Biotechnological Approaches to Enhancing Tropical Fruit Quality - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 380

higher plants (Krizek and Fletcher, 2005). Transgenic plants overexpressing genes such
as LEAFY or CONSTANS have been generated (Putterill et al., 2004). These genes are
sufficient to determine floral fate in lateral shoot meristems with the consequence that
flower development is induced precociously.
As mentioned above, most tree species have a long juvenile period of at least 5 years, and
the time to evaluate trees can be up to 20 years. This has hampered the development of new,
improved cultivars by traditional plant breeding and poses a challenge for the generation
of improved varieties when using plant tissue cultures techniques. Transgenic citrus plants
overexpressing the genes LEAFY or APETALA1 , which promote flower initiation in Ara-
bidopsis , have been produced (Pena et al., 2001). Both types of transgenic citrus produced
fertile flowers and fruits as early as the first year, and a shortening of their juvenile period
was detectable. Furthermore, expression of APETALA1 was as efficient as LEAFY in the
initiation of flowers, and did not produce any severe developmental abnormality. Both types
of transgenic trees flowered in consecutive years, and their flowering response was under
environmental control. In addition, zygotic and nucellar-derived transgenic seedlings had a
very short juvenile phase and flowered in their first spring, demonstrating the stability and
inheritance of this trait. These results have opened up new avenues for research in genetic
improvement of fruit trees.

18.4.7 Color and pigment metabolism

The external color of fruit is an important factor in consumer preference. The principal
pigments in many fruit are carotenoids and anthocyanins, which are synthesized via the
terpenoid and phenylpropanoid pathways, respectively. Pigment synthesis manipulation by
genetic means represents an interesting choice to modify the color of a fruit to make it
more attractive to the consumer. In some cases, the goal would be to increase the color of
the transgenic product. Color is particularly important in fruits to be employed for jams,
marmalades, pastes, and even wine.
There have been a number of attempts to increase the content of carotenoids in fruit
by genetic manipulation, not so much to alter the normal color of plant organs but for
other reasons (discussed below), and as expected, many of these attempts have resulted in
altered coloration of plant organs. However, there have been some examples of genetic ma-
nipulation of carotenoids to alter the color of plant organs. Phytoene synthase, an enzyme
induced during fruit ripening, catalyzes the dimerization of two molecules of geranylgeranyl
pyrophosphate to form phytoene, the first C40 carotene in the carotenoid synthesis path-
way (R omer and Fraser, 2005). Expression of phytoene synthase in antisense in tomatoes
produced pale yellow flowers and fruits that ripened to a yellow color (Bird et al., 1991).
Lycopene could not be detected in those fruits, although other ripening processes such as
polygalacturonase accumulation were unaffected. The carotenoid biosynthesis pathway has
also been modified in tobacco plants using the CrtO gene from the alga Haematococcus
pluvialis , encoding the -carotene ketolase (Mann et al., 2000). Transgenic plants accumu-

lated ketocarotenoids that changed the color of the nectary from yellow to red. The authors
speculate that plant transformation with this gene may be used in the future to change the
color of fruit.
Anthocyanins are flavonoid derivatives which are major secondary plant products well
known for the blue, red, and purple coloration they provide to flowers, fruits, and leaves.

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