A PDLP-NHL3 complex integrates plasmodesmal immune signaling cascades

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. This indicates 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 RBOHD to induce plasmodesmal closure. Further, we found that PDLP1, PDLP5 and CALS1 are common to microbe- and 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 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.

植物免疫系统依赖于对标记微生物威胁存在的分子的识别。该识别过程会触发信号转导，通过包含受体、小分子与酶在内的一系列分子机器介导一系列细胞应答。针对病原体识别的一类典型应答是通过胞间连丝(plasmodesmata)关闭来抑制细胞间通讯。我们此前的研究发现，尽管几丁质(chitin)与flg22可在胞间连丝质膜中触发特异性免疫信号级联反应，但二者均通过在胞间连丝处的胼胝质(callose)合成来实现胞间连丝关闭。这表明相关信号通路最终汇聚于胼胝质合成环节或其上游区域。为明确胞间连丝处的信号层级，并表征微生物激发子触发的信号转导中的汇聚位点，我们分析了不同激发子触发的胞间连丝应答对一系列胞间连丝信号转导机器的依赖性。研究发现，与几丁质类似，flg22通过呼吸爆发氧化酶同源物D(RBOHD)信号通路诱导胞间连丝关闭。进一步研究显示，PDLP1、PDLP5与CALS1是微生物与水杨酸(salicylic acid, SA)触发的应答所共有的核心组分，由此确定PDLPs可作为候选信号枢纽。为阐明PDLPs如何将信号传递至CALS1，我们筛选了PDLP5的互作蛋白，发现了NHL3，其同样是几丁质、flg22与水杨酸触发的胞间连丝应答以及PDLP介导的胼胝质合成激活所必需的蛋白。我们最终得出结论：PDLP-NHL3复合物可作为胞间连丝信号级联反应的整合节点，传递多种免疫信号以激活CALS1并介导胞间连丝关闭。