Biol 156 Discussion 2

  1. Syncitium
    Structure with 7 cells and 8 nucleii
  2. 4 types of cell
    • Egg cell
    • Synergid cells
    • Antipodal cells
    • Central cell
  3. Auxin maxima was discovered in...
    the distal tip of the nucellus. The auxin output was traced to megasporogenesis.
  4. GFP was used to determine when auxin was localized
    Auxin is localized (concentrated) in early development but is more dispersed during gametophyte development.
  5. Downregulation
    the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external variable. An increase of a cellular component is called upregulation.
  6. Downregulated Auxin Response Factors with microRNA.
    • Restricted down regulation of ARF to embryo sac.
    • Looked at GFP expression in developing embryos.
    • Cells lacked identity
  7. Determine origin of discrete auxin maxima
    • Looked at auxin efflux facilitators called PINs
    • No PIN expression in gametophyte, but there was PIN expression in nucellus until first stage
    • SO auxin flux may occur in nuccellus during early stages of development
    • Encode key enzymes in auxin biosynthesis
    • YUCCA expression first appears in micropylar region of nucellus outside embryo sac in first stage
    • Suggests auxin might be synthesized initially at the nucellus and subsequently at the micropylar pole of the embryo sac
  9. Distorted polarization of auxin
    • Cell identity problems. Too many synergid cell?
    • Would attract 2 pollen tubes
  10. Conclusions:
    1. Sphorophytic signals lead to auxin source specified within gametophyte at the micropylar pole that result in a gradient of auxin along the micropylar-chalazal axis

    2. Gametophytic auxin might not be the primary signal but instead might be the result of a patterning caused by unknown X factor which induces YUC expression in micropylar nuclei resulting in asymetric auxin distribution.
  11. Auxin Diffuses Rapidly...
    so seems to be controlled.
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Biol 156 Discussion 2