There are currently only five described types of virusoids and their associated helper viruses. An example of a helper virus is the subterranean clover mottle virus, which has an associated virusoid packaged inside the viral capsid. They are thus similar to viroids in their means of replication rolling circle replication and due to the lack of genes, but they differ in that viroids do not possess a protein coat. Viroids are said to move in an intracellular manner, cell to cell through the plasmodesmata, and a longdistance through the phloem. Virusoids are essentially viroids that have been encapsulated by a helper virus coat protein. Viroids are infectious plant pathogens that consist of a short stretch a few hundred nucleobases of highly complementary, circular, singlestranded rna without the protein coat that is typical for viruses. A second type of pathogenic rna that can infect commercially important agricultural crops are the virusoid s, which are subviral particles best described as nonselfreplicating ssrnas. The book discusses various strategies for stateoftheart methods for the detection and control of pathogens in their infected hosts and provides pivotal information from the discovery of viroids through the analysis of their molecular and biological properties, to viroid pathogenesis, host interactions, and rna silencing pathways. Sequenced viroids range from 246 to 375 nucleotides and possess extensive internal base pairing that results in the rna being rodlike and about 15nm long. For example, viruses can be killed, even crystallized like table salt however, they cant maintain a constant internal state homeostasis.
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