The double helix architecture of DNA was elucidated in 1953. Twenty years later, in 1973, the discovery of restriction enzymes helped to create recombi nant DNA molecules in vitro. The implications of these powerful and novel methods of molecular biology, and their potential in the genetic manipulation and improvement of microbes, plants and animals, became increasingly evi dent, and led to the birth of modern biotechnology. The first transgenic plants in which a bacterial gene had been stably integrated were produced in 1983, and by 1993 transgenic plants had been produced in all major crop species, including the cereals and the legumes. These remarkable achieve ments have resulted in the production of crops that are resistant to potent but environmentally safe herbicides, or to viral pathogens and insect pests. In other instances genes have been introduced that delay fruit ripening, or increase starch content, or cause male sterility. Most of these manipulations are based on the introduction of a single gene - generally of bacterial origi- that regulates an important monogenic trait, into the crop of choice. Many of the engineered crops are now under field trials and are expected to be commercially produced within the next few years. The early successes in plant biotechnology led to the realization that further molecular improvement of plants will require a thorough understanding of the molecular basis of plant development, and the identification and charac terization of genes that regulate agronomically important multi genic traits.

Advances in molecular biology and cell culture techniques have provided impetus to investigations of plant mitochondria. The organization of mitochondrial genomes has been intensely studied in maize, wheat, Oenothera, petunia, Brassica, and a few other species. These investigations have disclosed an unusually large and plastic genome, a unique organization based on a master chromosome and subgenomic chromosomes, and extra mitochondrial elements. The structural RNAs of plant mitochondria have furnished several new and exciting discoveries; they include the import of tRNAs into the mitochondria, editing of mRNAs, and the relaxed' nature of mitochondrial gene promoters. Cytoplasmic male sterility (CMS) is the most common mitochondrial gene mutation; it has, therefore, received extraordinary attention. Several mitochondrial gene mutations have been implicated in causing CMS, and attention is now focusing on the mechanism that causes pollen sterility, and how nuclear restorer genes interact with CMS genes to suppress sterility. Recently, a few other mitochondrial genes have been identified and characterized, which affect important mitochondrial fusions. Mitochondrial polypeptides, both nuclear and mitochondrial, are being studied to learn how they interact to form functional complexes, and how proteins are imported into the mitochondria. Protoplasm fusion experiments have provided a new and exciting means of recombining mtDNA that have generated interesting mutants, including CMS. Mitochondrial DNA replication is focusing on plasmid-like DNA and their origins of replication. Together, these studies have furnished insights into the origin of plant mitochondrial genomes and the relationshipsamong plant species. This volume describes these many new and exciting findings on plant mitochondria.