Ty‐1, a universal resistance gene against geminiviruses that is compromised by co‐replication of a betasatellite

Tomato yellow leaf curl virus (TYLCV), a begomovirus, causes large yield losses and breeding for resistance is an effective way to combat this viral disease. The resistance gene Ty‐1 codes for an RNA‐dependent RNA polymerase and has recently been shown to enhance transcriptional gene silencing of TYLCV. Whereas Ty‐1 was earlier shown to also confer resistance to a bipartite begomovirus, here it is shown that Ty‐1 is probably generic to all geminiviruses. A tomato Ty‐1 introgression line, but also stable transformants of susceptible tomato cv. Moneymaker and Nicotiana benthamiana (N. benthamiana) expressing the Ty‐1 gene, exhibited resistance to begomoviruses as well as to the distinct, leafhopper‐transmitted beet curly top virus, a curtovirus. Stable Ty‐1 transformants of N. benthamiana and tomato showed fewer symptoms and reduced viral titres on infection compared to wild‐type plants. TYLCV infections in wild‐type N. benthamiana plants in the additional presence of a betasatellite led to increased symptom severity and a consistent, slightly lowered virus titre relative to the high averaged levels seen in the absence of the betasatellite. On the contrary, in Ty‐1 transformed N. benthamiana viral titres increased in the presence of the betasatellite. The same was observed when these Ty‐1‐encoding plants were challenged with TYLCV and a potato virus X construct expressing the RNA interference suppressor protein βC1 encoded by the betasatellite. The resistance spectrum of Ty‐1 and the durability of the resistance are discussed in light of antiviral RNA interference and viral counter defence strategies.