Rice organelle genomics: Approaches to genetic engineering and breeding

Although organelle (mitochondria and plastid) genomes have less than 1% of the genes in the nucleus, they encode essential genes, such as those involved in energy production, respiration, and photosynthesis, and genes that control agronomically important characteristics such as cytoplasmic male sterility. Organelle genomes have high copy numbers in each cell (one to two orders of magnitude greater than in the nucleus) and are characterized by maternal inheritance. To know functions of genes encoded in the organelle genomes or to develop new plants adapted to various severe environments, genetic engineering of organelle genomes is one of the promising approaches. However, modifying the mitochondrial or plastid genomes in rice is presently impossible or difficult. Here, we discuss the characteristic features of these genomes and recent attempts at plastid transformation.