Savarni Tripathi

Identification of sources of resistance in Lycopersicon species to Tomato leaf curl geminivirus (ToLCV) by agroinoculation

A total of 90 genotypes of Lycopersicon species were tested forresistance to the Tomato leaf curlgeminivirus (ToLCV) by agroinoculation andthe vector whitefly (Bemisia tabaciGenn.) inoculation techniques underinsect-proof glasshouse conditions. Therate of infection in the inoculated plantswas determined by detection of the viralDNA in individual plants by the nucleicacid spot hybridization (NASH). Of the 38cultivars and 11 breeding lines of L.esculentum Mill. tested, none was highlyresistant or resistant while three andseven were moderately resistant whenexposed to the cloned virus DNAs byagroinoculation and whitefly inoculationrespectively. On the other hand, among the38 commercial cultivars screened, 16(42.1%) were highly susceptible in vectorinoculations and 31 (81.6%) inagroinoculation. Among the exoticcollection (EC) accessions six were highlyresistant, eleven resistant to whiteflyinoculation and none was highly susceptiblein either of the two tests, indicating thepresence of resistance among the ECaccessions. A higher degree of resistancewas observed in other species of Lycopersicon. While only one accession ofL. cheesmanii Riley was tested, itcould not be infected by either of the twomethods. L. pimpinellifolium (Jusl.)Mill. genotype EC 251580 was similarlyresistant. In L. peruvianum (L.)Mill., five EC accessions could not beinfected by whitefly inoculation, withthree of these being resistant and twomoderately resistant in agroinoculation.This study demonstrates the importance ofthe agroinoculation technique in the virusresistance screening programs andidentifies several good sources ofresistance to the Tomato leaf curlvirus in Lycopersicon species.

Isolation and characterization of an inducer protein (Crip-31) from Clerodendrum inerme leaves responsible for induction of systemic resistance against viruses

Abstract A novel protein Crip-31 responsible for systemic induced resistance (SIR) against CMV, PVY and ToMV infection in susceptible host Nicotiana tabacum, was purified from the leaves of Clerodendrum inerme. The inducer molecule Crip-31 is proteineous, basic in nature, having hydrophobic residues and 31 kDa in molecular mass. Minimum amount of purified preparation sufficient for systemic resistance induction was ∼25 μg ml−1. Treatment of plants with purified Crip-31 induced a very high degree of localized, as well as systemic resistance against CMV, PVY and ToMV infection. Resistance induction was detectable between 40 and 60 min after challenge inoculation with viruses on the systemic host (N. tabacum). The resistance inducing activity of this biomolecule was not affected with the proteinase K treatment. Its properties are close to ribosome inactivating proteins (RIPs) possessing antiviral properties.

Papaya ringspot virus-P: overcoming limitations of resistance breeding in Carica papaya L.

Abstract Papaya ringspot virus strain papaya (PRSV-P) is the main constraint in all papaya ( Carica papaya L.) cultivating areas of the world. The only successful option of managing PRSV-P infection via transgenic resistance is confined to Hawaii due to some technical and environmental issues; other viral management strategies have had limited success. Therefore, in the present scenario, introgression of the PRSV-P resistance gene into C. papaya from its wild relatives (species of Vasconcellea ) is the most optimistic option available with researchers. The earlier attempts at crossing C. papaya with species of Vasconcellea were not successful because the species chosen as the PRSV-P-resistant parent ( V. cauliflora) was genetically distant from C. papaya , which often led to embryo abortion. In the case of seed formation, or when this problem of incompatibility was overcome with an embryo rescue technique, the resultant F 1 hybrids were infertile. The approach was refined with the advancement of knowledge of genetic mapping of C. papaya vis-a-vis other species of the genus Vasconcellea with the help of molecular taxonomy. As a result, other PRSV-P-resistant species of Vasconcellea , which were genetically closer to C. papaya , were used in the breeding program. Adequate success was achieved in getting fertile F 1 hybrids by crossing C. papaya with V. quercifolia. These hybrids showed varying degrees of viral resistance, which was neither uniform nor stable. To overcome this problem, recent emphasis has shifted toward the use of PRSV-P-immune V. pubescens . The problem of incompatibility between C. papaya and V. pubescens was resolved by using V. parviflora as a bridge species. The screening of hybrids was accelerated with the development of molecular markers to establish hybridity of crosses and for PRSV-P resistance in V. pubescens . The putative resistance genes for PRSV-P in V. pubescens have been further sequenced and characterized. These efforts, and recently the advancement in papaya genomics, have raised hope for development of PRSV-P-resistant hybrids with commercially acceptable fruit quality in the near future.

Management of Viral Diseases Through Conventional Approaches in India

Viral diseases of crop plants cause enormous economic losses as most of the cultivated areas in India are in subtropical and tropical regions which provide congenial environment for multiplication of viruses and their vectors. Intensive agricultural practices for fulfilling the demand of food for the growing population has further contributed to the diseases and pests problems. Under favourable condition, the viral disease incidence can be as high as 100% resulting into serious losses to the farmers and consumers. The control of plant viral diseases has been challenging because of unavailability of effective direct method of control by chemical applications. Hence, indirect methods of managing viral diseases has been utilized such as use of modified cultural practices, use of virus-free planting materials, use of host resistance to viruses and their insect-vectors, cross protection, application of insecticides and oils for the control of virus viruses. Some success in viral disease management has been achieved by using a combination of these approaches in few crops. The range of conventional management approaches that have been studied and applied against plant viral diseases in India are summarized in this chapter.

Begomoviruses in India

Begomoviruses, a group of whitefly-transmitted single-stranded DNA viruses that are widely spread, cause significant economic losses in several important crops in tropical and subtropical regions of India. Begomoviruses have been known to be associated with and cause many diseases in cucurbitaceous, solanaceous, malvaceous vegetable and legume crops in most parts of the country. However, these viruses have emerged as a major threat to vegetable and legume production in India. Tomato, chillies, cucurbits, cotton, okra, legumes, papaya, and cassava are the most seriously affected crops. In recent decades, the most dramatic emergence of begomoviruses has been observed in tomato, chilli, and cucurbits throughout the country specially in tropical areas. The major factors responsible for the emergence of new viruses and their spread in the ecosystem are introduction of viruses, introduction of susceptible crops or genotypes, change in vector population, recombination in viruses, weather factors, and new intensive agricultural practices. This article presents the current understanding of begomovirus diseases in India and the driving forces for their emergence.

Resistance Against Papaya Ringspot Virus in Vasconcellea Species: Present and Potential Uses

Papaya (Carica papaya L.) is cultivated in all continents producing 12.41 million tonnes from 434,785 ha, but major share of its production comes from Asia, Central America and Africa. Yield of papaya varies from place to place mainly due to the widespread incidence of viral diseases. The genus Carica is more vulnerable to diseases due to narrow gene pool. Among various viral diseases affecting papaya cultivation, Papaya ringspot virus type Papaya (PRSV-P), is the most devastating one in all major papaya-growing areas. The use of PRSV-P resistant transgenic papaya cultivars has been limited to certain geographical regions. Other approaches of managing PRSV-P have only limited success. Therefore, the approach of introgression of PRSV-P resistance in papaya from PRSV-P resistant wild relatives (Vasconcellea species) by conventional breeding has become the only viable option. All PRSV-P resistant Vasconcellea species attempted to be used as a source of resistant gene(s) had certain limitations. V. cauliflora rarely produced hybrids, and most of them were infertile. V. quercifolia produced resistant hybrids which developed mild virus symptoms with age. V. cundinamarcensis was reported to be consistently resistant against PRSV-P infection under many geographical conditions. But it was not possible to transfer PRSV-P immunity to C. papaya because F1 hybrids were infertile females. The approach of bridge crossing is the latest strategy in developing PRSV-P resistance in C. papaya. V. parviflora which is cross-compatible with both V. cundinamarcensis and C. papaya was used as the bridge species. Stable homozygous hybrids of V. cundinamarcensis x V. parviflora were crossed with C. papaya to obtain PRSV-P resistant hybrid with marketable fruiting qualities. In addition to the genus Vasconcellea, other closely related genera of papaya (Cylicomorpha, Horovitzia, Jacarantia and Jarilla) should also be characterized and screened for variation in the PRSV-P resistance and their cross-compatibility with C. papaya to use them as wild sources in resistant breeding programme.

Viral Diseases of Banana and Their Management

Banana (Musa spp.) is one of the most important staple food crops which provide nutrition and income for the millions of farmers worldwide, especially in tropical regions. Asia being the major continent for banana production contributes more than half of the world banana production (101.9 million tonnes). Banana is vegetatively propagated using suckers or through tissue culture plants which grow, mature and fruit without seasonality throughout the year. Viral diseases are considered a major concern for banana production because of their effects on yield and quality as well as limitations to germplasm multiplication and the international germplasm exchange. There are many (about 20) different viruses reported to infect banana worldwide. However, the economically most important viruses are: Banana bunchy top virus (BBTV), Banana streak viruses (BSV), Banana bract mosaic virus (BBrMV) and Cucumber mosaic virus (CMV). Among these, BBTV and BSV are major threats for banana production. Of the two, BSV exist as episomal and endogenous forms and more widely spread worldwide than BBTV, though later is so far most economically damaging virus contributing to a yield reduction of up to 100 %. Due to lack of durable virus resistance in the Musa spp., measures such as phytosanitation, use of virus free planting material, strict regulation on movement of infected planting materials are effective means to control viral diseases in banana. Studies of several decades on the biology, epidemiology, survival, spread, sequence integration into the host genome of banana viruses and their integrated management strategies are summarized in this review.