Fungal LysM effectors: more than pathogen tools for host modulation?

Filamentous fungal effector biology has been studied as a mechanism to deregulate the immune response during colonisation of a host. During plant colonisation, LysM effectors like Mg3LysM from the wheat pathogen Mycosphaerella graminicola or Ecp6 from the tomato pathogen Cladosporium fulvum have been shown to block Chitin Triggered Immunity (CTI) through different strategies: while Mg3LysM binds chitin and protects the cell wall against chitinases, Ecp6 sequesters chitin fragments with ultrahigh affinity to avoid recognition. In nature, nonetheless, filamentous fungi also interact with a complex community of microorganisms inside and outside their host. Recent data revealed that the fungal LysM effector Mg1LysM from M. graminicola not also binds chitin monomers but it can bind N-Acetylmuramic acid as well, a bacterial cell wall component. In tests in vitro, MgLysMs have the capacity to prevent bacterial attachment to the hyphae, while conferring protection to the cell wall towards chitinases. Additionally, Mg1LysM protein prevents degradation of the hyphae in presence of antagonistic bacteria in vitro. This data suggest that LysM effectors may play a role in interaction with (antagonistic) bacteria protecting the cell wall against chitinases secreted by antagonist microbes or by scavenging chitin to avoid attraction of antagonistic microorganisms. Considering that around 300 LysM effector proteins have been predicted in more than 70 fungal species with different lifestyles including pathogens, endophytes, and saprophytes; it is interesting to explore what functions LysM effector proteins may have during interactions with other microorganisms in different environments.