Abstract
Studies on the ways in which viroids are transmitted are important for understanding their epidemiology and for developing effective control measures for viroid diseases. Viroids may be spread via vegetative propagules, mechanical damage, seed, pollen, or biological vectors. Vegetative propagation is the most prevalent mode of spread at the global, national and local level while further dissemination can readily occur by mechanical transmission through crop handling with viroid-contaminated hands or pruning and harvesting tools. The current knowledge of seed and pollen transmission of viroids in different crops is described. Biological vectors shown to transmit viroids include certain insects, parasitic plants, and goats. Under laboratory conditions, viroids were also shown to replicate in and be transmitted by phytopathogenic ascomycete fungi; therefore, fungi possibly serve as biological vectors of viroids in nature. The term “mycoviroids or fungal viroids” has been introduced in order to denote these viroids. Experimentally, known sequence variants of viroids can be transmitted as recombinant infectious cDNA clones or transcripts. In this review, we endeavor to provide a comprehensive overview of the modes of viroid transmission under both natural and experimental situations. A special focus is the key findings which can be applied to the control of viroid diseases.
Highlights
In 1971, Diener demonstrated that the causal agent of potato spindle disease, first described in the early 1920’s, is a small, highly structured, covalently closed circular RNA known as potato-spindle-tuber viroid (PSTVd) [1,2]
Identifying the modes of transmission of viroids such as asexual vegetative propagation, mechanical damage, sexual transmission via seed or pollen, or by vectors to new hosts are important for determining viroid-disease epidemiology, and for developing appropriate cultural control measures
PSTVd was transmitted by the aphid Macrosiphum euphorbiae in a non-persistent manner [73], and persistently by the green peach aphid when the viroid was encapsidated in potato-leafroll-virus (PLRV) particles [74,75,76]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. In 1972, Semancik and Wallace reported similar findings for the causal agent of citrus exocortis disease [3]. For such an unconventional agent the term viroid, suggested by Diener, was adopted in. The unassigned viroids in the genus Apscaviroid are apple-fruit-crinkle viroid (AFCVd) and grapevine-latent viroid (GLVd), in the genus Coleviroid are coleus-blumei viroid 5 (CbVd-5). Coleus-blumei viroid 6 (CbVd-6), and in the genus Pospiviroid is portulaca-latent viroid (PoLVd). Two novel viroids have been described, apple-chlorotic-fruit-spot viroid (ACFSVd), a tentative new viroid in the genus Apscaviroid [9,10], and coleus-blumei viroid. Identifying the modes of transmission of viroids such as asexual vegetative propagation, mechanical damage, sexual transmission via seed or pollen, or by vectors to new hosts are important for determining viroid-disease epidemiology, and for developing appropriate cultural control measures
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