Resistance and Tolerance to Viroid Infection

Viroids are known the smallest plant pathogens, and although their genome sequences do not encode proteins, they can cause disease in economically important crops. In order to control viroid diseases and mitigate their damage, genetic resources used for breeding of the viroid-resistant crop have been searched, but the practical resistant trait has not been found in almost all viroid-crop combinations, as well as the tolerant trait. Due to the difficulty in exploiting naturally occurring resistance or tolerance, various effective strategies have been devised to control viroid diseases using non-transforming or transforming techniques. Meanwhile, extensive findings related to viroid resistance and tolerance may lead to confer resistance or tolerance to viroid infection by combining with the recently emerged new technologies (e.g., spray-induced gene silencing and genome-editing technologies), which are believed to be more environmentally viable and acceptable to the general public than previously reported approaches. In particular, some genome-modified crops produced by the latter technology are already on the market. In this review, we comprehensively summarize the current status about investigation of naturally occurring genetic traits for viroid resistance and tolerance, accumulating knowledge about host factors involved in viroid pathogenicity, and various basic technologies developed to try to possible viroid disease control strategies. Furthermore, we discuss prospects and challenges for the achievement of more effective, practical, and sustainable disease control of viroid.