Somatic embryogenesis
Somatic embryogenesis is the process of single cell or group of cells
that undergo the sequence of events of embryogensis (i.e., all the
stages of embryogenesis) leading to the regeneration of plantlets.
The capability of the somatic plant cell of a culture to produce
embryoids is k/a embryogenic potential
Embryoid is a small ,well-organized structure comparable to the
sexual embryo, which is produced in tissue culture of dividing
embryogenic potential
In somatic embryogenesis, the embryos regenerate from
somatic cells, tissues or organs either de novo or directly from
the tissues (adventive origin)
Somatic embryogenesis is a useful regeneration pathway for many
monocots and dicots, but is especially useful for the grasses
Types of embryogenesis
zygotic embryogenesis – the result of normal pollination and
fertilization
somatic embryogenesis – embryos from (cultured) sporophytic cells,
that is embryos arise indirectly
Somatic embryogenesis differs from organogenesis in the
embryo being a bipolar structure rather than monopolar
The embryo arises from a single cell and has no vascular
connections with the maternal callus tissue or the cultured explant
Further, induction of somatic embryogenesis requires a single
hormonal signal, while in the organogenesis two different
hormonal signals are needed to induce first a shoot organ, then a
root organ
Two routes to somatic embryogenesis:
1. Direct embryogenesis
the embryo initiate directly from the explant tissue
through ″pre-embryogenic determined cells ″
(PEDC)
such cells are found in embryonic tissues (e.g.
scutellum of cereals), hypocotyls and nucellus
2. Indirect embryogenesis
cell proliferation i.e. callus from explant takes place
from which embryos are developed
the embryo arise from ″induced embryogenic
determined cells ″ (IEDC)
For some species any part of the plant body
serves as an explant for embryogenesis (e.g.
carrot) whereas in some species only certain
regions of the plant body may respond in culture
Floral and reproductive tissues in general have
proven to be excellent source of embryogenic
material
The presence of auxin always essential,
cytokinins in some species
The composition of the culture medium controls the process
auxin (usu. 2,4-D) added causes induction, the formation of
embryogenic clumps or proembryogenic masses (PEMs)
(induction medium)
auxin is deleted and the clumps become mature embryos
(maturation medium)
L-glutamine play a special role
Substantial amount of reduced nitrogen ( NH4
+)
Addition of activated charcoal to the medium is useful
(→ lower phenylacetic acid and benzoic acid compounds,
which inhibit somatic embryogenesis
Culturing methods
solid medium
liquid medium (often called suspension cultures)
Two cell types are usually present in
embryogenic suspension cultures
large, highly vacuolate, freely dispersed cells
small, densely cytoplasmic, aggregated cell clumps
Stages of development
early cell division doesn't follow a fixed pattern,
unlike with zygotic embryogenesis
later stages are very similar to zygotic embryos
(dicot pattern)
globular stage (multicellular)
heart-shaped stage (bilateral symmetry) – bipolarity
torpedo-shaped stage – consists of initial cells for the
shoot/root meristem
Following fertilization, the zygote undergoes an asymmetrical
cell division that gives rise to a small apical cell that becomes
the embryo and a large basal cell (called the suspensor) that
functions to provide nutrients from the endosperm to the
growing embryo. Asymmetrical cell division also seems to be
important in the development of somatic embryos, and while
failure to form the suspensor cell is lethal to zygotic embryos, it
is not lethal for somatic embryos. From the eight cell stage
(octant) in the zygotic embryo, 'embryo patterning' is apparent;
however, somatic embryos at this stage may be quite variable,
therefore zygotic and somatic embryos become most
comparable from the globular stage.
In the globular stage, the embryo develops radial
patterning through a series of cell divisions. The globular
embryo can be thought of as two layers of inner cells
with distinct developmental fates; the apical layer will go
on to produce cotyledons and shoot meristem, while the
lower layer produces the hypocotyl and root meristem.
Bilateral symmetry is apparent from the heart stage;
provascular cells will also differentiate at this stage. In
the subsequent torpedo and cotyledonary stages of
embryogenesis, the embryo completes its growth by
elongating and enlarging.
Artificial seeds (Synthetic seeds)
Encapsulated embryos
Redenbaught et al.,1984 discovered hydrogel, like, sodium
alginate could be use to produce single embryoid artificial seeds.
Method for making artificial seeds
Establishment of callus cultures
Induction of somatic embryogenesis
Maturation of SEs
Encapsulation of SEs
Test for embryoid to plant conversion
Green house and field planting
To coat SEs 2 standard methods –
1. Gel complexation via dropping procedure
Isolated SEs are mixed with 0.5% to 5%(w/v) sodium
alginate and dropped into 30-100μM Calcium nitrate
solution. Surface complexation begins immediately and
the drops are gelled completely within 30 minutes
2.Molding:Isolated SEs are mixed in a temperaturedependent
gel such as Gel-riteTM and placed in the well
of a microliter plate and it forms gel when it cools down
storage is a great limitation
Potential applications to crop improvement
haploids for breeding, mutant screening
germplasm storage and preservation
𐂫 "artificial seeds "Artificial seeds"
clonal propagation – theoretically much faster, easier to
mechanize than micropropagation
Results have not lived up to the promise
somatic embryos are highly susceptible to desiccation
"conversion" rates are low
shelf life is short
encapsulation techniques have yet to be perfected
Not used often in plant propagation, because there is a high
probability of mutations arising
the method is usually rather difficult
the changes of losing regenerative capacity become greater with
repeated subcultures
induction of embryogenesis is often very difficult or impossible with
many plant species
a deep dormancy often occurs
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