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| AGRICULTURE TRIBUNE | Monday, January 6, 2003,
Chandigarh, India |
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Artificial seeds on the way Know the pest-killer Bt gene ‘Seed villages’ for reliability at low cost Quality seed is where story begins |
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Artificial seeds on the way
ARTIFICIAL seeds have been proposed as a new low-cost, high-efficiency propagation system. They have opened new avenues in storage and delivery of new plant lines produced through biotechnological advances. The main advantage of the system is the combination of high-volume production and low-cost propagation. Encapsulation of cells, embryo, somatic tissue and somatic embryo have been attempted in several crops and has became increasingly popular as a simple way of handling cells, tissue and embryo and protecting them against external gradients and as an efficient storage and delivery system. The acceptance of the technology, however, depends on the value of the propagated crop and the cost of competing products, the use of artificial seeds also depends on advances in areas of faster delivery system and development of superior crop lines through genetic engineering. The concept The concept of somatic embryo encapsulation to produce an analogue to true seeds is based on the similarity of somatic embryos (SE) with zygotic embryos in terms of morphology, physiology and biochemistry. The gross morphology of somatic embryos is very similar in appearance to zygotic embryos. An important difference between somatic and zygotic embryos is that the latter typically ceases growth, becoming quiescent or dormant as water is lost, storage tissue matures, and the seed coat hardens. This arrested growth phase is a major factor accounting for efficient storage and handling qualities of natural seed. A similar arrested growth phase induced during somatic embryo development will be essential to match the efficiency of seed propagation and would be a pivotal step in the development of artificial seed technology. The term artificial seed was coined by Murashige in 1977. The technique was later perfected by Redenbaugh and his coworkers from Plant Genetics Incorporation, California, and Kitoo and Janick from Purdue University. The technology was patented in 1988 by Redenbaugh et al. In India the technique of encapsulation of somatic embryos is used in many modern research laboratories. Advantages
—Economy in space, medium inputs, better cost-benefit ratio. —Potential delivery system. —Alternative to high-cost vegetative propagation technologies. —Direct sowing is possible with the seed sized propagules, thus eliminating the acclimatisation step normally required in transplanting of mericlones. —Uniformity in production is possible because somatic embroys are genetically identical; by contrast zygotic embryos contain unpredictable mixture of parental genes. Because of relative ease of producing large number of artificial seeds, they can be used in monocultures as well as mixed genotype methods of commercial planting. Production Artificial seeds are basically of two types, hydrated seeds and desicated seeds. Hydrated seeds: The system consists of somatic propagules (embryo or shoot buds) encapsulated in a hydrogel like sodium alginate, gelrite, agar, carregenan, etc. The most common gelling agent is sodium alginate with complexing agent as calcium salts. Somatic embryos are sieved from the suspensions cultures, mixed with sodium alginate and dropped in calcium chloride solution. When sodium alginate drops come in contact with calcium chloride solution, surface complexion begins and firm round beads are formed, each containing 1-2 propagules. These beads are allowed to stand for 60-90 minutes and washed with water, air dried and stored at 4°C. Axillary, apical/adventitious buds can also be encapsulated in this method. The buds are trimmed to the smallest possible size and mixed with sodium alginate. In the second method, propagules are mixed in temperature-dependent gels such as Agar, or Gelrite and gelled to form capsules by lowering the temperature. Desicated artificial seeds: In this method the artificial seeds are produced by coating a mixture of somatic embroys or shoot buds with polyethylene g1ycol (PEG). The coated mixture is then allowed to dry for severa1 hours on teflon surface under sterile conditions. The dried mixture in the form of wafers is then p1aced in vitro in the culture medium, allowed to re-hydrate and stored for embryo survival and conservation. Hydrophobic coating: New compounds have been tried for hydrophobic coating of artificial seeds. A recent compound, EL vax 4260, manufactured by Dupont has shown significant impediments to capsu1e drying. This compound also reduces the stickiness and beads can be planted using a seed ptanter. Conventionally the beads can be rolled in talcum powder to make them flowable and store for a short period in culture room. Applied Artificial seeds have been successfully produced in the following crops for commercial use: Alfalfa, Apium graveolens, Cymbidium (orchid), Daucus carota, Dendrobium (orchid), Dioscorea ftoribunda, Gossypium hirsutum, Medicago sativa, Morus indica, Picorrhiza kursoa, Pogostemon patchouli, Phalaenopsis (orchid), Rheum emodi, Santalum album, Spathoglottis plicata (orchid), and Neem (Azadirachta). The concept of artificial seed is in its infancy. There is an immense potential for the use of this technique for the propagation and preservation of elite germplasm. Once the technique is
perfected it will be a boon to the seed production industry. Use of
artificial seeds will become popular for green house production.
Additional studies, however, will be needed to understand hardening
treatments, coating, duration of storage, embryo conversion frequency,
and most important the economics of artificial seeds as compared to
natural seeds. |
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Know the pest-killer Bt gene INSECT damage to crop plants results in heavy yield losses worldwide. Pest management has been revolutionised by the introduction of pesticides, but on the contrary it has also increased the cost of inputs and their excessive use has proved to be non-environment friendly. In the present agriculture scenario Bt technology, involving bacterial toxin (Bt) gene from Bacillus thuringiensis can be visualised as an important tool against lepidopteran insect pests. The term “Bt” alone is sufficient to raise a layman’s eyebrows, as bacteria are always associated with ill health. But it is important for us to know that it is Bacillus thuringiensis, a friendly gram-positive soil bacterium that has developed an evolutionary relationship with insects. It was first discovered by a Japanese scientist Ishiwata in 1902. He isolated a bacterium from diseased silkworm larvae, which he named Bacillus sotto. Later in 1912, a German scientist while investigating the insecticidal activity in the district of Thuringia in East Germany named the bacterium Bacillus thuringiensis. It was first used as a commercial insecticide (bacterial spray) in France in 1938 and then in the USA in 1950. In this era of biotechnology an important approach to protect our crops from insects is to engineer plants with the Bt gene to make them synthesise the toxin protein instead of spraying bacteria on them. Such plants are referred to as Bt transgenics. The mechanism of action of Bt is very interesting. Bacillus thuringiensis produces insecticidal crystal proteins. The crystals are aggregates of large proteins (about 130-140 KDa in size), that is actually a protoxin, i.e., it must be activated before it has any effect. The crystal protein is highly insoluble in normal conditions and is entirely safe to humans, higher animals and most of the insects. However, it is solubilised in reducing conditions of high pH, around 9.5, the conditions normally found in the mid-gut of lepidopteran larvae. Therefore, Bt protein is a highly specific insecticidal agent. On intake of crystal proteins by the insect larvae under alkaline conditions, the protoxin is cleaved to produce a toxin called delta-toxin or Bt toxins of about 60 KDa. These delta toxins bind to the specific receptors present in the gut membrane that result in pore formation in the epithelial cells of the mid-gut of larvae, disrupting the osmotic equilibrium of the cells. Such cells swell and burst, eventually leading to the death of insect larvae. Two most popular methods of introducing Bt gene into plants are the ‘particle gun’ and Agrobacterium method. Particle gun is also known as gene gun or the biolistic gun method. This method involves the acceleration of DNA (Bt gene) into cells of the target plant. For this DNA (Bt gene) is first coated over 0.2-0.7um gold or tungsten particles and then accelerated into the target plant cells by using a particle gun machine attached to a helium gas cylinder. It can be used to transform shoot apical meristems, leaf blades, pollen cultured cells, root and shoot sections, from which we can further regenerate transformed plants through tissue culture. The Agrobacterium method of plant transformation involves Agrobacterium tumefaciens , which is a soil-dwelling bacteria and is capable of causing tumour or gall formation in plants in nature on infection. The resident plasmid of the Agrobacterium tumefaciens is modified so that it lacks the undesirable tumour inducing genes but contains a foreign gene, Bt gene, in the T-DNA region of the plasmid. On infection of the plant cells with this Agrobacterium the T-DNA part of the bacterial plasmid gets transferred and integrates with plant chromosomes. In the process of transformation the DNA is delivered only to a fraction of cells involved in the experiment and gets stably integrated into the genome of only a fraction of the cells that receive DNA and finally the introduced Bt gene(s) are expressed in still fewer cells. The transformed cells or plants are then identified through the screenable marker gene or the selectable marker gene that has been attached to the Bt gene during construct designing. The selectable markers allow the selective multiplication of transformed cells by killing the non-transformed cells, whereas screenable markers only enable the identification of the transformed cells without addition of a selective compound in the tissue culture medium in which the cells are growing. The growing transformed cells are then selected and transferred to regeneration medium for regenerating the transformed plants. The confirmation of the transgenic plants carrying Bt is then carried by several independent tests. |
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‘Seed villages’ for reliability at low cost IN view of the low replacement rate in crops such as wheat and paddy (8 per cent on an average), the traditional seed supply system is important. Assuming a 5 to 10 per cent annual growth of improved seeds sales, traditional supply will be still vital. A method to improve this system of supply must be developed by stimulating the concept of “seed village” involving the village panchayat and women. In India women account for at least one half of the agricultural labour. Moreover, a serious constraint faced by small and marginal farmers is non-availability of quality seeds of improved varieties at the proper time and place. Even if the seeds are produced in the village (which is far cheaper) these farmers will find it difficult to retain the produce because they need money. They, therefore, sell it soon after harvest. To alleviate the situation, bank credit may be arranged either to the individual farmers or to village panchayats by hypothecating the seed for sowing. Alternatively, a cooperative society may be formed in the village for seed production, distribution and sale. The concept of seed village is not new. It started in the sixties, but was forgotten when the dwarf wheat from Mexico was introduced in India. The main task was to convince the farmers about its higher yield potential. Mexican dwarf wheat yielded far more than the Indian wheat at that time which generated a great demand for wheat seed. In each revenue division of a district, seed production can be organised by the Primary Agricultural Cooperative Societies (PACs) in a cluster of three to four nearby villages to avoid isolation problems and also to have effective monitoring of seed production plots. The seed should be harvested separately and threshed, cleaned, graded and tested for germination. Care should be taken to avoid physical admixture of cultivars. In each cluster a processing plant of at least 1 tonne/hour capacity should be installed or a mobile processing plants for cleaning and grading of the seeds can be taken on rent either by the village panchayat or the State Department of Agriculture. If this is not possible, cleaning can be done manually. Either all or a few selected farmers of the cluster can be given genetically pure seed, which can be multiplied initially under the supervision of experts. One part of seed can be exchanged for 1.5 parts of the grain. For a crop like paddy, a community nursery can be prepared for transplanting. Training farmers in seed production and testing should form an important element of the programme. Farmers can be taught to produce seeds of oilseeds, pulses, cereals and vegetables. It is also necessary to prepare a seed map of the country indicating areas suitable for economic seed production of different crops. It is advisable to take up seed production in semi-arid areas under irrigation, preferably in the rabi season. The basic seed map prepared can be modified from time to time indicating new areas for seed production. The seed production areas must be, as far as possible, closer to the states requiring seeds and seed production must be economical. By involving the village in seed production, the high-yield varieties of crop plants can be spread faster. It is the quickest way to saturate an entire nation with quality seeds of improved varieties. In this kind of venture, investment is modest when compared to the establishment/ expansion of the seed corporation. The concept of exchange of seed for grain can also be developed. |
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Quality seed is where story begins SEED is the most vital input in crop production and quality seed is indispensable for a bumper crop. Technically speaking quality seed means seed or any propagating material possessing genetic purity, physical purity, good germination, good seed and seedling vigour, optimum moisture content, freedom from diseases and pests. The ideal is to minimise pest and disease losses. Unfortunately the innate capacity of quality seed has not been utilised to the desired level in our region. The annual seed replacement rate for wheat is 4.6 per cent and for paddy is 6.7 per cent against a minimum recommended replacement rate of 10 per cent for each of the two major crops. With the ever-increasing pressure on land, the role of quality seed of good varieties cannot be understated. It is estimated that 10-15 per cent increase in production can be achieved by replacing farmer-saved seeds with quality seeds of improved varieties. These days agriculture has to be practised commercially so that it remains sustainable. It has emerged as an industry with farmers looking for profit along with fulfilling the basic need of food. Mechanisation along with increased use of fertilisers, better irrigation facilities, better management of insect pests and disease through IPM (Integrated Pest Management) and IDM (Integrated Disease Management) and control of weeds have resulted in sustained higher yields. But all these inputs respond better if we use quality seed of recognised, better-performing crop cultivars. Recently the trend has shifted towards cultivation of hybrids. Hybrid cultivars are better performers as compared to other types of varieties. Maize was the first important crop in which hybrids were first exploited on commercial lines in the 1930s and 40s in the USA. By 1944 about 80 per cent area of maize in the USA was under hybrids. Later on other cross-pollinated crops like jwar and bajra were included in hybrid programmes. In India four maize hybrids, namely Ganga, Ganga1, Ranjit and Deccan were released in 1961. With refinement of technologies in production strategy of hybrids their role in boosting agricultural production became more apparent. Earlier hybrids were developed in cross-pollinated crops like maize, bajra, muskmelon, brinjal, chilli, etc., but now they have been developed in self-pollinated crops like wheat and rice also. The development of hybrids in self-pollinated crops is difficult and demands more technical skills as well as resources. Therefore, they are likely to be expensive. Unless the hybrid seed of such crops are capable of out yielding the conventional varieties to an extent that they bring more profit, these hybrids should not be adopted on large scales. Hybrid rice is cultivated on a large scale in China and many hybrids are now being released for commercial cultivation in India as well. But the production of hybrid seeds should be done in a schematic fashion following proper guidelines so that the seed produced can maintain its superior performance. Its production, undertaken both by the public sector as well as the private sector, should be stringently monitored by state seed certification agencies. It should be ensured that they are not befooling the farmers by selling spurious seeds under the garb of hybrid seeds. |
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The essential advantages
of artificial seeds are: