11. Symptomless trees also represent a greater danger in terms of spread of the viroid . 12. Viroid RNA does not code for any protein.13. It serves as a model system for research on the satellite RNAs and viroids . 14. It is however thought that RY motifs are essential for establishing a viroid infection. 15. The viroid enters the cell via its plasmodesmata. 16. Infected but symptomless trees have a higher concentration of viroid particles than those showing symptoms. 17. That is because both viroids have 64 % of their sequence of nucleotides in common. 18. Related to but distinct from the RNA viruses are the viroids and the RNA satellite viruses. 19. Viroids thus are true circular RNAs.20. The amount ( titre ) of viroid particles present in avocado trees varies a great deal.
