SYNTHETIC SEEDS AND SECONDARY METABOLITES

Synthetic seeds

            To make the production of somatic embryos practically applicable a delivery system must be available by which the embryos survive ‘sowing’, develop further (for which nutrients are required) and finally germinate. To achieve this embryos formed in vitro are coated and then known as synthetic or artificial seeds.

            Two systems have been developed for automated handling of these synthetic seeds:

1. by encapsulation: a gel (eg. Calcium alginate), containing nutrients is coated around the somatic embryo. An encapsulation machine has already been developed which can produce and sort 6,000 single embryo capsules per hour.
2. sowing the somatic embryos by fluid drilling: this process has been developed by seeds firms who wanted pre-germinated seeds to achieve homogeneous germination in the field. The embryos are also packed in a gel containing additives before being sown in soil.

The production of ‘seeds’ by coating somatic embryos and obtaining plants from these encapsulated embryos is far from easy because:

1.      The development of artificial seeds that are stable for several months requires chemical and physical procedures for making the embryo quiescent.

2.      The artificial seed needs to be protected against desiccation when stored under dry conditions. If drying-out takes place, then the seed may enter a dormant phase. The embryos also need to be protected and stabilized during transport, storage and sowing (as is the case for normal seeds).

3.      Recovery of plants from encapsulated somatic embryos is often very low; this is due to:

a.       Incomplete embryo formation

b.      Difficulties to arrest growth

c.       Difficulties in creating an artificial endosperm within the capsule (the embryo must be provided with nutrients).

4.      The embryos must be protected (against e.g. micro-organisms) by the use of antibiotics and fungicides. However, at a later stage microorganisms may be advantageous (mycorrhizas).

The problems of encountered with the production and application of synthetic seeds are far from being solved. They are bound to be more expensive than ordinary seeds, although this initial price difference may be compensated:

1.      By higher output due to the selection of super genotypes.

2.      Because male and female parental strains no longer need to be maintained.

3.      Because research into male sterility may be unnecessary.

4.      Because ‘seed production’ becomes possible in plant species where no (real) seeds are formed.

5.      Because plant breeding is changing dramatically (e.g. By the application of genetic manipulation).

Secondary metabolites:

            During the plant metabolic pathways, some chemicals are produced but which may not be of direct use to the plant are called as secondary metabolites. These chemicals may be alkaloids, terpenoids, flavonoids or host of glycosides.

Uses of tissue culture techniques for production of secondary plant metabolites

            Plant cells in culture offer many advandages over intact plants for secondary metabolites production and their biosynthetic studies. These are as follows:

1. Plant cells are relatively easy to grow and can be kept under strict controlled nutritional and environmental conditions. Hence the uncertainities of climate and soil can be avoided.
2. Cells are cultured aseptically, devoid of many microorganisms or insects etc.
3. Suspension cultures, offers a very effective way of incorporating precursors which are often difficult to admininster to the plant growing in nature.
4. The technology is now available for the relatively large scale production of plant suspensions, in batch culture, closed continuous cultures system, open continuous culture systems etc. And may also eventually provides an efficient means of producing commercially important plant products.

However, in order to realize the industrial applications of plant cell culture for medicinal compounds production, it is essential to satisfy the following conditions as minimum requirements:

1.      Rate of cell growth and biosynthesis should be high enough to give a good production of the final product in a short period of time.

2.      The cultured cells should be genetically stable to give a constant yield of a plant.

3.      The metabolites should be accumulated in cells without being catabolized rapidly of preferably; they should be realized into the liquid medium.

4.      Production cost including the culture medium, precursor and chemical extraction should be low enough to be profitable to the industries.

Some secondary metabolites detected in plant tissue culture.

Secondary plant products	Stock plant
Flavones and flavonoids
Quercetin	Lycopersicon esculantum
Negretin	Solanum tuberosum
Chalcones and deoxyflavones
Daidzein	G. max
Pisatin	Pisum sativum
Steroidal alkaloids
Tomatin	L. esculentum
Diosgenin	S. nigrum