Viroids are unique infectious agents, distinct from bacteria and viruses, that primarily affect plants. They are the smallest known infectious entities, challenging traditional definitions of life with their simple structure. Discovered in 1971 by Theodor Otto Diener, viroids cause numerous plant diseases. Their study has revealed a form of infection based solely on genetic material.
Unique Characteristics
Viroids are composed exclusively of a single-stranded ribonucleic acid (RNA) molecule, typically forming a closed circular structure. This RNA is notably small, ranging from approximately 246 to 467 nucleotides in length, making them significantly smaller than even the smallest viruses. A defining feature of viroids is the complete absence of a protein coat, or capsid, a protective layer found in all viruses. Additionally, viroid RNA does not encode any proteins. Despite their single-stranded nature, viroids exhibit extensive internal base pairing, causing them to fold into a compact, often rod-like, three-dimensional structure.
Distinguishing Viroids from Viruses
A fundamental difference between viroids and viruses lies in their composition and structure. Viruses consist of genetic material, either DNA or RNA, encased within a protein coat called a capsid, and some even have an additional outer membrane. In contrast, viroids are simply naked RNA molecules, lacking a protective protein covering. This structural simplicity means viroids do not encode proteins, unlike viruses which possess genes for their capsid proteins and other functions necessary for replication.
Their replication strategies also differ significantly. Viruses often encode enzymes for their own replication, or they hijack the host’s machinery to synthesize viral proteins and replicate their genetic material. Viroids, however, rely entirely on the host cell’s enzymes for their replication. They utilize host RNA polymerase II, an enzyme typically involved in synthesizing messenger RNA from DNA, to copy their own RNA through a process called rolling-circle replication. This reliance on host machinery sets viroids apart from viral pathogens.
How Viroids Cause Plant Diseases
Viroids are exclusively plant pathogens, causing a range of diseases in various crops. Examples include Potato Spindle Tuber Viroid (PSTVd) and Citrus Exocortis Viroid (CEVd). PSTVd can lead to symptoms such as stunted plant growth, elongated and spindly potato tubers, and reduced yields. In tomatoes, PSTVd infection can result in stunted growth, mottled or yellowed foliage, and small, misshapen fruit that may fail to ripen.
Citrus Exocortis Viroid (CEVd) causes diseases in citrus plants, characterized by stunted growth, bark scaling, and cracking on susceptible rootstocks like trifoliate orange. While some citrus varieties may carry the viroid without showing symptoms, others can experience severe stunting and reduced productivity. The mechanism by which viroids cause disease without encoding proteins is thought to involve interference with the plant’s own gene expression processes. Evidence suggests that viroids can trigger RNA silencing pathways in the host, where small RNA molecules derived from the viroid interfere with the plant’s normal messenger RNA, disrupting essential cellular functions and leading to disease symptoms. This interference can impact plant development and metabolism.