The Questions of Developmental Biology

If the pollen and the stigma are compatible, the pollen takes up water (hydrates) and the pollen
tube emerges. The pollen tube grows down the style of the carpel toward the micropyle (Figure
20.10). The tube nucleus and the sperm cells are kept at the growing tip by bands of callose (a
complex carbohydrate). It is possible that this may be an exception to the "plant cells do not
move" rule, as the generative cell(s) appear to move ahead via adhesive molecules (Lord et al.
1996). Pollen tube growth is quite slow in gymnosperms (up to a year), while in some
angiosperms the tube can grow as rapidly as 1 cm/hour.
Calcium has long been known to play an essential role in pollen tube growth (Brewbaker and
Kwack 1963). Calcium accumulates in the tip of the pollen tubes, where open calcium channels
are concentrated (Jaffe et al. 1975; Trewavas and Malho 1998). There is direct evidence that
pollen tube growth in the field poppy is regulated by a slow-moving calcium wave controlled by
the phosphoinositide signaling pathway (Figure 20.11; Franklin-Tong et al. 1996). Cytoskeletal
investigations show that organelle positioning during pollen tube growth depends on interactions
with cytoskeletal components. This must link to signaling, but the specifics are still unknown (Cai
and Cresti 1999).
Genetic approaches have been useful in investigating how the growing pollen tube is guided
toward unfertilized ovules. In Arabidopsis, the pollen tube appears to be guided by a longdistance
signal from the ovule (Hulskamp et al. 1995; Wilhelmi and Preuss 1999). Analysis of
pollen tube growth in ovule mutants of Arabidopsis indicates that the haploid embryo sac is
particularly important in the long-range guidance of pollen tube growth. Mutants with defective
sporophyte tissue in the ovule but a normal haploid embryo sac appear to stimulate normal pollen
tube development.
While the evidence points primarily to the role of the gametophyte generation in pollen tube
guidance, diploid cells may make some contribution. Two Arabidopsis genes, POP2 and POP3,
have been identified that specifically guide pollen tubes to the ovule with no other apparent effect
on the plant (Wilhelmi and Preuss 1996, 1999). These genes function in both the pollen and the
pistil, thus implicating the sporophyte generation in the guidance system
Fertilization
The growing pollen tube enters the embryo sac through the micropyle and grows through one of
the synergids. The two sperm cells are released, and a double fertilization event occurs (see
review by Southworth 1996). One sperm cell fuses with the egg, producing the zygote that will
develop into the sporophyte. The second sperm cell fuses with the bi- or multinucleate central
cell, giving rise to the endosperm, which nourishes the developing embryo. This second event is
not true fertilization in the sense of male and female gametes undergoing syngamy (fusion). That
is, it does not result in a zygote, but in nutritionally supportive tissue. (When you eat popcorn,
you are eating "popped" endosperm.) The other accessory cells in the embryo sac degenerate after
fertilization.
The zygote of the angiosperm produces only a single embryo; the zygote of the gymnosperm, on
the other hand, produces two or more embryos after cell division begins, by a process known as
cleavage embryogenesis. Double fertilization, first identified a century ago, is generally restricted
to the angiosperms, but it has also been found in the gymnosperms Ephedra and Gnetum,
although no endosperm forms. Friedman (1998) has suggested that endosperm may have evolved
from a second zygote "sacrificed" as a food supply in a gymnosperm with double fertilization.
Investigations of the most closely related extant relative of the basal angiosperm, Amborella,