Plant species-specific recognition of long and short ß-1,3-linked glucans is mediated by different receptor systems.

Plants survey their environment for the presence of potentially harmful or beneficial microbes. During colonization, cell surface receptors perceive microbe-derived or modified-self ligands and initiate appropriate responses. The recognition of fungal chitin oligomers and the subsequent activation of plant immunity are well described. In contrast, the mechanisms underlying ß-glucan recognition and signaling activation remain largely unexplored. Here, we systematically tested immune responses towards different ß-glucan structures and show that responses vary between plant species. While leaves of the monocots Hordeum vulgare and Brachypodium distachyon can recognize longer (laminarin) and shorter (laminarihexaose) ß-1,3-glucans with responses of varying intensity, duration and timing, leaves of the dicot Nicotiana benthamiana activate immunity in response to long ß-1,3-glucans, whereas Arabidopsis thaliana and Capsella rubella perceive short ß-1,3-glucans. Hydrolysis of the ß-1,6 side-branches of laminarin demonstrated that not the glycosidic decoration but rather the degree of polymerization plays a pivotal role in the recognition of long-chain ß-glucans. Moreover, in contrast to the recognition of short ß-1,3-glucans in A. thaliana, perception of long ß-1,3-glucans in N. benthamiana and rice is independent of CERK1, indicating that ß-glucan recognition may be mediated by multiple ß-glucan receptor systems.

The receptor kinase FER is a RALF-regulated scaffold controlling plant immune signaling.

In plants, perception of invading pathogens involves cell-surface immune receptor kinases. Here, we report that the Arabidopsis SITE-1 PROTEASE (S1P) cleaves endogenous RAPID ALKALINIZATION FACTOR (RALF) propeptides to inhibit plant immunity. This inhibition is mediated by the malectin-like receptor kinase FERONIA (FER), which otherwise facilitates the ligand-induced complex formation of the immune receptor kinases EF-TU RECEPTOR (EFR) and FLAGELLIN-SENSING 2 (FLS2) with their co-receptor BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1 (BAK1) to initiate immune signaling. We show that FER acts as a RALF-regulated scaffold that modulates receptor kinase complex assembly. A similar scaffolding mechanism may underlie FER function in other signaling pathways.

Verticillium dahliae LysM effectors differentially contribute to virulence on plant hosts.

Chitin-binding lysin motif (LysM) effectors contribute to the virulence of various plant-pathogenic fungi that are causal agents of foliar diseases. Here, we report the LysM effectors of the soil-borne fungal vascular wilt pathogen Verticillium dahliae. Comparative genomics revealed three core LysM effectors that are conserved in a collection of V. dahliae strains. Remarkably, and in contrast with the previously studied LysM effectors of other plant pathogens, no expression of core LysM effectors was monitored in planta in a taxonomically diverse panel of host plants. Moreover, targeted deletion of the individual LysM effector genes in V. dahliae strain JR2 did not compromise virulence in infections on Arabidopsis, tomato or Nicotiana benthamiana. Interestingly, an additional lineage-specific LysM effector is encoded in the genome of V. dahliae strain VdLs17, but not in any other V. dahliae strain sequenced to date. Remarkably, this lineage-specific effector is expressed in planta and contributes to the virulence of V. dahliae strain VdLs17 on tomato, but not on Arabidopsis or N. benthamiana. Functional analysis revealed that this LysM effector binds chitin, is able to suppress chitin-induced immune responses and protects fungal hyphae against hydrolysis by plant hydrolytic enzymes. Thus, in contrast with the core LysM effectors of V. dahliae, this lineage-specific LysM effector of strain VdLs17 contributes to virulence in planta.