A Bacterial Effector Co-opts Calmodulin to Target the Plant Microtubule Network

The bacterial pathogen Pseudomonas syringae depends on effector proteins secreted by its type III secretion system for the pathogenesis of plants. The majority of these effector proteins are known suppressors of immunity, but their plant targets remain elusive. Using Arabidopsis thaliana as a model host, we report that the HopE1 effector uses the host calcium sensor, calmodulin (CaM), as a co-factor to target the microtubule-associated protein 65 (MAP65), an important component of the microtubule network. HopE1 interacted with MAP65 in a CaM-dependent manner, resulting in MAP65-GFP dissociation from microtubules. Transgenic Arabidopsis expressing HopE1 had reduced secretion of the immunity protein PR-1 compared to wild–type plants. Additionally, Arabidopsis map65-1 mutants were immune deficient and were more susceptible to P. syringae. Our results suggest a virulence strategy in which a pathogen effector is activated by host calmodulin to target MAP65 and the microtubule network, thereby inhibiting cell wall-based extracellular immunity. [Display omitted] •HopE1 interacts with the eukaryotic calcium sensor, calmodulin (CaM)•CaM activates HopE1 to target MAP65, dissociating it from the microtubule network•Arabidopsis expressing HopE1 or lacking MAP65-1 are more susceptible to P. syringae•Targeting MAP65 results in inhibition of cell wall-based extracellular immunity Guo et al. find that the Pseudomonas syringae effector HopE1 uses plant calmodulin as a co-factor to target MAP65, dissociating it from microtubules. Arabidopsis plants expressing HopE1 or lacking MAP65-1 secrete less PR-1, an immunity-related protein, suggesting that the pathogen may target MAP65 to inhibit cell wall-based extracellular immunity.