Tomato Yellow Leaf Curl Virus Disease incited by Tomato yellow leaf curl virus (TYLCV) causes huge losses in tomato production worldwide and is caused by different related begomovirus species. Breeding for TYLCV resistance has been based on the introgression of multiple resistance genes originating from several wild tomato species. In this study we have fine-mapped the widely used Solanum chilense–derived Ty-1 and Ty-3 genes by screening nearly 12,000 plants for recombination events and generating recombinant inbred lines. Multiple molecular markers were developed and used in combination with disease tests to fine-map the genes to a small genomic region (approximately 70 kb). Using a Tobacco Rattle Virus–Virus Induced Gene Silencing approach, the resistance gene was identified. It is shown that Ty-1 and Ty-3 are allelic and that they code for a RNA–dependent RNA polymerase (RDR) belonging to the RDRγ type, which has an atypical DFDGD motif in the catalytic domain. In contrast to the RDRα type, characterized by a catalytic DLDGD motif, no clear function has yet been described for the RDRγ type, and thus the Ty-1/Ty-3 gene unveils a completely new class of resistance gene. Although speculative, the resistance mechanism of Ty-1/Ty-3 and its specificity towards TYLCV are discussed in light of the function of the related RDRα class in the amplification of the RNAi response in plants and transcriptional silencing of geminiviruses in plants. Tomato yellow leaf curl virus and related begomoviruses cause major economic damage to tomato production in tropical and subtropical regions around the world. Because cultivated tomato is inherently susceptible to these viruses, breeders have incorporated several resistance alleles from wild tomato relatives. Among these are the commercially important alleles, Ty-1 and Ty-3, which were introgressed from the wild tomato relative Solanum chilense. These genes were originally mapped to different regions on chromosome 6, but recent findings suggest they may rather be alleles of the same gene. Here, we describe the precise mapping of Ty-1 and Ty-3 to a common chromosomal region, and we show that Ty-1 and Ty-3 are alleles that code for an RNA–dependent RNA polymerase of a class for which no function had been described before. Thus, Ty-1/Ty-3 unveils a completely new class of resistance genes. These results will be useful to breeders who utilize these genes in their breeding programs, and further studies should shed new light on the mechanism by which this gene functions.

To establish a successful infection plant viruses have to overcome a defence system composed of several layers. This review will overview the various strategies plants employ to combat viral infections with main emphasis on the current status of single dominant resistance (R) genes identified against plant viruses and the corresponding avirulence (Avr) genes identified so far. The most common models to explain the mode of action of dominant R genes will be presented. Finally, in brief the hypersensitive response (HR) and extreme resistance (ER), and the functional and structural similarity of R-genes to sensors of innate immunity in mammalian cell systems will be described.

The accumulation of two proteins, the nucleocapsid (N) protein and a non-structural (NSs) protein both encoded by the S RNA of tomato spotted wilt virus (TSWV), was followed in larvae during development and in adults of Frankliniella occidentalis after ingesting the virus for short periods on infected plants. The amounts of both proteins increased, as shown by ELISA and Western blot analysis, within 2 days above the levels ingested, indicating multiplication of TSWV in these insects. Accumulation of these proteins and of virus particles was further confirmed by in situ immunolabelling of the salivary glands and other tissues of adult thrips. The accumulation of large amounts of N and NSs protein, the occurrence of several vesicles with virus particles in the salivary glands and the massive numbers of virus particles in the salivary gland ducts demonstrate that the salivary glands are a major site of TSWV replication. The occurrence of virus particles in the salivary vesicles is indicative of the involvement of the Golgi apparatus in the maturation of the virus particles and its transport to the salivary ducts.

The complete nucleotide sequence of the large (L) genome segment of tomato spotted wilt virus (TSWV) has been determined. The RNA is 8897 nucleotides long and contains complementary 3′ and 5′ ends, comprising 62 nucleotides at the 5′ end and 66 nucleotides at the 3′ end. The RNA is of negative polarity, with one large open reading frame (ORF) located on the viral complementary strand. This ORF corresponds to a primary translation product of 2875 amino acids in length, with a predicted M r of 331500. Comparison with the polymerase proteins of other negative-strand viruses indicates that this protein most likely represents the viral polymerase. The genetic organization of TSWV L RNA is similar to that of the L RNA segments of Bunyamwera and Hantaan viruses, animal-infecting representatives of the Bunyaviridae.

Significance: Tomato yellow leaf curl virus disease causes enormous yield losses in tomato production worldwide and is caused by different begomoviruses, with tomato yellow leaf curl virus (TYLCV) as the most important one. Recently, we have cloned to our knowledge the first resistance gene against TYLCV, namely Ty-1 , which encodes an RNA-dependent RNA polymerase and has unveiled a new class of resistance genes. Here we provide data that indicate that Ty-1 confers resistance to TYLCV and other geminiviruses by enhancing transcriptional gene silencing, a part of the antiviral RNAi defense mechanism. Furthermore, we show that this resistance is compromised by infection with cucumber mosaic virus, another important and widely distributed RNA virus, likely due to its ability to suppress RNAi at different levels.