Chitin perception in plasmodesmata identifies subcellular, context-specific immune signalling in plants

Abstract

14The plasma membrane (PM) that lines plasmodesmata has a distinct protein and lipid 15 composition, underpinning specific regulation of these connections between cells. The 16 plasmodesmal PM can integrate extracellular signals differently from the cellular PM, but it 17 is not known how this specificity is established or how a single stimulus can trigger 18 independent signalling cascades in neighbouring membrane domains. Here we have used the 19 fungal elicitor chitin to investigate signal integration and responses at the plasmodesmal PM. 20 We found that the plasmodesmal PM employs a receptor complex composed of the LysM 21 receptors LYM2 and LYK4 which respectively change their location and interactions in 22 response to chitin. Downstream, signalling is transmitted via a specific phosphorylation 23 signature of an NADPH oxidase and localised callose synthesis that causes plasmodesmata 24 closure. This demonstrates the plasmodesmal PM deploys both plasmodesmata-specific 25 components and differential activation of PM-common components to independently 26 integrate an immune signal. 27 28 93that plasmodesmata are regulated independently of other immune responses, suggesting that 94 there is a critical requirement for a cell to finely tune connectivity to its neighbours. 95 4 Results 96Chitin-triggered plasmodesmata closure is dependent on LYK4 and LYK5 97 We previously identified that LYM2 is a GPI-anchored, LysM receptor protein that is 98 resident in the plasmodesmal PM (Faulkner et al., 2013). As LYM2 has no intracellular 99 domains we reasoned that it must interact with other proteins to initiate downstream signals 100 that result in plasmodesmal responses. Ligand perception by LysM RKs and RPs often 101 involves multiple members of the LysM protein family: chitin perception in rice involves 102 both the RP CHITIN ELICITOR BINDING PROTEIN (OsCEBiP) and the RK CHITIN 103 ELICITOR RECEPTOR KINASE 1 (OsCERK1) (Kaku et al., 2006; Hayafune et al., 2014); 104 peptidoglycan perception in Arabidopsis involves the RK CERK1, and RPs LYM1 and 105 LYM3 (Willmann et al., 2011); and PM chitin perception in Arabidopsis involves CERK1 106 (also called LYK1) and the RKs LYK4 and LYK5 (Cao et al., 2014). Thus, we hypothesised 107 that LYM2 might partner with a LysM RK for signalling. The Arabidopsis LysM RK family 108 consists of 5 members: CERK1/LYK1, LYK2, LYK3, LYK4 and LYK5. To narrow down 109 plasmodesmata signalling candidates we screened publicly available data sets for LYK gene 110 expression. Comparing data sets from seedlings (GSE74955, Yamada et al., 2016; 111 GSE78735, Hillmer et al., 2017) and mature leaves (eFP browser, Winter et al. 2007) we 112 identified variable expression patterns for the LYK family members (Fig. S1). Thus, we 113 performed RT-PCR to identify members of the family expressed in mature Arabidopsis 114 leaves where we assay for and detect LYM2 function. Only transcripts from CERK1, LYK3, 115LYK4 and LYK5 were detected in mature leaves grown in our conditions, eliminating LYK2 116 f...

Dare to change, the dynamics behind plasmodesmata-mediated cell-to-cell communication

“…Stimuli-dependant re-organisations of receptor-like activities at plasmodesmata are therefore frequent events and may in fact be a common strategy to modulate symplastic trafficking. In a similar fashion, the immune fungal elicitor chitin induces redistribution of the chitin-receptor complex at both PM and plasmodesmata [14]. Upon chitin sensing at the PM, the CHITIN ELICITOR RECEPTOR KINASE 1 (CERK1) interacts with LysM receptor-like kinase LYK5, which initiates intracellular defence responses [14,17].…”

“…In a similar fashion, the immune fungal elicitor chitin induces redistribution of the chitin-receptor complex at both PM and plasmodesmata [14]. Upon chitin sensing at the PM, the CHITIN ELICITOR RECEPTOR KINASE 1 (CERK1) interacts with LysM receptor-like kinase LYK5, which initiates intracellular defence responses [14,17]. Simultaneously, LYSIN MOTIF DOMAIN CONTAINING GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PROTEIN (LYM2) accumulates to plasmodesmata, where it associates with LYK4 to promote localised callose synthesis [14].…”

The Role of Plasmodesmata-Associated Receptor in Plant Development and Environmental Response

“…LYM2 locates to PM and PD but the mechanism of LYM2-mediated plasmodesmal closure remains unknown. Recently, Faulkner's group provided the evidence that LYK4 and LYK5 (LysM-CONTAINING RECEPTOR-LIKE KINASE4 and 5, respectively) were also involved in response to chitin-triggered plasmodesmal closure, raising the question of how LYM2, LYK4, and LYK5 integrate to regulate PD permeability in response to chitin [43]. However, based on the subcellular localization study, LYK5 and LYK4 are mainly localized to PM at the steady condition and only LYK4 is strongly accumulated at PD-PM in the presence of chitin.…”

“…Typical PD-RLPs contain the unique extracellular ligand-binding domain, a single transmembrane domain and a short cytoplasmic tail [21,36,42]. In another case, a PD-RLP uses a glycosylphosphatidylinositol (GPI) anchor to attach the extracellular membrane instead of transmembrane domain [23,43,44]. Meanwhile, PD-RLKs carry out an extracellular domain, a single transmembrane domain, and an intracellular kinase domain [32,43].…”

“…In another case, a PD-RLP uses a glycosylphosphatidylinositol (GPI) anchor to attach the extracellular membrane instead of transmembrane domain [23,43,44]. Meanwhile, PD-RLKs carry out an extracellular domain, a single transmembrane domain, and an intracellular kinase domain [32,43]. In general, extracellular domains of RLKs/RLPs recognize the ligands with high specificity and selectivity; they subsequently modulate the activation of the cytoplasmic kinase domain and the Table 1.…”