A promising opportunity of Applied Biotechnology: Synthetic Seed of Rice
Authors: Parthvi Patel and Dimpy Patel
Project Assistant at Nanotechnology Department
Anand Agricultural University, Anand, Gujarat

Rice is a tremendous food and is an excellent source of carbohydrates and energy. Production of synthetic seeds endowed with high germination rate under in vitro and in vivo conditions bears immense potential as an alternative of true seeds. Encapsulation technique for producing synthetic seeds has become an important asset in micropropagation. Botanically seed is a mature ovule along with its food storage in the form of either endosperm or cotyledon. The essential part is the embryo contained within the integuments, but it may be used less critically to describe planting materials. In terms of seed science, seed can be described as any propagating material used for raising a crop. Whereas, synthetic seed could be defined as artificially encapsulated somatic embryos, shoot buds, cell aggregates or any other tissue that can be used for sowing as a seed and that possesses the ability to be converted into a plant under in vitro or ex-vitro conditions, and that retains this potential even after storage (Capurno et al., 1998). Synthetic seed technology can also help in germplasm storage and transportation of elite genotypes.

Types of synthetic seeds

1. Desiccated synthetic seed
Desiccation can be achieved either slowly over a period of one or two weeks sequentially using chambers of decreasing relatively humidity, or quickly by unsealing the Petri dishes and leaving them on the bench overnight to dry. Such types of synthetic seeds are produced only in plant species whose somatic embryos are desiccation-tolerant. For example, Carrot. They used polyoxyethylene, which is readily soluble in water, dries to form a thin film, does not support the growth of microorganisms and is non-toxic to the embryo, leading to the production of desiccated synthetic seed.

2. Hydrated synthetic seed
Hydrated seeds are producing in those plant species where the somatic embryos are recalcitrant and susceptible to desiccation. Encapsulation of somatic embryos in hydrogel capsules produces hydrated synthetic seeds. The commonly used technique to induce artificial seed is isnotropic gelation of sodium alginate by calcium ions.

3. Double layered synthetic seed
somatic embryo encapsulation, sodium alginate is mainly used but, this is extremely permeable with loss of the nutritive substances or dehydration risk during preservation and transport causing harmful effects on the synthetic seed conversion and on the plantlet growth. In order to overcome these problems now a days double coat encapsulation procedure in M.26 apple rootstock. The inner layer contained a large quantity of sucrose. To prevent the diffusion of sucrose from artificial seeds to non-sterilized substrate, artificial seeds were enveloped in a dialysis membrane. The enveloped artificial seeds germinated quickly in non-sterilized vermiculite.

Encapsulating materials for rice synthetic seed

1 Embryogenic synthetic seeds
The method of somatic embryogenesis, in which the somatic cells or tissues develop into differentiated embryos, produces somatic embryos and each fully developed embryo is capable of developing into a plantlet. Embryos can be obtained either directly from cultured explants, anther or pollen, callus and isolated single cell in culture. For synthetic seed production, embryos may be obtained from the following sources.

• Somatic embryos:
The quality of the artificial seed depends on the temporal qualitative supply of growth regulators and nutrients along with an optional physical environment. The sources of somatic embryos of rice are mature seed derived callus, root culture, immature inflorescence culture, immature embryo culture etc.

2 Gametic Embryos:
• Androgenic embryos
Production of artificial seeds from androgenic embryos resulting from androgenic callus is very little. So, Haploid plant breeding has been found to be well familiar in many crops. The initiation of pollen embryogenesis, which genetically differs from zygotic embryogenesis, may be used for synthetic seed production. In rice, rapid and frequent mass multiplication of androgenic embryos and embryo like structure of IR 72 an elite indica cultivar. These embryos and embryo like structures can be used as source of synthetic seed production.

• Microspore culture
Haploid plants have capability to regenerating from microspore culture. Anther culture may correlate with production of diploid plantlets from anther wall or from other parts of anther other than pollen. Thus anther derived plantlets are with various ploidy levels. This can be avoided by culturing isolated pollen. Microspore culture usually produces homogenous population, while anther culture could constitute a heterogenous population.

• Ovule culture
Haploid plants have been successfully developed from culture of female gametophytic cells, that is, the egg nucleus or ovum. It was considered as an alternative means of haploidy as well as the expression of totipotency of female gametophytic cells in angiospermic plants. Chinese scientists could regenerate haploid plants in rice, wheat, sunflower, sugar beet and onion by culturing female gametophytic cells. The appropriate time of embryo sac for its culture is uninucleate to mature stage. But it may differ species to species, such as nearly mature (1~2 days before anthesis), embryo sac mother cell to megaspore tetrad stages are also suitable for culture. The explant for development of gametophytic haploid may be the ovary, isolated ovule or even unhusked flower.

References:
Capuano G., Piccioni E., and Standardi A., 1998, J. Hortic. Sci. Biotechnol., 73: 299-305


About Author / Additional Info:
We have pursued M.Sc. Integrated Biotechnology and currently working at Anand Agriculture University