engleski

Severe symptoms of tristeza may be determined by minor genome variants within the Citrus tristeza virus population

Citrus tristeza virus (CTV), is a member of the family Closteroviridae, is the most widespread and most destructive viral pathogen of citrus. CTV isolates display a high level of molecular and biological variability. From the field source (parental host, Fukumoto Navel orange), CTV was bud transmitted to the seedlings of Citrus wilsonii Tanaka. As a result of genetic bottleneck induced by the bud-inoculation, the C. wilsonii seedlings displayed different types of symptoms. All successfully grafted seedlings showed severe symptoms of stem pitting and seedling yellows. On the other hand, a few seedlings in which inoculated buds did not survive displayed CTV symptoms of mild intensity. Since complex CTV population structure was detected in the parental host, the aim of this work was to investigate the population structure of the virus transmitted to C. wilsonii analyzing its coat protein gene (CP), to correlate it with symptom intensity, and to examine possible bottlenecks during the virus transmission. The CP gene sequence of the predominant genotype was identical in parental and grafted plants and clustered to the phylogenetic group 5 encompassing severe CTV reference isolates. In C. wilsonii seedlings displaying severe symptoms, the low-frequency variants clustering to other “ severe” phylogenetic groups were detected, as well. In both cases, the transmission through the buds led to a reduction in nucleotide diversity, resulting in the occurrence of genetic bottlenecks. Surprisingly, indicator plants inoculated with buds taken from unsuccessfully grafted C. wilsonii seedlings displayed no severe symptoms despite the presence of phylogenetic group 5 genomic variants. The results suggest that the appearance of severe symptoms in this case is probably induced by a complex CTV population structure found, and not directly by the predominant genomic variant. This means that either symptoms are directly induced by virus variants of other genotypes present in the population in spite of their low frequency, or that they are the result of the synergistic interaction of group 5 genomic variants with variants of other genotypes present in the population.

CTV; genomic variants; genetic bottleneck

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