It seems that viroids prevent the correct cutting out of the introns. Viroids occur mainly in warm climates and cause significant loss of profit as the causative agents of the potato disease or the Cadang-Cadang disease of palms. The virus concentration within plant cells is high, although a virus like the TMV does not harm the host seriously. Infected cells contain often voluminous virus crystals. Plants are far from being defenceless against viruses.
Hypersensitivity is an effective protection.
We know it already from fungus infections. It is based on the dying of cells in the immediate surrounding of the primary site of infection, thus stopping the spreading of the virus. In the case of Nicotiana tabacum , the genetic cause of hypersensitivity has been analyzed. One Nicotiana gene product guarantees hypersensitivity against all TMV strains, while another affects only some TMV strains. The symptom a virus causes at the primary site of infection is called primary symptom .