Despite their small size, viroids have a complex genome in which the mechanism of pathogenicity and symptom induction remains unclear (Owens and Hammond, 2009). Species of the family have a rod-like secondary structure (Keese and Symons, 1985). Complex interactions between structural domains are determinant of symptom development and pathogenicity in 1996; Visvader and Symons, 1986) particularly in the members of the genus (Gora ., 1996; Visvader and Symons, 1996; Sano ., 1992). Members of the genus are restricted to woody plants and because of this, , studies on the relationship between their RNA secondary structural domains and their biological properties are very limited. (AGVd), a member of the genus is symptomless in grapevine, but induces stunting and mottling on tomato plants. (GYSVd1), on the other hand, induces yellow speckle symptoms in developed leaves of grapevine under favorable conditions. It is naturally restricted to grapevine (Hadidi ., 2003). The aim of this study was to evaluate the effect of exchanging distinct domains of AGVd with their corresponding parts from GYSVd1on symptom expression and identifying the pathogenicity determinants in a member of the genus .

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