Fusarium graminearum forms mycotoxin producing infection structures on wheat.

Marike J Boenisch,Wilhelm Schäfer

doi:10.1186/1471-2229-11-110

Abstract

BackgroundThe mycotoxin producing fungal pathogen Fusarium graminearum is the causal agent of Fusarium head blight (FHB) of small grain cereals in fields worldwide. Although F. graminearum is highly investigated by means of molecular genetics, detailed studies about hyphal development during initial infection stages are rare. In addition, the role of mycotoxins during initial infection stages of FHB is still unknown. Therefore, we investigated the infection strategy of the fungus on different floral organs of wheat (Triticum aestivum L.) under real time conditions by constitutive expression of the dsRed reporter gene in a TRI5prom::GFP mutant. Additionally, trichothecene induction during infection was visualised with a green fluorescent protein (GFP) coupled TRI5 promoter. A tissue specific infection pattern and TRI5 induction were tested by using different floral organs of wheat. Through combination of bioimaging and electron microscopy infection structures were identified and characterised. In addition, the role of trichothecene production for initial infection was elucidated by a ΔTRI5-GFP reporter strain.ResultsThe present investigation demonstrates the formation of foot structures and compound appressoria by F. graminearum. All infection structures developed from epiphytic runner hyphae. Compound appressoria including lobate appressoria and infection cushions were observed on inoculated caryopses, paleas, lemmas, and glumes of susceptible and resistant wheat cultivars. A specific trichothecene induction in infection structures was demonstrated by different imaging techniques. Interestingly, a ΔTRI5-GFP mutant formed the same infection structures and exhibited a similar symptom development compared to the wild type and the TRI5prom::GFP mutant.ConclusionsThe different specialised infection structures of F. graminearum on wheat florets, as described in this study, indicate that the penetration strategy of this fungus is far more complex than postulated to date. We show that trichothecene biosynthesis is specifically induced in infection structures, but is neither necessary for their development nor for formation of primary symptoms on wheat.

Highlights

The mycotoxin producing fungal pathogen Fusarium graminearum is the causal agent of Fusarium head blight (FHB) of small grain cereals in fields worldwide
Screening of infection and TRI5 induction in real time We observed similar stages of infection on all tested floral organs infected with the wild type 8/1 and the TRI5prom::green fluorescent protein (GFP) strains using a MZFLIII microscope
The results demonstrate that the used TRI5prom::GFP reporter strain produced high amounts of DON after GFP fluorescence was visually detected during plant infection

Summary

Introduction

The mycotoxin producing fungal pathogen Fusarium graminearum is the causal agent of Fusarium head blight (FHB) of small grain cereals in fields worldwide. The role of mycotoxins during initial infection stages of FHB is still unknown. We investigated the infection strategy of the fungus on different floral organs of wheat (Triticum aestivum L.) under real time conditions by constitutive expression of the dsRed reporter gene in a TRI5prom::GFP mutant. The role of trichothecene production for initial infection was elucidated by a ΔTRI5-GFP reporter strain. Fusarium graminearum Schwabe (teleomorph Gibberella zeae (Schwein) Petch) is the main causal agent of Fusarium head blight (FHB) disease of small grain cereals and cob rot of maize [1,2,3]. DON production enables the fungus to spread from infected florets into the wheat rachis [9,10,11,12,13]

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