A Pseudomonas syringae Effector Inactivates MAPKs to Suppress PAMP-Induced Immunity in Plants

Abstract

Pathogen-associated molecular patterns (PAMPs) elicit basal defense responses in plants, and, in turn, pathogens have evolved mechanisms to overcome these PAMP-induced defenses. To suppress immunity, the phytopathogenic bacterium Pseudomonas syringae secretes effector proteins, the biochemical function and virulence targets of which remain largely unknown. We show that HopAI1, an effector widely conserved in both plant and animal bacterial pathogens, inhibits the Arabidopsis mitogen-activated protein kinases (MAPKs) activated by exposure to PAMPs. HopAI1 inactivates MAPKs by removing the phosphate group from phosphothreonine through a unique phosphothreonine lyase activity, which is required for HopAI1 function. The inhibition of MAPKs by HopA1 suppresses two independent downstream events, namely the reinforcement of cell wall defense and transcriptional activation of PAMP response genes. The MAPKs MPK3 and MPK6 physically interact with HopAI1 indicating that they are direct targets of HopAI1. These findings uncover a mechanism by which Pseudomonas syringae overcomes host innate immunity to promote pathogenesis.

Arabidopsis MPK3 and MPK6 Play Different Roles in Basal and Oligogalacturonide- or Flagellin-Induced Resistance against Botrytis cinerea

“…Hydrogen peroxide production was measured in response to water or flg22 in leaf disks of these plants and compared to that of wild-type and atrbohD knock-out (KO) plants, used as positive and negative controls, respectively. AtrbohD encodes a NADPH-oxidase required for elicitorinduced production of ROS (Torres et al, 2002;Zhang et al, 2007;Galletti et al, 2008). As shown in Figure 7, both 640.1 and 640.2 plants produced wild-type-like levels of hydrogen peroxide upon stimulation with 10 nM flg22, whereas, as expected, the oxidative burst was completely abolished in leaf disks from atrbohD plants.…”

Dual Roles of Reactive Oxygen Species and NADPH Oxidase RBOHD in an Arabidopsis-Alternaria Pathosystem

“…(5) Most interestingly, as has been pointed out in two recent papers, pathogen-or host-associated molecular pattern elicitors such as flagellin (flg22) or oligogalacturonides do induce a rapid, transient, RBOHDdependent oxidative burst in Arabidopsis leaves detected by chemiluminescence or xylenol orange methods (Zhang et al, 2007;Galletti et al, 2008). When we reproduced flg22 treatment as described by Zhang et al (2007), DAB staining in our hands was not sensitive enough to detect this ROS response, which may indicate that early ROS accumulation occurs in a lower concentration range (M. Pogány and J. Durner, unpublished data).…”

Pseudomonas syringae type III effector HopAF1 suppresses plant immunity by targeting methionine recycling to block ethylene induction

“…T3Es commonly contain sequences addressing specific eukaryotic subcellular localizations (10,11) and enzymatic activities (12)(13)(14)(15)(16)(17) that disrupt and/or suppress PTI. Known targets of T3Es include plasma membrane-localized receptor complexes (13,(18)(19)(20)(21)(22)(23), downstream MAPK cascades (24,25), the stability of defenserelated transcripts (26), phytoalexin biosynthesis (27), and vesicle trafficking (28).…”