New Hosts and Comparison of Biological and Molecular Characteristics of Fig, Mulberry, and Citrus Isolates of Hop Stunt Viroid in Iran

Abstract Background and Aims: Hop stunt viroid (HSVd, genus Hostuviroid, family Pospiviroidae) has a wide host range among trees and herbaceous plants. The objectives of this study were to compare biological and physical characteristics of three isolates of HSVd from mulberry, fig and citrus and search for other hosts of this viroid in Iran. Materials and Methods: Plant samples were collected from Yazd, Isfahan, Fars and Tehran provinces and examined for their possible infection by hop stunt viroid (HSVd) using reverse transcription- polymerase chain reaction (RT-PCR). The experimental host range of the viroid was determined by injection of infected sap into greenhouse grown seedlings and subsequent assay of inoculated plants by RT-PCR. Fig, mulberry and citrus isolates of the viroid were compared with respect to host range and molecular characteristics. Nucleic acids were isolated from plant tissues by CTAB method and subjected to RT-PCR using HSVd specific primers. The PCR products were sequenced and the most stable secondary structures for the three mentioned isolates were predicted and compared.  Results: Among 120 samples collected from different regions and hosts, 13 samples from mulberry, fig, apple, quince, apricot, peach and citrus were infected by HSVd. The mulberry samples showed vein clearing and leaf deformation symptoms while usually no specific symptoms were observed in the fig samples. Apple, quince, apricot, peach and citrus samples were also symptomless hosts. In greenhouse tests, mulberry isolate induced more severe symptoms. Comparison of primary and secondary structures of the viroid isolates showed a closer similarity between the fig and mulberry isolates while the latter were less similar to citrus isolate. Conclusions: Sequence variation and structural differences were observed amon the isolates studied. Most differences of these isolates were in the pathogenicity, variable and terminal right regions which may determine the type andseverity of symptoms. Many trees are infected without showing apparent symptoms.