Viroids are unique infectious agents that, unlike viruses, consist solely of a single short strand of RNA, forming a closed ring structure. These acellular obligate intracellular parasites are known to infect only plant cells, causing various diseases. Viroids enter host plant cells primarily through wound sites, which can result from physical damage, such as that caused by insects.
Once inside the plant cell, viroids replicate using the host's RNA-dependent RNA polymerase, an enzyme similar to the replicase found in RNA viruses. Notably, viroids do not encode any proteins; instead, they function similarly to small interfering RNAs (siRNAs). This means that viroids can inhibit the expression of plant genes by blocking the translation of host mRNA. As a result, the absence of essential proteins can lead to disease symptoms in the infected plants.
In summary, viroids are circular RNA molecules that disrupt normal plant gene expression, leading to detrimental effects on plant health. Ongoing research continues to explore the complexities of viroids and their interactions with host plants, as many questions about their mechanisms and impacts remain unanswered.