4.2.5 Seed Germination
Germinationis the process wherein the embryo imbibes water and returns to growth after
dormancy.Imbibitionis the uptake of water by the embryo within the seed. During this
process, the embryonic tissues are loosened and the seed coat usually splits, allowing
more water to penetrate the embryo. Once the embryonic cells are rehydrated, the metabolic
processes of germination can begin.
Several common requirements are shared by very diverse types of seeds, including
temperature and moisture. Some seeds have a light requirement, and some also require a
cold pretreatment called stratification. These processes promote the increase and/or
action of a plant hormone calledgibberellic acid(GA). GA action is generally considered
as antagonistic to ABA and is considered to be the dormancy-breaking hormone. One well-
characterized action of GA is the induction ofa-amylase production that breaks down
stored starches in grain seeds. Germination can occur underground (in the dark) or above
ground (in the light). Either way, the major result of germination is the expansion of the
already preformed embryo (Koornneef et al. 2002).
4.2.6 Photomorphogenesis
Imbibition of a seed allows dormant cells to expand and for new cell division to occur
within the embryo. The specific type of growth is influenced heavily by the presence or
absence of light. Light is the most influential signal from the environment that plants per-
ceive. When a seed germinates above ground, or in the presence of light, it immediately
responds to light with an elegant and complex developmental response calledphotomor-
phogenesis. If a seed germinates underground or in the absence of light, it undergoes a
brief and specific developmental pathway calledskotophotomorphogenesis. The purpose
of this dark developmental pathway is assumed to be the alteration of growth in the seedling
that increases its chance of encountering light, a signal required for the further development
of the seedling.
When germination occurs in the dark, the seedling develops into what is called anetio-
latedseedling, which is characterized by increased hypocotyl growth, an apical hook (in
dicots), unexpanded cotyledons, and no chlorophyll synthesis. These adaptations to dark
can allow for the elongating hypocotyl to push the SAM and cotyledons up through the
soil to encounter light. The apical hook thus can protect the new SAM, and chlorophyll syn-
thesis is not needed until light is encountered.
When the seedling encounters light, the elongation of the hypocotyl slows, the apical
hook uncurls, and thecotyledonsexpand and begin to assemble functional chloroplasts
containing chlorophyll. Transcription of genes encoding the chlorophyll a/b binding pro-
teins and part of the Rubisco complex are rapidly upregulated. Thus, if a seed germinates
in the presence of light, itshypocotylwill be much shorter than that of an etiolated seedling.
The apical meristem will then give rise to the first pair of true leaves that differ in structure
from the cotyledons and containtrichomes, or hairs.
The light receptor required for red light signal transduction is calledphytochrome, which
is composed of an open-chain tetrapyrole pigment calledphytochromobilinand a protein
dimer of 240 kDa. This pigment/protein complex allows for the perception of red light
by absorption of either red or far-red light. Phytochrome is distributed throughout many
different cell types in the plant, and more recent evidence suggests that it traffics from
the cytosol to the nucleus in response to light, where it interacts with transcription
4.2. EMBRYOGENESIS AND SEED GERMINATION 91