Grafting to manage infections of top stunting and necrogenic strains of cucumber mosaic virus in tomato

Cucumber mosaic virus (CMV) lists among the most important etiological agents of tomato diseases. Some isolates of CMV function as helper virus for replication, encapsidation and transmission of satellite RNAs (satRNA), which may exacerbate symptoms induced by CMV in certain hosts. Outbreaks of CMV strains supporting hypervirulent variants of satRNAs are recurrent in tomato with devastating effects on crop production and efficient control measures are still unavailable. In this study, we examined the dynamics of infection of the CMV strains tomato top stunting (TTS) and 77 supporting replication of satRNA variants that codetermine top stunting (TTS-satRNA) and necrotic (77-satRNA) phenotypes in two tomato cultivars denoted Solanum lycopersicum Manduria (Sl-Ma) and S. lycopersicum UC82 (Sl-UC). Sl-Ma but not Sl-UC recovered from disease symptoms induced by CMV-TTS while both the cultivars succumbed to the infection of CMV-77 and its necrogenic satRNA. Ability to recover of the Sl-Ma plants was transmitted by grafting to the susceptible genotype Sl-UC. More interestingly, recovery was observed also against the challenge inoculation of CMV plus 77-satRNA in plants grafted on Sl-Ma and in self-grafted plants of both the Sl-Ma and Sl-UC cultivars. Analysis of small RNAs and genes of the defence plant response based on RNA interference (RNAi) suggested that RNAi is involved in the recovery of Sl-Ma against CMV with hypervirulent satRNAs and in scions grafted on this rootstock. The response of Sl-Ma to the inoculation of CMV-77 plus 77-satRNA was compared with that of the transgenic tomato line S. lycopersicum transgenic line UCTC5.9.2 that expresses constitutively the benign variant of the satRNA denoted Tfn-satRNA. Comparative analysis suggested that the response may operate via similar mechanisms, which involve RNAi, the graft and the presence of the satRNA.