The most deleterious variants in tissue culture raised plants are those that effect yields, quality and carry infection of viruses which are difficult to diagnose. To bridge the gap, the Department of Biotechnology, Govt. of India has established recognized test centres for testing of tissue culture raised plants for quality and freedom from viruses.
The participating institutions and contact persons are:
Indian Institute of Horticultural Research,
Institute of Himalayan Bioresource Technology,
National Chemical Laboratory, Pune
SPIC Science Foundation, Chennai
The Energy and Resources Institute, New Delhi
Testing for Virus and
Generation of virus-free planting material is an ideal strategy to confine these viruses and also to facilitate the movement of materials across the domestic and international boundaries. Tissue culture is an useful approach for generating virus-free planting material. In order to minimise the risk of inadvertent propagation of virus infected plants and introduction of somaclonal variability, the tissue culture raised plants need to be thoroughly indexed for freedom from viruses and checked for quality. Careful indexing based on recent biotechnological methods such as imunoprobes, nucleic acid probes and polymerase chain reaction (PCR) would ensure phytosanitary safety during the movement of planting materials. Similarly, molecular testing will ensure quality control.
The National Facility for Virus Diagnosis and Quality Control of Tissue Culture Raised Plants (NFVD & QC), is aimed at providing virus testing and quality testing support to the tissue culture industry to ensure that only (a) virus-free mother plants are used for micropropagation, and (b) virus-free and quality tested tissue culture plants are supplied to the growers. Hence, not only the risk of inadvertent introduction and subsequent spread of viruses would be minimised but also the quality of plants would be assured. This will be achieved by developing a biophysical, immunological and molecular technique based virus diagnosis programme and molecular based quality control programme of the plant species of interest. The testing protocols will not only detect the known viruses, but also the unknown viruses.
The test for virus and quality will have direct impact on developing a vibrant tissue culture industry as the demand for certified tissue culture raised plants will grow exponentially.
Various immunological and molecular techniques like ELISA (Malathi et al, 1989; Jain et al, 1998), immuno-electron-microscopy (Ahlawat and Varma, 1997), DIBA, Western blotting (Bhat et al, 1996, 1997), immunofluorescence (Varma et al, 1993), nucleic acid hybridization using radio-labeled probes (Raj and Singh, 1996; Mandal et al, 1997) and PCR (Varma et al, 1999) have been used successfully for the diagnosis of virus infections in a variety of plant species. Of these techniques, ELISA, immuno electron microscopy, dot-blot and PCR are the most sensitive and reliable techniques which can be very effectively used for the purpose of diagnostic services to tissue culture industry in the country.
For nucleic acid hybridization, c-DNA probes are also available for the gemini-, poty-, tobamo-, and badnaviruses (Varma and Ramachandran, 1996). Electrophoresis and direct electron-microscopy (Ramachandran et al, 1992; Viswanathan et al, 1998) are also being commonly used particularly for detecting viroids and unknown viruses.
The viruses for which diagnostic probes (antibodies and cDNA probes) have been developed are known to infect a variety of plant species like: banana, brinjal, cassava, citrus, Frenchbean, garlic, gladioli, groundnut, lilies, mungbean, onion, rice, soybean, sugarcane, tobacco, tomato, urdbean, ornamental plants etc.
Citrus (Citrus spp.) is the third largest fruits crop grown in India, with a great potential for export if raised from desirable virus free planting material to increase production and maintain quality. Its cultivation in India extends to over 0.37 million hectares with an annual production of 2.98 million tonnes. The productivity of citrus fruits in India is comparatively low owing to many biotic stresses of which viruses play a significant role. In India, more than 15 virus and virus like diseases occur on citrus (Singh, 1996). Of these, three viral diseases namely tristeza, ringspot and mosaic, and citrus greening disease caused by a BLO are widely distributed in the country. While tristeza and greening present worldwide the ringspot and mosaic diseases are of specific concern in India. Tristeza disease is caused by a closterovirus which has been detected from all parts of the country. The severe strain of CTV is most commonly distributed and result in tree decline (Chakraborthy et al., 1993). Mild strains of the virus are also reported, but they were not found useful for cross protection (Chakraborthy et al., 1993). capillovirus is associated with the ringspot disease, and a double stranded DNA virus belonging to badnavirus group is associated with the mosaic disease (Byadagi et al., 1993; Byadagi and Ahlawat, 1995; Ahlawat et al, 1996). Citrus greening disease, caused by a fastidious bacterium, Liberobacter asiaticum (LA) is present throughout the country and has been the major cause in nearly wiping out citrus orchards in Coorg, Karnataka (Varma et al., 1993). Often these pathogens are found in mixed infections (Ahlawat, 1997).
Banana is another important commercial fruit being grown in all parts of the country. The estimated area under banana is 3.92 m ha with production of 10.4 m tonnes/annum. This crop is adversely affected by the bunchy top disease which has spread throughout the country. The causal virus has been cloned for developing c-DNA diagnostic probes (Bhat and Varma, 1996). Another important viral disease of banana is infectious chlorosis which is caused by cucumber mosaic virus. This disease is also widely distributed throughout the country causing serious economic losses (Summanwar and Marathe, 1982). Other two viral diseases namely the bract mosaic (Rodoni et al., 1997) and streak mosaic (Selvarajan et al., 1996) are localized and are of minor importance, but these are spreading fast in the absence of appropriate diagnostic systems and healthy planting material certification programmes.
With the discovery of viroids as potential plant pathogens several diseases which were earlier attributed to viral infections are now known to be caused by viroids. Viroids are single stranded circular RNA molecules with low molecular weight, devoid of proteins. Since they are free infectious RNAs it is not possible to develop protein based diagnostic reagents. To date about 43 of them were found in vegetatively propagated crops like citrus, potato, grapes, hops, apple and avocado etc. In India, research on viroids started only recently and the diseases caused by viroids have been recorded in tomato (Mishra et al., 1991), potato (Khurana et al., 1989), citrus (Ramachandran et al., 1993), and coleus (Ramachandran et al., 1992).
In India, viral diseases of ornamental plants are common. Most of these plants are infected by the viruses belonging to cucumovirus (Sang and Varma, 1975), potyvirus (Singh et al., 1970; Roy and Ramachandran, 1988), and tobamovirus (Varma and Gibbs, 1977). In orchids, infection by CyMV is very common (Varma and Ramachandran, 1994).
Somaclonal variants of pigeonpea were analysed using RAPD (Prasannalatha et al., 1999) and the polymorphic fragment scored for an identifiable locus like white seed, high seed mass, low Helicoverpa incidence etc. RFLPs were reported in studies on regenerated maize, rice, potato, coffee, populus and others (Rani et al., 1998; Mohan et al., 1997).
Microsatellite primed-Polymerase Chain Reactions (MP-PCR) were used to study genetic fidelity of regenerated shoots of Eucalyptus, populus and coffee species (Rani et al., 1995; Rani et al., 1998). Polymorphisms could not be detected in bread wheat genotypes using 23 simple sequence repeats indicating the inadequacy of the molecular analysis (Varshney et al., 1998). In Populus deltoides six of the off-types were reported through RAPD analysis of 32 randomly selected plants from a population of 500 individual plants. The plants were derived from nodal segments (Rani et al., 1995). Similarly, in sugar beet Munthali et al. (1996) reported 0.05% somaclonal polymorphism. Existence of distinct genetic differences among several variants of peach was reported by Hashmi et al. (1997). Rani and Raina (1998) reported minor variation in RAPD banding pattern among the regenerated plants of Eucalyptus tereticornis and Eucalyptus camadulensis derived from enhanced axillary branching.
From the contrasting reports on the use of molecular marker techniques for analysis of micropropagated plants it is evident that the marker system chosen for analysis needs to be evaluated for its suitability for the application before it can be further used (Gupta and Varshney, 1999). TERI is a leading group to standardize the technique of AFLP in India and utilize it for studying the genetic diversity in Azadirachta indica (Singh et al., 1999). Attempts have also been made to compare the various techniques of genetic typing such as RAPD, RFLP and AFLP. Results have demonstrated that even though RFLP and RAPD markers are useful in studying both intra and inter-specific variation, the extent of polymorphism generated by AFLP makers is several times higher. Hence, AFLPs may be the method of choice for discriminating very closely related genotypes (such as the micropropagated plants) as compared to other molecular markers.
Importance of the work
In India, banana streak virus severely infects 'Mysore' and Cavendish group of bananas. Many plants remain under symptomless normal condition, but, under stress like high temperature, nutrients they develop severe symptoms. The yield loss due to severe infection of BSV is reported to be more than 48 per cent (Annual Report NRCB, Trichy). The BSV infected plants have poorly filled fingers, sometimes seediness has also been observed. Seeded fruits are unfit for marketing. The disease is assuming a major threat to banana industry owing to its fast spread, the tissue cultured plants supplied by some of the firms were found to have cent per cent infection in Maharashtra (Cavendish) and Tamil Nadu (Poovan) (Selvarajan and Singh, 1997). Sensitive and reliable techniques for the detection of banana viruses in planting material are not available. It is necessary to develop virus specific diagnostic kits for the reliable and sensitive detection of these most common viruses so that tissue culture raised plants could be tested.
Like banana, other plant species, which are being micropropagated by the tissue culture industry in the country, are also affected by a number of viruses (Table 1). For value addition, it is important that all the tissue culture raised plants are certified for freedom from viruses. Apart from the plant species listed in Table 1, plants like citrus and oil palm are also being propagated through tissue culture intervention. Gradually, more and more plant species will be taken up for micropropagation through tissue culture.
Several plants are produced annually through tissue culture techniques. These plants are planted in large numbers in the fields either for evaluation purposes or for routine plantation. In either case it is important to maintain the genetic fidelity of the plants. Since sophisticated techniques like the AFLP have been developed and found useful, in this study we propose to use them to maintain a strict quality check of the plants produced at the MTP. This service will be made available not only to R & D centers developing tissue culture protocols but also to the established tissue culture industries for certification of planting material. Establishing clonal uniformity for horticultural species is very important as for most species it takes 3-4 years to bear fruits and certification in terms of disease free nature and expected yields are very important for the grower.