A silent war rages between crops and the diseases that attack them.Â Plant diseases have caused severe losses to humans by loss in productivity of economically important crops worldwide. Many cultural practices can be utilized to manage the occurrence, intensity or severity of plant diseases. Strategies for management of viral diseases normally include control of vector population using insecticides, use of resistant cultivars. However, each of the above methods has its own limitations. Increased understanding of the molecular biology of virus infection is starting to bear fruits, enabling specific strategies to be designed for virus resistance in crops. Over the past few decades, the tools of molecular biology have permitted rapid advances in our understanding of plant viruses and their replicative strategies. Arising from this hypothesis a recent and controversial technique in developing disease resistant plants is the insertion of genes from other organisms into plants to impart some characteristic. Thus, genetic transformation has opened up the possibility of an entirely new approach of genetic engineering towards controlling plant virus diseases.
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Groundnut (Arachis hypogaea L.) is a member of Leguminosae family and the Papillionacea subfamily comprises most important edible oil seed crop in the world. The major groundnut-producing countries of the world are India, China, Nigeria, Senegal, Sudan, Burma and the USA. Out of the total area of 18.9 million hectares and the total production of 17.8 million tonnes in the world, these countries account for 69% of the area and 70% of the production. India is ranked first in both, area under cultivation and production in the world. About 7.5 million hectares is used for cultivation annually and the production is about 6 million tonnes. Four states Gujarat, Andhra Pradesh, Tamil Nadu and Karnataka cover 70% of the area and 75% of the production of groundnut. Groundnut is the king of oilseeds and accounts for 29% of total area and 36% of total production in India, while in Gujarat state, groundnut contributes 64% of area and 64% of
production of oilseed crops. The productivity level is less than half of major groundnut growing countries and around one-third of the best of world levels.
Recently, groundnut has been gaining importance as a food crop due to its high content of digestible proteins (22-30%), vitamins (E, K & B group), minerals (Phosphorus, calcium, magnesium and potassium) and phytosterols (Savage and Keenan, 1994). The oil content of the seed varies from 44 to 50 per cent, depending on the varieties and agronomic conditions. Groundnut oil is an edible oil. It finds extensive use as a cooking medium both as refined oil and Vanaspati Ghee. It is also used in soap making, and manufacturing cosmetics and lubricants, olein stearin and their salts. Kernels are also eaten raw, roasted or sweetened. They are rich in protein and vitamins A, B and some members of B2Â group. Their calorific value is 349 per 100 grams. The H.P.S. type of groundnut kernels is exported to foreign countries. The residual oilcake contains 7 to 8 per cent of N, 1.5 per cent of PÂ 2O5Â and 1.2 per cent of K2O and is used as aÂ fertilizer. It is an important protein supplement in cattle and poultry rations. It is also consumed as confectionary product. The cake can be used for manufacturing artificial fiber. The haulms (plantÂ stalks) are fed (green, dried or silaged) to livestock. Groundnut shell is used as fuel for manufacturing coarse boards, cork substitutes etc. Groundnut is also of value as rotation crop. Being a legume with root nodules, it can synthesize atmospheric nitrogen and therefore improveÂ soil fertility.
Botanically cultivated groundnut can be classified into two subspecies which mainly differ in their branching pattern: subspeciesÂ hypogaeaÂ with alternate branching and subspecies fastigiata with sequential branching. Each subspecies is again divided into two botanical varieties; subsp.Â hypogaeaÂ into var.hypogaeaÂ (Virginia) and var.hirsuta; and subsp.Â fastigiataÂ into var.Â fastigiata (Valencia), var.Â vulgarisÂ (Spanish), var.Â peruviana, and var.Â aequatoriana. In trade, the bold-seeded types are referred to as Virginia, the small seeded as Spanish, and a third type Runner is also recognized. The flowers are born in the axils of the leaves mostly near the base of plant and have generally yellow petals. It is a self pollinated crop. After fertilization stalk of ovary elongates and forms peg which contains fertilized ovules at the tip. The growth of peg is positively geotropic until it penetrates soil to some depth (7 cm). The tip then becomes diageotropic and ovary starts developing into a fruit called pod which contains seeds. Generally it takes about 60 days from fertilization to full pod maturity.
CLIMATE AND SOIL
Always on Time
Marked to Standard
Groundnut is grown throughout the tropics and its cultivation is extended to the subtropical countries lying between 45 degrees N and 35 degrees S and upto an altitude of 1000 metres. The crop can be grown successfully in places receiving a minimum rainfall of 1,250 mm. The rainfall should be well distributed well during the flowering and pegging of the crop. The total amount required for presowing operations (preparatory cultivation) is 100 mm; for sowing, it is 150 mm and for flowering and pod development an evenly distributed rainfall of 400-500 mm is required. The groundnut crop, however, cannot stand frost, long and severe draught or water stagnation. Groundnut is grown on wide variety ofÂ soilÂ types. However, the crop does best on sandy loam and loamyÂ soilsÂ and in the blackÂ soilsÂ with good drainage. Heavy and stiff clays are unsuitable for groundnut cultivation as the pod development is hampered in theseÂ soils.
Groundnut yield is low in most Asian countries, owing to a number of biotic and abiotic stresses, including its cultivation on marginal lands, moisture stress and frequent droughts, disease and pest attacks, low input use, etc. In addition, low output prices reduce incentives for farmers to invest in productivity enhancing technologies such as improved seeds, fertilizers and pesticides. Groundnut being a rainfed crop, its yield is largely determined by the quantum and temporal distribution of rainfall. It performs well even under low rainfall conditions if the rainfall is evenly distributed during the growing period. Moisture stress at critical growth stages can reduce yield substantially (Dhandhalya and Shiyani 2009). Irrigation is limited to a very small proportion of the total groundnut area. Hence, groundnut yield is uncertain and production is riskier, discouraging farmers from investing in technology, inputs and irrigation. Lack of adoption of improved technologies is one of the main factors limiting improvement in groundnut yield. A majority of farmers grows traditional varieties that are adapted to local agro-ecological conditions, but have low genetic yield potential. In recent years, a number of new varieties with higher yield, better tolerance to drought and higher resistance to insect pests and diseases have been released by national and international agricultural research systems. However, their adoption is constrained by a lack of access to seed (seed requirement of groundnut is as high as 80 kg /ha -120 kg ha-1) and outturn is low and uncertain. Poor storage conditions and low use of seed treatment chemicals reduce seed quality. Seed multiplication and delivery systems too are poor. Private sector participation in seed multiplication and delivery is limited because of high seed requirement and a low multiplication factor. The public sector too has not shown much interest in multiplication and distribution of seed.
Groundnut is susceptible to a large number of fungal, viral and bacterial diseases, though all of them may not be economically important (Roy and Shiyani 2000). Diseases such as rust, early leaf spot, late leaf spot and bacterial wilt can cause considerable yield loss. Several insect pests such as the tobacco caterpillar, gram pod borer and leaf miner are main insects responsible for reducing groundnut yield. Aflatoxin contamination (by the fungi Aspergillus flavus and Aspergillus parasiticus) is an important constraint affecting groundnut quality in most Asian countries. It is a major health risk to both humans
and animals and importing countries place strict restrictions on acceptable aflatoxin levels. In addition to biophysical constraints, domestic and international trade policies have acted as disincentives to groundnut production.
One of the major production constraints in production of groundnut is reduction in yield caused by the various diseases caused by viruses. Although natural infection of more than 30 plant viruses representing 14 groups have been recorded on groundnut in different countries (Sreenivasulu 2005). Peanut bud necrosis virus (PBNV), Tobacco streak virus (TSV), Peanut mottle virus (PeMoV) and Indian peanut clump virus (ICPV) are the main viruses which attack the groundnut crop in India. Among viral diseases Bud necrosis is the most destructive disease of groundnut in many groundnut growing areas of India and is potentially damaging in other countries.
PEANUT BUD NECROSIS DISEASE
The incidence of peanut or groundnut bud necrosis disease (PBND) was reported in the annual report of Indian Agricultural Research Institute (IARI), India in 1949 and later by Chohan (1972, 1974); Ghanekar et al (1979) and the name given 'Bud Necrosis' by Reddy et al (1968). Groundnut bud necrosis, which is emerging as one of the most important viral diseases on several crops was thought initially to be caused by a strain of tomato spotted wilt virus (Reddy et al, 1983), but is now classified as a distinct virus in the genus Tospo and species groundnut bud necrosis (Reddy et al, 1992). In India, groundnut is grown in an area of 5.47 million ha with a production of 5.51 million tons (Anon., 2010).The disease is reported to occur in all prominent groundnut-growing areas of India. In Karnataka, the disease was first reported from Dharwar (Siddramaiah et al, 1977). The incidence of bud necrosis of groundnut ranges from 5 to 80 per cent in different parts of the Indian subcontinent. Worldwide it causes a loss of over one billion dollars annually (Goldbach, 1994, Moyer 1999, Prins et al 1995). Losses due to PBND have been estimated at over 89 million US $ per annum (Anon., 1992). The yield loss due to PBNV in India was estimated to be more than 80 per cent (Das Gupta et al., 2003). PBNV is the type species of genus Tospovirus and family Bunyaviridae. PBNV is the type species of genus Tospovirus and family Bunyaviridae. Tospovirus constitute the only genus of plant-infecting viruses in the family Bunyaviridae (Fauquet et al., 2005). The virus is vectored by thrips, Frankliniella occidentalis, F. schultzei, F. fusca, Thrips tabaci and Scirtothrips dorsalis which infest over 800 plant species, both dicots and monocots, in more than 80 plant families. Solanaceae and compositae families have the largest numbers of susceptible plant species (Prins, 1996).
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Symptoms produced by peanut bud necrosis virus (PBNV) in groundnut are difficult to distinguish, if at all, from those caused by tomato spotted wilt virus (TSWV). Initial symptoms appear on young quadrifoliolates as mild chlorotic mottle or spots, which develop into necrotic and chlorotic rings and streaks. Necrosis of the terminal bud, a characteristic symptom, occurs on crops grown in the rainy and post rainy seasons, when ambient temperatures are relatively high. Secondary symptoms are stunting, axillary shoot proliferation, and malformation of leaflets. If plants are infected early, they are stunted and bushy. If plants older than 1 month are infected, the symptoms may be restricted to a few branches or to the apical parts of the plants. Due to the severity of the symptoms, the virus causes severe losses to the groundnut crop, especially when plants are infected before they are a month old. Seeds from such plants are small, shriveled, mottled, and discolored. Late-infected plants may produce seed of normal size. However,
testae on such seed are often mottled and cracked.
Until 1990, PBND in India was reported to be caused by TSWV (Reddy et al. 1991). High-quality antisera became available for the detection of tospoviruses, to which the group TSWV belongs, only during the late 1980s. Data from serological comparisons and subsequently from sequencing of nucleic acids revealed the existence of several distinct tospoviruses (German et al. 1992, de Avila et al. 1993). In 1992, the virus causing PBND was identified as a distinct tospovirus and named PBNV. With ELISA as well as Western blots, PBNV was shown to be serologically distinct from TSWV and Impatiens necrotic spot virus (INSV) (Reddy et al. 1992). PBNV contains three RNA species of about 9.0 kb (1RNA), 5.0 kb (mRNA), and 3.0 kb (sRNA) (D.V.R. Reddy and S. Gowda).
Peanut bud necrosis virus can be transmitted by mechanical sap inoculations if care is taken to extract the virus only from young infected leaflets with primary symptoms. Extracts should be prepared in neutral phosphate buffer containing an antioxidant such as merceptoethanol, and must be kept cold throughout the inoculation process.
Amin et al. (1981) reported that the virus causing PBND in India is transmitted by Frankliniella schultzei and Scirtothrips dorsalis. Subsequent investigations, which involved accurate identification of thrips, showed that in fact Thrips palmi transmits PBNV, and not F. schultzei or S. dorsalis, which are also present on the plants. Further experiments showed that T. palmi could acquire PBNV as larvae and transmit it as adults. Maximum transmission (100%) was obtained when there were 10 adults per plant. The majority of individual adult thrips transmitted the virus for more than half of their life period, indicating the degree of erratic transmission. Cowpea was found to be the best host for rearing and multiplying T. palmi under laboratory conditions (Vijaya Lakshmi 1994, Wightman et al. 1995).
MANAGEMENT OF PBND
Several cultural practices such as adjustments to sowing dates, sowing at the recommended rate, adopting measures to maintain plant population, intercropping with fast-growing cereal crops such as maize and pearl millet can reduce the incidence of PBND. These practices have been shown to reduce infestation by T. palmi. Roguing of infected plants, especially during early stages of plant growth, should be avoided because this practice creates gaps in the field and can increase PBND incidence. Excellent progress has been made in the identification of sources of field resistance to PBND.
Although many high-yielding PBNV resistant varieties have been developed, they are medium-maturing types. Some of the field resistant genotypes such as ICGV 86388, show resistance to PBNV and less colonization by vector thrips compared with susceptible genotypes (Buiel et al. 1995, Dwivedi et al. 1995). Cultivars such as ICGS 11, Kadiri 3, and ICGS 44 are field resistant to PBND (Reddy et al. 1995). Conventional breeding programs to develop resistance are effective, but also protracted and expensive, and the resistance can be circumvented by virus variation. Alternative and additional approaches to protecting plants from infection with viruses have made use of the techniques of plant transformation and regeneration. These techniques have enabled the introduction of DNA sequences from foreign organisms into the plant genome.
The development of transformation and regeneration systems has allowed the introduction of viral and other useful genes into peanut germplasm (Yang et al. 1998; Rohini and Rao, 2001). Genetic transformation of peanut is an alternative for the improvement of the crop, allowing the transfer of individual genes which confer agronomic traits such as pest and virus resistance or enhancement of protein quality of the seeds (Mansur et al., 1995).
Here genetically engineered resistance can be obtained to PBNV, an enveloped virus with a negative strand RNA genome, by transforming groundnut with the gene encoding the viral nucleocapsid protein. This approach may be useful for producing plant resistant to infection by other negative strand viruses as well.
In view of the above considerations, the studies on PBNV were undertaken with the following objectives:
To develop transgenic groundnut plants resistant to PBNV.
To characterize the putative transformants for integration, expression and inheritance of the introduced gene.
To carry out evaluation of the transgenic plants for resistance to PBNV under glasshouse conditions.
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