Somatic embryogenesis and artificial seed production

Somatic Embryogenesis and
Artificial Seed Production
By-
arvind yadav,721

In vitro somatic embryogenesis (SE)
• First demonstrated in 1958 by Reinert and Steward.
• There are two ways by which SE could be obtained –
i) Indirect SE, where first the callusing is induced from the explant by
rapid cell division and later the callus give rise to SE and
ii) Direct SE, somatic embryos are developed directly from the explant
without an intermediate callus phase
 In dicotyledonous plants, the somatic embryos passes through the globular,
heart, torpedo and cotyledonary stages (as in Zyogtic embryogenesis).
 The only major difference is that somatic embryos do not pass through
the desiccation and dormancy phases, but rather continue to participate
in the germination process.
(i) globular, (ii) early heart shape, (iii) late heart shape, (iv) torpedo shape, (v)
early dicot, and (vi) fully developed dicot embryo

Steps involved in Somatic embryogenesis
i. Induction of embryo
ii. Embryo development
iii. Embryo maturation

Organogenesis versus embryogenesis
i. Embryo is a bipolar structure rather than a monopolar one.
ii. The embryo arises from a single cell and has no vascular connection with
maternal callus tissue or the cultured explant.
During organogenesis shoots or roots develop from a group of cells
resulting into chimera formation which later establish a strong connection
with the maternal tissue.
iii. Induction of somatic embryogenesis requires a single hormonal signal to
induce a bipolar structure capable of forming a complete plant,
In organogenesis, it requires two different hormonal signals to induce
shoot first and then root organ.

Factors affecting somatic embryogenesis
 Genotype and type of explant
 Growth regulators
 Nitrogen source

Genotype and type of explant
 Like organogenesis, SE is also genotype dependent for a
given species and significant variations in response between
cultivars have been observed in several plants like, wheat,
barley, soyabean, rice, alfalfa etc.
 Genotypic variations could be due to endogenous levels of
hormones, therefore, if the species has not shown SE
previously, then it is required to test number of different
cultivars of that species.

What tissue should be used as an explant in a
particular species to induce SE?
 The explant selection is much more important than the media selection for SE
process.
 Immature zygotic embryos : the best explant as cells that are already in
embryonic state.
 In Azadirachta indica (neem): The globular embryos did not show any
response.
 The older embryos germinated, formed calli or differentiated three types of
organized structures, viz. shoots, somatic embryos and neomorphs (abnormal
or embryo-like structures with varied morphology).
 Often the same explant differentiated more than one kind of regenerants.
 The most responsive stage of embryos was early dicotyledonous,
followed by torpedo shaped embryos.

Growth regulators
 Auxin: Auxin plays a major role in the development of somatic embryos.
 Require a synthetic auxin for the induction of SE followed by transfer to
an auxin-free medium for embryo differentiation.
 2,4-D is the most commonly used auxin for the induction of SE.
 Other auxins, NAA, IBA, picloram (4-Amino-3,5,6-trichloro-2-
pyridinecarboxylic acid) and IAA
 A naturally occurring auxin IAA is a weak auxin and more readily broken
down compare to 2,4-D and NAA.
 The auxins, particularly 2,4-D, in the concentration range of 0.5 – 1.0 mgl-
1 (proliferation or induction medium), stimulates the formation of
localized group of meristematic cells in the callus called ‘proembryogenic
masses' (PEMs), which are cell clusters within cell population competent
to form somatic embryos
Embryogenic callus with PEMs (indicated by arrows) in the induction medium

Auxin:Growth regulators
• In repeated subcultures on the proliferation medium, the embryogenic cells
continue to multiply without the appearance of embryos.
• However, if the PEMs are transferred to a medium with a very low level of
auxin (0.01-0.1 mgl-1 ) or no auxin in the medium (embryo development
medium ; ED medium), they develop into embryos.
Note:
• The presence of an auxin in the proliferation medium seems essential for the
tissue to develop embryos in the ED medium.
• The tissues maintained continuously in auxin-free medium would not form
embryos.
• Therefore, the proliferation medium is called the ‘induction medium' for SE
and each PEMs as an unorganized embryo.

Growth regulators: Cytokinin
• Only few reports of somatic embryo induction and development in cytokinin
containing medium.
• Cytokinin, in general, induced SE directly without the callusing of explant.
• In most cases, TDZ is used as cytokinin, a herbicide, which mimics both auxin
and cytokinin effects on growth and differentiation.
• The other cytokinins are also used when zygotic embryos are used as the
explant source. The most commonly used cytokinins are BAP and Zeatin.
• The cytokinin e.g. 2,4-D results into the formation of SE as well as
Neomorphs.
• Neomorphs are suppressed embryos with green, smooth, shiny surface and
solid interior. Although they were epidermal in origin like somatic embryos
with heart shape notch but showed monopolar germination and no clear cut
radicular region

Nitrogen source
 The most important nutrient of the culture medium is nitrogen which
affects SE significantly.
 The form of nitrogen have a strong influence on the induction of SE.
 Often the presence of ammonium or some other source of reduced nitrogen
is required, such as glycine, glutamate or casein hydrolysate.
 The ratio of ammonium to nitrate has also been shown to affect
SE.
 In few cases, the calli initiated on a medium with KNO3 as the sole source
of nitrogen failed to form embryos upon removal of auxin. However, the
addition of a small amount (5mM) of nitrogen in the form of NH4Cl in
the presence of 55mM KNO3 allowed embryo development.

Embryo maturation and germination
 Germination of somatic embryos can occur only when it is mature enough to have functional shoot
and root apices capable of meristematic growth.
 Somatic embryos show poor germination quality with respect to their convertibility into complete
plantlets. This is because these embryos do not go through ‘embryo maturation' phase which is
the characteristic of seed or zygotic embryos.
 During this phase, accumulation of embryo-specific reserve food materials and proteins imparts
desiccation tolerance to seed embryos and thereby promote their normal development for
germination.
 Abscisic acid (ABA), which prevents precocious germination and promotes normal development of
embryos by is reported to promote embryo maturation in several species.
 A number of other factors, such as temperature, shock, osmotic stress, nutrient deprivation and high
density inoculums, can substitute for ABA, presumably by inducing the embryos to synthesize the
hormone.
 ABA is known to trigger the expression of genes which normally express during the drying down
phase of seeds. Probably the products of these genes impart desiccation tolerance to the embryos.

Secondary somatic embryogenesis
 Secondary SE is a process in which new somatic embryos are
proliferated from originally formed primary somatic embryos.
 Secondary SE have some advantage over primary somatic embryogenesis,
such as high multiplication rate, long term repeatability and
independency of an explant source.
 Secondary SE also overcomes post fertilization barriers of the embryo,
immature embryos of interspecific plants from incompatible crosses may
be rescued by culturing them for secondary SE.
 It can also be used for the production of somatic embryos of species in
which the embryos are the reservoir of important secondary
metabolites.

Synchronization of embryo development
 Generally, the differentiation of somatic embryos in semi-solid medium or liquid
medium is highly asynchronous which adversely affect the germination rate.
 Synchronization of embryo development is very important for artificial seed
technology.
 Of the several approached tried to achieve this, the most effective method are the
physical separation of embryogenic stages and use of growth regulators to
physiologically synchronize the development.
 The other alternative methods are the fractionation of embryos of different
stages by filtration of suspension through meshes of different sizes or by
gradient centrifugation.
 Besides, the most effective method to achieve synchronous development of
somatic embryos is the use of substances that would induce reversible
cessation of embryo development at a particular stage.
 ABA at low concentration is the most satisfactory substance for the purpose.
For example, in carrot it inhibits the growth of roots and enhances suspension
with torpedo shaped embryos.

Production of synthetic seeds or artificial seed
 Although it is possible to use naked embryos for large scale planting, it
would be beneficial to convert them into ‘synthetic seeds' or ‘synseeds' for
large scale clonal propagation at commercial level.
 This can be achieved by encapsulating the viable somatic embryos in a
protective covering.

The coating material should have several qualities:
 Non-damaging to the embryos.
 Mild enough to protect the embryos and allow germination but it
must be sufficiently durable for rough handling during
manufacture, storage, transportation and planting.
 Contain nutrients, growth regulators and other components
necessary for germination.
 The quality of somatic embryo should be good enough, they all are
of uniform stage with reversible arrested growth and with high rate
of conversion to plantlets.

Types of synthetic seeds are produced:
 I. Desiccated synthetic seeds
 II. Hydrated synthetic seeds

I. Desiccated synthetic seeds
 It involves encapsulation of somatic embryos followed by their
desiccation and can be prepared by following methodology:
The polyox is readily soluble in water and dries to thin film.
It does not support the growth of microorganism and is non toxic to the
embryos.

II. Hydrated synthetic seed
 Several methods have been examined to produce hydrated artificial seeds
of which Ca-alginate encapsulation has been the most widely used. It can
be prepared by following steps:

Applications of somatic embryogenesis
 To speed up the clonal propagation of plants being bipolar in nature.
 Being single cell in origin, there is a possibility to automate large scale
production of embryos in bioreactors and their field planting as synthetic
seeds.
 The bipolar nature of embryos allows their direct development into complete
plantlet without the need of a rooting stage as required for plant regeneration
via organogenesis.
 Epidermal single cell origins of embryos favor the use of this process for plant
transformation.
 It can also be used for the production of metabolites in species where
embryos are the reservoir of important biochemical compounds.
 Efficient transport and storage.
 To overcome embryo abortion, seed dormancy and self-sterility in plants.

Limitations of somatic embryogenesis
 Complete conversion into plantlets or poor germination of
embryos.
 Compared to other plant species active research on somatic
embryogenesis involving forest trees has been very slow.
 The lack of knowledge controlling somatic embryogenesis, the
synchrony of somatic embryo development and low frequency of
true to type embryonic efficiency are responsible for its reduced
commercial application.
 To obtain a complete conversion into plantlets it is necessary to
provide optimum nutritive and environmental conditions.

Somatic embryogenesis and artificial seed production

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