11.3—
Mitochondrial Autonomy
Research on the biogenesis of plant mitochondria has lagged far behind that on mitochondria from animal and fungal cells. A summary of the position is given here in the hope of stimulating readers to undertake work in this field; recent reviews have appeared on the biogenesis of both animal and fungal mitochondria (Borst, 1971, 1972), and of those in plants (Boulter et al., 1972; Leaver & Harmey, 1973; Leaver, 1975).
The first point to emphasize is that mitochondria constitute a much smaller compartment in plant cells compared to chloroplasts. The amount of mitochondrial DNA and ribosomes is correspondingly much smaller as a percentage of the total cellular complement. The technical difficulties of extracting small quantities of intact, uncontaminated mitochondria from plant tissues make studies on their genetic system more difficult than in the case of chloroplasts. Another difference from chloroplasts is that the DNA and ribosomes of mitochondria are more variable in their properties between species, so the extrapolation of results must be done with caution.
In a range of higher plants the density of mitochondrial DNA is constant at 1.706–1.707 g cm–3 , whereas the nuclear DNA from the same species varies in density between 1.691–1.702 g cm––3 . In many organisms mitochondrial DNA has been isolated in circular form; in contrast to the situation for chloroplasts (see 11.2.1.5), the contour length of the circles varies with the species. Thus, animals have mitochondrial DNA circles of about 5 µm contour length; this corresponds to a molecular weight of about 107 , or 15,000 base pairs. In Ascomycetes the contour length of circular mitochondrial DNA is in the range 20–26 µm, while in higher plants lengths of 30 µm have been reported. The small size of the mitochondrial DNA in animals means that it can code for only a few components. Hybridization studies in Xenopus indicate that mitochondrial ribosomal RNA and at least some mitochondrial transfer RNA are encoded in mitochondrial DNA. The remaining mitochondrial DNA is sufficient to code for only about ten proteins each of molecular weight 50,000. However, kinetic complexity measurements on mitochondrial DNA from higher plants indicate that it has between six and ten times the coding potential of animal mitochondrial DNA. This observation raises the possibility that mitochondria from plants possess a higher degree of autonomy than those from animals, and this reinforces the plea that more research be carried out with plant cells.
The sedimentation coefficients of mitochondrial ribosomes and their component ribosomal-RNAs vary greatly with the species. Animal mitochondria contain so-called 'miniribosomes' which sediment between 55s and 60s, while fungal mitochondria contain 70–74s ribosomes. Leaver and Harmey (1972) have shown that mitochondrial ribosomes from several higher plants sediment at 77s–78s, while their major RNA components sediment at 25s and 18s. Although mitochondrial ribosomes differ in size in different species, they are all similar
in that protein synthesis by them is sensitive to the same set of antibiotics which inhibit prokaryote and chloroplast ribosomes.
Protein synthesis by isolated mitochondria has been studied extensively in animals and fungi, but hardly at all in higher plants. Several proteins of the inner, but not the outer, mitochondrial membrane are synthesized by animal and fungal mitochondria. These proteins have been identified as subunits of the ATPase complex, cytochrome b, and cytochrome c oxidase; the other subunits of these insoluble complexes are synthesized by cytoplasmic ribosomes. This evidence suggests the synthesis of mitochondrial membranes requires the co-operative activity of both mitochondrial and nuclear genomes. This idea of the co-operation between the several genetic systems in eukaryotic cells is already familiar to the reader from section 11.2.5. The same problem of the transport of proteins across the bounding membranes applies to mitochondria as well as to chloroplasts. In contrast to chloroplasts, mitochondria from animal and fungal cells synthesize no soluble proteins; whether higher plant mitochondria synthesize any soluble proteins is unknown.
No protein has so far been rigorously identified as being encoded in mitochondrial DNA in any organism. A simple presumption is that those proteins synthesized by isolated mitochondria are encoded in mitochondrial DNA. It is likely that the integrative controls between nuclear and mitochondrial genomes in higher plants differ from those in animals which contain less than one sixth as much DNA. Why do plant mitochondria apparently contain so much more genetic information to perform the same functions as animal mitochondria? Answering this question is a challenge which must be taken up if our understanding of the origin of this organelle in plants is to improve.