Functional analysis of LysM effectors secreted by fungal plant pathogens

Chitin is a homopolymer of N-acetyl-d-glucosamine (GlcNAc)that is abundantly present in nature and found as a major structural component in the fungal cell wall. In Chapter 1,the role of chitin as an important factor in the interaction between fungal pathogens and their plant hosts is discussed. As plants do not produce chitin, they evolved to recognize fungal chitin as a non-self molecule by plasma membrane receptors that can activate host immune responses to stop fungal growth.To overcome those host immune responses, fungal pathogens secrete effector molecules that manipulate host physiology, including immune responses, to support colonization. The chitin-binding Lysin motif (LysM) effector Ecp6 from the fungal tomato pathogen Cladosporium fulvumwas previously demonstrated to contribute to virulence through interfering with the activation of chitin-induced host immune responses. Subsequently, LysM effector genes were found in the genomes of many fungal species. In Chapter 2 we describe the functional characterization of LysM effectors of the plant pathogenic fungi Mycosphaerella graminicola, Magnaporthe oryzae and Colletotrichum higginsianum, which cause leaf blotch disease of wheat, rice blast disease and anthracnose disease on Brassicaceae, respectively. This functional analysis revealed that the ability to perturb chitin-induced immunity is conserved among LysM effectors of these fungal plant pathogens. In addition, two LysM effectors that are secreted by M. graminicolawere found to protect fungal hyphae against cell wall hydrolytic enzymes from plants, demonstrating that LysM effectors can contribute to virulence of fungal plant pathogens in multiple ways. The M. graminicola LysM effector Mg3LysM and C. fulvum Ecp6 both contain three LysM domains and show a high overall similarity. However, whereas Mg3LysM can protect fungal hyphae against plant-derived cell wall hydrolytic enzymes, Ecp6 does not have this capacity. Chapter 3describes a functional analysis of the contribution of LysM domains of Mg3LysM to its protection ability. To this end a series of chimeric proteins were produced in whichLysM domains of Mg3LysM were swapped with the corresponding LysM domain of Ecp6.Analysis of these chimeras indicated that protection against the hydrolytic activity of plant enzymes is mediated by the concerted activity of LysM1 and LysM3 in Mg3LysM. LysM effectors do not only occur in foliar fungal plant pathogens, but also in soil-borne pathogens that infect their host through the roots. In Chapter 4, LysM effectors of the fungal soil-borne vascular wilt pathogen Verticillium dahliaeare described. Comparative genomics of eleven V. dahliae strains revealed that four LysM effectors are found in the core genome, which are referred to as core VdLysM effectors. Intriguingly, for none of the core LysM effector genes expression could be monitored during host colonization, and targeted deletion could not reveal a role in virulence, suggesting that the core LysM effectors do not act as virulence factors during host colonization. In addition to the core genome, V. dahliaestrains generally carry lineage-specific (LS) genomic regions. Interestingly, an additional LysM effector gene (Vd2LysM) was found in an LS region of V. dahliaestrain VdLs17 that is absent in all other sequenced V. dahliaestrains. Remarkably, the LS effector Vd2LysM was found to contribute to virulence of strain VdLs17. Like the previously characterized plant pathogen LysM effectors, also Vd2LysM was found to bind chitin and suppress chitin-induced immune responses. These results indicate that Vd2LysM interferes with chitin-induced immunity during host colonization by V. dahliaestrain VdLs17. Thus far, LysM effectors were demonstrated to contribute to virulence of various fungal plant pathogens through their ability to interfere with host immune responses. However, the presence of LysM effector genes in the genomes of non-pathogenic fungi and fungi with a saprophytic lifestyle suggests that LysM effectors contribute to fungal physiology in other manners as well. In Chapter 5we investigated the hypothesis that LysM effectors play a role in the interaction of fungi with other microbes in the environment, which could even be relevant for plant pathogenic fungi that encounter other microbes at the site of host infection. To investigate this hypothesis, assays were developed that allow to assess the attachment and antagonistic effects of particular bacterial species on fungi by employing the fungus Trichoderma viride, as this species is known to have accessible cell wall chitin upon growth in vitro. Assays to assess bacterial attachment and antagonistic activity in the absence or presence of LysM effectors indicate that LysM effectors play a role in the protection of fungi against bacterial competitors. In Chapter6, the major results described in this thesis are discussed and a perspective on the (potential) roles of LysM effectors in fungi with different lifestyles, including pathogenic as well as non-pathogenic fungi, is presented.