Three Cycles of Continuous Propagation of a Severe PSTVd Strain NicTr-3 in Solanum lycopersicum cv. Rutgers Resulted in Its Attenuation and Very Mild Disease Symptoms in Potato

Abstract

Potato spindle tuber viroid (PSTVd) is a small infectious non-coding circular RNA causing diseases of important crops, including tomato and potato. The symptoms vary from mild to severe depending on the viroid strain, the host plant variety, and the environmental conditions. The molecular mechanisms underlying the development of particular disease phenotypes remain elusive. The PSTVd strain NicTr-3 causes severe disease symptoms on Solanum lycopersicum cv. Rutgers and Solanum tuberosum cv. Colomba. It was found that after three cycles of propagation in tomato cv. Rutgers, infection of potato cv. Colomba with NicTr-3 resulted in very mild symptoms including smaller size of tubers and delayed leaf senescence. Sequence analysis of PSTVd clones isolated from tomato inoculum revealed the presence of mutated variants of genomic RNA. Transcriptome analysis carried out on leaves showed a considerable difference between infected and healthy plants at 14 dpi and 30 dpi. Interestingly, the response of potato plants to the attenuated PSTVd strain revealed a large number of DEGs associated with initiation of dormancy with a considerable increase in the vegetation period. The second vegetative generation (tuber-derived plantlets from control healthy and PSTVd-infected plants) was characterized by similar phenotypes and transcriptomes. Thus, here we describe a case of attenuation of a severe PSTVd strain after continuous propagation in tomato. It would be of interest to consider the attenuated viroid strains as a potential biocontrol agent or vaccine against this type of pathogenic RNA.