The fungal-specific β-glucan-binding lectin FGB1 alters cell-wall composition and suppresses glucan-triggered immunity in plants

Abstract

β-glucans are well-known modulators of the immune system in mammals but little is known about β-glucan triggered immunity in planta. Here we show by isothermal titration calorimetry, circular dichroism spectroscopy and nuclear magnetic resonance spectroscopy that the FGB1 gene from the root endophyte Piriformospora indica encodes for a secreted fungal-specific β-glucan-binding lectin with dual function. This lectin has the potential to both alter fungal cell wall composition and properties, and to efficiently suppress β-glucan-triggered immunity in different plant hosts, such as Arabidopsis, barley and Nicotiana benthamiana. Our results hint at the existence of fungal effectors that deregulate innate sensing of β-glucan in plants.

FGB1 and WSC3 are in planta‐induced β‐glucan‐binding fungal lectins with different functions

“…Additionally, the fungal transformant m5, harboring a Si FGB1:GFP fusion construct under the control of the S. indica FGB1 promoter, was used as control. Si FGB1:GFP was previously shown to localize to the fungal CW and to additionally be one of the most abundant proteins secreted by S. indica in the culture filtrate (Wawra et al ., ). The Si WSC3:GFP fluorescence signal co‐localized with the signal of the chitin‐binding lectin WGA‐AF594 at the CW for transformant T 3 (Fig.…”

“…Whereas gold‐conjugated WGA labels for chitin was constant in all fungal samples (Figs f, S3), the amount of gold labels for β 1‐3‐glucan was significantly increased in the Si FGB1:GFP overexpressing transformant m5 compared to the control (Figs g, S3). The data obtained for the transformant m5 corroborate previous results obtained by NMR where the ratio between chitin and glucan was found to be altered in the CW of this fungal transformant (Wawra et al ., ). The amount of gold labels for β 1‐3‐glucan was significantly reduced in the CWs of the Si WSC3:GFP overexpressing transformant T 3 which could result from alteration in glucan composition or in availability of β 1‐3‐glucan to the antibodies.…”

“…S4e). This suggests a specificity of Si WSC3 for an oligosaccharide or polysaccharide of fungal origin as it was shown previously for Si FGB1 (Wawra et al ., ). Therefore, the heterologously produced Si WSC3‐His was used for ITC analysis to determine its affinity to different oligo‐ and polysaccharide ligands possibly found in fungal CWs.…”

“…S3 for more details). Schematic representations of FGB1 and WSC3: red, predicted signal peptide; blue, FGB1 carbohydrate‐binding domain as predicted from alignment analysis (Wawra et al ., ); green, WSC domain as predicted by SMART ( http://smart.embl-heidelberg.de). Box–whisker plots: horizontal lines, median; circles, outliers; whiskers, minimum and maximum of 1 st and 4 th quartile.…”

“…(). Additionally, a homokaryotic S. indica transformant carrying a geneticin resistance gene was used as reference for experiments including the S. indica homokaryotic transformant m5 expressing FGB1:GFP (Wawra et al ., ; GFP, green fluorescent protein). The Colletotrichum tofieldiae strain Ct61 (Hiruma et al ., ) was kindly provided by Paul Schulze‐Lefert from the Max Planck Institute for Plant Breeding Research, Cologne and Soledad Sacristán from the Universidad Politécnica de Madrid, and propagated on solid CM supplemented with 1.5% agar in darkness at 25°C.…”