Cytopathic Effects Incited by Viroid RNAs and Putative Underlying Mechanisms

Abstract

Viroids are infectious agents identified only in plants so far. In contrast to viruses, the genome of viroids is composed of a tiny circular RNA (250-400 nt) not coding for proteins, but containing in its compact structure all the information needed for parasitizing the transcriptional and RNA trafficking machineries of their hosts. Viroid infections are frequently accompanied by cellular and developmental disorders that ultimately result in macroscopic symptoms. The molecular events linking the structural domains of viroid RNAs with cellular and macroscopic alterations remain largely unexplored, although significant progress has been lately achieved in one specific viroid-host combination, highlighting the ability of viroids to strongly interfere with their host RNA regulatory networks. Cytopathic effects induced by nuclear-replicating viroids, which were investigated since early studies on viroids, consist in irregular proliferations of cell membranes (paramural bodies or plasmalemmasomes), cell wall distortions, and chloroplast malformations. Different alternatives have been proposed regarding how these cytological alterations may influence the onset of macroscopic symptoms. Recently, the cytopathology and histopathology incited by a chloroplast-replicating viroid have been investigated in depth, with defects in chloroplast development having been related to specific molecular events that involve RNA silencing and impairment of chloroplast ribosomal RNA maturation. On this basis, a tentative model connecting specific cytopathologic alterations with symptoms has been put forward. Here, early and more recent studies addressing this issue will be reviewed and reassessed in the light of recent advances in the regulatory roles of small RNAs.

Keywords: RNA silencing; cell wall; chloroplast; non-coding RNAs; pathogenesis; plasmalemmasome.

Figure 1

Cytopathic effects associated to viroid infection. Plasmalemmasomes in tomato infected by PSTVd (A), and in avocado infected by ASBVd (B). Normal and malformed chloroplasts with irregularly stacked thylakoids and wide interspaces (denoted with arrows) in healthy and PSTVd-infected tomato [(C,D) respectively], and in peach healthy and infected with a PLMVd latent variant [(E,F) respectively). Bar = 50 nm. (A,C,D) have been reproduced (with modifications) from Hari (; Copyright The American Phytopathological Society), and (B) from Da Graça and Martin, ; Copyright John Wiley and Sons), in all cases with permission.

Figure 2

Association of cytopathic effects and biochemical lesions with macroscopic symptoms in viroid infections. (A) Schematic representation of the secondary structure predicted for a PLMVd variant inducing PC, with the nucleotides forming the PC-associated insertion denoted by a black background. (Inset) Two PLMVd (-) sRNAs mapping at this insertion target for cleavage the mRNA coding for the chloroplastic heat-shock protein 90 (cHSP90) as predicted by RNA silencing; arrow marks the predicted and validated cleavage site (Navarro et al., 2012b). (B) Accumulation of plastid rRNAs in GF-305 peach leaves infected by latent (PC-P1.149 and PC-C40Δ) and PC-inducing (PC-C40) variants as revealed by Northern-blot hybridizations with cDNA probes specific for 23, 16, 5, and 4.5S rRNAs; the impaired accumulation and processing of plastid rRNAs is associated with the albino phenotype (Rodio et al., 2007). (C) Severe chlorosis induced by a PC-inducing variant of PLMVd and (D) altered plastid with rudimentary thylakoid membranes (arrows) observed in chlorotic areas. (E) Severe chlorosis induced by ASBVd and (F) altered plastid with rudimentary thylakoid structures (arrows), slightly more organized than in PC, in the corresponding bleached areas. Bar = 50 nm. (A,B,F) have been reproduced with permissions from Navarro et al., ; Copyright John Wiley and Sons), Rodio et al., ; Copyright American Society of Plant Biologists, www.plantcell.org) and Da Graça and Martin (1981), respectively, in all cases with permission.