Interdependence of Pathogenicity and Replicability with Potato Spindle Tuber Viroid

Abstract

After the unexpected appearance of lethal symptoms on tomato plants infected with the PSTVd strain Intermediate Di, viroids were isolated and sequenced. It was found that a new strain, named RG 1, had been generated spontaneously in our greenhouse. In a different series of plant passages two new strains, named QF A and QF B, were detected which coexisted with the wild-type strain Di. Strains QF A and QF B showed intermediate symptoms when inoculated separately. In order to confirm the working hypothesis that the more pathogenic strain outcompetes the less pathogenic strain but strains of similar pathogenicity might coexist in the host, strains of different pathogenicity were mixed for inoculation in a ratio from 1:1 to 1:100 (more pathogenic:less pathogenic). The concentrations of the individual strains were determined 6 weeks postinfection with the method of nondenaturing polyacrylamide gel electrophoresis, and the working hypothesis was confirmed. The total concentrations of viroids in infected plants were very similar, irrespective of whether severe, intermediate, or mild strains or mixtures of different strains were present. The mutations in all new strains (3 in RG 1, 2 in QF A, 3 in QF B) were located in the so-called virulence-modulating region. The mutations of strain RG 1 influenced dramatically the thermodynamic stability of the native rod-like structure, as determined experimentally by temperature-gradient gel electrophoresis. Since during replication a multihairpin structure is generated transiently which is transformed afterwards into the rod-like structure, a lower thermodynamic stability of the rod-like structure leads to a higher accumulation of the transient structure. It is assumed that the transient structure, which is active in replication as shown earlier, is essential also in pathogenesis. This model explains the experimentally determined interdependence between pathogenicity and replicability of PSTVd strains.

On the role of RNA silencing in the pathogenicity and evolution of viroids and viral satellites

“…This ruled out the possibility that the symptoms are due to the generation of infectious PSTVd. The symptoms in hpPSTVd plants did not appear to be as strong as those described for plants infected with the natural RG1 strain (6,15). This is probably due to lower levels of siRNA being produced from the hpPSTVd transgenes than from replicating PSTVd.…”

“…We reasoned that if RNA silencing mediates viroid and satellite symptoms, then the production of dsRNA or siRNAs, corresponding to viroid or satellite sequences, from a nonreplicating hpRNA should give symptoms that mimic those of viroid or satellite infection. To test this, we transformed tomato plants with a transgene designed to express an hpRNA that contains sequences corresponding to a virulent strain (RG1) of PSTVd (15). A total of five transgenic lines containing the hpPSTVd construct ( Fig.…”

Effect of citrus hosts on the generation, maintenance and evolutionary fate of genetic variability of citrus exocortis viroid

“…This polymorphic population structure arises as a result of (i) the high mutation rates inherent to the cellular DNA-dependent RNA polymerases involved in viroid replication subverted to replicate an RNA template and (ii) the diverse and fluctuating selective pressures imposed by the different host species. Surveys of diversity have been performed for different viroid species, including pospiviroidae such as citrus exocortis viroid (CEVd) (Visvader & Symons, 1985;Gandía et al, 2005Gandía et al, , 2007, citrus dwarfing viroid (formerly citrus viroid III) (Owens et al, 2000), citrus bent leaf viroid (Foissac & Duran-Vila, 2000;Gandía & Duran-Vila, 2004), potato spindle tuber viroid (PSTVd) (Gó ra et al, 1994;Gruner et al, 1995;Gó ra-Sochacka et al, 1997), hop stunt viroid (HSVd) (Kofalvi et al, 1997;Palacio-Bielsa et al, 2004) and grapevine yellow speckle viroid 1 (Rigden & Rezaian, 1993;Polivka et al, 1996), as well as avsunviroidae such as chrysanthemum chlorotic mottle viroid (Navarro & Flores, 1997; Codoñer et al, 2006), peach latent mosaic viroid (Hernández & Flores, 1992;Ambró s et al, 1998Ambró s et al, , 1999 and avocado sunblotch viroid (Rakowski & Symons, 1989). CEVd is a member of the genus Pospiviroid within the family Pospiviroidae.…”

Respuesta De Distintos Genotipos De Cítricos Y Géneros Afines a La Infección Con Viroides

“…Sin embargo, estudios posteriores sugieren también la participación de otros dominios en el proceso patogénico Reanwarakorn y Semancik, 1998;Qi y Ding, 2003). Por otro lado, aunque se han reportado excepciones Visvader y Symons, 1985), no se ha encontrado una relación directa entre los niveles de acumulación de los viroides y la expresión de síntomas (Schnölzer et al, 1985;Visvader y Symons 1986;Gruner et al, 1995;Rodio et al, 2006). Así, se ha propuesto que la patogénesis viroidal en miembros de la familia Pospiviroidae podría depender de interacciones moleculares específicas entre secuencias o motivos de estructura secundaria y/o terciaria y factores celulares que resultarían en la manifestación de síntomas, pero los mecanismos específicos siguen siendo poco conocidos.…”