Viroids and Virusoids

Viroids infect plant cells, and more than twenty-five kinds in two families are known. Viroid RNA is 246 to 375 nucleotides long and it folds to form rodlike structures with nucleotide base pairing (in which A pairs with U, C Pospiviroidae, or virusoid RNA (1), is copied by RNA polymerase (2), gives a negative strand first (3), and then a positive strand (4). This is cleaved and recircularized to make viroid or virusoid RNA (5). Avsunviroidae, or virusoid RNA, is copied to make negative strand RNA (2, 6). Following ribozyme cleavage the negative circular RNA (7) is copied to make positive strand RNA (8). Ribozyme cleavage and recircularization results in formation of genomic RNA. Negative strand RNA is shown in thin lines, positive strand RNA is shown in thick lines. pairs with G). The potato spindle tuber viroid and tomato plant macho viroid, both members of the family Pospiviroidae, replicate in the cell nucleus. A cellular enzyme, RNA polymerase, copies the circular RNA of the viroid to make a linear, repeated copy of the genome in complementary or "negative sense." This RNA is copied again to make another linear, repeated "positive sense" RNA. Cellular enzymes cut this second copy of RNA at each place where the genome begins a repetition, yielding multiple copies of the genome. These copies then reassume a circular shape to make new viroid RNA.

The members of the second family of viroids, the Avsunviroidae, replicate in cell chloroplasts. Two are known, the avocado sunblotch viroid and the peach latent mosaic viroid. In both of these species, RNA polymerase makes a long, linear negative sense RNA. This RNA contains a catalytic ribozyme sequence, which cleaves itself. The negative sense RNA resumes its customary circular shape and is copied to form linear positive sense RNA. The ribozyme again cleaves this RNA, yielding linear genomic units that again recircularize, forming viroids.

The differing replication strategies of these two groups reflect different evolutionary origins. Viroids move through the plant in the phloem and plasmodesmata, which are part of the plant's circulatory system. They propagate by mechanical means, vegetative reproduction, and possibly via seeds and insects. They cause plant diseases following interactions with proteins. For example, when they interact with an enzyme that impairs protein synthesis, the growth of the host plant may be stunted. This can have severe economic consequences.