Viroids - The smallest and indeed the simplest pathogenic agents known have also been proposed as plant gene vectors. They are small, 300-400 bases long, circular, single stranded (although they undergo extensive base pairing) and consist of naked RNA. These nonprotein coding viroids replicate in the host using host enzymes, probably host RNA polymerase II.

They are mechanically transmissible, able to move throughout the sap and infect other parts of the plant; some may also be transmitted through the seed. They infect a wide variety of tropical (mainly) plants. They are certainly associated with the nucleus and may replicate in it