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Definition

Viroids are small (250-400 nucleotides), circular, single-stranded, naked low-molecular-weight ribonucleic acids (RNA) that can infect plant cells, replicate themselves, and cause disease.  These sub-viral agents are found in low concentrations, yet are highly infectious.

Discussion

Viroids have a smaller genome than viruses. They are sometimes referred to as "DNA-dependent RNA".Viroids are biotrophs (obligate pathogens) that infect plants systemically. It is not known how viroids cause disease. Viroids consist of nucleic acid only (no protein coat).They consist of single-stranded RNA (ssRNA). T ransmission of viroids occurs through mechanical means, grafting, dodder, seeds and pollen. Viroids are found only in the nucleus of infected plants, and are found only with the chromatin fraction.Viroids have no consistent morphology, and replicate de novo. Viroids are heat-resistant.

 

Theories of the origin of viroids:

 

1)  viroids are degenerative viruses.

 

2)  viroids are abnormal nuclear RNA.

 

3)  viroids are primitive precursors of today's viruses.

 

To date, 10 plant diseases have been shown to be caused by viroids. Some examples are: coconut cdang-cdang, tomato bunchy-top, and potato spindle-tuber. Viroids do not infect humans or animals.

 

Symptoms of plant disease caused by viroids are similar to those plant disease symptoms caused by viruses.