Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 243

The exact time and place of tomato domesti-
cation is not clearly known; however, by the
fifteenth century the crop had reached a fairly
advanced stage of domestication. Tomato was
exported to Europe during that time, and sub-
sequent further intensification of the process
occurred. Since the twentieth century, human
beings have created a huge array of morpho-
logically different cultivars and forms from the
single species S. lycopersicum via plant breeding
(Foolad et al. 2008). Worldwide efforts aimed at
domestication, research, and breeding activities
have resulted in modern tomato varieties (mostly
hybrids) developed in all shapes, colors, and
sizes. Modern tomato genomics knowledge has
transformed breeding from an individually based
activity to multidisciplinary teamwork, focused
on exploiting genes from a tremendous tomato
germplasm, for high efficiency breeding (Bai and
Lindhout 2007).
Improvement in tomato cultivars is necessi-
tated by increased tomato production in response
to global demand for this commodity. In 2009,
the total net economic value of tomato was
∼

produced and marketed. Although tomato does
not rank high in nutritional value, by virtue of
volume consumed, it contributes significantly to
the dietary intake of vitamins A and C, essen-
tial minerals, and other nutrients. For example,
tomato ranks first among all fruits and vegeta-
bles as a source of vitamins, minerals, and phe-
nolic antioxidants in the U.S. diet (Rick 1980;
Vinson et al. 1998). In addition, fresh and pro-
cessed tomatoes are the richest sources of the
antioxidant lycopene, which arguably protects
cells from oxidants that have been linked to can-
cer (Ashrafi et al. 2011). Market demands require
high global production output of tomato. In order
to provide growers and consumers with high
quality tomatoes, breeders have to overcome a
number of issues threatening tomato production.

Low Genetic Diversity within the
Cultivated Species

Crop plant genomes have evolved under human
selection, which often has led to loss-of-function
mutations, such as loss of seed dispersal through
shattering in grains, loss of seed dormancy, and
loss of long branches (Sim et al. 2009). At the
same time, some agriculturally desirable charac-
teristics result from gain-of-function mutations.
Examples include disease and pest resistance,
high fruit nutritional quality, and large size of
fruit and seed (Foolad 2007; Sim et al. 2009).
During and following its domestication, the cul-
tivated tomato has undergone intensive selec-
tion and a few genetic bottlenecks occurred,
resulting in narrow genetic variation within the
cultigen. For example, tomatoes that were origi-
nally introduced to Europe by Spanish explor-
ers furnished the entire genetic base for the
modern tomato cultivars throughout the world
(Foolad 2007). The low genetic diversity in the
cultivated tomato is reflected by a low level
of isozymes and DNA markers polymorphism
across tomato breeding lines and commercial
cultivars (Labate and Baldo 2005). It is estimated
that only about 5% of the total genetic varia-
tion within Solanum section Lycopersicon (i.e.,

Int.$56 billion, which ranked it fourth among
all crop species (after rice, wheat, and soybeans)
and first among all vegetable crops (FAOSTAT
2011). Although a tropical plant, tomato is grown
in almost every region of the world, from the
tropics to within a few degrees of the Arctic
Circle. When outdoor production is restricted
because of cold temperatures, the tomato is
grown in greenhouses. Major tomato-producing
countries (in descending order of tonnage as of
2009) include China, the United States, India,
Turkey, and Egypt, followed by Italy, Iran, Spain,
Brazil, Mexico, Russian Federation, and Uzbek-
istan (FAOSTAT 2011). In North America, pro-
duction takes place in the U.S., Canada, and
Mexico, comprising a total of approximately
310,000 ha.
Tomatoes are an important part of a diverse
and balanced diet. In addition to tomatoes that
are consumed as raw vegetables or added to other
food items, a variety of processed products such
as pastes, juices, sauces, and soups are mass-