program after fertilization. For example, the enlarged ovary under a decaying rose flower is
called arose hip, and like citrus fruits, contains high levels of vitamin C. Fruit development
is a strategy thought to attract animals that will eat the fruit and disperse the seeds far from
the plant. Animals and plants have coevolved, with animals trying to get the most nutrients
(through digestion) from the fruit and seeds, and the plant evolving processes designed to
facilitate seed dispersal in contrast to seed digestion. This coevolution may account for the
incredible diversity of fruit and seed types.
Fruit development requires both fertilization and growth of the embryo within the seed;
thus seed and fruit development are related. For example, in some species lopsided fruit
will result when fertilization of ovules on one side of the ovary is defective. The seeds
developing from fertilized ovules are thought to signal to the surrounding fruit via their
production of growth hormones, such as auxin and cytokinin. There are physiological
conditions, however, that will override the requirement for these seed-derived hormones.
The process of fruit development in the absence of seed development is calledpartheno-
carpy, which is a desirable trait for certain fresh fruit. Some commercial “seedless”
varieties, like the seedless watermelon, actually have very tiny, partially developed seeds.
In contrast, certain true seedless grape varieties undergo parthenocarpic fruit development
in the absence of fertilization of the ovules. Studies on parthenocarpic fruit will lead to a
better understanding of the processes that accompany fertilization. One useful tool will
be thefwf(fruit without fertilization) mutant fromArabidopsis, which is a facultative
parthenocarp, setting seed in a normal way when pollinated, but also forming short seedless
fruit when left unpollinated. It is thought that the FWF protein acts as an inhibitor of fruit
development and that this inhibition is released after fertilization. Better understanding of
FWF function awaits cloning of the gene (Giovannoni 2001).
4.2.4 Embryogenesis
As described earlier,embryogenesisbegins after the 1Negg cell and 1Nsperm nuclei fuse
together, forming a 2Nembryo. Plant embryogenesis differs significantly from animal
embryo development in its lack of cell migration and substantial cell specification. For
example, the mature plant embryo within the seed does not contain cells specified to
become flower cells or gamete-producing cells. These differentiation events will occur
later in development, well after seed germination. Instead, plant embryogenesis will
result in the acquisition of bilateral symmetry, an apical/basal or shoot/root axis, and
the three types of tissue.
The first cell division of the plant embryo results in an asymmetric division giving rise to
a small upper, terminal cell and a larger, lower basal cell (Fig. 4.4). This establishes a longi-
tudinal or an apical/basal axis in the embryo (Weijers and Jurgens 2005). The upper cell
always gives rise to the embryo proper, while the lower cell gives rise to thesuspensor
and thehypophysis, forming part of root meristem, root initial cells, and root cap. The sus-
pensor is a highly specialized and terminally differentiated tissue that connects the embryo
to the embryo sac and maternal ovule tissue. It functions as a conduit for nutrients and
senesces after the heart stage of embryo development. This short-lived unique organ con-
sists of only 7–10 cells total inArabidopsis thaliana.
The upper cell of the two-cell embryo undergoes two more cell divisions, passing
through the four- and eight-cell stages, in which a gain of embryo mass occurs. Further
cell divisions result in mass of cells on top the suspensor referred to as theglobular-
stage embryo(Fig. 4.4). More cell divisions result in development of the heart-stage
4.2. EMBRYOGENESIS AND SEED GERMINATION 89