The apple scar skin, cleus blemei and coconut cadang-cadang viroids are examples of these subviral agents. Identified in 1971 by plant pathologist Theodor Otto Diener, viroids represent the smallest and simplest infectious pathogens known as of 2015.
Viroids consist solely of short stands of circular, single-stranded RNA molecules. Unlike more complex viruses, they lack a protein coat. Viroids do not code for any proteins after invading host cells. Instead, they use RNA polymerase II, normally synthesized as a messenger RNA, to recreate their RNA template, enabling their replication. Some viroids are also capable of reproduction through cleavage and ligation after growing large, intermediate RNA molecules.
Most viroids infect plants, including their namesake coconut and apple trees. The potato spindle tuber viroid can cause significant crop damage to potato yields by causing tubers to elongate and crack. Other common viroid infection symptoms include stunting and leaf epinasty. The activation and mutation of viroids is an ongoing research area as of 2015, with pathologists attempting to find vectors of viroid infection and causes of viroid activation.
Viroids are regarded as living relics predating the evolution of DNA and protein, representing a crucial intermediate step for the evolution of life from inanimate matter.