Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

386 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
to obtain and maintain morphogenesis in calli for a long time in fruit species such as apple,
cherry, and olive (Baldoni and Rugini, 2001).
Work with tropical crops lags far behind that with herbaceous species. Genetic transformation
of perennial fruit crops has generally depended on embryogenic systems, and
therefore regenerants of the woody species. Many species must pass through a period of
juvenility before they can be properly evaluated. Two alternatives have been utilized to
overcome this limitation: (1) invigorating plant material through grafting of mature buds
onto juvenile stock plants (Cervera et al., 1998); (2) constitutive expression of either the
LEAFY or APETALA 1 genes from Arabidopsis thaliana to shorten the juvenile phase and
promote precocious flowering (Peña et al., 2001). Both of these innovations could stimulate
more transformation attempts with perennial species.
It would be advisable to use new selectable markers instead of the traditional ones and
reduce the percentage of loss, which can be as high as 40% in many fruit crops such as apple,
pear, banana, citrus, and grape (Gómez Lim and Litz, 2004). It would even be better to get rid
of marker genes altogether and to employ one of the several methods available to generate
marker-free transgenic plants (Ebinuma et al., 1997; Daniell et al., 2001; Huang et al., 2004;
Wang et al., 2005). Clearly, these approaches would help improve public acceptance and
perception of transgenic plants.
The major hindrances that have stymied genetic transformation studies with tropical/subtropical
fruit, however, concern lack of regeneration protocols for elite (mature phase)
selections and the relative absence of molecular studies. The latter reflects the state of the
science in many developing countries where tropical fruit crops are grown on a large scale
and the relative severity of production and postharvest problems of the crop. Biotechnology
studies involving fruit crops everywhere are generally underfunded, and national and
international agencies should perhaps consider more support for research with these plants.
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