Abstract
Chitin is a major structural component of fungal cell walls and acts as a microbe‐associated molecular pattern (MAMP) that, on recognition by a plant host, triggers the activation of immune responses. To avoid the activation of these responses, the Septoria tritici blotch (STB) pathogen of wheat, Zymoseptoria tritici, secretes LysM effector proteins. Previously, the LysM effectors Mg1LysM and Mg3LysM were shown to protect fungal hyphae against host chitinases. Furthermore, Mg3LysM, but not Mg1LysM, was shown to suppress chitin‐induced reactive oxygen species (ROS) production. Whereas initially a third LysM effector gene was disregarded as a presumed pseudogene, we now provide functional data to show that this gene also encodes a LysM effector, named Mgx1LysM, that is functional during wheat colonization. While Mg3LysM confers a major contribution to Z. tritici virulence, Mgx1LysM and Mg1LysM contribute to Z. tritici virulence with smaller effects. All three LysM effectors display partial functional redundancy. We furthermore demonstrate that Mgx1LysM binds chitin, suppresses the chitin‐induced ROS burst, and is able to protect fungal hyphae against chitinase hydrolysis. Finally, we demonstrate that Mgx1LysM is able to undergo chitin‐induced polymerization. Collectively, our data show that Z. tritici utilizes three LysM effectors to disarm chitin‐triggered wheat immunity.
Highlights
Plants deploy an effective innate immune system to recognize and appropriately respond to microbial invaders
An important part of this immune system involves the recognition of conserved microbe-associated molecular patterns (MAMPs) that are recognized by cell surface-localized pattern recognition receptors (PRRs) to activate pattern-triggered immunity (PTI) (Cook et al, 2015; Jones and Dangl, 2006; Thomma et al, 2001)
We show that the ∆Mgx[1] strains caused similar levels of necrosis as the WT and ∆Mg1 strains, and that the ∆Mg1-∆Mgx[1] strains shows no apparent decrease in disease development, suggesting that these two lysin motifs (LysMs) effectors are dispensable for virulence of Z
Summary
Plants deploy an effective innate immune system to recognize and appropriately respond to microbial invaders. An important part of this immune system involves the recognition of conserved microbe-associated molecular patterns (MAMPs) that are recognized by cell surface-localized pattern recognition receptors (PRRs) to activate pattern-triggered immunity (PTI) (Cook et al, 2015; Jones and Dangl, 2006; Thomma et al, 2001). PTI includes a broad range of immune responses, such as the production of reactive oxygen species (ROS), ion fluxes, callose deposition and defence-related gene expression (Altenbach and Robatzek, 2007; Boller and Felix, 2009; Jones and Dangl, 2006). Plants secrete hydrolytic enzymes, such as chitinases, as an immune response to target fungal cell wall chitin in order to disrupt cell wall integrity, and to release chitin molecules that act as a MAMP that can be recognized by PRRs that carry extracellular lysin motifs (LysMs) to activate further immune responses against fungal invasion (Felix et al, 1993; Kombrink and Thomma, 2013; Sánchez-Vallet et al, 2015). Homologs of the crucial components of these complexes have been identified in wheat (Lee et al, 2014)
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