Analysis of Two in Planta Expressed LysM Effector Homologs from the FungusMycosphaerella graminicolaReveals Novel Functional Properties and Varying Contributions to Virulence on Wheat    

Abstract

Secreted effector proteins enable plant pathogenic fungi to manipulate host defenses for successful infection. Mycosphaerella graminicola causes Septoria tritici blotch disease of wheat (Triticum aestivum) leaves. Leaf infection involves a long (approximately 7 d) period of symptomless intercellular colonization prior to the appearance of necrotic disease lesions. Therefore, M. graminicola is considered as a hemibiotrophic (or necrotrophic) pathogen. Here, we describe the molecular and functional characterization of M. graminicola homologs of Ecp6 (for extracellular protein 6), the Lysin (LysM) domain-containing effector from the biotrophic tomato (Solanum lycopersicum) leaf mold fungus Cladosporium fulvum, which interferes with chitin-triggered immunity in plants. Three LysM effector homologs are present in the M. graminicola genome, referred to as Mg3LysM, Mg1LysM, and MgxLysM. Mg3LysM and Mg1LysM genes were strongly transcriptionally up-regulated specifically during symptomless leaf infection. Both proteins bind chitin; however, only Mg3LysM blocked the elicitation of chitin-induced plant defenses. In contrast to C. fulvum Ecp6, both Mg1LysM and Mg3LysM also protected fungal hyphae against plant-derived hydrolytic enzymes, and both genes show significantly more nucleotide polymorphism giving rise to nonsynonymous amino acid changes. While Mg1LysM deletion mutant strains of M. graminicola were fully pathogenic toward wheat leaves, Mg3LysM mutant strains were severely impaired in leaf colonization, did not trigger lesion formation, and were unable to undergo asexual sporulation. This virulence defect correlated with more rapid and pronounced expression of wheat defense genes during the symptomless phase of leaf colonization. These data highlight different functions for MgLysM effector homologs during plant infection, including novel activities that distinguish these proteins from C. fulvum Ecp6.

A fungal avirulence factor encoded in a highly plastic genomic region triggers partial resistance to septoria tritici blotch

“…It is possible that the role of Avr3D1 is more pronounced under field conditions or at different developmental stages, for example in adult plants. An additional hypothesis to explain the apparent dispensability of Avr3D1 is that functional redundancy masks phenotypic effects in the knockout mutants (Marshall et al ., 2011; Win et al ., 2012; Mirzadi Gohari et al ., 2015; Rudd et al ., 2015). …”

“…The genetic basis of Z. tritici virulence is poorly understood as a result of its largely quantitative nature (Hartmann et al ., 2017; Stewart et al ., 2018). Two highly conserved lysin motif (LysM) effectors, Mg1LysM and Mg3LysM, prevent fungal recognition and shield the fungal cell wall from degradation by host hydrolytic enzymes (Marshall et al ., 2011). The other known effectors of Z. tritici , Zt80707, AvrStb6 and Zt_8_609, are rapidly evolving small secreted proteins (Poppe et al ., 2015; Hartmann et al ., 2017; Zhong et al ., 2017; Kema et al ., 2018).…”

Chitin and chitin-related compounds in plant–fungal interactions

“…However, functional homologs of Ecp6 were found in Zymoseptoria tritici (Mg3LysM) and Magnaporthe oryzae (Slp1) (De Jonge et al 2010; Marshall et al 2011; Mentlak et al 2012). Interestingly, the Mg3LysM effector of Z. tritici unites the abilities of both Avr4 and Ecp6 to sequester chitin oligomers and protect fungal cells from hydrolysis (Marshall et al 2011). …”

Purification, cDNA cloning, and characterization of LysM-containing plant chitinase from horsetail (Equisetum arvense)

“…The LysM domain-containing effector protein MgLysM of fungus Mycosphaerella graminicola blocked the elicitation of chitin-induced plant defenses and also protected fungal hyphae against plant-derived hydrolytic enzymes. 35) EaChiA, having no antifungal activity, but a strong chitin-binding activity and high homology to fungal LysM protein, may contribute to symbiosis with mycorrhizal fungi by preventing plant defense response through binding to the chitin in the fungal cell walls and protect them against other chitinases. Chitinase-active fractions without EaChiA were obtained from the chitinase purification process in this study ( Supplementary Fig.…”