The potato spindle tuber viroid (PSTVd) was the first

viroid Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economi ...

to be identified.Discovery of Viroids
PSTVd is a small, single stranded

circular RNA Circular RNA (or circRNA) is a type of single-stranded RNA which, unlike linear RNA, forms a covalently closed continuous loop. In circular RNA, the 3' and 5' ends normally present in an RNA molecule have been joined together. This feature confer ...

molecule closely related to the chrysanthemum stunt viroid. Present within the viroidal RNA is the Pospiviroid RY motif stem loop common to its genus. The natural hosts are

potato The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae. Wild potato species can be found from the southern Unit ...

es (''Solanum tuberosum'') and

tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...

es (''Solanum lycopersicum''). All potatoes and tomatoes are susceptible to PSTVd and there is no form of natural resistance. Natural infections have also been seen in

avocado The avocado (''Persea americana'') is a medium-sized, evergreen tree in the laurel family (Lauraceae). It is native to the Americas and was first domesticated by Mesoamerican tribes more than 5,000 years ago. Then as now it was prized for i ...

s and infections in other solanaceous crops have been induced in the laboratory. Until 2017 PSTVd was thought to be unable to infect ''

Solanum sisymbriifolium ''Solanum sisymbriifolium'' is commonly known as vila-vila, sticky nightshade, red buffalo-bur, the fire-and-ice plant, litchi tomato, or Morelle de Balbis. The small edible fruits are red on the outside and yellow inside. It grows inside a spi ...

''. Then in May seeds exported by a Dutch company were noticed to be infected. These seeds were shipped from the company, but had been originally bred to their specifications in two Asian countries. ''Pstv'' also causes Tomato bunchy top and is seed transmitted in tomato.

Discovery

It was discovered by

Theodor Otto Diener Theodor Otto Diener (born 28 February 1921) is a Swiss-American plant pathologist. In 1971, he discovered that the causative agent of the potato spindle tuber disease is not a virus, but a novel agent, which consists solely of a short strand of ...

, who is a plant pathologist at the U.S Department of Agriculture's Research Center in Beltsville, Maryland, and discovered it in 1971. He named it viroid because it is 1/80th size of a virus.

Strains and their symptoms

Different strains of PSTVd exist and symptoms range from mild to severe. Mild strains produce no obvious symptoms. Symptoms in severe strains are dependent on environmental conditions and are most severe in hot conditions. Symptoms may be mild in initial infections but become progressively worse in the following generations. Common symptoms of severe infections include color changes in the foliage, smaller leaves and spindle-like elongation. Sprouting also occurs at a slower rate than in unaffected potatoes. Infected tomatoes are slower to show symptoms which include stunted growth with a ‘bunchy top’ caused by shortened inter-nodes. Leaves become yellow or purple and often become curled and twisted. Necrosis eventually occurs in the veins of the bottom and middle leaves and the top leaves decrease in size. Fruit ripening is also affected leading to hard, small, dark green tomatoes. Long distance spread of PSTVd usually occurs via infected seeds but transmission via

aphid Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...

s (''

Myzus persicae ''Myzus persicae'', known as the green peach aphid, greenfly, or the peach-potato aphid, is a small green aphid belonging to the order Hemiptera. It is the most significant aphid pest of peach trees, causing decreased growth, shrivelling of the ...

'') also occurs but only in the presence of PLRV ( potato leaf roll virus). Mechanical transmission also occurs once it has been introduced to an area.

Primary and secondary structure of PSTVd

PSTVd comprises 359

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...

Course of infection

Systemic movement within the infected host is through the

phloem Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This tran ...

.p.114, " In the first experimental investigation on viroid systemic movement within an infected plant, Palukaitis (99) showed that PSTVd moved from a photoassimilate-source leaf into sink organs in a pattern consistent with phloem transport. This was supported by in situ localization of PSTVd in the phloem (144)."

References

Bibliography

* Agrios, George N., 1936- ''Plant pathology'' (3rd edition) San Diego: Academic Press, 1988.
Singh, R.P; Fletcher J.D.; "Background of disease (potato spindle tuber) and method of control"
''Agriculture and Agri food Canada, Crop & Food Research New Zealand'' Retrieved November 15, 2007

* "Pathogen and plant damage (potato spindle tuber)" Retrieved November 15, 2007, from George N. Agrois, Plant Pathology (3rd Ed) San Diego: Academic Press,(1988). {{Taxonbar, from=Q1134442 Viroids Viral plant pathogens and diseases Potato diseases Tomato diseases