Herbivore-associated elicitors: FAC signaling and metabolism Gustavo Bonaventure, Arjen VanDoorn, Ian T. Baldwin  Trends in Plant Science  Volume 16, Issue 6, Pages 294-299 (June 2011) DOI: 10.1016/j.tplants.2011.01.006 Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure 1 Schematic representation of the systems used by plants to perceive and transduce signals generated by microbes and folivorous insects. In Arabidopsis, the FLS2 receptor for flagellin binds the BIK1 kinase and upon flagellin binding it, associates with BAK1 and releases BIK1. The BAK1/FLS2 complex activates two MAPK cascades and thereby downstream WRKY transcription factors to regulate gene expression. Calcium-dependent protein kinases (CDPKs) and their dependent TFs are also activated upon flagellin perception. Ca2+ influx also causes activation of NADPH oxidases and production of reactive oxygen species (ROS; e.g. H2O2). NPR1 interacts with bZIP transcription factors to regulate the expression of WRKY genes and WRKYs in turn regulate the expression of the NPR1 gene. In tomato, the Avr9 effector interacts via the HABS (high affinity binding site) with the Cf-9 receptor activating the influx of Ca2+, NADPH oxidases and production of ROS, CDPKs and MAPKs (including SIPK and WIPK). Activation of these protein kinases leads to the activation of TFs and gene expression. In wild tobacco (N. attenuata), the perception of FACs occurs via unknown mechanisms but it has been shown that volicitin binds to a proteinaceous fraction from membranes suggesting a membrane-bound receptor type of interaction. FAC perception leads to the activation of MAPK cascades (including SIPK and WIPK) and CDPKs. Herbivore folivory induces the generation of ROS and Ca2+ influx. WRKY3, WRKY6 and NPR1 play central roles in the orchestration of responses against herbivores in N. attenuata. Trends in Plant Science 2011 16, 294-299DOI: (10.1016/j.tplants.2011.01.006) Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure 2 New insights into FAC signaling in N. attenuata leaves. Feeding by M. sexta larvae on N. attenuata leaves deposits FACs on the wound surface which are perceived by still unknown cellular components (red circle with question mark). One of the major FACs, 18:3-Glu, in M. sexta OS is rapidly metabolized at the wound surface to generate oxygenated and other derivatives [27]. Lipoxygenase 2 (LOX2), a chloroplast localized enzyme, comes into contact with 18:3-Glu upon tissue disruption and oxygenates 18:3-Glu to form active (13-oxo-18:3-Glu) and inactive (13-OH-18:3-Glu) elicitors of some defense responses [34]. Perception of FACs and/or its modified forms triggers specific signaling events such as the differential activation of the MAPKs SIPK and WIPK together with NPR1. Activation of SIPK and NPR1 induces the release of 18:3 from membrane glycerolipids via the activation of GLA1 and the rapid conversion of this fatty acid into 13S-OOH-18:3 by LOX3 [18]. This hydroperoxide is rapidly used for the synthesis of OPDA by AOS and AOC, a conversion stimulated by activation of WIPK [24]. Perception of FACs also induces the specific activation of gene expression with a group of genes only activated when wounds come in contact with FACs but not in their absence (i.e. wounding alone). A substantial fraction of these genes correspond to putative regulatory components such as transcription factors and protein kinases [33] and might correspond to regulators of early responses against M. sexta caterpillars. SIPK and WIPK also contribute to the induction of some of these FAC responsive genes (Gilardoni, Baldwin and Bonaventure; unpublished results). A second group of genes is induced by mechanical damage and the induction heightened by FACs, probably via the enhanced JA production after this stimulus. Abbreviations: AEPs, Avr-9 rapidly elicited proteins; C6 and C12: 6- and 12-carbon derivatives of the HPL pathway; HPL, hydroperoxide lyase; JA, jasmonic acid; MT, monoterpenes; OPDA, (9S,13S)-12-oxo-phytodienoic acid. Trends in Plant Science 2011 16, 294-299DOI: (10.1016/j.tplants.2011.01.006) Copyright © 2011 Elsevier Ltd Terms and Conditions