Abstract: 【Objective】 A suitable season and a long time are needed for the symptom visualization by grafting onto woody indicator plant to identify the viroids infecting citrus. Moreover, the system is not well developed for genetic transformation and function identification in citrus indicator plants, thus, it is difficult to study the interaction between viroids and their host in such plant. Therefore, it will be important to find suitable herbaceous plants as indicator and experimental host for viroid idenfication and study on the interaction between viroids and their host. In this study, five viroids including Citrus exocortis viroid (CEVd), Citrus bent leaf viroid (CBLVd), Hop stunt viroid (HSVd), Citrus dwarfing viroid (CDVd), and Citrus bark cracking viroid (CBCVd) have been reported to be the major ones infecting citrus trees in China. The objective of this study is to identify whether the five viroids infect tomato (Solanum lycopersicum), and then to clarify whether the tomato can be adopted as the herbraceous and experimental host plant for the five viroids.【Method】 After enzyme-digested with Nde I at 37℃ into linearized form, the pGEM-T plasmids containing the dimerized cDNA of each wild type of five citrus viroids (CEVd.188, 370 nt in length; CBLVd.032, 328 nt in length; HSVd.cit106, 296 nt in length; CVd-III.072, 295 nt in length; CVd IV Ca, 285 nt in length) were in vitro transcribed with T7 RNA polymerase at 37℃, the obtained dimeric RNAs were inoculated into tomato leaves of S. lycopersicum vars. Alisa Craig and Rutgers by mechicanlly rubbling manners, and incubated in a greenhouse at 24℃. New developed tomato leaves were collected and extracted their total RNAs after 60 days post inoculation (dpi), and subjected to detection of the inoculated viroids by RT-PCR. The resulted RT-PCR products were analyzed on agarose gel, purified by excising the target bands from the gel, ligated with pMD18-T vector, and transformed into competent cells of Escherichia coli DH5α. After idetification by PCR, positive clones were choicely sequenced and their cDNA sequences were aligned using software DNAMAN version 6.0. 【Result】After transcribed based on the lineralized plasmids, analyses on agarose gel revealed the target RNA bands with expect sizes for all the plasmids. After inoculation, RT-PCR identification of the inoculated viroids revealed that the infectivity significantly varied among the five viroids. Of which, CEVd, CDVd and HSVd infected most inoculated seedlings, with total infectivity of 7/8, 5/8 and 7/8, respectively. Sequencing five to ten clones chosen in random showed that each parental sequence could be found in the progeny sequences, and their progeny sequences had similarities more than 99.7% with the parental sequences, with varibilities less than 0.3% (less than 10 mutations). While CBLVd and CBCVd could not infect the inoculated seedlings or had a vey low infectivity (1/8) for the latter. 【Conclusion】 CEVd, CDVd and HSVd can infect S. lycopersicum vars. Alisa Craig and Rutgers, which could be acted as herbaceous indicator and experimental plants. Whereas it is difficult to infect vars. Rutgers and Alisa Craig for CBLVd and CBCVd. For the inoculated seedlings of var. Alisa Craig, they all were significantly dwarfed than the control seedlings inoculated with buffer for all these five viroids; whereas for the inoculated seedlings of var. Rutgers, only the ones inoculated with CEVd showed the dwarfed effect compared with the controls.

Key words: citrus viroids, tomato, herbraceous indicator, infecious RNA