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, 8 (5), e1002684

The Ustilago Maydis Effector Pep1 Suppresses Plant Immunity by Inhibition of Host Peroxidase Activity

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The Ustilago Maydis Effector Pep1 Suppresses Plant Immunity by Inhibition of Host Peroxidase Activity

Christoph Hemetsberger et al. PLoS Pathog.

Abstract

The corn smut Ustilago maydis establishes a biotrophic interaction with its host plant maize. This interaction requires efficient suppression of plant immune responses, which is attributed to secreted effector proteins. Previously we identified Pep1 (Protein essential during penetration-1) as a secreted effector with an essential role for U. maydis virulence. pep1 deletion mutants induce strong defense responses leading to an early block in pathogenic development of the fungus. Using cytological and functional assays we show that Pep1 functions as an inhibitor of plant peroxidases. At sites of Δpep1 mutant penetrations, H₂O₂ strongly accumulated in the cell walls, coinciding with a transcriptional induction of the secreted maize peroxidase POX12. Pep1 protein effectively inhibited the peroxidase driven oxidative burst and thereby suppresses the early immune responses of maize. Moreover, Pep1 directly inhibits peroxidases in vitro in a concentration-dependent manner. Using fluorescence complementation assays, we observed a direct interaction of Pep1 and the maize peroxidase POX12 in vivo. Functional relevance of this interaction was demonstrated by partial complementation of the Δpep1 mutant defect by virus induced gene silencing of maize POX12. We conclude that Pep1 acts as a potent suppressor of early plant defenses by inhibition of peroxidase activity. Thus, it represents a novel strategy for establishing a biotrophic interaction.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Characterization of plant defense responses to Δpep1 infections.
(A) Appressorium (arrow) and penetrating hypha (arrow heads) of SG200RFP 24 hpi. Aniline blue staining indicates callose deposition at point of penetration which is suppressed upon establishment of biotrophy and intracellular growth. (B) Penetration attempts of SG200Δpep1RFP 24 hpi. Staining reveals formation of papillae below appressoria (arrows) and surrounding short invading hyphae. White bars: 20 µm. (C) TEM analysis of SG200 hypha in maize epidermis cell, stained with cerium chloride (CeCl3). The plant plasma membrane (arrow heads) surrounds the invading hypha (H). Magnification of the biotrophic interface shows no signs of CeCl3 staining (inset). V: plant vacuole, C: plant cytosol, UCW: Ustilago maydis cell wall, PCW: plant cell wall, M: mitochondrium, chevrons: tonoplast. (D) TEM picture of SG200Δpep1 invasion hypha (H) in maize epidermis cell. CeCl3 staining localizes at the biotrophic interface (arrows), revealing accumulation of ROS (magnification, upper right). Tonoplast (chevrons) is ruptured (arrow heads), indicating an induced hypersensitive response of the penetrated cell (inset, lower left). Black bars: 1 µm.
Figure 2
Figure 2. Regulation of JA and SA associated maize marker genes in response to U. maydis wild type versus SG200Δpep1 infections.
Expression levels of SA/JA marker genes were determined by quantitative real-time PCR. The expression values represent three biological replicates and are shown relative to GAPDH expression in each sample. Leaf samples of mock, SG200 or SG200Δpep1 infected plants were taken after 2 dpi. Expression levels in mock infected plants were set to 1 and relative expression of marker genes was calculated for SG200 (light grey bars) and SG200Δpep1 (dark grey bars) samples. (A) Expression of SA marker genes atfp4, pox12 and pr1 24 hours after infection with strain SG200Δpep1 or SG200, respectively. (B) Expression of JA marker genes cc9 and bbi after infection with strain SG200Δpep1 or SG200, respectively. Data represent three biological replicates. P values have been calculated by an unpaired t test. Error bars show SEM. * P≤0.05.
Figure 3
Figure 3. Inhibition of the elicitor triggered oxidative burst in maize leaves.
(A) Luminol based readout to determine H2O2 production in maize leaf discs. The oxidative burst was elicited by the addition of chitosan (2.5 mg/ml) one minute after starting the measurement. Concentrations of recombinant Pep1, Pep1IA and GFP proteins: 10 µM. (B) Quantification of elicitor triggered H2O2 production in maize leaf discs. The bars represent the integrated signal intensity of the average of 6 independent samples over the first 5 min after elicitation. (C) Quantification of chitosan induced H2O2 production in maize leaf discs based on xylenol orange staining. The peroxidase inhibitor SHAM (2 mM), NADPH-oxidase inhibitor DPI (5 µM) as well recombinant Pep1 protein (10 µM) cause a significant reduction of H2O2. Heat inactivated Pep1 (Pep1IA) does not influence the elicitor triggered oxidative burst. Data represent three biological replicates. P values have been calculated by an unpaired t test. Error bars show SEM. * P≤0.05.
Figure 4
Figure 4. Scavenging of reactive oxygen species suppresses maize penetration resistance to the Δpep1 mutant.
(A) Aniline blue staining of SG200Δpep1RFP attempting to penetrate maize epidermis cell, 24 hpi. A papilla is formed at the point of penetration (arrow). (B) SG200Δpep1RFP penetrating the maize epidermis of plants treated with 5 µM ascorbate. Arrows mark penetration sites. The invading hyphae succeed in cell to cell penetrations (arrow heads); in some cases, proliferation of the hypha could be observed (chevron). (C) SG200Δpep1RFP on maize leaves expressing PIN1-YFP. Enhanced autofluorescence of penetrated cells as well as cells surrounding the penetration event indicate a HR reaction of the plant. (D) SG200Δpep1RFP is able to penetrate PIN1-YFP expressing maize leaves after the treatment with 5 µM ascorbate without eliciting enhanced autofluorescence. Bars: 20 µm. (E) Quantification of the length of intracellularly growing hyphae of SG200Δpep1 in maize leaves treated with 5 µM ascorbate compared to mock treated leaves. The addition of ascorbate leads to an average 6-fold increase of hyphal length. (F) Quantification of maize epidermal cells expressing visual signs of HR per penetration attempt by SG200Δpep1. The ascorbate treated plants show a ∼45% decrease in HR symptoms compared to the mock treated plants. Data represent three biological replicates. P values have been calculated by an unpaired t test. Error bars show SEM. * P≤0.05.
Figure 5
Figure 5. Pep1 directly inhibits peroxidase activity.
(A) Measurement of HRP activity using an in vitro DAB assay. Dark coloration indicates peroxidase activity, visualized by the precipitation of DAB. The addition of purified GFP as well as heat inactivated Pep1 (Pep1IA) does not interfere with HRP activity. Addition of 5 µM native Pep1 results in reduced DAB precipitation, indicating suppressed HRP activity. (B) Quantification of DAB based HRP activity assay. Different concentrations of Pep1 were added to the assay solution at two different relevant pH values. Pep1 exhibits the ability of concentration dependent suppression of HRP activity at pH 6.5 and 7.5. (C) Far Western blot shows physical interaction of HRP with Pep1 (18.5 kDa) but not GFP (29.8 kDa) (upper panel). As a loading control, a separate gel was equally loaded and stained with coomassie blue (lower panel). (D) Peroxidase activity of maize apoplastic fluid was determined in a quantitative DAB assay. Recombinant native Pep1 and heat inactivated Pep1 (Pep1IA) were added to the reaction in respective concentrations. Data represent three biological replicates. P values have been calculated by an unpaired t test. Error bars show SEM. * P≤0.05.
Figure 6
Figure 6. In vivo interaction of Pep1 with POX12.
Confocal images in (A) and (B) show N. benthamiana epidermal cells expressing BiFC constructs. (A) A plant cell co-expressing pSPYCE_C and pSPYNE_Pep1. Blue and red channels show apoplastic co-localization of the respective signals. No complementation of fluorescence is observed in the YFP channel. (B) A cell co-expressing pSPYCE_POX12 and pSPYNE_Pep1. Both signals co-localize in the apoplast. The YFP channel exhibits YFP fluorescence with the same localization pattern indicating restoration of the YFP complex due to direct interaction of POX12 and Pep1. Bars: 25 µm. (C) Yeast-Two-Hybrid experiment confirming interaction of Pep1 and POX12. Mutation of the putative active site of POX12 (POX12m) did not abolish interaction with Pep1.
Figure 7
Figure 7. Silencing of pox12 suppresses maize penetration resistance to the Δpep1 mutant.
(A) (left panel): Aniline blue staining of control plants (BMV-YFPsi) show formation of papillae at points of SG200Δpep1 penetration attempts. SG200Δpep1 is arrested directly upon penetration. (right panel): pox12-silenced (BMV-POX12si) maize plants infected with SG200Δpep1. Strain SG200Δpep1 successfully penetrates epidermal cells (arrows), shows cell to cell penetrations (arrow heads) and reaches the mesophyll layer (M, chevron) without eliciting visible plant defense responses. Bars: 10 µm (B) Quantification of intracellular hyphae length of U. maydis SG200Δpep1 on pox12-silencing plants compared to control plants. Silencing of pox12 led to a significant, ∼10-fold increase in length of intracellular SG200Δpep1 hyphae. (C) pox12 expression was quantified by quantitative real-time PCR using leaf samples of 8 independent pox12-silenced plants (BMV/POX12si) and 7 control plants (BMV/YFPsi) 48 h after U. maydis SG200Δpep1 infection (for details see methods section). Relative expression of pox12 in BMV/YFPsi control plants was averaged and set to 1. Data represent three biological replicates. P values have been calculated by an unpaired t test. Error bars show SEM. * P≤0.05.

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