Viroids are sub-viral infectious entities that infect plants. They are acellular infectious agents like the prions that lack the essential features of a virus; and which are capable of directing their own replication since they have small naked RNA as their genome. Unlike the prions which lack nucleic acids (inclusive of DNA or RNA), viroids contain single-stranded RNA (ssRNA) molecules. The ssRNA serves as the genome of viroids. Viroids are generally plant-pathogenic infectious particles that comprise small, naked and circular ssRNA molecules. They lack protein coat since they are naked ssRNA molecules. The ssRNA molecule of viroids depends entirely on the infected host cell for its replication since they lack genes that encode protein biosynthesis. The ssRNA molecule of viroids rarely code for any protein molecule. Viroids can exist outside of their plant host cell unlike viruses. Viroids cause significant damage to crop plants. And this damage caused by viroids on crop plants leads to considerable commercial damage to agricultural products, a factor that can impact negatively on food security especially in places where viroid infection occur. Viroids are transmissible from plants to plants, and they are the smallest known pathogens that contain a nucleic acid molecule (particularly ssRNA).
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Some of the viroids that causes plant diseases include: tomato apical stunt viroid (TASV) -which causes disease in tomato; coconut cadang-cadang viroid (CCCV) – which causes diseases in coconut and oil palm plants; potato spindle tuber viroid (PSTV) – which causes disease in potato; tomato bunchy top viroid (TBTV) – which causes disease in the leaves of tomato plants; cucumber pale fruit viroid (CPFV) – which causes disease in cucumber plants; citrus exocortis viroid (CEV) – which causes disease in citrus plants; and hop stunt viroid (HSV) – which causes disease in hop plants, grapevine, plum and peach plants. Cocaviroid, Popsiviroid, Coleviroid, Hostuviroid and Pelamoviroid are some examples of viroid families or genera that cause disease in plants.
The symptoms of plant infection by viroids is usually seen as plant discolouration, leave malfunction, fruit malfunction and stunted growth of particular crop plants. The transmission of viroids amongst crop plants usually occurs during crop propagation such as in grafting of plants and through other mechanical damage to the plant. Viroids can also be transmitted through pollen grains and seeds amongst plant population. The mechanical damage that causes viroids to penetrate food crops or plants can be caused by farming implements such as contaminated cutlasses, hoes and diggers. Some insects especially aphids are notorious in transmitting plant viruses (i.e., viroids) through insect bites; and this occurs when the insects feed on plant saps. Viroids are not known to cause disease in animals; and none have also been identified to infect bacterial cells like the bacteriophages.
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