embryo, so called because of the characteristic heart shape of the embryo. This heart
shape results because differentiation of cells has occurred, with some cells beginning
to acquire shoot apical meristem (SAM) identity in the cleft of the heart, and two lateral
domains giving rise to cells destined to form the cotyledons of the embryo. In addition,
the root apical meristem (RAM) becomes specified at this stage. With the development
of the SAM, RAM, and cotyledons, the embryo is now beginning a change to
bilateral symmetry.
After the heart stage, organ expansion and further cell divisions result in the lengthening
of the embryonic cotyledons into the “torpedo” stage (Fig. 4.4). At this point, two patterns
have been established: the apical/basal patterns, which allows for shoot vs. root devel-
opment; and the radial pattern, which gives rise to the three types of tissue: (1) theproto-
derm(which gives rise to the epidermis), which divides anticlinally; (2) a middle layer, the
ground meristem(which gives rise to the cortex and endodermis); and an inner layer, the
procambium layer(which gives rise to the vascular tissue) (Willemsen and Scheres 2004).
The last stage before the mature embryo stage is the “walking stick” stage, so called
because the developing cotyledons have folded down over the SAM. To mature, the
embryo must enter a dehydration phase in which metabolism pauses. In the dehydrated
state the embryo within its seed coat is waiting for the appropriate environmental con-
ditions suitable for seedgermination. The plant hormoneabscisic acid(ABA) is required
for initiating dehydration and establishing seed dormancy. Without an ABA source or a
functioning ABA signal transduction pathway, embryos can germinate “precociously”
inside a fruit. Thus the study of ABA signaling pathways and the genes turned on
by these pathways is directly relevant to the understanding and manipulation of
seed germination.
Figure 4.4.Embryo development. (a) Schematic of embryo stages. [Reprinted from Lenhard and
Laux (1999), with permission by Elsevier Science Ltd.] (b) Scanning electron micrograph of
Arabidopsisembryos in the globular and heart stages. The white lines indicate the cell division
planes. [Reprinted from Costa and Dolan (2000), with permission from Elsevier Science Ltd.]
90 PLANT DEVELOPMENT AND PHYSIOLOGY