Plant tissue culture

Plant genetic resources can be conserved by two important methods, namely In-situ and Ex-Situ conservation. The ex-situ conservation includes methods like seed storage, DNA storage, pollen storage, in vitro conservation, field gene bank, and botanical gardens. The following article is about the pathways of plant tissue culture namely organogenesis and somatic embryogenesis.

Whereas in-situ conservations include strategies using the tools of biotechnology that are increasingly being applied toward the conservation of plant genetic resources. These include (a) in vitro conservation and (b) in vitro propagation and re-introduction of plants to their natural habitats.

In vitro preservation of germplasm is a safe method for protecting the species by reducing the risk of natural vagaries (Borthakur et al. 1999). Plant tissue culture facilitates the accomplishment of a large number of uniform plants irrespective of the season and serves as an alternative source of seed materials.

Pathways of cultured cells and tissues

The cultured cells and tissue can take several pathways to produce a complete plant. Among these, the pathways that lead to the production of true-to-type plants in large numbers are the popular and preferred ones for commercial multiplication. The following terms have been used to describe various pathways of cells and tissue in culture (2).

Organogenesis

In this pathway, groups of cells of the apical meristem in the shoot apex, axillary buds, root tips, and floral buds are stimulated to differentiate and grow into shoots and ultimately into complete plants.

It is one of the widely used methods employed for in vitro plant regeneration. Plant growth regulator regimes can be used to manipulate the morphogenetic response of plants under in vitro cultures.

There are two types of shoot organogenesis, direct and indirect. Direct shoot organogenesis is the production of shoots from explants directly, while indirect shoot organogenesis refers to the formation of shoots indirectly from an intermediary callus that first develops on the explants.

Somatic Embryogenesis

Bipolar structures are formed in somatic embryogenesis is a process in which a b, resembling a zygotic embryo, develops in vitro from a non-zygotic cell without vascular connection to the original tissue.

Somatic embryos (SEs) are used to study the regulation of embryo development and are a tool for large-scale vegetative propagation. Embryos formed in cultures are referred to as embryoids, supernumerary embryos, adventives embryos or accessory embryos. Of these, the term embryoid is used most widely (2).

There are two different patterns of the origin of somatic embryos from in vitro-grown explants

1) Direct production of somatic embryos from the cells of the explant called the pre-embryogenic-determined cells (PEDC).

2) Indirect production of somatic embryos from unorganized callus tissue mass called the induced embryogenic determined cells (IEDC).

Organogenesis and somatic embryogenesis

Direct production of somatic embryos	Indirect production of somatic embryos
These embryos are formed from the cells of the explant called the pre-embryogenic-determined cells (PEDC)	In this embryos are formed from unorganized callus tissue mass called the induced embryogenic determined cells (IEDC).
Somatic embryos are presumed to originate from explant cells that require only in vitro environment to be released from some repressive condition composed by the organization of the explants.	IEDC pattern not only requires the release of the previously differentiated state through mitotic cell divisions. But also induction of the new pattern of cell divisions to form organized embryos.
Here the embryogenic pathway is predetermined and the cells appear to only wait for the synthesis of an inducer (or removal of an inhibitor) to resume independent mitotic division in order to express their potential.	IEDCs, on the other hand, require redetermination to the embryogenic state by exposure to specific growth regulators such as 2,4-D. Once the embryogenic state has been reached both cell types proliferate in a similar manner as embryogenic-determined cells (EDCs)
Such cells are found in embryogenic tissues, certain tissues of young in vitro-grown plantlets, the nucellus, and the embryo sac (within the ovules of mature plants).	These can be induced from any type of tissue-like somatic cells or adventitious parts.

Somatic embryos are produced as adventitious structures directly on explants of zygotic embryos, from callus and suspension cultures. Somatic embryos and synthetic seeds (embryos encapsulated in artificial endosperm).

It holds the potential for large-scale clonal propagation of superior genotypes of heterogeneous plants. They have also been used in commercial plant production and for the multiplication of parental genotypes in large-scale hybrid seed production.

The earliest report of somatic embryogenesis in vitro was in 1957 with aseptic cultures of Oenanthe aquatica. Followed by that of carrot by Reinert 1958, Steward et al. 1958.

Somatic embryogenesis of woody plants was achieved in the 1960s with Citrus spp., Biota orientalis, Santalum album, and Zamia integrifolia.

Advantages of somatic embryogenesis

Somatic embryogenesis, the development of embryos from somatic cells, is analogous to the development of zygotic embryos. And results in the production of a complete somatic seedling (embling) with the potential to grow into a whole plant. It carries a low risk of genetic diversity. It potentially provides many advantages:

1. Mass production

A large number of plantlets can be produced inexpensively. This is the most commercially attractive application of in vitro somatic embryogenesis.

Quick and easy scale-up can be achieved via liquid culture thus creating large-scale mechanized or automated culture systems.

2. No or less mutation

The plants derived from somatic embryos are less variable than those derived via organogenesis. This may reflect an intolerance of somatic embryos to a mutation in any of the numerous genes that must be necessary for ontogeny to be successfully completed.

3. True copy to the mother plant

The morphological and physiological similarity of somatic embryos to zygotic embryos means that they are almost complete propagules in themselves. That is with embryonic roots, shoots, and leaves (or at least cotyledons) and the “program” to make a complete plant. This is unlike other clonal propagation systems, no separate shoot growth or rooting steps are required for plantlet production.

4. Germplasm conservation

Long-term germplasm conservation via cryopreservation can be utilized.

Somatic embryos that originate from single cells and subsequently regenerate mostly genetically uniform plants are good candidates for genetic resource (germplasm) conservation.

5. Synthetic seed

Manufactured seeds can be used for embling establishments. Synthetic seeds, consisting of somatic embryos enclosed in a protective coating, have been proposed as a ‘low-cost-high-volume’ propagation system.

6. Genetic transfer

It also provides a means of genetic transfer the multiplication of genetically transformed cells and mass production of transgenic trees. The transformed cell lines can then be induced to form an unlimited number of transformed somatic embryos through repetitive embryogenesis.

7. Synthesis of metabolites

The repetitive embryogenesis system is of potential use in the synthesis of metabolites such as pharmaceuticals and oils.