PLANT MITOCHONDRIAL BIOLOGY

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PLANT MITOCHONDRIAL BIOLOGY

• A. Structure
• outer membrane
• inner membrane
• intermembrane space
• Matrix

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?Mitochondrion from Bean root tip
Mit. from transfer cell? (part of the phloem
vascular tissue transports sugar, hormones,
amino acids, RNA!)
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B. Functions

• respiration (TCA cycle, etc.)
• ATP synthesis (and export)
• other oxidation pathways
• own genetic system

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C. Reproduction

• by division (fission)
• generally smaller than chloroplasts, but also can
  fuse transiently to form giant mitochondrion
• can be many mitochondria per cell

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Plant Mt DNA Genetics

• circular, usually (see below)
• no histones
• low copy number per organelle
• inherited uniparentally (usually)
• Conifers both parents
• Angiosperms maternal (same as cpDNA)
• Chlamydomonas minus (-)
• cpDNA inherited from the () parent

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Mt DNA more variable in size and overall
structure than CpDNA
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- Linear molecule - Inverted repeat at the ends,
with 3 extensions that are identical (not
complementary)
Other tRNAs must be imported from cytoplasm!
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Maize (Zea mays) Multiple, related circles
master circle of 570 kb (now known to be 700
kb) and subgenomic circles. Subgenomic circles
derive from master circle by recombination at
direct repeats.
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(No Transcript)
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Liverwort (Marchantia polymorpha) mtDNA - based
on complete sequence - 1 circle of
180,000 bp
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MtDNA in selected Green Algae (chlorophytes) and
Land Plants (streptophytes)

• Genome expanded in size (but not gene ) during
  the evolution of land plants from green algae.
• And intron type switched from Group I ? II (and
  increased).

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Size Trends in Evolution of MtDNA

• fungi ? man economization
• green algae ? higher plants
• Not a single trend, shrank in some lines, then
  got large again in others

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Transfer RNAs in angiosperm mitochondria

• Interesting origins, not all are encoded in the
  organelle, some are transported in from the
  cytoplasm!
• 2 groups of Mt-encoded ones
• Chloroplast homologous (apparently came from Cp
  DNA) or Cp-derived.
• Vertically inherited mitochondrial genes.

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Where did the excess DNA in certain angiosperm Mt
DNAs come from?

• A lot of the DNA must be non-coding dont have
  many more genes than liverwort Mt DNA.
• There are a lot of Cp-DNA sequences
• promiscuous DNA", integrates by illegitimate
  recombination
• There are also nuclear DNA sequences
• e.g., Oenothera nuclear 18S rrn gene in Mt DNA

• Some of these genes are transcribed, but not
  likely that they function, except the tRNAs.

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DNA transferred from Cp to Mt DNA in rice.
Fig. 6.29
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Mitochondrial Gene Expression

• Transcription
• A consensus promoter of 11 bp, contains CRTA
  sequence (R purine) within 20 bp of
  transcription start site.
• Phage-like RNA polymerase
• Single, large catalytic subunit
• Small specificity factor protein

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Mitochondrial RNAs

• Transcripts not polyadenylated or capped
• Many transcripts edited
• RNA Editing
• discovered in trypanosome mitochondria
• Common in plant mitochondria, also occurs in some
  cp genes of higher plants, and in some nuclear
  genes (in mammals)
• Definition any process, other than splicing,
  that results in a change in an RNA such that it
  differs from the sequence of the DNA template

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Determined by comparing sequences of cDNAs
(copies of Mt RNAs) with the corresponding
genomic genes.
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Plant Mt RNA editing

• Prominent in angiosperms (none in liverwort).
• Most transcripts are edited.
• Most events are C ? U.
• Preferential editing of coding regions
• Editing produces translatable mRNAs, and restores
  conserved amino acids (functional protein).
• Some transcripts incompletely edited (editing
  intermediates).

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Possible mechanism for plant Mt editing
Deamination of cytosine (to uracil) by a cytidine
deaminase
NH2
O
N
N
H20
O
O
N
N
Cytosine
Uracil
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Plant mt RNA Editing Mechanism (cont.)

• Cytidine deaminases are known, and in fact one
  is involved in ApoB editing in mammals.
• How are editing sites recognized?
• No guide RNAs (such as those involved in editing
  in Trypanosome mitochondria) have yet been found
  in angiosperm mitochondria.

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More Evolution

• Liverwort Mt genome is clearly an ancestral
  (primitive) land plant mt genome
• no RNA editing
• no trans-splicing
• full complement of tRNAs
• no cpDNA sequences
• Suggests all are recently evolved characters