Browsing by Author "Van Wyk, Luka"

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    Identification of early indicators of CTV induced stem pitting to aid the selection of CTV cross protection sources.
    (Stellenbosch : Stellenbosch University, 2024-03) Van Wyk, Luka; Maree, H. J. ; Bester, Rachelle; Stellenbosch University. Faculty of Agrisciences. Dept. of Genetics.
    ENGLISH ABSTRACT: Citrus tristeza virus (CTV) is the most impactful viral pathogen of citrus, having caused the death of more than 100 million citrus trees. CTV elicits several disease syndromes, but continues to affect citrus production through stem pitting disease phenotypes of varying severity. These phenotypes reduce citrus tree productivity and result in smaller, unmarketable fruit. Stem pitting is incompletely characterised, and the mechanisms underlying its induction remain elusive. This poses limitations regarding our understanding of CTV-induced stem pitting and the management thereof. This study aimed to better understand the plant-pathogen interactions underlying stem pitting in CTV-infected citrus over the course of disease onset from an untargeted perspective. To this end, high-throughput sequencing and ultra-performance liquid chromatography-mass spectrometry were used to profile the transcriptome and metabolome of ‘Mexican’ lime and ‘Duncan’ grapefruit plants infected with T3-KB, T68, and RB isolates of CTV. This profiling was carried out prior to infection, early post-infection, and after symptom development, with the goal of identifying determinants of severe CTV-induced stem pitting. It was shown that secondary metabolite profiles of ‘Mexican’ lime plants were able to clearly discern between CTV infections at ten months post-infection. These profiles indicated a common dysregulation of carbohydrate metabolism over disease onset and provided a putative identification of the furcatin molecule as a major driver of differentiation. Similarly, differential gene expression analysis implicated a carbohydrate metabolism gene, along with seven others, as significantly different in severely pitted, T3-KB infected ‘Mexican’ lime plants. These genes were consistently upregulated over the course of disease onset. This provided a candidate metabolic pathway for further study and eight candidate genes to be validated as determinants of CTV-induced stem pitting severity. The characterization of hypersensitive response and systemic acquired resistance in citrus due to CTV infection was also discussed. It was speculated that determinants of stem pitting severity may lie earlier in the infection progression of CTV within the hypersensitive response of citrus. Carbohydrate metabolism was also putatively associated as a factor of systemic acquired resistance in response to CTV infection. This emphasized the importance of the temporal component of citrus’ response to CTV infection that has only been studied in a limited capacity thus far. The utility of complementary transcriptomic and metabolomic analyses was also demonstrated, and the findings therefrom contribute to the understanding of plant responses to CTV over time.