Browsing by Author "Van Zyl, Karlien"

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    Genetics of fumonisin biosynthesis and resistance to fusarium verticillioides in maize
    (Stellenbosch : Stellenbosch University, 2018-12) Van Zyl, Karlien; Viljoen, Altus; Rose, Lindy J. ; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Fusarium ear rot (FER) is a serious disease of maize (Zea mays L.) caused by the hemi- biotrophic fungus Fusarium verticillioides. The fungus also produces toxic secondary metabolites, known as fumonisins, in the grain that pose serious animal and human health risks. The role that fumonisins play during F. verticillioides infection is unclear, but they are believed to suppress host basal defence and facilitate necrotrophic proliferation. Disease severity and fumonisin contamination of maize are influenced by host and fungal genetics, and by plant stresses imposed by warm and dry climates. Resistance to F. verticillioides is quantitatively controlled and strongly influenced by the environment. No maize cultivar exists that is immune to FER and fumonisin contamination. The availability of the full genome sequences of both maize and F. verticillioides, however, makes it possible to investigate plant and fungal genetic responses during infection. A cluster of genes responsible for fumonisin biosynthesis in F. verticillioides has been identified, but its regulation is not yet well understood. Random insertional mutagenesis was thus performed to potentially identify genes governing fumonisin production. A mutant was obtained that produced significantly more fumonisins than its wild-type strain. Functional annotation of the single insertion site in the mutant strain showed that it was in a non-protein- coding area of F. verticillioides chromosome 10. The integration potentially causes transcriptional interference of the downstream gene encoding a F. verticillioides 7600 hypothetical protein (FVEG_08564). In future this gene needs to be inactivated by targeted mutagenesis to confirm its function, and the expression of fumonisin in maize grain infected by the mutant determined. Information on the expression of F. verticillioides genes in maize kernels during infection is limited. This is due to the small amount of fungal RNA produced in maize, which makes transcriptome sequencing unreliable. A targeted approach to study the expression of genes involved in fumonisin production in planta was, therefore, employed. A reverse-transcription quantitative PCR (RT-qPCR) assay was first optimized, and then used to study the relative expression of two fumonisin biosynthesis (FUM) genes in maize kernels. The expression of these genes was also correlated with fumonisins levels. A positive but non-significant correlation was obtained between FUM1 and FUM19 gene expression and fumonisin concentration. This finding was in conflict to a significantly positive correlation between FUM1 and FUM19, and fumonisin production, in vitro. The disparity could be attributed to factors affecting FUM gene expression and mycotoxin production in planta, such as host and pathogen genotype, the climate and kernel maturation. The RT-qPCR used can be a valuable tool to further investigate fungal genes expressed in maize kernels. Resistance to F. verticillioides in maize is controlled by many genes that are expressed to protect the plant from early infection, through colonization, to fumonisin production. These defence-related genes are present in both resistant and susceptible genotypes, but their induction is more rapid and stronger in resistant than susceptible plants. When transcript profiles of resistant and susceptible South African maize inbred lines were studied over a 52- day time period, genes associated with pathogen recognition and redox homeostasis were most strongly induced in the resistant than in the susceptible inbred line. During the necrotrophic phase of infection the plant responded by activating jasmonic acid/ethylene signalling and genes that modulate programmed cell death. The study provides novel insights into the upstream host recognition processes over the course of F. verticillioides infection and gene expression during the latter stages of infection.