In this article we will discuss about the meaning and methods of artificial seed.
Meaning of Artificial Seed:
Somatic embryos are not enclosed by seed coats and due to microbial contamination and desiccation they are not able to survive if these are sown directly into field soil. The aim of somatic embryos encapsulation is to produce an analog to true seeds.
The procedure is to mix the somatic embryos in a substance to make it well protected in that matrix and it is rigid enough to allow for rough handling. So, the artificial or synthetic seeds are the seed like structures where the somatic embryos derived from tissue culture are encapsulated by hydrogels and can be used directly to be sown in soil as a substitute of natural seeds (Fig. 18.6).
These will germinate normally into plants. This covering helps to give physical protection, avoid desiccation and encapsulation should carry nutrients, growth regulators to help in germination. Antibiotics may be added to avoid contamination and also it should be durable and non-toxic.
The difference between the seed with zygotic embryo and artificial seed with somatic embryo are:
Natural Seeds:
1. Hard seed coat present.
2. Embryos are much protected within cotyledons or endosperm.
3. Embryos undergo controlled desiccation by the maternal tissue and have a natural dormancy period.
4. The natural seeds have their own storage reserves like endosperm or cotyledons to provide food during germination.
Synthetic Seeds:
1. No seed coat, only encapsulated.
2. Embryos are not protected within any kind of maternal tissue.
3. Embryos do not pass through any kind of desiccation and they the do not have any dormancy period.
4. The artificial seeds do not have their own storage tissue, the nutrients or growth regulators can be supplied within the encapsulating material.
Methods of Artificial Seed Production:
For making synthetic seeds the somatic embryos should be coated or encapsulated in protective covering and handled in such a condition so that, these can be delivered in field condition with good germination rate (Fig. 18.7).
The synthetic seeds can be produced using the following procedures:
(i) Hydrated condition, using sodium-alginate or calcium alginate hydrogels as the encapsulation material;
(ii) Using desiccated embryos in coated condition;
(iii) Using polymer, self-breaking capsules releasing nutrient;
(iv) Using fluid drilling method.
Radenbergh et. al. (1986) developed hydrated artificial seeds by mixing somatic embryos with sodium-alginate, followed by dropping into a solution of CaCl2 to form calcium-alginate beads. About 30-55% embryo had the germination capacity producing seedlings.
Kin and Janick (1989) developed the desiccated artificial seeds applying synthetic seed coats. They mixed equal volume of embryo suspension and 5% solution of polyethylene oxide, a water soluble resin, which subsequently dried to form strip containing multiple embryos. The embryos used here are desiccated by hardening treatments with 12% sucrose and using 10-6 M ABA followed by chilling.
Onishi et. al. (1994) and Sakamoto et. al. (1995) developed the synthetic seeds using automated encapsulation process by using coating material Elvax 4260 (ethylene vinyl acetate acrylic acid tetrapolymer). It helped to remove the stickyness of coating material and maintain uniform structure of beads.
Many other techniques have been developed recently which help to release the nutrient within the bead, self-breaking beads, pharmaceutical type of capsules. Cellulose acetate mini-plugs have recently been developed to promote good sowing quality of synthetic seeds i.e. good germination rate in field condition.
Gray (1987) used the technique of desiccation of somatic embryos and then storage at normal temperature. When rehydrated again germinated to produce viable plantlet but not in good amount.
Senratna et. al. (1990) used ABA at the last stage of embryo development which promoted their tolerance to desiccation. Such desiccated embryos had good germination ability either in vitro or in soil condition.
Thorpe and Stasolla (2001) concluded that heat-shock treatments, osmotic stress, nutrient depletion also can induce a degree of desiccation tolerance comparable to ABA treatment and have no detrimental effect on germination of embryos.
Potential Uses of Synthetic Seeds:
(i) In some plants it requires a long time to reach at reproductive phase and seed production. In these cases the synthetic seeds can be helpful to get the propagules in a short period.
(ii) Generally the reproduction phase in a plant is season dependent, in these cases the somatic embryos and the synthetic seeds can be produced at any time as required.
(iii) In case of natural seeds there is a particular dormancy period, but in artificial seeds there is no dormancy period, thus it is more helpful for propagation. It can reduce the life cycle period of a plant.
(iv) Artificial seeds can also be helpful where there is no successful seed production after sexual hybridization. The somatic embryos can be obtained from somatic hybrids obtained through protoplast fusion.
(v) Artificial seeds are helpful in case of meiotically unstable genotypes, where the normal seed set is of low frequency.
(vi) In cases where the embryo germination is difficult, the artificial seed can provide the beneficial adjuvants i.e., growth promoting seed substances, plant nutrients, etc. through the artificial coats.
(vii) In genetic manipulation of crop plants the production of artificial seed may be useful.
(viii) For somatic hybrid plant production through protoplast culture and fusion the artificial seed technology is helpful.
(ix) In the production of genetically modified (GM) crops through transgenesis, the synthetic seed production may be applied.
(x) In general, synthetic seed technology is useful for rapid easiest way of non- conventional method of propagation in crop plants.