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Abstract
Rice tungro disease (RTD), caused by the coinfection of rice tungro bacilliform virus (RTBV) and rice tungro spherical virus, is one of the most important viral diseases of rice in South and Southeast Asia. The disease remains one of the major threats to sustainable rice production in many countries. The lack of resistance genes to RTBV-the causal agent of tungro disease-makes it even more difficult to manage RTD. In this review, we summarize previous and current research efforts to genetically engineer rice in order to increase the crop's resistance to tungro disease, including the use of pathogen-derived resistance and of host genes that confer RTD resistance and/or that restrict feeding by the insect vector. The prospects of developing rice cultivars with durable resistance to RTD are also discussed.
Keywords Rice tungro disease * Genetic engineering * Rice tungro bacilliform virus * Rice tungro spherical virus * Green leafhopper * Pathogen-derived resistance * Host factor
Introduction
More than 90% of the world's rice is both produced and consumed in Asia. Consequently, the stability of rice production has strong economic and social effects on residents in Asian countries. Sustainable rice production is threatened by a variety of factors, including biotic stressors, such as insects, fungi, bacteria, viruses, and weeds, and abiotic stresses, such as drought, flooding, and extreme temperatures. Because of the insidious nature of viral pathogens, it is likely that they are responsible for far greater losses than is generally recognized (Shands and Stoner 1998). Rice tungro disease (RTD) is an important viral disease of rice in South and Southeast Asia, estimated to cause a worldwide annual loss in rice production of approximately US $1.5 billion (Herdt 1988). More recently, it has been estimated that RTD causes annual losses of 5% to 10% of the rice yields in South and Southeast Asia.
RTD is widespread in Asian countries, including the Philippines, India, Indonesia, Malaysia, Bangladesh, Nepal, Pakistan, Sri Lanka, Vietnam, China, and Japan (Anjaneyulu et al. 1995; Azzam et al. 2000; Azzam and Chancellor 2002; Muralidharan et al. 2003). Introduction of the semi-dwarf, high-yielding rice cultivare Taichung Native 1 and IR 8, both released by the International Rice Research Institute, into South and Southeast Asia disclosed the devastating nature of RTD because of...