Theodor Otto Diener

Theodor Otto Diener (born February 28, 1921) is the Swiss-American plant pathologist who, in 1971, 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 single-stranded RNA without a protein capsid, eighty times smaller than the smallest viruses. He proposed to name it and similar agents to be discovered viroids. Viroids displace viruses as the smallest infectious agents known.

Diener was born in Zürich, Switzerland. He attended the Swiss Federal Institute of Technology, from which he graduated with a Dr.sc.nat.ETH degree in 1946. After graduation, he worked as a research assistant at the Swiss Federal Experiment Station for Viticulture and Horticulture at Wädenswil, where he discovered on leaves of a cherry tree the first occurrence in more than 100 years of a rust fungus (Puccinia cerasi), a fungus which is common south of the Alps, but is rarely seen in the north.

In 1949, he emigrated to the United States, where, after a brief tenure at the Rhode Island State College, he accepted a position as Assistant Plant Pathologist at Washington State University's outlying Irrigation Experiment Station in Prosser, where he showed that an unusual amino acid, pipecolic acid, accumulates only in peach leaves bearing symptoms of Western-X-Disease. and that injection of the amino acid into healthy peach seedlings resulted in abnormalities which strikingly resembled disease symptoms, thus indicating that pipecolic acid is intimately associated with the disease's molecular pathogenesis.

In 1959, Diener joined the US Department of Agriculture's Agricultural Research Service Pioneering Laboratory for Plant Virology at the Agricultural Research Center in Beltsville, Maryland, where he pursued the cause of the potato spindle tuber disease. This led to the unexpected discovery of the causative agent, a small RNA molecule, eighty times smaller than the smallest known viruses, for which he proposed the term viroid. Later, viroids were characterized as single stranded covalently closed circular RNA molecules occurring as highly base-paired rod-like structures. Viroids, together with viroid-like satellite RNAs have been officially endorsed by the International Committee for Virus Taxonomy (ICTV) as a novel order of subviral agents, which, in its 2014 publication, encompassed 2 families, 8 genera and 32 species.

In 1989, Diener hypothesized that the unique properties of viroids make them more plausible candidates as "living relics" of a hypothetical, pre-cellular RNA World than are Introns or other RNAs then considered as such. In 2016, Diener reevaluated his hypothesis, with the result that both reviewers agreed that Diener's hypothesis was still valid, but that alternative hypotheses positing a more recent origin of viroids from cellular RNAs needed also to be considered.

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