Plant and animal intracellular nucleotide-binding, leucine-rich repeat (NLR) immune receptors detect pathogen-derived molecules and activate defense. Plant NLRs can be divided into several classes based upon their N-terminal signaling domains, including TIR (Toll-like, Interleukin-1 receptor, Resistance protein)- and CC (coiled-coil)-NLRs. Upon ligand detection, mammalian NAIP and NLRC4 NLRs oligomerize, forming an inflammasome that induces proximity of its N-terminal signaling domains. Recently, a plant CC-NLR was revealed to form an inflammasome-like hetero-oligomer. To further investigate plant NLR signaling mechanisms, we fused the N-terminal TIR domain of several plant NLRs to the N terminus of NLRC4. Inflammasome-dependent induced proximity of the TIR domain in planta initiated defense signaling. Thus, induced proximity of a plant TIR domain imposed by oligomerization of a mammalian inflammasome is sufficient to activate authentic plant defense. Ligand detection and inflammasome formation is maintained when the known components of the NLRC4 inflammasome is transferred across kingdoms, indicating that NLRC4 complex can robustly function without any additional mammalian proteins. Additionally, we found NADase activity of a plant TIR domain is necessary for plant defense activation, but NADase activity of a mammalian or a bacterial TIR is not sufficient to activate defense in plants.

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 & 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 (

The receptors for the neuropeptides vasoactive intestinal polypeptide and pituitary adenylate cyclase-activating polypeptide are strong activators of adenylate cyclase, but recent evidence suggests that they can elicit a number of additional intracellular signals. Some of these are likely to be downstream of the conventional adenylate cyclase pathway, but it is now clear that others reflect novel primary coupling events of the receptors.

A BSTRACT : To investigate the phospholipase D (PLD) responses of the VIP/PACAP receptors, VPAC 1 and VPAC 2 , and the PACAP-specific PAC 1 receptors (short and "hop" intracellular loop 3 (i3) splice variants), stable CHO cell lines expressing similar levels of each wildtype receptor were generated (except for the VPAC 1 receptor clone which showed considerably lower expression and lesser responses in signalling assays). All clones caused activation of PLD in response to agonists, as monitored by [ 3 H]phosphatidylbutanol production. The PLD responses of the PAC 1 "hop", but not the "null" receptor, were sensitive to the ARF inhibitor, brefeldin A (BFA) (as were VPAC 1 and VPAC 2 responses). Chimeric constructs of VPAC 2 receptors containing i3 of either PAC 1 hop or PAC 1 null receptors were transiently expressed in COS 7 cells and PLD responses were measured. Only the PLD response of the hop construct was sensitive to BFA. This suggests that i3 motifs in certain Group II GPCRs may play a key role in determining their linkage to ARF-dependent PLD activation.

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