Browsing by Subject "Foxy-2"

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    Impacts of the fungal bio-control agent Fusarium oxysporum f.sp. strigae on plant beneficial microbial communities in the maize rhizosphere
    (2016) Musyoki, Mary Kamaa; Cadisch, Georg
    Striga hermonthica causes severe yield reduction in cereal crop production in Sub-Saharan Africa. Intergrated Striga management has been proposed as one of the best options to control striga. Along this line, the use of biocontrol agent (BCA) Fusarium oxysporum f.sp. strigae (Foxy-2) has been proven as an effective and environmental friendly management strategy. It is well established that a prerequisite for a successful BCA is sufficient risk assessment analysis. Towards this direction, Foxy-2 was assessed for its potential non-target impacts on the abundance, community structure of bacterial and archaeal nitrifying prokaryotes as well as enzymatic activities of proteolytic bacteria. Maize rhizosphere soils treated with or without Foxy-2, Striga and high quality organic residues (i.e., Tithonia diversifolia) as N source were evaluated by quantitative polymerase chain reaction (qPCR) and terminal restriction fragment length polymorphism (TRFLP). It was observed that Foxy-2 had a promoting effect on archaeal abundance under controlled conditions in sandy soils. Furthermore, crop growth stage, seasonality and soil type had a strong effect on abundance and community structure of nitrifying prokaryotes over Foxy-2 inoculation. In addition proteolytic enzymatic activities analysis showed that Foxy-2 did not affect their activities. Correlation analysis also showed that abundance and community structure on nitrifying communities positively correlated with extractable organic carbon, extractable organic nitrogen and soil pH, while proteolytic enzymatic activities correlated with extractable organic nitrogen and soil ammonium. It was concluded that Foxy-2 is compatible with nitrifying prokaryotes and proteolytic enzymatic activities.
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    Molecular perspectives on the ecologically inconsistent effectiveness of the mycoherbicide Fusarium oxysporum f. sp. strigae against Striga hermonthica
    (2022) Anteyi, Williams Oyifioda; Rasche, Frank
    Cereals are a major staple that is crucial for food security in sub-Saharan Africa (SSA). Sadly, the obligate hemiparasitic witchweed, Striga spp., especially Striga hermonthica (Delile) Benth., is a major biotic constraint to cereal production in SSA, causing enormous crop yield losses estimated at US$10 billion annually. Fusarium oxysporum f. sp. strigae (Fos) is the most renowned fungal biological control agent (BCA) for specifically and significantly tackling S. hermonthica under agricultural systems. Field surveys, however, have revealed the inconsistent effectiveness of Fos isolates against S. hermonthica in differing zones of SSA (i.e., West Africa, East Africa). This daunting phenomenon is a critical challenge that affects Fos reliability and deters its use for S. hermonthica management. The inconsistent effectiveness of Fos against S. hermonthica was presumably ascribed to the interactions that occur between the differing location-specific ecological factors of the pathosystem i.e., abiotic (climate, moisture, or soil physico-chemistry) or biotic (S. hermonthica, Fos isolate, or the plant microbiome). Without doubt, the diversity of a host or pathogen is a primary determinant of the innate susceptibility or virulence of the host or pathogen, respectively. In terms of S. hermonthica diversity, genomic variation of individuals, or regional genetic variation of the sampling zone, were the two major forces suspected. However, the important determiner out of the two forces was unknown. Besides, despite the suppression/death that Fos causes to S. hermonthica, the physiological damage S. hermonthica initiates to an infested cereal crop is mostly irreversible. Hence, in examining strategies for circumventing the main problem of Fos inconsistent effectiveness against S. hermonthica, and the physiological consequences of S. hermonthica on the host cereal crop, the integration of other (non-Fos inoculum) BCA were suggested as possible means for improving the efficiency of S. hermonthica biocontrol. For example, by utilizing a bioherbicide cocktail of Fos and plant growth promoting rhizobacteria (PGPR), or Striga seed germination-inhibiting fungal toxins. Apart from the popular reputation of PGPR in enhancing crop health and growth, certain PGPR strains (especially Bacillus subtilis isolate GB03) have been earlier reported for their highly-promising potential of antagonizing S. hermonthica development. Similarly, certain fungal extracellular metabolites (exometabolites), especially of Fusarium origin, were reported to completely inhibit S. hermonthica seed germination in vitro at very low concentrations (≤ 1 mM). Unfortunately, knowledge of the microbe (Fos)–microbe (PGPR) interaction, their localization and ecological niche, for enabling their expected synergistic impact of simultaneously suppressing S. hermonthica and enhancing the Striga-infected cereal crop biomass, was unknown. Also, it was unknown if highly potent/efficient Striga seed germination-inhibiting fungal exometabolites will consistently suppress S. hermonthica in planta. Thus, in the context of genetic diversity in S. hermonthica, the PhD study focused on gaining (molecular) insights into the inconsistent effectiveness of Fos against S. hermonthica; including the examination of some strategies for improving S. hermonthica biocontrol efficiency, precisely by integrating PGPR, or Striga seed germination-inhibiting Fusarium exometabolites, into a S. hermonthica biocontrol system. The first research examined the molecular genetic basis, underlying the variable susceptibility of S. hermonthica populations sampled from differing zones of SSA (West Africa, East Africa) to contrasting Fos isolates (Foxy-2, FK3). Regardless of sampling zone, the S. hermonthica populations displayed divergent susceptibility patterns to the Fos isolates i.e., a S. hermonthica class was susceptible to both Foxy-2 and FK3, while the other class was susceptible to either Foxy-2 or FK3. This manifestation correlated with nucleotide mutations at certain loci. Thus, genomic variation in S. hermonthica is a superior determinant of the inconsistent effectiveness of Fos isolates, rather than the S. hermonthica sampling zone. The second research examined the impact of coinoculating Fos and a PGPR (B. subtilis isolate GB03) into a S. hermonthica-sorghum parasitic system. Notwithstanding the colocalization of Fos and GB03 in common ecological niches of diseased S. hermonthica shoot (mainly in flavonoid-rich regions), GB03 thwarted Fos suppressive activity against S. hermonthica. Interestingly, a novel, alternative Fos entry route into S. hermonthica (through the trichome) was discovered. The coinoculation of Fos and GB03 presented no added advantage for S. hermonthica control. Finally, the third research screened a set of highly phytotoxic Fusarium exometabolites against S. hermonthica seed germination (in vitro) and incidence (in planta). This was to identify the most potent/efficient Fusarium exometabolite for S. hermonthica biocontrol. Among the tested exometabolites, diacetoxyscirpenol (DAS) was the most potent/efficient to completely suppress S. hermonthica both in vitro and in planta. Fos, however, did not produce DAS, due to underexpression of key genes necessary for Fusarium trichothecene biosynthesis. In conclusion, owing to the obligate outcrossing mating system in S. hermonthica, genomic variation is an inevitable phenomenon. This, therefore, plays a crucial role in the variable susceptibility of S. hermonthica to Fos. The newly discovered Fos (direct) entry route into S. hermonthica (trichome entry), elucidates a novel paradigm to the infection mechanism occurring under the S. hermonthica (host)–Fos (pathogen) interaction, in addition to the previously reported indirect, rhizosphere-transmission. Thus, this novel phyllosphere-transmission, paves the way for further research that exploit this alternative Fos infection route for better S. hermonthica biocontrol. Lastly, considering the potency and broadscale efficacy against diverse S. hermonthica populations, the exometabolite DAS could serve as a new agent for a more efficient S. hermonthica biocontrol. Though, further examination of its specific mode of action against the target weed (S. hermonthica), as opposed to non-target organisms, is required.
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    The biocontrol agent Fusarium oxysporum f. sp. strigae - Monitoring its environmental fate and impact on indigenous fungal communities in the rhizosphere of maize
    (2016) Zimmermann, Judith; Cadisch, Georg
    The fungal biocontrol agent (BCA) Fusarium oxysporum f. sp. strigae (Fos) has proven to be effective in the suppression of the parasitic weed Striga hermonthica, which causes substantial yield losses in cereals in Sub-Saharan Africa. A prerequisite for widespread implementation of the biocontrol technology is the official registration of the BCA Fos by country authorities in Sub-Saharan Africa. The FAO and OECD institutions established international registration regulations to ensure the environmental safety of microbial BCAs. The present thesis aimed on assessing the potential of the BCA Fos to meet these registration requirements and was, therefore, based on the following two major objectives: (1) A specific DNA-based monitoring tool for Fos was developed which allows following its population kinetics in soils as driven by contrasting environmental impacts, such as soil type, plant growth stage and seasonality. (2) Risk assessment studies were conducted to assess potential side effects of Fos inoculation on non-target soil microorganisms.