The Questions of Developmental Biology

of hormones. In scarlet runner beans, younger embryos without a suspensor can survive in culture
if they are supplemented with the growth hormone gibberellic acid (Cionini et al. 1976).
As the establishment of apical-basal polarity is one of the key achievements of embryogenesis, it
is useful to consider why the suspensor and embryo proper develop unique morphologies. Here
the study of embryo mutants in maize and Arabidopsis has been particularly helpful.
Investigations of suspensor mutants (sus1,sus2, and raspberry1) of Arabidopsis have provided
genetic evidence that the suspensor has the capacity to develop embryo-like structures (Figure
20.17; Schwartz et al. 1994; Yadegari et al. 1994). In these mutants, abnormalities in the embryo
proper appear prior to suspensor abnormalities. Earlier experiments in which the embryo
proper was removed also demonstrated that suspensors could develop like embryos (Haccius
1963). A signal from the embryo proper to the suspensor may be important in maintaining
suspensor identity and blocking the development of the suspensor as an embryo. Molecular
analyses of these and other genes are providing insight into the mechanisms of communication
between the suspensor and the embryo proper.
Maternal effect genes play a key role in establishing embryonic pattern in animals (see Chapter
9). The role of extrazygotic genes in plant embryogenesis is less clear, and the question is
complicated by at least three potential sources of influence: sporophytic tissue, gametophytic
tissue, and the polyploid endosperm. All of these tissues are in close association with the
egg/zygote (Ray 1998). Endosperm development could also be affected by maternal genes.
Sporophytic and gametophytic maternal effect genes have been identified in Arabidopsis, and it is
probable that the endosperm genome influences the zygote as well. The first maternal effect gene
identified, SHORT INTEGUMENTS 1 (SIN1), must be expressed in the sporophyte for normal
embryonic development (Ray et al. 1996). Two transcription factors (FBP7 and FBP11) are
needed in the petunia sporophyte for normal endosperm development (Columbo et al. 1997). A
female gametophytic maternal effect gene, MEDEA (after Euripides' Medea, who killed her own
children), has protein domains similar to those of a Drosophila maternal effect gene
(Grossniklaus et al. 1998). Curiously, MEDEA is in the Polycomb gene group (see Chapter 9),
whose products alter chromatin, directly or indirectly, and affect transcription. MEDEA affects an
imprinted gene (see Chapter 5) that is expressed by the female gametophyte and by maternally
inherited alleles in the zygote, but not by paternally inherited alleles (Vielle-Calzada et al. 1999).
How significant maternal effect genes are in establishing the sporophyte body plan is still an
unanswered question.
Radial and axial patterns develop as cell division and differentiation continue (Figure 20.18; see
also Bowman 1994 for detailed light micrographs of Arabidopsis embryogenesis). The cells of
the embryo proper divide in transverse and longitudinal planes to form a globular stage embryo
with several tiers of cells. Superficially, this stage bears some resemblance to cleavage in
animals, but the nuclear/cytoplasmic ratio does not necessarily increase. The emerging shape of
the embryo depends on regulation of the planes of cell division and expansion, since the cells are
not able to move and reshape the embryo. Cell division planes in the outer layer of cells become
restricted, and this layer, called the protoderm, becomes distinct. Radial patterning emerges at
the globular stage as the three tissue systems (dermal, ground, and vascular) of the plant are
initiated. The dermal tissue (epidermis) will form from the protoderm and contribute to the outer
protective layers of the plant. Ground tissue (cortex and pith) forms from the ground meristem,
which lies beneath the protoderm. The procambium, which forms at the core of the embryo will
give rise to the vascular tissue (xylem and phloem), which will function in support and transport.
The differentiation of each tissue system is at least partially independent. For example, in the