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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.
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