Zymoseptoria tritici LysM effectors protect cell wall chitin against host chitinases

Zymoseptoria tritici (syn. Mycosphaerella graminicola) causes Septoria tritici blotch, a major disease on wheat. Understanding the mechanisms by which this filamentous fungus colonizes wheat is therefore highly relevant. Two main phases can be discriminated in the infection by this pathogen. In the first phase, Z. tritici grows slowly and without developing macroscopic symptoms. Immediately after the first necrotic spots appear, fungal growth spikes and pycnidia begin to form pycnidia which are essential for epidemic development. Although the fungus grows slowly in the first phase, it still needs to protect itself against the resistance responses of the plant. In order to achieve this, Z. tritici produces two LysM effectors, Mg1LysM and Mg3LysM. Mg3LysM is an essential virulence factor that prevents the activation of chitin-triggered immunity, as shown for its homologs in C. fulvum and M. oryzae. Both Z. tritici LysM effectors have the additional function of protecting fungal cell walls from host chitinases, which would otherwise inhibit fungal growth. LysM effectors are small secreted proteins which contain at least one LysM domain and bind chitin, but it remains to be determined how they exert their biological function. Mg1LysM and Mg3LysM may directly inhibit the function of chitinases or may bind to the cell wall to prevent access of chitinases. The high-resolution crystal structure of Mg1LysM revealed its capacity to oligomerize in the presence of chitin, indicating that it may wrap around the fungal cell wall and thus prevent chitinases from gaining access to the cell wall chitin.