Only during the last years began the interest in plant mitochondria to grow. But the analyses of the mitochondria of other sources like fungi (Saccharomyces cerevisiae, Neurospora crassa), ciliates and vertebrates offer enough data to give a general impression.
They contain genetic information (mtDNA).
They harbour all the components necessary for protein synthesis
(ribosomes, tRNA, etc.)
Beside the outer membrane do they carry an inner membrane,
the site of energy conversion.
According to the endosymbiontic theory have both types of organelles developed from procaryotes that began to live in symbiosis with primitive eucaryotic cells.
All questions of the previous section are here important, too. Mitochondria themselves synthesize only a small part of the proteins they require, most stem from the cytosol. The human mitochondrial genome has been sequenced, the mtDNA (one molecule) contains 165,691 base pairs (S. ANDERSON et al., 1981). Many of the results concerning vertebrate and yeast mtDNA are not applicable to higher plants. The mtDNA of higher plants is extremely complex. Its molecular weight is between 200,000 and 2,400,000. A range of different molecules can be found in plants. Besides large, linear molecules occur also circular molecules of differing size. Many molecules are incomplete and recombination seems to be a common event. It has already been mentioned that the genetic code of mitochondria differs from the general one.
During the last years has special attention been paid to the cytoplasmic male sterility (CMS) phenomenon since this inhibition of the pollen development that is very common in higher plants is used for the production of hybrid seeding material. In maize are at least three CMS varieties known: C, T and S. Only plants with cytoplasm of the type N (= normal) produce fertile pollen. The mtDNA of the C - plasma differs from that of the N plants in numerous restructurings. In this way have hybrid genes, i.e. genes that consist partially of an ATPase gene and partially of a cytochrome - oxidase gene been developed. The mtDNA of the T variety displays an even higher degree of restructuring. In the mitochondria of the S variety were two linear plasmids found. Plasmids are additional small DNA molecules typical for bacteria. During the last years did they become indispensable tools of genetic engineering. They were detected in eucaryotic cells, too: in fungi, where they are localized mainly in the mitochondria and in higher plants. It seems as if the hybrid proteins produced in the CMS varieties would be deposited in the inner mitochondrial membrane and would impair its function.
The activity of a number of enzymes can be detected in the cytosol and the mitochondria simultaneously (and sometimes in chloroplasts, too). The respective enzymes have the same specificities but differ in their structure (amino acid composition). This guarantees that one type is transported into one organelle, another into a second organelle while a third type may remain in the cytosol. Enzymes of the same activity and specificity but of different structures and reaction kinetics are called isoenzymes. Though the exact protein - chemical data of most isoenzymes are not known, is it still known that they differ in this respect because they show different behaviours in gel electrophoresis and / or isoelectric focusing.
Mitochondria develop by division, too. While in animal cells only the interactions of the nuclear and the mitochondrial genomes have to be regarded, are we confronted with an interaction between nuclear, chloroplast and mitochondrial genomes in green plants. Especially the inner mitochondrial membrane can only be passed selectively even by low molecular weight metabolites. The carriers within this membrane cause the selectivity and control the transport rate.
But the most important feature of mitochondria is the respiratory or oxydative phosphorylation. It consists of two functional parts, the electron transport chain and ATP synthesis, the original phosphorylation. ATP synthesis is catalyzed by an ATP synthethase that is located within the inner mitochondrial membrane. This enzyme functions only, if the membrane is intact, i.e. if it is impermeable for protons and if it separates two compartments from each other so that a proton gradient can be maintained. The gradient fuels the phosphorylation.
The malate - oxaloacetate shuttle is characteristic for plant cells. It transports redox equivalents intracellularly. The oxaloacetate transporter (OA) is decisive for this process (according to H. W. HELDT and U.T. FLÜGGE, 1986).
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de