Principles of Plant Genetics and Breeding

and cell walls are degraded. There is an accumulation of
lycopene (red pigment in tomato), sugars, and various
organic acids. Ripening is a complex process that
includes fruit color change and softening.
Ripening in tomato has received great attention
because it is one of the most widely grown and eaten
fruits in the world. Ethylene plays a key role in tomato
ripening. When biosynthesis of ethylene is inhibited,
fruits fail to ripen, indicating that ethylene regulates
fruit ripening in tomato. The biosynthesis of ethylene is
a two-step process in which s-adenosyl methionine
(SAM) is metabolized into aminocyclopropane-1 carboxylic
acid (ACC), which in turn is converted to ethylene.
Knowing the pathway of ethylene biosynthesis,
scientists can manipulate the ripening process by either
reducing the synthesis of ethylene or reducing the
effects of ethylene (i.e., plant response).
In reducing ethylene biosynthesis, one successful
strategy by Agritope of Oregon has been the cloning of
a gene that hydrolyzes SAM, called SAM hydrolase,
from a bacterial virus. After bioengineering the gene
to include, among other factors, a promoter that
initiates expression of the gene in mature green fruits,
Agrobacterium-mediated transformation was used to
produce transgenic plants. The effect of the chimeric
gene was to remove (divert) SAM from the metabolic
pathway of ethylene biosynthesis. The approach adopted
by researchers was to prevent the ACC from being
converted to ethylene. A gene for ACC synthase was
isolated from a bacterium and used to create a chimeric
gene as in the Agritope case.
The technology of antisense has been successfully
used to develop a commercial tomato that expresses
the antisense RNA for ACC synthase and ACC oxidase.
United States Department of Agriculture (USDA)
scientists pioneered the ACC synthase work, while
scientists from England in collaboration with Zeneca
pioneered the ACC oxidase work. Because transgenic
tomatoes with an incapacitated ethylene biosynthetic
pathway produced no ethylene, they failed to ripen on
their own, unless exposed to artificial ethylene sources
in ripening chambers. The technology needs to be
perfected so that fruits can produce some minimum
amount of ethylene for autocatalytic production for
ripening over a protracted period.
The “FlavrSavr” tomato
Another application of antisense technology is in preventing
an associated event in the ripening process, fruit
softening, from occurring rapidly. Vine-ripened fruits
BREEDING COMPOSITIONAL TRAITS AND ADDED VALUE 413
are tastier than green-harvested and forced-ripened
fruits. However, when fruits vine ripen before harvesting,
they are prone to rotting during shipping or have
a short shelf-life in the store. It is desirable to have fruits
ripen slowly. In this regard, the target for genetic engineering
is the enzyme polygalacturonase (PG). This
enzyme accumulates as the fruit softens, along with
cellulases that breakdown cell wall cellulose and pectin
methylesterase that together with PG break the pectic
cross-linking molecules in the cell wall. Two pleiotropic
mutants of tomato were isolated and studied. One
mutant, never ripe (Nr), was observed to soften slowly
and had reduced accumulation of PG, while the second
mutant, ripening inhibitor (rin), had very little accumulation
of PG throughout the ripening process. This
and other research evidence strongly suggested a strong
association between PG and fruit ripening. PG is biosynthesized
in the plant and has three isoenzymes (PG1,
PG2, PG3).
This technology was first successfully used by Calgene
to produce the “FlavrSavr” tomato, the first bioengineered
food crop, in 1985. The protocol has been previously
described. This pioneering effort by Calgene
flopped for several reason, among which was the poor
decision to market a product intended for tomato processing
as a fresh market variety.
Breeding cooking and processing qualities
Cooking food changes its texture, color, taste (palatability),
and digestibility, among other changes. The heat
treatment applied during cooking breaks down some
toxic compounds in food, where applicable. What is
considered to be a good cooking or processing quality
depends on the product and the culture in which the
product is used. As previously cited, for example, some
cultures prefer sticky rice, others non-sticky rice for certain
food preparations. Similarly, some potato cultivars
are suitable for frying, others for baking, and yet others
for cooking.
Similarly, canning or processing quality is an important
breeding objective in crops that are grown for that
purpose. It is desirable for canned produce to retain its
texture and color to an appreciable degree. Some cultivars
remain firm and of good color, whereas others crack
or become mushy after canning.
Other products are crushed, ground, or milled during
processing. In corn, for example, milling may be dry
or wet. For dry milling white endosperm and semihard
kernel is preferred, while wet milling (for starch and oil)
requires softer kernels.