (R)


  Activates phytochrome, GA biosynthesis and germination