Plant cell biology 2

• why do chloroplasts move

  to avoid photo damage
• chloroplast photorelocation movement

  1. Photoperception: Plant blue-light photoreceptors perceive the light (phototropins)
  2. Signal transduction: Calcium signalling.
  3. Chloroplast movement: Motor dependent movement of chloroplasts (myosins, but a role for kinesins as well)
• cytoplasmic streaming features

  • occurs independently of chloroplast movement
  • depends on energy and requires F-actin
  • supports "tip growth" of pollen tubes
• what is the sliding theory of cytoplasmic streaming

  • A mobile endoplasm moves against a stationary exoplasm
  • The force is generated by myosin motors that move along actin cables (bundles of F-actin)
• what myosins do plants contain

  XI, XIII and VIII myosins
• cytokinesis in plants

  • During cytokinesis, microtubules form a plant-specific array
  • The endoplasmic reticulum crosses the cell plate
  • Plasmodesmata are formed during cytokinesis
  • Cell plate formation starts in the middle of the cell
• mitosis in plants

  • Cytokinesis is accompanied by de novo synthesis of plasma membrane and cell wall
  • An array of microtubules (the phragmoplast) guides and supports the formation of the cell plate
  • The microtubules guide secretory vesicles to the forming cell plate
  • Vesicles of the cell plate form cell wall in their interior; they fuse to give rise to the primary cell wall
  • Plasmodesmata are spared in the newly forming primary wall
  • Cell plate initiates in the cell centre and expands outward until it fuses with the existing plasma membrane at the cortical division zone
• what are stomata

  openings in the epidermis of plant leaves
• when do stomata open or close

  • Opening: light, low CO2, high temperature
  • Closure: low light (or darkness), high CO2 and high difference in water content in the air
• what makes stomata open

  • due to a swelling of the guard cells
  • Chloroplasts are providing the required ATP
  • Turgor pressure is generated by an increase of potassium in vacuoles
• what are trichomes

  "hair-like" extensions on plant leaves, outgrowths of the epidermis, Important sites of secondary metabolite production
• glandular trichomes

  Biosynthesise, secret and store secondary metabolites; defence against microbes, insects and animals
• non-glandular trichomes

  UV protection, plant defence against herbivores and mechanosensing
• xylem cells

  • Xylem consists of trachea, which are a string of dead and empty cells;
  • Their cell wall is enforced by lignin deposits
• phloem cells

  • Sucrose is transported from "sources" to "sinks" in the sieve tubes of the phloem
  • Sieve cells (=sieve elements) are forming a sieve tube, and companion cells
  • Sieve cells have lost ribosomes and most organelles (incl. most mitochondria, Golgi, nuclei, vacuole)
  • Sieve cells rely on companion cells for survival
  • The cells are separated by "sieve plates"
• what is the pressure flow hypothesis

  describes a mechanism by which osmotically generated intracellular pressure (turgor pressure) generates a bulk flow of sugar, water and other assimilate.
• who proposed the pressure flow hypothesis

  Ernst Münch in 1930