RNA editing in plant mitochondria is a post-transcriptional process involving the partial change of C residues into U. These C to U changes lead to the synthesis of proteins with an amino acid sequence different to that predicted from the gene. Proteins produced from edited mRNAs are more similar to those from organisms where this process is absent. This biochemical process involves cytidine deamination. The cytoplasmic male sterility (CMS) phenotype generated by the incompatibility between the nuclear and the mitochondrial genomes is an important agronomical trait which prevents inbreeding and favors hybrid production. The hypothesis that RNA editing leads to functional proteins has been proposed. This hypothesis was tested by constructing transgenic plants expressing a mitochondrial protein translated fom unedited mRNA. The transgenic "unedited" protein was addressed to the mitochondria leading to the appearance of mitochondrial dysfunction and generating the male sterile phenotype in transgenic tobacco plants. Male sterile plants were also obtained by expressing specifically a bacterial ribonuclease in the anthers. The economical benefits of artificially engineered male-sterile plants or carrying the (native) spontaneous CMS phenotype, implies the restoration to obtain fertile hybrids that will be used in agriculture. Restoration to fertility of transgenic plants was obtained either by crossing male-sterile plants carrying the "unedited" mRNA with plants carrying the same RNA, but in the antisense orientation or, in the case of plants expresing the ribonuclease, by crossing male-sterile plants with plants expressing an inhibitor specific of this enzyme. RNA editingGeneralities. The first time the word "RNA editing" was used, a dozen years ago, concerned the insertion or deletion of uridine residues in the mitochondrial RNAs of some trypanosomes. After that, RNA editing has been found in mitochondria and chloroplasts from land plants, in the mitochondria of some fungi, in the nuclear-cytoplasmic compartments from animal cells and in the genomes of some viruses. The RNA editing process involves in some cases the modification of residues in RNA or, in other cases, the insertion and/or deletion of nucleotides in messenger RNA (for some reviews see Adler and Hadjuk, 1994;Araya et al., 1994;Schuster and Brennicke, 1994;Scott, 1995;Smith et al., 1997;Stuart et al., 1997). The consequences of the RNA editing process are either the modification of the coded information for some amino acids or the generation of new initiation and/or termination codons. In organisms where RNA editing is active the protein sequence predicted from the gene may be different from that of the mRNA translated protein. While RNA editing is abondantly found in mitochondria and to a lesser extent in the chloroplast compartment of higher plants, it is not present in algae. RNA editing has also been descriibed in mitochondria from other organisms such as Physarum polycephalum, but not in yeast organelles. When detected in mammalian nuclear transcr...