Small Pathogenic RNA in Plants-The Viroids

Abstract

INTRODUCTION.3719Over the past several years, the low molecular weight plant pathogenic RNA, the viroids, originally proposed as the causal agent of potato spindle tuber disease (12) and visualized in extracts from the exocortis diseases (76), have been reviewed extensively (7,15,16,66). Most of these treatments have focused on evidence for the existence as well as physical characteriza tion of small pathogenic RNAs as a unique class of infectious molecules. Studies of the physical and structural features of the viroids recently have culminated in the elucidation of the primary sequence of nucleotides in the potato spindle tuber viroid (PSTV) (24 ). This feat of structural chemical analysis was significant from several perspectives. It provided the most precise molecular sizing to which various earlier observations of the struc ture and conformation could be related. It also suggested many physical chemical approaches for investigating the postulated unique conformation of this minimal infectious molecule. It did not, however, provide any dra matic insights into the biological activity, or especially the pathogenic activity, of this unique class of molecules.As with the historical development of plant virology, our understanding of the physical-chemical properties of the infecting viroid RNA has pro ceeded signifi cantly faster and with greater comprehension than our appre ciation of the biological interaction of the viroid with host cells. We have only the barest understanding of the possible mechanisms involved in viroid replication and pathogenesis. The viroid model does not lend itself to a correlation with virus nucleic acid operating at a minimal level. This inter pretation is promoted by the unique structural features of the viroid and its 461 0066-4286/79/0901-0461 $0 1.00 Annu. Rev. Phytopathol. 1979.17:461-484. Downloaded from www.annualreviews.org Access provided by New York University -Bobst Library on 02/04/15. For personal use only. Quick links to online content Further ANNUAL REVIEWS 462 SEMANCIK lack of messenger RNA activity. Thus we may anticipate that the biological and pathogenic activities of viroid RNA may present a view equally unique as are the physical and chemical characteristics of this important class of plant pathogenic molecules.This pape�' contains a discussion of some of the biological properties of viroid RNA with spec�al emphasis on host cell interactions. I hope this discussion will help identify observations that are relevant to both the process of replication and the process of pathogenesis. Recent advances in the physical description of viroid RNA are presented here principally for this purpose and also to resolve certain alternative hypotheses that have been proposed to account for the biological properties of the viroid. PHYSICAL PROPERTIES Molecular Size and ConformationDuring the earliest efforts to characterize viroid RNA, estimates of the molecular weight of potato spindle tuber viroid (PSTV) and citrus exocortis viroid (CEV) ranged from 50,000-60,000 (12) to 110,000 -1...

Correlation between structure and pathogenicity of potato spindle tuber viroid (PSTV)

“…On the basis of certain sequence homologies it has furthermore been argued that the viroid RNA and/or its complement interferes with the components and mechanisms involved in pre-mRNA splicing (Gross et al, 1982;Diener, 1981;Dickson, 1979) thereby perturbing normal mRNA processing. From the disease symptoms induced by viroid infections such as growth retardation and stunting of plants combined with curling and malformation of their leaves it has been concluded that viroids induce disturbances in the metabolism of plant growth hormones (Rodriguez et al, 1978;Semancik, 1979;Duran-Villa Fig. 4.…”

Viroid: A Useful Model for Studying the Basic Principles of Infection and RNA Biology

“…Expression of viroid symptoms has long been recognized to result from interactions between viroid RNAs and cellular factors that lead to changes in development and physiology of an infected host (Diener 1999;Flores et al 2004Flores et al , 2005Riesner and Gross 1985;Semancik 1979;Tabler and Tsagris 2004). In light of recent discoveries of the many regulatory roles of noncoding RNAs in gene expression, development, and pathogen-host interactions, elucidating the mechanisms of viroid pathogenicity should contribute to a broad understanding of the mechanisms of RNA-based regulation of gene expression.…”

Polyamines in Plants Infected by Citrus Exocortis Viroid or Treated with Silver Ions and Ethephon

“…Viroids are single-stranded, small, circular RNA molecules (about 105 D), which, despite their limited coding capacity, are able to replicate without the aid of a helper virus in host cells and produce developmental distortions in certain host plants (15,27).…”