No abstract

SUMMARYWhile diverse microbe-or damage-associated molecular patterns (MAMPs/DAMPs) typically trigger a common set of intracellular signalling events, comparative analysis between the MAMPs flg22 and elf18 revealed MAMP-specific differences in Ca 2+ signalling, defence gene expression and MAMP-mediated growth arrest in Arabidopsis thaliana. Such MAMP-specific differences are, in part, controlled by BAK1, a kinase associated with several receptors. Whereas defence gene expression and growth inhibition mediated by flg22 were reduced in bak1 mutants, BAK1 had no or minor effects on the same responses elicited by elf18. As the residual Ca 2+ elevations induced by diverse MAMPs/DAMPs (flg22, elf18 and Pep1) were virtually identical in bak1 mutants, a differential BAK1-mediated signal amplification to attain MAMP/DAMP-specific Ca 2+ amplitudes in wild-type plants may be hypothesized. Furthermore, abrogation of reactive oxygen species (ROS) accumulation, either in the rbohD mutant or through inhibitor application, led to loss of a second Ca 2+ peak, demonstrating a feedback effect of ROS on Ca 2+ signalling. Conversely, mpk3 mutants showed a prolonged accumulation of ROS but this did not significantly impinge on the overall Ca 2+ response. Thus, fine-tuning of MAMP/DAMP responses involves interplay between diverse signalling elements functioning both up-or downstream of Ca 2+ signalling.

Despite the fact that X-box binding protein-1 (XBP-1) is one of the main regulators of the unfolded protein response (UPR), the modulators of XBP-1 are poorly understood. Here, we show that the regulatory subunits of phosphotidyl inositol 3-kinase (PI3K), p85α (encoded by Pik3r1) and p85β (encoded by Pik3r2) form heterodimers that are disrupted by insulin treatment. This disruption of heterodimerization allows the resulting monomers of p85 to interact with, and increase the nuclear translocation of, the spliced form of XBP-1 (XBP-1s). The interaction between p85 and XBP-1s is lost in ob/ob mice, resulting in a severe defect in XBP-1s translocation to the nucleus and thus in the resolution of endoplasmic reticulum (ER) stress. These defects are ameliorated when p85α and p85β are overexpressed in the liver of ob/ob mice. Our results define a previously unknown insulin receptor signaling pathway and provide new mechanistic insight into the development of ER stress during obesity.The ER is a large membrane-enclosed cellular organelle in which secretory and membranebound proteins are folded into their final three-dimensional structures, lipids and sterols are synthesized, and free calcium is stored 1,2 . Conditions that interfere with proper functioning of the ER create a state defined as ER stress and lead to activation of the UPR3-5.The UPR is conveyed to the cell through three main signaling pathways. The first two pathways are initiated by type I transmembrane kinases, PKR-like endoplasmic reticulum kinase (PERK) and inositol requiring enzyme-1 (IRE1), and the third pathway launches with activation of a type II transmembrane protein called activating transcription factor-6 (ATF6) [1][2][3][4][5] . Activation of PERK during ER stress leads to phosphorylation of eukaryotic translation initiation factor-2α at Ser51 and consequently results in global translational attenuation 6,7 . IRE1, on the other hand, has both kinase and endoRNase activity [8][9][10][11] . The endoRNase domain of IRE1 splices the mRNA of a transcription factor called X-boxbinding protein-1 (XBP-1), removing a 26-bp segment from the full-length XBP-1 messenger RNA that creates a translational frame shift leading to the expression of a highermolecular-weight protein, XBP-1s 12-14 .

The sensing of microbe-associated molecular patterns (MAMPs) triggers innate immunity in animals and plants. Lipopolysaccharide (LPS) from Gram-negative bacteria is a potent MAMP for mammals, with the lipid A moiety activating proinflammatory responses via Toll-like receptor 4 (TLR4). Here we found that the plant Arabidopsis thaliana specifically sensed LPS of Pseudomonas and Xanthomonas. We isolated LPS-insensitive mutants defective in the bulb-type lectin S-domain-1 receptor-like kinase LORE (SD1-29), which were hypersusceptible to infection with Pseudomonas syringae. Targeted chemical degradation of LPS from Pseudomonas species suggested that LORE detected mainly the lipid A moiety of LPS. LORE conferred sensitivity to LPS onto tobacco after transient expression, which demonstrated a key function in LPS sensing and indicated the possibility of engineering resistance to bacteria in crop species.