Applications of Cell and Tissue Culture

Clonal propagation refers to the process of asexual reproduction by multiplication of genetically identical copies of individual plants. The vegetative propagation of plants is labour-intensive, low in productivity and seasonal. The tissue culture methods of plant propagation, known as 'micropropagation' utilizes the culture of apical shoots, axillary buds and meristems on suitable nutrient medium.The regeneration of plantlets in cultured tissue was described by Murashige in 1974. Fossard (1987) gave a detailed account of stages of micropropagation.

The micropropagation is rapid and has been adopted for commercialization of important plants such as banana, apple, pears, strawberry, cardamom, many ornamentals (e.g. Orchids) and other plants.The micropropagation techniques are preferred over the conventional asexual propagation methods because of the following reasons: (a) In the micropropagation method, only a small amount of tissue is required to regenerate millions of clonal plants in a year., (b) micropropagation is also used as a method to develop resistance in many species., (c) in vitro stock can be quickly proliferated as it is season independent,. (d) long term storage of valuable germplasm possible.

The steps in micropropagation method are: a) Initiation of culture - from an explant like shoot tip on a suitable nutrient medium, b) multiple shoots formation from the cultured explant, c) rooting of in vitro developed shoots and, d) transplantation - transplantation to the field following acclimatization.

The factors that affect micropropagation are: (a) genotype and the physiological status of the plant e.g. plants with vigorous germination are more suitable for micropropagation., (b) the culture medium and the culture environment like light, temperature etc. For example an illumination of 16 hours a day and 8 hours night is satisfactory for shoot proliferation and a temperature of 250C is optimal for the growth.

The benefits of micropropagation this method are:

a) rapid multiplication of superior clones can be carried out through out the year, irrespective of seasonal variations.

b) multiplication of disease free plants e.g. virus free plants of sweet potato (Ipomea batatus), cassava (Manihot esculenta)

c) multiplication of sexually derived sterile hybrids

d) It is a cost effective process as it requires minimum growing space.

Somaclonal variation

The genetic variations found in the in vitro cultured cells are collectively referred to as somaclonal variation and the plants derived from such cells are called as ‘somaclones’. It has been observed that the long-term callus and cell suspension culture and plants regenerated from such cultures are often associated with chromosomal variations. It is this property of cultured cells that finds potential application in the crop improvement and in the production of mutants and variants (e.g. disease resistance in potato).

Larkin and Scowcroft (1981) working at the division of Plant Industry, C.S.I.R.O., Australia gave the term 'somaclones' for plant variants obtained from tissue cultures of somatic tissues. Similarly, if the tissue from which the variants have been obtained is having gametophytic origin such as pollen or egg cell, it is known as 'gametoclonal' variation. They explained that it may be due to:

• a) Reflection of heterogeneity between the cells and explant tissue
• b) A simple representation of spontaneous mutation rate
• c) Activation by culture environment of transposition of genetic materials

Shepard et al. (1980) also contributed by screening about 100 somaclones produced from leaf protoplasts of Russet Burbank. They found that there was a significant amount of stable variation in compactness of growth habit, maturity, date, tuber uniformity, tuber skin colour and photoperiodic requirements.

Somaclonal Variations has been used in plant breeding programmes where the genetic variations with desired or improved characters are introduced into the plants and new varieties are created that can exhibit disease resistance, improved quality and yield in plants like cereals, legumes, oil seeds tuber crops etc.

Applications of Somaclonal Variations

• a) Methodology of introducing somaclonal variations is simpler and easier as compared to recombinant DNA technology.
• b) Development and production of plants with disease resistance e.g. rice, wheat, apple, tomato etc.
• c) Develop biochemical mutants with abiotic stress resistance e.g. aluminium tolerance in carrot, salt tolerance in tobacco and maize.
• d) Development of somaclonal variants with herbicide resistance e.g. tobacco resistant to sulfonylurea.
• e) Development of seeds with improved quality e.g. a new variety of Lathyrus sativa seeds (Lathyrus Bio L 212) with low content of neurotoxin.
• f) Bio-13 – A somaclonal variant of Citronella java (with 37% more oil and 39% more citronellon), a medicinal plant has been released as Bio-13 for commercial cultivation by Central Institute for Medicinal and Aromatic Plants (CIMAP), Lucknow, India.
• g) Supertomatoes- Heinz Co. and DNA plant Technology Laboratories (USA) developed Supertomatoes with high solid component by screening somaclones which helped in reducing the shipping and processing costs.

Production of virus free plants

The viral diseases in plants transfer easily and lower the quality and yield of the plants. It is very difficult to treat and cure the virus infected plants therefore the plant breeders are always interested in developing and growing virus free plants.

In some crops like ornamental plants, it has become possible to produce virus free plants through tissue culture at the commercial level. This is done by regenerating plants from cultured tissues derived from:

• a) Virus free plants
• b) Meristems which are generally free of infection - In the elimination of the virus, the size of the meristem used in cultures play a very critical role because most of the viruses exist by establishing a gradient in plant tissues. The regeneration of virus-free plants through cultures is inversely proportional to the size of the meristem used.
• c) Meristems treated with heat shock (34-36°C) to inactivate the virus
• d) Callus, which is usually virus free like meristems
• e) Chemical treatment of the media- attempts have been made to eradicate the viruses from infected plants by treating the culture medium with chemicals e.g. addition of cytokinins suppressed the multiplication of certain viruses.

Among the culture techniques, meristem-tip culture is the most reliable method for virus and other pathogen elimination.

Viruses have been eliminated from a number of economically important plant species which has resulted in a significant increase in the yield and production e.g. potato virus X from potato, mosaic virus from cassava etc. These virus free plants are not disease resistant so there is a need to maintain stock plants to multiply virus free plants whenever required.

Production of synthetic seeds

In synthetic seeds, the somatic embryos are encapsulated in a suitable matrix (e.g. sodium alginate), along with substances like mycorrhizae, insecticides, fungicides and herbicides. These artificial seeds can be utilized for the rapid and mass propagation of desired plant species as well as hybrid varieties. The major benefits of synthetic seeds are:

• a) They can be stored up to a year without loss of viability
• b) Easy to handle and useful as units of delivery
• c) Can be directly sown in the soil like natural seeds and do not need acclimatization in greenhouse

Mutant selection

An important use of cell cultures is in mutant selection in relation to crop improvement. The frequency of mutations can be increased several fold through mutagenic treatments and millions of cells can be screened. A large number of reports are available where mutants have been selected at cellular level. The cells are often selected directly by adding the toxic substance against which resistance is sought in the mutant cells. Using this method, cell lines resistant to amino acid analogues, antibiotics, herbicides, fungal toxins etc have actually been isolated.

Production of secondary metabolites

The most important chemicals produced using cell culture are secondary metabolites, which are defined as those cell constituents which are not essential for survival. These secondary metabolites include alkaloids, glycosides (steroids and phenolics), terpenoids, latex, tannins etc. It has been observed that as the cells undergo morphological differentiation and maturation during plant growth, some of the cells specialize to produce secondary metabolites. The in vitro production of secondary metabolites is much higher from differentiated tissues when compared to non-differentiated tissues.

The cell cultures contribute in several ways to the production of natural products. These are:

• a) A new route of synthesis to establish products e.g. codeine, quinine, pyrethroids
• b) A route of synthesis to a novel product from plants difficult to grow or establish e.g. thebain from Papaver bracteatum
• c) A source of novel chemicals in their own right e.g. rutacultin from culture of Ruta
• d) As biotransformation systems either on their own or as part of a larger chemical process e.g. digoxin synthesis

The advantages of in vitro production of secondary metabolites

• a) The cell cultures and cell growth are easily controlled in order to facilitate improved product formation.
• b) The recovery of the product is easy.
• c) As the cell culture systems are independent of environmental factors, seasonal variations, pest and microbial diseases, geographical location constraints, it is easy to increase the production of the required metabolite.
• d) Mutant cell lines can be developed for the production of novel and commercially useful compounds.
• e) Compounds are produced under controlled conditions as per the market demands.
• f) The production time is less and cost effective due to minimal labour involved.

Applications of secondary metabolites

Many of these secondary products especially various alkaloids are of immense use in medicine. The yield of these chemicals in cell culture, is though generally lower than in whole plants, it is substantially increased by manipulating physiological and biochemical conditions.

Shikonine is a dye produced by the cells Lithospermum erythrorhizon on a commercial scale. Besides this there are a number of secondary metabolite products that are being widely used for various purposes. Vincristine is used as anticancer agent, digoxin controls cardiovascular disorders, pyrethrins is an insecticide etc. The production of specialty chemicals by plants has become a multibillion industry.

Table showing plant species and secondary metabolites obtained from them using tissue culture techniques

Production of Somatic Hybrids and Cybrids

The Somatic cell hybridization/parasexual hybridization or protoplast fusion offers an alternative method for obtaining distant hybrids with desirable traits, especially between species or genera, which cannot be made to cross by conventional methods of sexual hybridization.

Somatic Hybridization

Somatic hybridization broadly involves in vitro fusion of isolated protoplasts to form a hybrid cell and its subsequent development into a hybrid plant. The process involves:

• a) Fusion of protoplasts
• b) Selection of hybrid cells
• c) Identification of hybrid plants

Various treatments are used to bring about protoplast fusion, including:

• a) Use of fusogens like NaNO₃
• b) High pH with high Ca²⁺ concentration
• c) Polyethylene glycol (PEG)
• d) Electrofusion

PEG treatment is the most widely used due to the following advantages:

• a) Results in reproducible high frequency of heterokaryon formation
• b) PEG fusion is non-specific and can be used for a wide range of plants
• c) Has low toxicity to cells
• d) Low formation of binucleate heterokaryons

Mechanism of Fusion

Fusion of protoplasts occurs in three phases:

• a) Agglutination
• b) Plasma membrane fusion
• c) Formation of heterokaryons

Agglutination is induced by PEG, high pH, and Ca²⁺. Membranes fuse at adhesion sites forming cytoplasmic bridges. This results in spherical heterokaryons.

Selection of Hybrid Cells

Selection methods include:

• a) Biochemical methods: Use of compounds in media, e.g., drug sensitivity and auxotrophic mutant selection
• b) Visual method: Identification of heterokaryons under light microscope, especially using chloroplast-deficient and green protoplasts
• c) Cytometric method: Use of flow cytometry and fluorescent-activated cell sorting

Applications of Somatic Hybridization

• a) Creation of hybrids with disease resistance (e.g., TMV, spotted wilt virus in tomato)
• b) Environmental tolerance (e.g., frost, cold, salt tolerance in tomato)
• c) Cytoplasmic male sterility via cybridization
• d) Quality characters (e.g., high nicotine content)

Chromosome Number in Somatic Hybrids

The chromosome number in somatic hybrids is usually higher than the sum of parental protoplasts. If it equals the total of both parents, it is a symmetric hybrid. Variations lead to asymmetric hybrids. In 1972, Carlson et al. produced the first interspecific somatic hybrid between Nicotiana glauca and N. langsdorffii. In 1978, Melchers et al. developed the first intergeneric somatic hybrids between Solanum tuberosum (potato) and Lycopersicon esculentum (tomato), known as ‘Pomatoes’ or ‘Topatoes’.

Limitations of Somatic Hybridization

• a) Somatic hybridization does not always produce plants that give fertile and visible seeds.
• b) There is genetic instability associated with protoplast culture.
• c) There are limitations in the selection methods of hybrids, as many of them are not efficient.
• d) Somatic hybridization between two diploids results in the formation of an amphidiploid which is not favourable therefore haploid protoplasts are recommended in somatic hybridization.
• e) It is not certain that a specific character will get expressed in somatic hybridization.
• f) Regenerated plants obtained from somatic hybridization are often variable due to somaclonal variations, chromosomal elimination, organelle segregation etc.
• g) Protoplast fusion between different species/genus is easy, but the production of viable somatic hybrids is not always possible.

Cybrids

The cytoplasmic hybrids where the nucleus is derived from only one parent and the cytoplasm is derived from both the parents are referred to as cybrids. The process of formation of cybrids is called cybridization. During the process of cybridization and heterokaryon formation, the nuclei are stimulated to segregate so that one protoplast contributes to the cytoplasm while the other contributes nucleus alone. The irradiation with gamma rays and X-rays and use of metabolic inhibitors makes the protoplasts inactive and non-dividing. Some of the genetic traits in certain plants are cytoplasmically controlled. This includes certain types of male sterility, resistance to certain antibiotics and herbicides. Therefore cybrids are important for the transfer of cytoplasmic male sterility (CMS), antibiotic and herbicide resistance in agriculturally useful plants. Cybrids of Brassica raphanus that contain nucleus of B. napus, chloroplasts of atrazinc resistant B. capestris and male sterility from Raphanus sativas have been developed.

In vitro plant germplasm conservation

Germplasm refers to the sum total of all the genes present in a crop and its related species. The conservation of germplasm involves the preservation of the genetic diversity of a particular plant or genetic stock for it’s use at any time in future. It is important to conserve the endangered plants or else some of the valuable genetic traits present in the existing and primitive plants will be lost. A global organization- International Board of Plant Genetic Resources (IBPGR) has been established for germplasm conservation and provides necessary support for collection, conservation and utilization of plant geneic resources through out the world. The germplasm is preserved by the following two ways:

(a) In-situ conservation - The germplasm is conserved in natural environment by establishing biosphere reserves such as national parks, sanctuaries. This is used in the preservation of land plants in a near natural habitat along with several wild types.

(b) Ex-situ conservation - This method is used for the preservation of germplasm obtained from cultivated and wild plant materials. The genetic material in the form of seeds or in vitro cultures are preserved and stored as gene banks for long term use.
In vivo gene banks have been made to preserve the genetic resources by conventional methods e.g. seeds, vegetative propagules, etc. In vitro gene banks have been made to preserve the genetic resources by non - conventional methods such as cell and tissue culture methods. This will ensure the availability of valuable germplasm to breeder to develop new and improved varieties

The methods involved in the in vitro conservation of germplasm are:

(a) Cryopreservation- In cryopreservation (Greek-krayos-frost), the cells are preserved in the frozen state. The germplasm is stored at a very low temperature using solid carbon dioxide (at -790C), using low temperature deep freezers (at -800C), using vapour nitrogen (at- 1500C) and liquid nitrogen (at-1960C). The cells stay in completely inactive state and thus can be conserved for long periods. Any tissue from a plant can be used for cryopreservation e.g. meristems, embryos, endosperms, ovules, seeds, cultured plant cells, protoplasts, calluses. Certain compounds like- DMSO (dimethyl sulfoxide), glycerol, ethylene, propylene, sucrose, mannose, glucose, praline, acetamide etc are added during the cryopreservation. These are called cryoprotectants and prevent the damage caused to cells (by freezing or thawing) by reducing the freezing point and super cooling point of water.

(b) Cold Storage- Cold storage is a slow growth germplasm conservation method and conserves the germplasm at a low and non-freezing temperature (1-90C). The growth of the plant material is slowed down in cold storage in contrast to complete stoppage in cryopreservation and thus prevents cryogenic injuries. Long term cold storage is simple, cost effective and yields germplasm with good survival rate. Virus free strawberry plants could be preserved at 100C for about 6 years. Several grape plants have been stored for over 15 years by using a cold storage at temperature around 90C and transferring them in the fresh medium every year.

(c) Low pressure and low oxygen storage- In low- pressure storage, the atmospheric pressure surrounding the plant material is reduced and in the low oxygen storage, the oxygen concentration is reduced. The lowered partial pressure reduces the in vitro growth of plants. In the low-oxygen storage, the oxygen concentration is reduced and the partial pressure of oxygen below 50 mmHg reduces plant tissue growth. Due to the reduced availability of O2, and reduced production of CO2, the photosynthetic activity is reduced which inhibits the plant tissue growth and dimension. This method has also helped in increasing the shelf life of many fruits, vegetables and flowers.

The germplasm conservation through the conventional methods has several limitations such as short-lived seeds, seed dormancy, seed-borne diseases, and high inputs of cost and labour. The techniques of cryo-preservation (freezing cells and tissues at -1960c) and using cold storages help us to overcome these problems.