Special Section: Transgenic Crops

Plant virus diseases cause severe constraints on the productivity of a wide range of economically important crops worldwide. In India the Green Revolution ushered in intensive agricultural practices and reduced varietal diversity, resulting in the emergence of viral diseases at an alarming pace in the cultivated crops. Some such diseases, which are especially relevant to India, along with their yield losses, are listed in Table 1. Strategies for the management of viral diseases normally include control of vector population using insecticides, use of virus-free propagating material, appropriate cultural practices and use of resistant cultivars. However, each of the above methods has its own drawback. Rapid advances in the techniques of molecular biology have resulted in the cloning and sequence analysis of the genomic components of a number of plant viruses. A majority of plant viruses have a single-stranded positivesense RNA as the genome. However, some of the most important viruses in tropical countries like India have single-stranded and double-stranded DNA genomes and RNA genomes of ambisence polarity, i.e. genes oriented in both directions. An excellent book is now available on the organization of plant viral genomes. Genome organization, electron-microscopic structures and symptoms caused by some of the viruses, referred to in this review, are briefly illustrated in Figure 1. Concomitantly, tremendous advances have taken place in our understanding of plant–virus interaction in the process of pathogenesis and resistance. This, along with associated advances in the genetic transformation of a number of crop plants, have opened up the possibility of an entirely new approach of genetic engineering towards controlling plant virus diseases. There are mainly two approaches for developing genetically engineered resistance depending on the source of the genes used. The genes can be either from the pathogenic virus itself or from any other source. The former approach is based on the concept of pathogen-derived resistance (PDR). For PDR, a part, or a complete viral gene is introduced into the plant, which, subsequently, interferes with one or more essential steps in the life cycle of the virus. This was first illustrated in tobacco by the group of Roger Beachy, who introduced the coat protein (CP) of tobacco mosaic virus (TMV) into tobacco and observed TMV resistance in the transgenic plants. The concept of PDR has generated lot of interest and today there are several host–virus systems in which it has been fully established. Non-pathogen-derived resistance, on the other hand, is based on utilizing host resistance genes and other genes responsible for adaptive host processes, elicited in response to pathogen attack, to obtain transgenics resistant to the virus. The use of non-PDR type of resistance, even though reported much less in the literature in comparison to PDR-based approaches, holds a better promise to achieve durable resistance. Various aspects of the above topics have been reviewed extensively.