The Plant Mitochondrial Transfer RNA Population: A Mosaic of Species with Different Genetic Origins

Plant mitochondria still contain a large (from 200 to over 2000 kbp), autonomously replicating genome, although a massive transfer of genes to the nucleus has probably occurred since their endosymbiotic formation (Palmer, 1990). The mitochondrial DNA encodes some of the polypeptides involved in the enzymatic complexes of the respiratory chain, but these organelles depend on the nuclear and cytosolic compartments for most of their proteins, including the aminoacyl-transfer RNA (tRNA) synthetases and the majority of the ribosomal proteins. Plant mitochondria also do not retain all the tRNA genes of the ancestral endosymbiotic genome (called “native” or “genuine” genes). The higher plant mitochondrial tRNA population is therefore of particular complexity. First, a number of chloroplast DNA sequences, some of them comprising tRNA genes, have been integrated into the plant mitochondrial DNA during evolution. Some of these chloroplast-originating tRNA genes are expressed in mitochondria and produce mature, functional “chloroplast-like” species. Second, even including the “chloroplast-like” tRNA genes, plant mitochondria do not contain a complete set of tRNA genes and import some species from the cytosol. The plant mitochondrial tRNA population appears therefore to be a mosaic of species with nuclear, chloroplast and genuine mitochondrial origins.