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.respiration (TCA cycle, etc.) 2.ATP synthesis (and export) 3.other oxidation pathways 4.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

Copyright restrictions may apply. Van Gestel, K. et al. J. Exp. Bot : ; doi: /jexbot/ Four stages of the formation of giant mitochondria in an elongated tobacco cell, densely packed for microscopy Cultured cells put under low Oxygen

Copyright ©2004 by the National Academy of Sciences Arimura, Shin-ichi et al. (2004) Proc. Natl. Acad. Sci. USA 101, Fig. 2. Mitochondrial fusion in onion bulb epidermal cells 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 Arimura, Shin-ichi et al. (2004) Proc. Natl. Acad. Sci. USA 101, Fig. 4. Mitochondria and mitochondrial nucleoids in tobacco suspension cultured cells (BY-2) Arrows indicate mitos lacking nucleoids. Movies 2-4 Movie 3 Movie 2

Plant Mt DNA & Genetics 1.circular, usually (see below) 2.no histones 3.low copy number per organelle 4.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

Other tRNAs must be imported from cytoplasm! - Linear molecule - Inverted repeat at the ends, with 3’ extensions that are identical (not complementary)

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.A lot of the DNA must be non-coding; don’t have many more genes than liverwort Mt DNA. 2.There are a lot of Cp-DNA sequences “promiscuous DNA", integrates by illegitimate recombination 3.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

Determined by comparing sequences of cDNA copies of mt RNAs with the corresponding genomic gene. Editing of Oenothera mitochondrial RNAs

Plant Mt RNA editing 1.Prominent in angiosperms (none in liverwort). 2.Most transcripts are edited. 3.Most events are C  U. 4.Preferential editing of coding regions 5.Editing produces translatable mRNAs, and restores conserved amino acids (functional protein). 6.Some transcripts incompletely edited (editing intermediates).

N N = O NH 2 N N = O O Cytosine Uracil H20H20 Possible mechanism for plant Mt editing: Deamination of cytosine (to uracil) by a cytidine deaminase

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