ABSTRACT
Viroids are covalently closed, circular RNA molecules (Hammond and Zhao, 2000). Viroids were the first circular RNAs to be discovered in nature. These are the smallest known infectious agents (Elena et al., 2001). Potato spindle tuber viroid was the first viroid reported, and it is widely prevalent in different potato growing areas. Citrus exocortis viroid is widespread in citrus production areas where trifoliate orange (Poncirus trifoliate) is used as root stock (Guo et al., 2002). Hop stunt viroid has a wide range of hosts. Mechanism of viroid pathogenesis in plants has been elucidated recently. This chapter describes the viroids and viroid-like satellite RNAs (virusoids).
