Fungal LysM Effector Functions
The plant immune system recognizes fungal invaders through recognition of fungal cell wall chitin, leading to the activation of immune responses that comprise the release of various types of antimicrobials, including chitinases to hydrolyze chitin. In turn, many fungal pathogens have been shown to secrete effectors that comprise carbohydrate-binding lysin motifs, known as LysM effectors, that can scavenge chitin oligomers to prevent chitin recognition by host plant cells. How this can work mechanistically was shown for the LysM effector Ecp6 that is secreted by the tomato leaf mold fungus Cladosporium fulvum. Two of its three LysMs are able to undergo a chitin-inducible intrachain LysM dimerization, resulting an ultrahigh-affinity chitin binding groove that competitively sequesters chitin oligomers from host immune receptors.
Crystal structure model of a monomer of the Cladosporium fulvum LysM effector Ecp6 with the three LysMs in three shades of blue with the chitin-binding loops shown in red. A chitin tetramer (green sticks) is tightly bound between LysM1 and LysM3 (figure adapted from Sánchez-Vallet et al. 2013).
However, not all LysM effectors display the ability to suppress chitin-triggered immunity in plant hosts. Rather, particular LysM effectors protect fungal hyphae against chitinase hydrolysis during host colonization. A molecular mechanism underpinning the protection of fungal cell walls against hydrolysis was recently proposed. The LysM effector Mg1LysM of the fungal wheat pathogen Zymoseptoria tritici was shown to simultaneously undergo chitin-induced oligomerization of chitin-independent Mg1LysM homodimers, leading to a supramolecular structure that confers protection to fungal cell walls against host chitinases.
Picture 1: Microscopic pictures of fungus grown in vitro in the absence (H2O) or presence of Mg1LysM in the absence (H2O) or presence of tomato hydrolytic enzymes (HE) that include chitinases, showing that Mg1LysM protects hyphae against degradation by chitinases.
Picture 2: Model for a chitin-induced polymeric structure of Mg1LysM homodimers based on a Mg1LysM crystal structure. Mg1LysM homodimers, shown with Mg1LysM monomers in red and green, polymerize in a substrate-dependent manner induced by chitin, shown as grey sticks (figure adapted from Sánchez-Vallet et al. 2020).
Interestingly, most fungal species contain LysM effector genes in their genomes. Thus, besides fungal plant pathogens, also fungal mutualists of plants such as arbuscular mycorrhizal fungi, but also pathogens of other taxa such as human and animal pathogens, as well as non-pathogenic species such as decay fungi, contain LysM effector genes. This finding suggests that LysM effectors play other roles besides dealing with host immune responses. Potentially, fungal LysM effectors also act in interactions with microbiota co-inhabitants.
LysM effectors may play roles during host colonization, as well as in other ecological niches (figure taken from Kombrink & Thomma 2013).
Most relevant literature:
Tian et al. (2021). Three LysM effectors of Zymoseptoria tritici collectively disarm chitin-triggered plant immunity. Molecular Plant Pathology (doi: 10.1111/mpp.13055)
Zeng et al. (2020). A lysin motif effector subverts chitin-triggered immunity to facilitate arbuscular mycorrhizal symbiosis. New Phytologist 225: 448-460.
Sánchez-Vallet et al. (2020). A secreted LysM effector protects fungal hyphae through chitin-dependent homodimer polymerization. PLoS Pathogens 16: e1008652.
Kombrink et al. (2017). Verticillium dahliae LysM effectors differentially contribute to virulence on plant hosts. Molecular Plant Pathology 18: 596-608.
Rovenich H, Zuccaro A, Thomma BPHJ (2016). Convergent evolution of filamentous microbes towards evasion of glycan-triggered immunity. New Phytologist 212: 896-901.
Takahara et al. (2016). Colletotrichum higginsianum extracellular LysM proteins play dual roles in appressorial function and suppression of chitin-triggered plant immunity. New Phytologist 211: 1323-1337.
Sánchez-Vallet et al. (2015). The battle for chitin recognition in plant-microbe interactions. FEMS Microbiology Reviews 39: 171-183.
Sánchez-Vallet et al. (2013). Fungal effector Ecp6 outcompetes host immune receptor for chitin binding through intrachain LysM dimerization. eLife 2: e00790.
Kombrink & Thomma (2013) LysM effectors: secreted proteins supporting fungal life. PLoS Pathogens 9: e1003769.
Mentlak et al. (2012). Effector-mediated suppression of chitin-triggered immunity by Magnaporthe oryzae is necessary for rice blast disease. The Plant Cell 24: 322-335.
Marshall et al. (2011). Analysis of two in planta expressed LysM effector homologs from the fungus Mycosphaerella graminicola reveals novel functional properties and varying contributions to virulence on wheat. Plant Physiology 156: 756-769.