Characterization of citrus tristeza virus variants and their influence on symptom expression in grapefruit

[摘要] ENGLISH ABSTRACT: Citrus tristeza virus (CTV), a member of the family Closteroviridae, was responsible for largescale destruction of citrus, especially in the Americas, due to tristeza disease and necessitated aproduction switch to less sensitive rootstocks. CTV however continues to affect citrus through thestem-pitting disease phenotype which is especially problematic in sweet orange, grapefruit andlime cultivars. In South Africa, the productive lifespan of grapefruit trees was severely affected bystem-pitting, requiring early tree replacement with an associated lag in production. This affect waslater mitigated by applying cross-protection, a management strategy using non-stem-pittingsources of CTV, but without prior knowledge of which CTV strains were responsible for stempittingor which strains were present in the cross-protection sources. To understand the disease andunravel mechanisms underlying cross-protection, it is necessary to characterise CTV strains toinvestigate both virus-host- and strain-interactions. The aim of this study was firstly to identifysingle-strain isolates belonging to different strains, to characterise them biologically and todetermine full-genome sequences. These characterised CTV isolates were further used in acomplementation study to investigate possible synergistic interactions affecting stem-pitting.Complete viral genomes of eight single-strain isolates were determined during the study. Twocommercial grapefruit cultivars, 'Star Ruby' and 'Marsh', were used in a glasshouse trial toevaluate the ability of specific strains to induce stem-pitting in single or mixed infections.Evaluation over four years showed that symptom expression of mild strains did not result in alteredsymptom expression when in combination with each other. Importantly demonstrating that therewas no additive effect on stem-pitting expression with multiple isolates. Relative quantitation ofthe strains in 'Marsh' and 'Star Ruby' plants indicated that the individual strain concentrationswere not significantly altered when in combination with the other strains. A valuable discovery made within this project was the characterisation of two variants of the T68 strain, derived fromthe same GFMS12 source, but displaying differences in stem-pitting severity in grapefruit. Thisfinding demonstrates the co-existence of severe and mild variants of the same strain in one sourceand provides an explanation for the presumed strain segregation event observed for the GFMS12cross-protection source that resulted in the discontinuation of the source for use in cross-protectionof grapefruit. The characterisation of these variants will further assist in the identification of thesequence determinants for stem-pitting in grapefruit.

[发布机构] Stellenbosch University