Plant Viruses and Viroids:  
                       Plant viruses resemble animal vi ruses in many respects: plant
viruses are morphologically similar to animal viruses, and they
have similar types of nucleic acid . In fact, some plant
vi ruses ca n multiply inside insect cells. Plant  viruses cause many
diseases of economically important crops, including beans (bean
mosaic virus), corn and sugarcane (wound tumor virus), and
potatoes (potato yellow dwarf virus). Viruses can cause color
change, deformed growth, wilting, and stunted growth in their
plant hosts. Some hosts, however, remain symptom less and only 
serve as reservoirs of infection .
Plant cells are generally protected from disease by an impermeable
cell wall. Viruses must enter through wounds or be assisted
by other plant parasites, including nematodes, fungi, and, most
often, insects that suck the plant's sap. Once one plant is infected,
it can spread infection to other plants in its pollen and seeds.
In laboratories, plant viruses are cultured in protoplasts (plant
cells with the cell walls removed) and in insect cell cultures.   

Some plant diseases arc caused by viroids. short pieces of
naked RNA, only 300 to 400 nucleotides long, with no protein
coat. The nucleotides arc often interna lly pai red, so the molecule
has a closed, fold ed, three-dimensional structure that presumably
helps protect it from attack by cellu lar enzymes. The
RNA does not code for any proteins. Thus far, viroids have
been conclusively identified as pathogens only of plants. Annually, infections by viroids, such as potato spindle tuber viriod, result in losses of million of dollars from crop damage.

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Cited By Kamal Singh Khadka
Msc Microbiology, TU
Professor In PU,PNC, RE-COST, NOVEL, LA.


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