Abstract
Fusarium Head Blight (FHB) is a fungal disease of cereals that threatens global food security, humanhealth and the economy. On wheat, FHB reduces grain yield, quality and contaminates the grain with
harmful mycotoxins such as deoxynivalenol (DON). However, the scale of the FHB mycotoxin
problem has not been quantified in Europe. Furthermore, there is no single control method to
prevent FHB, and for improved controls to be developed a better understanding of the biology of the
causal pathogen is needed. The dominant pathogen causing FHB of small grain cereals is the
ascomycete fungus Fusarium graminearum. One understudied aspect of plant pathogen infection is
micronutrient homeostasis, particularly of zinc which is essential for all organisms. In mammalian
pathogenic fungi, zinc acquisition is well known to be important in the face of host nutritional
immunity and for virulence. The aims of this work were therefore 1) to quantify and analyse the
changing scale and impact of the FHB mycotoxin problem in European wheat and 2) to determine
how F. graminearum maintains zinc homeostasis and to what extent this contributes to growth and
virulence.
The FHB mycotoxins in European wheat intended for food and animal feed were investigated by
analysing large, long term datasets from the European Food Safety Authority and the BIOMIN World
Mycotoxin Survey. This analysis revealed that DON was common (in 47% food wheat, and 64% of
feed wheat) and present at moderately high levels in food wheat and high levels in feed wheat.
Approximately 25% of food and 45% of feed wheat containing DON was co-contaminated with other
FHB mycotoxins, most commonly zearalenone and T-2. These raised concerns about the effects of
chronic, low-level exposure and potential synergy on human health, and high levels of mycotoxins
on livestock health. The economic cost of DON in Europe was estimated for the first time. 5% of
wheat intended for food between 2010-2019 exceeded the 750µg/kg DON limit. If these 75 million
tonnes of wheat were downgraded from food to livestock feed, the resulting loss in value would be
an estimated €3 billion: a significant economic cost. Peak years of high DON levels corresponded to
known FHB epidemic years. The DON levels in feed wheat in peak years were observed to be
increasing in EU countries at lower latitudes, perhaps attributable to changing agronomic practices
and climate.
Investigation of zinc homeostasis in F. graminearum revealed that its genome encodes nine ZIP zinc
transporters FgZrfDII-I, a putative zinc-scavenging zincophore FgPraA, and a transcription factor
FgZafA that is orthologous to those regulating ZIP expression in other fungi. The ZIPs were found to
have distinct functions and contributions to zinc homeostasis and virulence during wheat infection.
Of note, FgZrfB is a PM localised zinc importer that is essential in acidic zinc limiting conditions and is
the most important for virulence. The transcription factor FgZafA is important for tolerance of zinc
limitation because it upregulates expression of the PM localised ZIPs (except FgZrfE) to promote zinc
uptake. Such conditions are expected to be experienced by F. graminearum during wheat infection
and in this way FgZafA is important for virulence. FgZafA also acts as a “regulatory hub”: sensing the
low zinc environment in the wheat host to upregulate genes involved in tolerance of multiple other
stresses and virulence.
| Date of Award | 4 Dec 2023 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Neil Brown (Supervisor) & Samuel Sheppard (Supervisor) |