Browsing by Subject "Exudates"

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    Mechanistic aspects of the eco-physiology of Fusarium oxysporum f. sp. cubense TR4
    (2023) Were, Evans; Rasche, Frank
    Banana and plantain (Musa spp.), here termed as bananas, are a source of food security and income for more than 400 million people globally. Banana production is threatened by Fusarium wilt disease, caused by the soilborne root-infecting fungal pathogen Fusarium oxysporum f. sp. cubense (Foc). Foc Tropical Race 4 (Foc TR4) is considered the most virulent race of Foc and has gained notoriety due to its inexorable spread and devastating impact on banana cultivation. Host infection occurs when pathogen propagules, called chlamydospores, germinate and produce hyphae that penetrate host roots and subsequently invade host tissues. Infection occurs in a narrow zone of soil immediately adjacent to the roots, called rhizosphere. The rhizosphere is notable for the extensive interactions between roots, the microbiome, and soil physico-chemical factors. Banana rhizosphere interactions are poorly understood, yet profoundly influence infection and development of Fusarium wilt. It is speculated that a better understanding of banana rhizosphere interactions will improve management of Fusarium wilt through the reduction of the abundance and/or efficacy of inoculum or enhance the disease suppressiveness of soils. Hence, the overarching objective of this doctoral study was to contribute to the fundamental ecological understanding of banana rhizosphere interactions related to Foc. The first study of this thesis analysed literature from four electronic databases (AGRIS, CAB Direct, SciVerse Scopus, ProQuest) to bring together the relatively scant data available on banana rhizosphere interactions and to highlight the key knowledge gaps. Analysis of 2,281 publications revealed the complexity of banana rhizosphere interactions and the driving factors of Fusarium wilt, for which the mechanisms remain poorly understood. Data from the literature shows that management of Fusarium wilt through rhizosphere manipulation is a dominant element albeit with limited success in the field. Notably, the data from literature shows that biological control agents (bacterial and fungal strains) are highly effective in vitro and in the greenhouse with a mean efficacy of 77.1% and 73.5%, respectively, but efficacy remains below 25.0% under field conditions. The second study of this thesis provides empirical evidence for suppression of Foc TR4 by root-secreted phenolic acids of non-host plants. Hydroponic culture and targeted metabolite analysis of root exudates of two legumes, Desmodium uncinatum and Mucuna pruriens, identified phenolic compounds such as benzoic-, t-cinnamic-, and p-hydroxybenzoic acid with inhibitory potential. These phenolic compounds suppressed Foc TR4 by inhibition of chlamydospore germination, production of new spores, and hyphal growth, and specifically also the biosynthesis of fusaric acid and beauvericin toxins, which are essential in the biology of the fungus. The third study of this thesis provides empirical evidence that the process of chlamydospore germination in Foc TR4 is developmentally orchestrated and iron-dependent. Scanning electron microscopy showed that iron-starved chlamydospores are unable to form a germ tube and exhibit reduced metabolic activity. Moreover, germination exhibits plasticity regarding extracellular pH, where over 50% germination occurs between pH 3 and pH 11. This suggests that disease suppression by manipulation of soil pH may not necessarily act via alteration of iron bioavailability. The requirement for iron was further investigated by assessing the expression of two genes (rnr1 and rnr2) that encode ribonucleotide reductase (RNR), the enzyme that controls cell growth through DNA synthesis. Expression of rnr2 was significantly induced in iron-starved chlamydospores compared to the control. The fourth study assessed the production of microbial iron-sequestering metabolites (siderophores) as a potential mechanism to counteract iron starvation. Specifically, ferrichrome, a hydroxamate siderophore, was synthesized exclusively in the mycelia of iron-starved cultures, which suggests de novo biosynthesis. Moreover, amino acid precursors for siderophore biosynthesis (ornithine, arginine) were altered by iron starvation. Collectively, this doctoral thesis extends the fundamental understanding of the biology and ecology of Foc TR4 and provides a base for realizing the potential of rhizosphere manipulation for management of Fusarium wilt.