3. Results
3.2. The role of FgOS-2 in Fusarium graminearum
3.2.7. Osmotic stress leads to a strongly increased oxidative burst in FgOS-2-mutants
71 Table 14. ELISA analysis of ZEA production under different growth conditions in the wild type and ∆FgOS-2 mutant strains. All values were normalized against the amount of fungal material in the sample using qPCR (see Materials and Methods).
ZEA (µg kg-1 mycelium)
in vitro in planta
1 dpi 3 dpi 7 dpi
WT:PH1 0.19 (±0.006) 0.92 (±0.005) 1.34 (±0.006)
∆FgOS-2 0.33 (±0.053) 0.78 (±0.011) 0.25 (±0.053)
Table 15. Gene expression analysis of genes involved in ZEA biosynthesis. Quantitative real-time PCR results indicate up or down regulation in the ∆FgOS-2 mutant compared to the wild type (set at 1). Expression analysis was performed using two biological and three technical replicates. Gene expression was normalized against β-tubulin gene expression.
ZEA biosynthesis zeb1 zeb2 zea1
Toxin induction
medium 1,0629±0,088 1,4043±0,138 0,7358±0,018 in planta 0,3250±0,131 12,730±0,128 0,3310±0,073
3.2.7. Osmotic stress leads to a strongly increased oxidative burst in FgOS-2-mutants
72 Figure 43. ROS production assay in the wild type (WT:PH1) and FgOS-2 deletion strains. A.
Nitroblue tetrazolium (NBT) staining for reactive oxygen species (ROS) production in cultures of the wild type (WT:PH1) and ΔFgOS-2 mutant strains on CM and CM supplemented with 0.2 M NaCl after 3 days of growth. The dark colour indicates ROS production. The mutant showed a greater production of ROS upon osmotic stress compared to the wild type. B. Quantitative analysis of H2O2 production in both strains using samples obtained from NaCl-supplemented cultures (0.2 M). Amplex red peroxide/peroxidase assay (Invitrogen, Germany) revealed a higher level of H2O2 in the mutant under osmotic stress conditions. Error bars indicate the standard deviation (n=6).
The higher H2O2 production might be either due to an enhanced production or a decreased capability for decomposition. I therefore tested the gene expression of H2O2-producing enzymes, i.e. NADPH-oxidases (NOX), in a semi-quantitative reverse-transcription PCR (primers number refers to Table 4). Expression level of noxA, noxB and the regulator gene noxR was nearly the same between ∆FgOS-2 mutants and the wild type in all conditions tested (H2O2, NaCl, in planta; Fig. 44). Only the expression of a putative calcium-dependent NADPH-oxidase noxC was drastically down-regulated in planta in the deletion strains at 7 dpi (Fig. 44) but like in the wild type under in-vitro conditions.
WT:PH1 ΔFgOS-2
CM
CM 0.2 M NaCl NBT staining
CM
CM 0.2 M NaCl NBT staining
A
WT:PH1∆FgOS-2
B
CM CM/0.2M NaCL
H2O2 concentration (µM/mg)
0,0 0,2 0,4 0,6 0,8
CM CM
0.2 M NaCl H2O2concentration (µM H2O2mg-1mycelium)
0.0 0.2 0.4 0.6 0.8
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2 WT:PH1
∆FgOS-2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2 WT:PH1
∆FgOS-2
A B
73 Figure 44. NADPH-oxidase (Nox) expression assay in the wild type (WT:PH1) and FgOS-2 deletion strains. A semi-quantitative RT-PCR assay (30 PCR cycles, primer list: Supplementary Table 1) was performed on different putative Nox-related genes. CDNA was obtained from the samples as indicated after 3 dpi (in vitro) and 7 dpi (in planta). The expression of putative noxA, noxB and the regulatory gene noxR remained unchanged between the conditions and strains. The expression of a putative noxC gene was down-regulated during wheat infection in the mutant strain.
Quantitative real-time PCR verified this result and revealed a 218-fold down-regulation of noxC expression (Fig. 45B and Table 16) during plant infection. These results suggest that, under in-vitro conditions, the regulation of nox gene expression is independent of FgOS-2.
Maybe, however, regulation might occur through regulation of enzyme activity. In order to determine how FgOS-2 is involved in regulation of ROS decomposition, expression analysis and activity assays on fungal catalases were performed. Table 16 summarized the results and provides the relative expression levels (the wild-type expression level is set at 1). Under oxidative stress conditions induced by exogenous H2O2 (10 mM), catalase gene expression (Fig. 46B) and enzyme activity (Fig. 46D) and atf1 (Fig. 45A) gene expression was greater in FgOS-2 mutants than in the wild type. This might explain the increased growth observed for the mutants on agar plates supplemented with H2O2 (Fig. 31). However, expression of all catalase genes tested (cat1, cat2.1, cat2.2, cat3) and of the putative transcriptional regulator of catalase gene expression, atf1, was strongly down-regulated in planta and in the osmotic stress medium (0.8 M NaCl) in ΔFgOS-2 mutants (Fig. 46A, C and Fig. 45A). In order to determine whether or not this would lead to a decrease of individual catalase enzymes activity, I used 0.8 M NaCl-stressed mycelium and measured catalase activity in a native PAGE. Figure 46E clearly showed a complete loss of cat2.2-activity and a drastic down-regulation of cat1. Also the highly sensitive fluorometric catalase activity assay certified the overall decrease in catalase
WT
∆FgOS-2 wheatCMCM 0.8 M NaCl
WT
∆FgOS-2
WT
∆FgOS-2
CM 10 mMH2O2
WT
∆FgOS-2
74 activity in ∆FgOS-2-mutants compared to the wild type (Fig. 46D). Thus, FgOS-2 is involved in ROS catabolism under salt stress.
Figure 45. Gene expression analysis of the putative transcriptional regulator of ROS metabolic genes, atf1 (A) and the putative calcium-responsive NADPH-oxidase noxC (B) using cDNA obtained from inoculated wheat spikelets (7 dpi), H2O2 supplemented (10 mM) and NaCl-supplemented samples (0.8 M). Gene expression was assayed in the wild type and ∆FgOS-2 mutant strains (primer list:
Table 5). The wild-type expression level was set at 1. The atf1 and noxC gene expression in the ∆FgOS-2 mutant was reduced during the infection of wheat and under salt stress and was massively induced in the samples supplemented with H2O2. Error bars indicate the standard deviation. QRT-PCR was performed in triplicate.
Figure 46. Catalase expression and activity assay. A-C. Quantitative RT-PCR using cDNA obtained from inoculated wheat spikelets (7 dpi, A), H2O2 supplemented (10 mM, B) and NaCl-supplemented samples (0.8 M, C). Expression of four different, putative catalase genes was assayed in the wild type and ∆FgOS-2 mutant strains (primer list: Table 5). The wild-type expression level was set at 1. Catalase expression in the ∆FgOS-2 mutant was reduced during the infection of wheat and in the
in planta H2O2 NaCl
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 4,5 5,0 5,5 6,0 6,5
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
in planta H2O2 NaCl planta H2O2 NaCl
0,0 0,5 1,0 16,0 18,0 20,0
in planta H2O2 NaCl
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
A
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
B
0.0 0.2 0.4 0.6 0.8 1.0 1.2 4.5 5.0 5.5 6.0 6.5
0.0 0.5 1.0 16.0 18.0
atf1 20.0 noxC
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2 WT:PH1
∆FgOS-2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2 WT:PH1
∆FgOS-2
Cat2.1 Cat2.2
Cat1 Cat3 E
Cat 1 Cat 2.1 Cat 2.2 Cat 3 0
1 2 3 4 5
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
B
cat1 cat2.1 cat2.2 cat3
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
0 1 2 3 4 5
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
Cat 1 Cat 2.1 Cat 2.2 Cat 3 0,0
0,2 0,4 0,6 0,8 1,0 1,2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
A
cat1 cat2.1 cat2.2 cat3
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Cat 1 Cat 2.1 Cat 2.2 Cat 3 0,0
0,2 0,4 0,6 0,8 1,0 1,2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
C
cat1 cat2.1 cat2.2 cat3
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
0.0 0.2 0.4 0.6 0.8 1.0 1.2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
D
0 100 200 300 400 500 600 700
Specific enzyme activity [U mg-1]
CM 0.8 M NaCl 10 mM H2O2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2 WT:PH1
∆FgOS-2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2 WT:PH1
∆FgOS-2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2 WT:PH1
∆FgOS-2
TRI4 TRI5 TRI6 TRI10
Relative expression level
0,0 0,2 0,4 0,6 0,8 1,0 1,2
WT:PH1 DFgOS-2 WT:PH1
∆FgOS-2
75 Figure 46 continuance
medium supplemented with NaCl, but not in the samples supplemented with H2O2. Error bars indicate the standard deviation. QRT-PCR was performed in triplicate. D. Catalase activity assay using protein extract obtained from mycelium raised in CM with and without 2h-induction by 0.8 M NaCl and 10 mM H2O2, respectively. Total catalase activity was reduced in the ∆FgOS-2 mutant under no-stress and osmotic-no-stress conditions and elevated under oxidative no-stress conditions. The assay was performed using two biological and three technical replicates. E. Catalase activity staining using native protein extract from the wild type and the mutant strains cultures supplemented with NaCl (0.8 M).
Activity of all catalases was reduced in the mutant.
Table 16. Gene expression analysis of genes involved in ROS metabolism. Quantitative real-time PCR results indicate up or down regulation in the ∆FgOS-2 deletion strain compared to the wild type (set at 1). Expression analysis was performed using two biological and three technical replicates. Gene expression was normalized against β-tubulin gene expression.
ROS
metabolism cat1 cat2.1 cat2.2 cat3 atf1 noxC
NaCl 0,0296 (±0,001)
0,7236 (±0,032)
0,4286 (±0,01)
0,4426 (±0,058)
0,5150 (±0,038)
0,5601 (±0,020) H2O2 2,7323
(±0,179)
4,0107 (±0,226)
1,8003 (±0,124)
1,5177 (±0,181)
5,8421 (±0,455)
18,5494 (±0,98) in planta 0,0171
(±0,006)
0,4623 (±0,019)
0,4570 (±0,018)
0,0523 (±0,005)
0,0492 (±0,001)
0,0046 (±0,001)
Interestingly, supplementation of CM containing 0.2 M NaCl with up to 50 µg ml-1 of purified catalase partially restored the usually observed growth defect of ∆FgOS-2 mutants on osmotic medium (Fig. 47).
Figure 47. Growth performance of the FgOS-2 deletion and wild type (WT:PH1) strains on CM agar plates containing 0.2 M NaCl and increasing concentrations of purified catalase. After 5 days of growth pictures were taken and the diameter of the colonies was measured. The wild type colonized the whole plate. Growth of the mutant enhanced with increasing concentrations of catalase in the medium.
0 µg catalase 25 µg 50 µg
Colony diameter [mm]
45 50 55 60 65 70 75 80
Colony diameter [mm]
45 50 55 60 65 70 75 80
WT:PH1
control:
∆FgOS-2 on CM
control:
WT:PH1 on CM
0 µg µl-1 catalase
25 µg µl-1 catalase
50 µg µl-1 catalase
76 Taken together, FgOS-2 is a central regulator of all steps in the life cycle of F. graminearum. It is involved in nearly all developmental processes, such as perithecia formation, oxidative and osmotic stress tolerance, fungicide resistance, virulence, ROS metabolism and secondary metabolite production, including mycotoxins.
3.3. The role of the Activating Transcription Factor Atf1 in Fusarium graminearum