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

Abstract

Citrus exocortis viroid (CEVd) populations are composed of closely related haplotypes whose frequencies in the population result from the equilibrium between mutation, selection and genetic drift. The genetic diversity of CEVd populations infecting different citrus hosts was studied by comparing populations recovered from infected trifoliate orange and sour orange seedling trees after 10 years of evolution, with the ancestral population maintained for the same period in the original host, Etrog citron. Furthermore, populations isolated from these trifoliate orange and sour orange trees were transmitted back to Etrog citron plants and the evolution of their mutant spectra was studied. The results indicate that (i) the amount and composition of the within-plant genetic diversity generated varies between these two hosts and is markedly different from that which is characteristic of the original Etrog citron host and (ii) the genetic diversity found after transmitting back to Etrog citron is indistinguishable from that which is characteristic of the ancestral Etrog citron population, regardless of the citrus plant from which the evolved populations were isolated. The relationship between the CEVd populations from Etrog citron and trifoliate orange, both sensitive hosts, and those from sour orange, which is a tolerant host, is discussed. INTRODUCTIONViroids are small plant pathogens consisting of a naked single-stranded circular RNA molecule of 246-475 nt, that do not encode any proteins but are endowed with autonomous replication in their host plants. It is very likely that viroids have been co-evolving with their hosts since their origin, although many of them show a wide host range. The identification of viroids in wild and cultivated plants, as symptomless carriers, suggests that certain hosts may act as natural viroid reservoirs (Diener, 1995).Viroids are taxonomically classified into two families, Pospiviroidae and Avsunviroidae (Elena et al., 1991). These families mainly differ in three characteristics. First, all pospiviroidae present a central conserved region (CCR) in their rod-like secondary structure, whereas the avsunviroidae lack such a region. Second, the pospiviroidae rely on cellular factors to process their multimeric replication intermediates into unit-length molecules, while the avsunviroidae present hammerhead ribozyme structures that are able to self-cleave the multimeric forms. Third, the pospiviroidae replicate in the nucleus whereas the avsunviroidae replicate in the chloroplast (reviewed by Flores et al., 2005).Like most RNA and some DNA viruses, viroids replicate within their hosts as polymorphic populations composed of closely related sequence variants generally distributed around a predominant one. 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 ...

Caracterización del Viroide exocortis de los cítricos en diferentes condiciones de indexado

“…Las plantas fueron cultivadas de forma individual en macetas de plás-tico de 4 L y mantenidas en invernadero con 28-32 °C. Durante año y cuatro meses se registró el desarrollo de síntomas descritos por Roistacher (1991) y Bernard et al (2009), se detectó la presencia de CEVd por RT-PCR y se midió la altura del nivel del suelo hasta el brote más alto.…”

“…Los resultados de la secuenciación mostraron que los fragmentos obtenidos correspondían al Citrus exocortis viroid con 99 % de similitud y 99 % de cobertura con un tamaño de 310 pb indicando su presencia en las plantas indicadoras. Probablemente el alto valor de similitud encontrado se deba a que la composición genética del viroide suele no variar cuando es inoculado en el mismo tipo de hospedero de donde se aisló originalmente y a que los viroides se componen de haplotipos estrechamente relacionados (Bernard et al, 2009). La detección de viroides en cítricos por RT-PCR es una herramienta útil (Saponari et al, 2013;Papayiannis, 2014) que no representa una desventaja al diagnóstico biológico ya que las aportaciones del indexado están relacionadas con el estudio de la interacción planta patógeno en un ambiente controlado de desarrollo sequencing showed that the fragments obtained corresponded to the Citrus exocortis viroid with a similarity of 99% and 99% coverage with a size of 310 pb, indicating its presence in indicator plants.…”

“…Se presentaron diferencias altamente significativas (p ≤ 0.01) entre tratamientos, tiempo (ddi) y la interacción tratamiento* tiempo para las variables evaluadas en ambos procedimientos de indexado (Bernard et al, 2009). The detection of viroids in citruses by RT-PCR is a useful tool (Saponari et al, 2013;Papayiannis, 2014) which does not represent a disadvantage to the biological diagnosis, since the contributions of the indexing are related to the study of the plant-pathogen interaction in a controlled environment for the development of the disease.…”

“…The plants were planted individually in 4L plastic pots and kept in a greenhouse at 28-32 °C. For one year and four months, a record was kept of symptoms described by Roistacher (1991) and Bernard et al (2009), the presence of CEVd was detected using RT-PCR and the height was measured from ground level to the tallest sprout.…”

“…Key words: tissue culture, callus, pospoviroid, graft, inoculation, Citrus exocortis viroid (CEVd) is a pathogen that causes symptoms of stunting, bark sloughing and cracking, leaf epinasty and cracks in the petiole (Bernard et al, 2009); it affects different species of citrus, such as Poncirus trifoliata and its hybrids, rangpur lime (Citrus limonia), lemon (C. limon) and citron (C. medica) (Lin et al, 2015a), and has various hosts such as the common grapevine (Vitis vinifera), tomato (Lycopersicum esculentum), broad bean (Vicia faba), cucumber (Cucumis sativus), velvet plant (Gynura aurantiaca), chrysanthemum (Crysanthemum morifolium), impatiens (Impatiens walleriana), verbena (Verbena x hybrida) (Škorić et al, 2001;Singh et al, 2009;Palukaitis, 2014) and petunia (Petunia spp.) (Van Brunschot et al, 2014).…”