A Review of the Interactions between Wheat and Wheat Pathogens: Zymoseptoria tritici, Fusarium spp. and Parastagonospora nodorum

Abstract

Zymoseptoria tritici is a hemibiotrophic pathogen which causes Septoria leaf blotch in wheat. The pathogenesis of the disease consists of a biotrophic phase and a necrotrophic phase. The pathogen infects the host plant by suppressing its immune response in the first stage of infection. Hemibiotrophic pathogens of the genus Fusarium cause Fusarium head blight, and the necrotrophic Parastagonosporanodorum is responsible for Septoria nodorum blotch in wheat. Cell wall-degrading enzymes in plants promote infections by necrotrophic and hemibiotrophic pathogens, and trichothecenes, secondary fungal metabolites, facilitate infections caused by fungi of the genus Fusarium. There are no sources of complete resistance to the above pathogens in wheat. Defense mechanisms in wheat are controlled by many genes encoding resistance traits. In the wheat genome, the characteristic features of loci responsible for resistance to pathogenic infections indicate that at least several dozen genes encode resistance to pathogens. The molecular interactions between wheat and Z. tritici, P. nodorum and Fusarium spp. pathogens have been insufficiently investigated. Most studies focus on the mechanisms by which the hemibiotrophic Z. tritici suppresses immune responses in plants and the role of mycotoxins and effector proteins in infections caused by P. nodorum and Fusarium spp. fungi. Trichothecene glycosylation and effector proteins, which are involved in defense responses in wheat, have been described at the molecular level. Recent advances in molecular biology have produced interesting findings which should be further elucidated in studies of molecular interactions between wheat and fungal pathogens. The Clustered Regularly-Interspaced Short Palindromic Repeats/ CRISPR associated (CRISPR/Cas) system can be used to introduce targeted mutations into the wheat genome and confer resistance to selected fungal diseases. Host-induced gene silencing and spray-induced gene silencing are also useful tools for analyzing wheat-pathogens interactions which can be used to develop new strategies for controlling fungal diseases.

Keywords: Fusarium; Parastagonospora nodorum; Zymoseptoria tritici; anatomical barriers; pattern-triggered immunity.

Figure 1

Metabolic pathway in wheat tissues infected with Z. tritici (adapted from Kettles and Kanyuka (2016) [19]). Ch-chitin, CEBiP-chitin elicitor-binding protein, CERK1-chitin elicitor response kinase 1, LysM-lysine motif, MAPK-mitogen-activated protein kinase, NIP-necrosis-inducing proteins.

Figure 2

Metabolic pathway of the three main stages of infection caused by F. graminearum in wheat (adapted from Chetouhi et al. (2016) [50]). AOX: alternative oxidase, E: gene expression unchanged; E+: gene up-regulation; E−: gene down-regulation; PCD: programmed cell death; PR: pathogenesis-related genes; TCA: tricarboxylic acid; UGT: UDP-glucuronosyltransferase. Modifications: emphasis on gene expression profiles using “+” and “−“ indicators, simplification of graphic presentation.