Cytological and immunocytochemical studies on responses of wheat spikes of the resistant Chinese cv. Sumai 3 and the susceptible cv. Xiaoyan 22 to infection by Fusarium graminearum

Fusarium head blight (FHB), predominantly caused by Fusarium graminearum and F. culmorum, is one of the most destructive diseases of wheat, reducing grain yield and quality of kernels. In diseased kernels trichothecne mycotoxins accumulate, which are harmful to human and animal health. Pathogen development and host responses to infection by F. graminearum were investigated in wheat spikes of the resistant cv. Sumai 3 and susceptible cv. Xiaoyan 22 to infection by means of electron microscopy and immunogold labelling techniques. The infection process of the pathogen in wheat spikes was similar in both the resistant and susceptible cultivars, but the pathogen developed more slowly in the resistant cv. Sumai 3 compared to the susceptible cv. Xiaoyan 22, indicating that fungal spread was restricted to the spike tissues of the resistant cultivar. The formation of thick-layered appositions and papillae was essentially more pronounced in the infected host tissues of the resistant cultivar than in the susceptible one. beta-1,3-glucan was detected in the appositions and papillae. Immunogold labelling studies demonstrated that labelling densities for lignin, thionins and hydroxyproline-rich glycoproteins (HRGP) over the cell walls of the infected tissues of the susceptible wheat cv. Xiaoyan 22 only slightly increased whereas these compounds intensely accumulated in the host cell walls of the infected wheat spikes of the resistant cv. Sumai 3. The labelling densities for the two plant hydrolases, beta-1,3-glucanase and chitinase, increased slightly in the infected wheat spike tissues of the susceptible cv. Xiaoyan 22, whereas higher labelling densities of both enzymes were found in the infected wheat spikes from the resistant cv. Sumai 3. Immunogold labelling of the Fusarium toxin DON in the infected wheat spike tissues showed that labelling densities in spike tissues for DON in the resistant cv. Sumai 3 were significantly lower than those in the susceptible cv. Xiaoyan 22. The significance of the induced morphological (e.g. thick-layered wall appositions and papillae) and chemical defence constituents (e.g. beta-1,3-glucanase, chitinase, lignin, thionin and HRGP) in resistance to FHB as well as the possible role of DON as an aggressiveness factor in translation of transcripts of defence response genes and spreading of F. graminearum and F. culmorum are discussed.