Doctoral Degrees (Genetics)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/551

Browse

Browsing Doctoral Degrees (Genetics) by Author "Aldrich, Dirk Jacobus"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Characterisation of citrus tristeza virus-induced stem pitting in citrus
    (Stellenbosch : Stellenbosch University, 2023-12) Aldrich, Dirk Jacobus; Maree, Hans Jacob; Bester, Rachelle; Burger, Johan Theodorus; Stellenbosch University. Faculty of Agrisciences. Dept. of Genetics.
    ENGLISH ABSTRACT: Citrus tristeza virus (CTV) is the most important viral pathogen of citrus and causes several disease syndromes in different citrus hosts. CTV-induced stem pitting leads to substantial economic losses in sensitive citrus varieties, including grapefruit. The exact mechanisms of stem pitting development in CTV-infected citrus remain unclear. This study aimed to utilise CTV infectious clone mutants in a reverse genetics approach to study stem pitting induction. A panel of recombinant CTV clones was generated focussing on the open reading frames (ORFs) implicated in stem pitting induction and pathogenicity, namely p33, p18, p13 and p23. ORF replacements from severe- and mild-pitting South African CTV isolates were introduced into the mild-pitting infectious clone (CTV-fl6 - genotype T36) to determine if severe stem pitting could be induced. Stem pitting assessments were complemented with the determination of virus concentration ratios in ‘Mexican’ lime and ‘Duncan’ grapefruit to relate stem pitting outcomes to virus concentration. The various infectious clone and wild type CTV infections yielded a broad range of stem pitting outcomes and specific ORF replacements associated with enhanced stem pitting were identified. Plant responses to different stem pitting pressures were further assessed by untargeted metabolite profiling and the quantitation of the stress-responsive phytohormones, abscisic acid, jasmonic acid and salicylic acid. In both citrus hosts, the metabolite profiling yielded fourteen statistically significant compounds that differed between stem pitting groups. These compounds were mainly phenolic acids and phenolic glycosides and are known to function as antioxidant and stress signalling molecules. Significant differences in phytohormone content were also found between test groups, particularly in plants that were severely stunted. Stem pits were also characterised at the molecular anatomical level using a combination of known and novel techniques to better understand the nature of the xylem and phloem tissues impacted by severe pitting. Established methods such as biological staining and fluorescence microscopy were used to visualise CTV-induced stem pitting and virus localisation. The utility of two novel technologies that have not previously been used to study CTV-induced stem pitting, namely high-resolution CT scanning and serial block face scanning electron microscopy was also evaluated. This study contributed to our understanding of CTV-induced stem pitting in citrus. Importantly, the use of CTV infectious clones for South African CTV research was established, which can allow for targeted follow-up experiments to further characterise the local citrus-CTV pathosystem. This can also provide novel opportunities for potentially employing CTV infectious clones as a crop protection tool against other important citrus diseases on the African continent.