The Questions of Developmental Biology

keule mutant of Arabidopsis, the dermal system is defective while the inner tissue systems
develop normally (Mayer et al. 1991).
The globular shape of the embryo is lost as cotyledons ("first leaves") begin to form. Dicots have
two cotyledons, which give the embryo a heart-shaped appearance as they form. The axial body
plan is evident by this heart stage of development. Hormones (specifically, auxins) may mediate
the transition from radial to bilateral symmetry (Liu et al. 1993). In monocots, such as maize,
only a single cotyledon emerges.
In many plants, the cotyledons aid in nourishing the plant by becoming photosynthetic after
germination (although those of some species never emerge from the ground). In some cases
peas, for example the food reserve in the endosperm is used up before germination, and the
cotyledons serve as the nutrient source for the germinating seedling. Even in the presence of
a persistent endosperm (as in maize), the cotyledons store food reserves such as starch, lipids, and
proteins. In many monocots, the cotyledon grows into a large organ pressed against the
endosperm and aids in nutrient transfer to the seedling. Upright cotyledons can give the embryo a
torpedo shape. In some plants, the cotyledons grow sufficiently long that they must bend to fit
within the confines of the seed coat. The embryo then looks like a walking stick. By this point,
the suspensor is degenerating.
The shoot apical meristem and root apical meristem are clusters of stem cells that will persist
in the postembryonic plant and give rise to most of the sporophyte body. The root meristem is
partially derived from the hypophysis in some species. All other parts of the sporophyte body are
derived from the embryo proper. Genetic evidence indicates that the formation of the shoot and
root meristems is regulated independently. This independence is demonstrated by the dek23
maize mutant and the shootmeristemless (STM) mutant of Arabidopsis, both of which form a root
meristem but fail to initiate a shoot meristem (Clark and Sheridan 1986; Barton and Poethig
1993). The STM gene, which has been cloned, is expressed in the late globular stage, before
cotyledons form. Genes have also been identified that specifically affect the development of the
root axis during embryogenesis. Mutations of the HOBBIT gene in Arabidopsis (Willemsen et al.
1998), for example, affect the hypophysis derivatives and eliminate root meristem function.
The shoot apical meristem will initiate leaves after germination and, ultimately, the transition to
reproductive development. In Arabidopsis, the cotyledons are produced from general embryonic
tissue, not from the shoot meristem (Barton and Poethig 1993). In many angiosperms, a few
leaves are initiated during embryogenesis. In the case of Arabidopsis, clonal analysis points to the
presence of leaves in the mature embryo, even though they are not morphologically well
developed (Irish and Sussex 1992). Clonal analysis has demonstrated that the cotyledons and the
first two true leaves of cotton are derived from embryonic tissue rather than an organized
meristem (Christianson 1986).
Clonal analysis experiments provide information on cell fates, but do not necessarily indicate
whether or not cells are determined for a particular fate. Cells, tissues, and organs are shown to be
determined when they have the same fate in situ, in isolation, and at a new position in the
organism (see McDaniel et al. 1992 for more information on developmental states in plants).
Clonal analysis has demonstrated that cells that divide in the wrong plane and "move" to a
different tissue layer often differentiate according to their new position. Position, rather than
clonal origin, appears to be the critical factor in embryo pattern formation, suggesting some type
of cell-cell communication (Laux and Jurgens 1994). Microsurgery experiments on somatic carrot