Resistance and tolerance to viroid infection: status and prospects

Viroids are the smallest known plant pathogens. Although their genome sequences are not known to encode any peptides, they can nonetheless replicate and cause disease in several plant species. One of the goals of research into viroids is to safeguard crops from viroid diseases and hence limit the economic losses to acceptable levels. In order to control viroid disease epidemics, and to mitigate the damage they cause, various different approaches have been used. These approaches include the development of viroid-resilient plants through breeding programs and the grafting of high-yield varieties onto viroid-resistant rootstocks. Furthermore, biotechnological approaches—such as the transgenic expression of ribonuclease, ribozymes, antisense RNA, and hairpin RNA—have been tested against various viroid-host combinations. In this review, we summarize the current understanding of both the natural genetic resources available with either viroid resistance or disease tolerance and the gene-editing technologies that is available for enhancing viroid resistance/tolerance. Additionally, we discuss currently available technologies with which plant viral diseases can be managed (e.g., spray-induced gene silencing and genome editing) in the context of their application to future viroid research.