A PDLP-NHL3 complex integrates plasmodesmal immune signaling cascades

Abstract

The plant immune system relies on the perception of molecules that signal the presence of a microbe threat. This triggers signal transduction that mediates a range of cellular responses via a collection of molecular machinery including receptors, small molecules, and enzymes. One response to pathogen perception is the restriction of cell-to-cell communication by plasmodesmal closure. We previously found that while chitin and flg22 trigger specialized immune signaling cascades in the plasmodesmal plasma membrane, both execute plasmodesmal closure via callose synthesis at the plasmodesmata. Therefore, the signaling pathways ultimately converge at or upstream of callose synthesis. To establish the hierarchy of signaling at plasmodesmata and characterize points of convergence in microbe elicitor-triggered signaling, we profiled the dependence of plasmodesmal responses triggered by different elicitors on a range of plasmodesmal signaling machinery. We identified that, like chitin, flg22 signals via RESPIRATORY BURST OXIDASE HOMOLOGUE D (RBOHD) to induce plasmodesmal closure. Further, we found that PLASMODESMATA-LOCATED PROTEIN 1 (PDLP1), PDLP5, and CALLOSE SYNTHASE 1 (CALS1) are common to microbe- and salicylic acid (SA)-triggered responses, identifying PDLPs as a candidate signaling nexus. To understand how PDLPs relay a signal to CALS1, we screened for PDLP5 interactors and found NON-RACE SPECIFIC DISEASE RESISTANCE/HIN1 HAIRPIN-INDUCED-LIKE protein 3 (NHL3), which is also required for chitin-, flg22- and SA-triggered plasmodesmal responses and PDLP-mediated activation of callose synthesis. We conclude that a PDLP–NHL3 complex acts as an integrating node of plasmodesmal signaling cascades, transmitting multiple immune signals to activate CALS1 and plasmodesmata closure.

Role of PMR4 and PDLP1 in priming of early acting penetration defense by resistance-inducing β-amino acids

“…Interestingly, PDLP1 and PDLP5 have recently been shown to be part of a protein complex with the immune regulatory protein NDR1/HIN1-LIKE 3 (NHL3), which mediates CalS1-dependent callose and defense-related plasmodesmatal closure. 41 However, PDLP5, unlike PDLP1, did not co-localize with Hpa haustoria, while NHL3 was not identified as a direct interactor with PDLP1, suggesting that PDLP1 acts through a different signaling complex to regulate Hpa -induced callose. 22 Our finding that BABA primes PDLP1 localization at the onset of PMR4-dependent penetration resistance provides further evidence for a specific defense regulatory role of this protein in the Arabidopsis- Hpa interaction.…”

Plasmodesmal closure elicits stress responses

“…Plasmodesmal closure is an early reaction, initiated within 30 minutes upon perceiving pathogen-derived molecules and stress-associated signals including SA and hydrogen peroxide (Xu et al 2017;Cheval et al 2020;Tee et al 2023b), and is critical for the execution of a full response. Thus, we reason that plasmodesmal closure regulates a range of downstream processes.…”

“…The decrease in plasmodesmal aperture in response to stress is a process activated by signaling cascades mediated by specific machinery. For example, LYSM-CONTAINING GPI-ANCHORED PROTEIN 2 (LYM2; Faulkner et al 2013) and CALMODULIN-LIKE 41 (CML41;Xu et al 2017) specifically mediate plasmodesmal responses to the microbial elicitors chitin and flg22 respectively, and 3 PLASMODESMATA LOCATED PROTEIN 5 (PDLP5) integrates these responses, as well as plasmodesmal responses triggered by the defense hormone salicylic acid (SA; Wang et al 2013;Tee et al 2023b). The observation that lym2 mutants cannot close their plasmodesmata in response to chitin but execute normal chitin-triggered mitogenactivated protein kinase (MAPK) activation and apoplastic ROS production (Faulkner et al 2013) indicates that plasmodesmal signaling cascades can act independently of other responses.…”

Distributing Plant Developmental Regulatory Proteins via Plasmodesmata

“…Several family members are typically encoded in a plant genome, and different members can have a tissue or context-specific role at plasmodesmata [ 68 , 72 , 73 , 74 ]. Several accessory proteins with regulatory functions have been identified that can integrate internal and external cues and relay them toward enzymes affecting callose homeostasis [ 68 , 75 ]. Such changes typically take place on the timescale of minutes.…”