Chloroplast Biology

1
Pioneers in chloroplast plant molecular biology
Ruth Sager Lawrence Bogorad
Jean-David Rochaix (Harvard) (Harvard)
(Geneva)
Discovered chloroplast genetics (Chlamydomonas)
and DNA
Chloroplast genes in corn demonstrated light
regulation
Chloroplast gene function nuclear control
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Chloroplast Biology

• I. Structure
• double-membrane envelope
• stroma large soluble interior
• thylakoid membrane system
• intrathylakoid space or lumen

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From Gunning Steer
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Isolated Spinach chloroplast
envelope
thylakoid membrane
stroma
From Hoober
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High magnification of thylakoid membranes
From Hoober
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Stroma of Avena (rye) plastid at high
magnification to show ribosomes
S aggregates of a glucosidase used in defense
against fungi, substrate is in vacuole
(avenacoside), product is a toxic saponin
From Gunning Steer
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• II. Functions
• many important biochemical (anabolic) pathways,
  e.g.,
• photosynthesis
• starch synthesis
• fatty acid synthesis
• amino acids synthesis
• pigment synthesis
• nucleotide synthesis
• nucleic acids and protein synthesis
• sulfur and nitrogen assimilation
• 2. own genetic system
• Indicates that pathway involves a chloroplast
  encoded gene in at least some organisms

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Many of the biosynthetic pathways are regulated,
to peak during the light period of L-D cycle.
Chlorophyll a and b synthesis during a 24 hour
light-dark cycle (Chlamydomonas). A similar
result was obtained for carotenoids and other
chloroplast lipids. (Janero and Barnett, 1982)
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III. Reproduction

• all plant and eukaryotic algal cells have
  plastids
• chloroplasts form by division semi-autonomous
• Involves proteins (Fts) similar to those that
  mediate cell division in bacteria

Cyanidioschyzon chloroplast dividing
From Miyagishima et al.
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IV. Development

• There are several forms of plastids
• 1. Proplastids - precursor of all plastids, found
  in meristems
• 2. Etioplasts - form in shoots of dark-grown
  plants, distinctive internal structure
• 3. Chloroplasts - in all green tissues
• 4. Amyloplasts - prominent in roots, store
  starch, colorless
• 5. Chromoplasts - in mature fruit, lots of
  carotenoids, little chlorophyll

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Plastid development in dark-grown barley
Mature Etioplast
Oldest
Youngest
Proplastid
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Amyloplasts from Glycine (soybean) root-cap
(peripheral cell)
S - starch grains Also have some thylakoid
membranes (star) close-by.
From Gunning Steer
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From U. Wisconsin Botany Dept.
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Young chromoplast from developing tomato fruit
Stars mark lycopene crystals many plastoglobuli
From Gunning and Steer
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Plastid development is plastic mostly under
nuclear control. Shoots
light proplastids etioplasts chloroplasts
chromoplasts Roots proplastids
amyloplasts
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V. Chloroplast Genetics

• 1. Inheritance is typically uniparental, usually
  maternal.
• Multiple mechanisms involved, not well
  understood
• - in Chlamydomonas (next slide), the paternal
  (-) cpDNA is destroyed, and the maternal ()
  cpDNA is preferentially replicated
• - in some land plants, the paternal plastids are
  excluded during fertilization or absent from the
  sperm cell
• 2. Essentially all plastids have DNA, usually the
  same DNA throughout the organism (homoplasmy).
• 3. The DNA sequence does not change during
  differentiation.
• There are exceptions to the last 2 statements.

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Chlamydomonas life cycle has sexual and asexual
reproduction. For sex, there are 2 mating types,
mt and mt-,
mt- cpDNA destroyed
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Digestion of cpDNA of the mt- parent in a young
zygote of Chlamydomonas revealed by fluorescence
staining of DNA.
0 Minutes 10 minutes
Nishimura, Yoshiki et al. (1999) Proc. Natl.
Acad. Sci. USA 96, 12577-12582
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From Mauseth, 1998
Acetabularia green alga, fossils known, many
species likely extinct A single giant cell (5
cm), 1 nucleus, 1 x 106 chloroplasts 30 of
chloroplasts dont have DNA!
nucleus
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Chloroplast DNA (cpDNA)

• General features
• double-stranded, circular molecule
• no histones, but have bound proteins (e.g., Hu),
  organized into nucleoids
• G-C content typically less than nuclear DNA
• multiple copies (30-100) per plastid (i.e., all
  cp genes are multi-copy)
• can be 10-20 of the total DNA in leaves

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relaxed cpDNA molecule from lettuce
From Kolodner Tewari