Allele-Specific Virulence Attenuation of the Pseudomonas syringae HopZ1a Type III Effector via the Arabidopsis ZAR1 Resistance Protein

Abstract

Plant resistance (R) proteins provide a robust surveillance system to defend against potential pathogens. Despite their importance in plant innate immunity, relatively few of the ∼170 R proteins in Arabidopsis have well-characterized resistance specificity. In order to identify the R protein responsible for recognition of the Pseudomonas syringae type III secreted effector (T3SE) HopZ1a, we assembled an Arabidopsis R gene T–DNA Insertion Collection (ARTIC) from publicly available Arabidopsis thaliana insertion lines and screened it for plants lacking HopZ1a-induced immunity. This reverse genetic screen revealed that the Arabidopsis R protein HOPZ-ACTIVATED RESISTANCE 1 (ZAR1; At3g50950) is required for recognition of HopZ1a in Arabidopsis. ZAR1 belongs to the coiled-coil (CC) class of nucleotide binding site and leucine-rich repeat (NBS–LRR) containing R proteins; however, the ZAR1 CC domain phylogenetically clusters in a clade distinct from other related Arabidopsis R proteins. ZAR1–mediated immunity is independent of several genes required by other R protein signaling pathways, including NDR1 and RAR1, suggesting that ZAR1 possesses distinct signaling requirements. The closely-related T3SE protein, HopZ1b, is still recognized by zar1 Arabidopsis plants indicating that Arabidopsis has evolved at least two independent R proteins to recognize the HopZ T3SE family. Also, in Arabidopsis zar1 plants HopZ1a promotes P. syringae growth indicative of an ancestral virulence function for this T3SE prior to the evolution of recognition by the host resistance protein ZAR1. Our results demonstrate that the Arabidopsis resistance protein ZAR1 confers allele-specific recognition and virulence attenuation of the Pseudomonas syringae T3SE protein HopZ1a.

The Arabidopsis ZED1 pseudokinase is required for ZAR1-mediated immunity induced by the Pseudomonas syringae type III effector HopZ1a

“…1C; ref. 17). Similarly, in the zed1-6 knockout, PtoDC3000(hopZ1a) growth is similar to PtoDC3000(Ev) (Fig.…”

“…We previously demonstrated that HopZ1a confers a growth advantage in the absence of ZAR1 when it is delivered by P. syringae pathovar cilantro (Pci0788-9), a strain closely related to a virulent strain of P. syringae pathovar maculicola (PmaES4326) (17). To determine whether HopZ1a would also confer a growth advantage in zed1, we infiltrated wild-type Col-0 or the zar1-1 or zed1-6 mutants with Pci0788-9 carrying hopZ1a or empty vector.…”

“…In the plant pathogen Pseudomonas syringae, the HopZ1 subfamily is represented by three closely related alleles, HopZ1a, HopZ1b, and HopZ1c, that diversified under pressure from the host immune system (10). The most phylogenetically basal representative of this subfamily, HopZ1a, is recognized in Arabidopsis by the ZAR1 NB-LRR protein (16,17) as well as by unidentified proteins in rice, Nicotiana benthamiana, sesame, and soybean (10). Like YopJ, HopZ1a is an acetyltransferase and can promote pathogen growth in Arabidopsis plants lacking the ZAR1 NB-LRR protein.…”

“…Like YopJ, HopZ1a is an acetyltransferase and can promote pathogen growth in Arabidopsis plants lacking the ZAR1 NB-LRR protein. The virulence and immuneeliciting functions of HopZ1a both require the cysteine residue in the acetyltransferase catalytic triad (16,17). It is likely that ZAR1 evolved to recognize an ancestral virulence activity of HopZ1a; however, the molecular relationship between virulence and immunity-eliciting functions remain to be established (17,18).…”

Maize Homologs of CCoAOMT and HCT, Two Key Enzymes in Lignin Biosynthesis, Form Complexes with the NLR Rp1 Protein to Modulate the Defense Response

“…In contrast, the same NLR can also interact with different host cofactors targeted by different pathogen effectors. In Arabidopsis, the NLR ZAR1 is required for the recognition of the HopZ1a effector from P. syringae (Lewis et al, 2010). ZAR1 is activated upon acetylation of the pseudokinase ZED1, by HopZ1a (Lewis et al, 2013).…”