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PLANT MITOCHONDRIAL BIOLOGY
A. Structure
1. outer membrane
2. inner membrane
3. intermembrane space
4. Matrix
Mitochondrion from Bean root tip
Mit. from transfer cell
(part of the phloem vascular
tissue: transports sugar,
hormones, amino acids,
RNA!)
B. Functions
1.
2.
3.
4.
respiration (TCA cycle, etc.)
ATP synthesis (and export)
other oxidation pathways
own genetic system
C. Reproduction
1. by division (fission)
2. generally smaller than chloroplasts,
but also can fuse transiently to form
“giant” mitochondrion
3. can be many mitochondria per cell
Four stages of the formation of giant mitochondria in an elongated tobacco cell, densely packed
for microscopy
Cultured cells put under low Oxygen
Van Gestel, K. et al. J. Exp. Bot. 2002 53:1215-1218; doi:10.1093/jexbot/53.371.1215
Copyright restrictions may apply.
Fig. 2. Mitochondrial fusion in onion bulb epidermal cells
Arimura, Shin-ichi et al. (2004) Proc. Natl. Acad. Sci. USA 101, 7805-7808
The arrowheads point to a constriction that remains after fusion.
In b, the fused mito. divides again, at the constriction.
Movie 1.
Copyright ©2004 by the National Academy of Sciences
Fig. 4. Mitochondria and mitochondrial nucleoids in tobacco suspension cultured cells (BY-2)
Arrows
indicate
mitos lacking
nucleoids.
Movies 2-4
Movie 2
Movie 3
Arimura, Shin-ichi et al. (2004) Proc. Natl. Acad. Sci. USA 101, 7805-7808
Copyright ©2004 by the National Academy of Sciences
Plant Mt DNA & Genetics
1.
2.
3.
4.
circular, usually (see below)
no histones
low copy number per organelle
inherited uniparentally (usually)
–
–
–
Conifers: from both parents
Angiosperms : maternal (same as cpDNA)
Chlamydomonas: minus (-)
•
cpDNA inherited from the (+) parent
Mt DNA more variable in size and overall structure than CpDNA
- Linear molecule
- Inverted repeat at the
ends, with 3’ extensions
that are identical (not
complementary)
Other tRNAs must be imported from cytoplasm!
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.
Liverwort (Marchantia
polymorpha) mtDNA:
- based on complete
sequence
- 1 circle of 180,000 bp
MtDNA in selected Green Algae (chlorophytes) and Land
Plants (streptophytes)
1. Genome expanded in size (but not gene #) during the
evolution of land plants from green algae.
2. And intron type switched from Group I  II (and
increased).
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
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:
1. Chloroplast homologous (apparently came
from Cp DNA) or Cp-derived.
2. Vertically inherited mitochondrial genes.
Where did the excess DNA in certain angiosperm
Mt DNAs come from?
1.
2.
3.
A lot of the DNA must be non-coding; don’t 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.
DNA
transferred
from Cp to
Mt DNA in
rice.
Fig. 6.29
Mitochondrial Gene Expression
Transcription
1. A consensus promoter of 11 bp, contains
CRTA sequence (R = purine) within 20 bp
of transcription start site.
2. Phage-like RNA polymerase
– Single, large catalytic subunit
– Small specificity factor protein
Mitochondrial RNAs
1. Transcripts not polyadenylated or capped
2. Many transcripts “edited”
3. 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
Editing of Oenothera
mitochondrial RNAs
Determined by comparing
sequences of cDNA copies of mt
RNAs with the corresponding
genomic gene.
Plant Mt RNA editing
1.
2.
3.
4.
5.
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).
6. Some transcripts incompletely edited (editing
intermediates).
Possible mechanism for plant Mt editing:
Deamination of cytosine (to uracil) by a
cytidine deaminase
NH2
O
N
N
H20
=O
N
Cytosine
=O
N
Uracil
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.
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 these features (in angiosperms) are
recently evolved characters