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.

Abstract

Tomato yellow leaf curl virus (TYLCV) and related begomoviruses are a major threat to tomato production worldwide and, to protect against these viruses, resistance genes from different wild tomato species are introgressed. Recently, the Ty-1 resistance gene was identified, shown to code for an RNA-dependent RNA polymerase and to be allelic with Ty-3. Here we show that upon TYLCV challenging of resistant lines carrying Ty-1 or Ty-3, low virus titers were detected concomitant with the production of relatively high levels of siRNAs whereas, in contrast, susceptible tomato Moneymaker (MM) revealed higher virus titers but lower amounts of siRNAs. Comparative analysis of the spatial genomic siRNA distribution showed a consistent and subtle enrichment for siRNAs derived from the V1 and C3 genes in Ty-1 and Ty-3. In plants containing Ty-2 resistance the virus was hardly detectable, but the siRNA profile resembled the one observed in TYLCV-challenged susceptible tomato (MM). Furthermore, a relative hypermethylation of the TYLCV V1 promoter region was observed in genomic DNA collected from Ty-1 compared with that from (MM). The resistance conferred by Ty-1 was also effective against the bipartite tomato severe rugose begomovirus, where a similar genome hypermethylation of the V1 promoter region was discerned. However, a mixed infection of TYLCV with cucumber mosaic virus compromised the resistance. The results indicate that Ty-1 confers resistance to geminiviruses by increasing cytosine methylation of viral genomes, suggestive of enhanced transcriptional gene silencing. The mechanism of resistance and its durability toward geminiviruses under natural field conditions is discussed.