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Published on July 3, 2008; 10.1104/pp.108.120808

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Received April 11, 2008
Accepted June 29, 2008

Viroid-induced symptoms in Nicotiana benthamiana plants are dependent of RDR6 activity

Gustavo Gomez , German Martinez , and Vicente Pallas *

Instituto de Biologia Molecular y Celular de Plantas, Consejo Superior de Investigaciones Cientificas-UPV, CPI, Edificio 8 E, Av. de los Naranjos s/n, 46022 Valencia, Spain

* Corresponding author; email: vpallas{at}ibmcp.upv.es.

Viroids are small self-replicating RNAs that infect plants. How these non-coding pathogenic RNAs interact with hosts to induce disease symptoms is a long-standing unanswered question. Recent experimental data has led to the suggestive proposal of a pathogenic model based on the RNA silencing mechanism. However, evidences of a direct relation between key components of the RNA silencing pathway and the symptom expression in infected plants remains elusive. To address this issue, we used a symptomatic transgenic line of Nicotiana benthamiana that expresses and processes dimeric forms of Hop stunt viroid (HSVd). These plants were analyzed under different growing temperature conditions, and were used as stocks in grafting assays with the rdr6i-Nb line, in which the RNA-dependent RNA polymerase 6 (NbRDR6) is constitutively silenced. Here we show that the symptom expression in N. benthamiana plants is independent of HSVd accumulation levels, but dependent on an active state of the viroid-specific RNA silencing pathway. The scion of rdr6i-Nb plants remained asymptomatic when grafted onto symptomatic plants, despite an accumulation of a high level of mature forms of HSVd indicating the requirement of RDR6 for the viroid-induced symptom production. In addition, the RDR6 requirement for symptom expression was also observed in wild type N. benthamiana plants mechanically infected with HSVd. These results provide biological evidence of the involvement of the viroid-specific RNA silencing pathway in the symptoms expression associated with viroid pathogenesis.






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