Agriculture Reference
In-Depth Information
imperative. Fruits are also ideal choices for production of biopharmaceuticals since this can
be achieved in a totally contained atmosphere such as a greenhouse.
18.4.11 The alternate oxidase
The alternate oxidase is an enzyme involved in the cyanide-resistant respiratory pathway,
and it transfers electrons from the ubiquinone pool to oxygen without energy conservation.
The enzyme can use reductants (electron donors) that are produced in excess and cannot be
used efficiently by the cytochrome pathway, preventing the formation of reactive oxygen
species from an overreduced ubiquinone pool, and thus may be involved in acclimation
to oxidative stresses (Umbach et al., 2005) and to low temperatures (Fiorani et al., 2005).
In addition, the alternate oxidase may act as an important mitochondrial “survival protein”
against programmed cell death (Robson and Vanlerberghe, 2002). It has also been studied in
thermogenic species, and its activity correlated with heat production, necessary to volatilize
foul-smelling compounds to attract insect pollinators. There is a significant participation of
this pathway in the climacteric of many fruit. A cDNA coding for the mango alternate oxidase
has been identified, and by northern blot analysis the message was detected in unripe fruit
and shown to increase substantially in ripe fruit (Cruz Hernandez and Gomez Lim, 1995).
These results showed, for the first time, the participation of this enzyme in fruit ripening at
the molecular level and were subsequently confirmed by an independent group (Considine
et al., 2001). The temperature in ripe mango pulp is up to 10 C higher than in unripe pulp,
and this has been attributed to the activity of the alternate oxidase (Kumar et al., 1990).
This extra heat might also serve to volatilize aroma-giving compounds. Unfortunately, no
additional studies have been performed in other ripening fruit.
18.4.12 Genetic stability
Considering the time and effort invested in transferring a gene in long-lived perennials such
as fruit trees, it is essential that stable patterns of gene expression are maintained for long
periods of time. Although fruit trees are normally vegetatively propagated, the transgene
should also expressed in the progeny. There have been several studies to address this issue
both with marker genes (Vain et al., 2002; James et al., 2004) or with genes conferring novel
agronomic traits, such as rolABC from A. rhizogenes in transgenic kiwi plants of staminate
GTH and pistillate Hayward cultivars (Baldoni and Rugini, 2001). After 12 years, the
staminate rolABC plants maintained the same morphology and the offspring (transgenic
staminate X normal pistillate) was transgenic in 50% of plants. The cherry rootstock Colt,
containing RiT-DNA , which seems to modify the scion vigour, showed stability after 4 years
in the field (Baldoni and Rugini, 2001). R. Scorza and coworkers have performed extensive
analyses on transgenic Prunus domestica carrying the plum pox virus coat protein (PPV-
CP), uidA and nptll genes. Gene expression has been stable in the greenhouse for over 5 years
and the progeny, produced from hybridization of transgenic plants carrying plum pox virus
coat protein, expressed the transgenes (Ravelonandro et al., 1997). These results seemed
predictable in the light of a study in Arabidopsis carried out to search for transcriptome
changes associated with expression of transgenes regulated by constitutive promoters (El
Ouakfaoui and Miki, 2005). Insertion and expression of the marker genes, uidA and nptII ,
did not induce changes to the expression patterns of the approximately 24,000 genes that
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