Apples over-expressing a fruit-ripening enzyme lacked flowers and
had malformed stomata and altered composition of cell walls.
Apple (Malus domestica, Borkh.), cultivar Royal Gala.
Fruit-specific apple polygalacturanase (PG) gene (MdPG1) derived
from the apple cultivar Golden Delicious. PG is known as a fruit-specific
enzyme associated with fruit ripening (pectin degradation) and other
processes. The gene was fused to the cauliflower mosaic virus (CaMV-35S)
promoter so that the gene would be expressed in all parts of the plant.
Create transgenic apple trees that overexpress the enzyme PG "with
the expectation that disruption of cell wall metabolism would occur
in ripening fruit" (p. 123), since PG is known to break down pectin,
which is an important component of cell walls.
The trees did over-express the enzyme PG in their leaves.
The transgenic trees formed no flowers and fruits in contrast to
unmanipulated trees. The leaves of the transgenic trees were abnormal
in a variety of ways:
The authors conclude that this manipulation "has led to a range of
novel phenotypes including disrupted leaf organization, perturbed
water relations, malformed and malfunctioning stomata, silvery leaves,
and changes in leaf abscission. The modification of apple trees by this
single fruit gene, therefore, has offered a new and unexpected perspective
on a number of physiological and developmental processes" (p. 126).
The leaves were silver-colored and brittle; they wilted and broke
Microscopically, there were more air spaces between cells.
The stomata, which let air pass into and out of leaves, were
frequently malformed and did not close at night, which caused higher
transpiration (loss of moisture). There were also holes next to the
stomata, which is not normally the case.
Biochemically, there was less cell wall material, a changed
composition of the cell walls, and less pectin on cell wall surfaces.
Atkinson R. G., R. Schröder, I. Hallett, D. Cohen et al. (2002).
"Overexpression of Polygalacturonase in Transgenic Apple Trees Leads to a
Range of Novel Phenotypes Involving Changes in Cell Adhesion," Plant
Physiology vol. 129, pp. 122-33.
The Horticulture and Food Research Institute of New Zealand.
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Copyright 2008 The Nature