Here is a term paper on ‘Meristem Culture’ for class 11 and 12. Find paragraphs, long and short term papers on ‘Meristem Culture’ especially written for school and college students.
Term Paper # 1. Introduction to Meristem Culture:
Many plant species which propagate vegetatively are systematically infected by virus, fungi, bacteria and nematodes. In order to ensure high and sustainable yield, it is vital to produce disease-free stock plants. Unless large-scale population of pure inoculum of test pathogens are available, it is difficult to pursue the establishment of pathogenicity. Tissue culture techniques have made possible to carry out such experiments/tests in laboratory within short span of time.
Morel and Martin (1952) applied tissue culture techniques for elimination of viral infection in plants. They successfully obtained for the first time a virus-free plant (Dahlia) through shoot meristem culture. Several virus-free plants are regenerated in vitro such as Pisum sativum, Trifolium repens and Citrus sp., Dactylis glomerata (from mild mosaic mottle viruses) and Loliurn multiflorum (from ryegrass mosaic virus).
An increased resistance to leaf blight disease caused by Phytophthora infestans and Alternaria solani in clonal populations generated from mesophyll cell protoplasts of the potato cultivar – ‘Russet Burbank’ was demonstrated.Tissue culture techniques are employed for mass rearing of nematodes in vitro and screening of resistant breeding materials, nematicides and fungicides. Toxin resistant callus plants have also been obtained. Carlson (1973) obtained disease resistant plant by treating tissues to be cultivated with toxins, then by regenerating plants from the stable cell clones.
Similarly, chlorosis resistant plants are regenerated after treating haploid tobacco cells with an analog to a bacterial toxin. Disease resistant plants induced by these techniques are sugarcane, clover, tobacco, corn, potato, etc. Some of the important reports in this regard are presented in the Table 7.1.
Leaves of potato re-generants, produced from calli and selected for resistance to culture filtrate of P. infestans, showed greater resistance to this filtrate than the leaves of control. Different factors like temperature, inoculum density, phytochrome affect the expression of resistance in cultured plants.
The expression of race specific resistance in tissue culture is demonstrated for potato in response to P. infestans. Besides, other systems in which expression of disease resistance in tissue cultures have been studied include tomato/P. infestans system, tobacco/Pseudomonas sp., soybean/P. megasperma var. sojal system, tobacco/T.M.V. system etc.
Term Paper # 2. Methods for Producing Virus-Free Plants:
There are five methods employed for producing virus-free plants:
(i) Meristem culture,
(ii) Heat treatment,
(iii) Heat treatment followed by meristem culture,
(iv) Adventitious shoot formation followed by meristem culture, and
(v) Grafting of meristems on virus-free root stocks (also called ‘micro-grafting’).
Virus-free plants are sometimes also obtained by callus or protoplast culture. Meristem culture is often used to obtain plants free from bacteria or fungi.
(i) Meristem Culture:
Limasset and Cornuet (1949) observed that in systematically infected plants virus concentration decreased as they approached the apical meristem. In apical meristem, no virus was detectable in 50% of the materials tested. Accordingly, Morel and Martin (1952) suggested isolation of the apical meristem of a systematically infected plant in vitro to obtain a virus-free plant. They were successful in proving this hypothesis and obtained virus-free Dahlia plants through meristem culture of the infected plants. Meristem culture is also used extensively in germplasm preservation of endangered crop species.
The meristem is a dome of actively dividing cells, about 0.1 mm in diameter and 0.25 mm long. Meristem of both main shoot and auxiliary buds are used for obtaining virus- free plants. Shoots of all angiosperms and gymnosperms grow by virtue of their apical meristems.
The apical meristems are first formed during embryo development and they remain (except in dormant buds) in an active state of division throughout the vegetative phase of the plant. The totipotencies of the plant cells form the basis of meristem culture technique.
In principle, all shoot meristems of a plant may act as starting material but the chance of success is dependent on the type of bud or shoot (terminal or axillary) and/or the position (basal or terminal) of the bud. After sterilization of shoot buds apical meristem (0.3 – 0.5 mm) is removed using forceps and scalpel.
Isolation of larger meristems (with more leaf primordia) makes the chance of getting virus-free plants very small. The isolated meristem is immediately transferred on to the surface of solid medium (which is preferred) and incubated in the culture room. In the media, usually low salt and high vitamins are used for the growth of isolated meristem. The pH lies between 5.4 and 6.0, and sugar concentration is 2-5% (w/v).
The in vitro culture of meristem and shoot tip involves several phases such as initiation of culture and establishment of explant, growth and differentiation, proliferation of shoots and finally plantlet formation by rooting of shoots. All the conditions of nutrients, plant growth regulators and meristem are chosen empirically.
The meristem culture is influenced by factors like lateral position of the meristem as in Chrysanthemum sp., concentration and composition of plant growth regulators, pre-condition of the plants from which meristems are obtained, pH of the medium, and addition of gibberellic acid. Use of filter paper with liquid medium has been found better for growing meristem compared to static medium.
The percentage of virus-free plants can also be dependent on season, as shown in case of carnation. Shoots of potato and carnation roots best when meristem is isolated in the spring. The low success (percentage) of isolated meristems that develop into virus-free plants could be due to infection, damage, drying out and browning.
(ii) Heat Treatment:
This method is particularly efficient against viruses and mycoplasmas attacking fruit trees, cassava and sugarcane. Axillary buds in case of woody plants are made virus-free with heat treatment of 37-38°C. Similarly, virus-free grapevines have been obtained by treating shoots in vitro for 21 days at 35°C.
(iii) Meristem Culture and Heat Treatment:
Prior treatment of plants with heat treatment increases the chance of obtaining more virus-free plants, especially when there is more than one virus present. Such treatment depends upon the precondition of plant, type of virus infection and its population, and the treatment itself.
Experiments with Chrysanthemum cv. Blanche portivine supreme showed increase in virus-free plants from 9 to 90% by heat treatment of plant material on 10th or 30th day. Such treatment of heat is helpful in eliminating certain viruses which are difficult to be eliminated by normal meristem culture e.g., in potato infected with PVX, PVS.
Plants are subjected to heat therapy prior to excision of meristem; by this way the virus concentration gets reduced and/or the virus-free zone is enlarged. The length of heat treatment (35 – 38°C) varies from 5-10 weeks. This procedure has been successfully employed with potatoes, carnation, strawberry and Chrysanthemum. Morel suggested the storage of potato tubers at 37 – 38°C for a month before taking up meristem culture.
(iv) Adventitious Shoot Formation Followed by Meristem Culture:
Mori et al. (1982) showed that adventitious shoots obtained in vitro from TMV infected leaf of tobacco were virus-free. This method of obtaining virus-free plants has also been successful with Lily and hyacinth.
(v) Micro-Grafting:
Using this technique, Murashige et al. (1972) and Navarro et al. (1975) eliminated two viruses while working with citrus. Later, this was also employed successfully with other plants like peach, apple and eucalyptus.
Regeneration of Virus-Free Plants from Callus and Protoplasts:
Cooper’s experiment demonstrated that after a few sub- culturing tobacco callus can escape virus infection. Isolation of protoplasts from dark green zones of tobacco leaves infected by TMV produced virus-free plants.
Eradication of Pathogens Other than Virus:
Meristem culture can also get rid of bacteria (Erwinia, Pseudomonas, Bacillus) and fungi (Fusarium, Rhizoctonia, Verticillium). Baker & Phillips (1962) obtained fungal-free plants of carnation by meristem culture – Fusarium roseum f. cerialis was eliminated, as a result. Theiler (1981) produced bacteria-free plants of Pelargonium which were initially infected with Xanthomonas pelargonii. Some mycoplasma-like bodies cause aster yellow disease in carrot. The repeated transferring of explants excised from these plants, in cultures eradicated this pathogen.
Term paper # 3. Mechanism of Virus Elimination in Meristems:
(1) Virus inhibiting effect of plant growth regulators could be responsible for virus-free apical meristem, as apical meristem has higher concentration of plant growth regulators. This hypothesis has been ignored.
(2) Rapid cell division may lead to virus-free cells. Normal nucleo-proteins are synthesized first, and viral nucleoprotein is synthesized during cell elongation. So there is a competition between cell division and viral multiplication as evinced by experiments with tobacco callus.
There is no general reason available to define the cause of virus elimination; this is due to the fact that some preferential viruses are only eliminated in meristem culture.
Term Paper # 4. Effect of Virus Elimination:
Following effects can be observed in virus-free plants raised through meristem culture:
(1) Increased growth and vigour of plants e.g., in rhubarb, cauliflower, Chrysanthemum.
(2) Improvement in rooting of stem cutting e.g., in Pelargonium.
(3) Increased flower size e.g., Chrysanthemum, Narcissus.
Term Paper # 5. Virus Indexing:
The availability of a rapid, effective and sensitive method for virus indexing is of paramount importance for success of a programme aimed at production of virus-free plants. All the plant materials need to be certified for virus-free status for initial exchange. Biological assays are used for virus indexing, e.g., sap of plants to test virus infection (sap transmission test). TEM serology or combinations of both are also used. Enzyme linked immunosorbent assay (ELISA) has been extensively used for virus indexing along with rDNA technology and nucleic acid hybridization techniques.
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