skip to main content

Title: Arabidopsis Abscisic Acid Repressor 1 is a susceptibility hub that interacts with multiple Pseudomonas syringae effectors

Pathogens secrete effector proteins into host cells to suppress host immunity and promote pathogen virulence, although many features at the molecular interface of host–pathogen interactions remain to be characterized. In a yeast two‐hybrid assay, we found that thePseudomonas syringaeeffector HopZ1a interacts with the Arabidopsis transcriptional regulator Abscisic Acid Repressor 1 (ABR1). Further analysis revealed that ABR1 interacts with multipleP. syringaeeffectors, suggesting that it may be targeted as a susceptibility hub. Indeed, loss‐of‐functionabr1mutants exhibit reduced susceptibility to a number ofP. syringaestrains. The ABR1 protein comprises a conserved APETALA2 (AP2) domain flanked by long regions of predicted structural disorder. We verified the DNA‐binding activity of the AP2 domain and demonstrated that the disordered domains act redundantly to enhance DNA binding and to facilitate transcriptional activation by ABR1. Finally, we compared gene expression profiles from wild‐type andabr1plants following inoculation withP. syringae, which suggested that the reduced susceptibility ofabr1mutants is due to the loss of a virulence target rather than an enhanced immune response. These data highlight ABR1 as a functionally important component at the host–pathogen interface.

more » « less
Author(s) / Creator(s):
 ;  ;  
Publisher / Repository:
Date Published:
Journal Name:
The Plant Journal
Page Range / eLocation ID:
p. 1274-1292
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Shan, Libo (Ed.)

    The plant pathogenPseudomonas syringaesecretes multiple effectors that modulate plant defenses. Some effectors trigger defenses due to specific recognition by plant immune complexes, whereas others can suppress the resulting immune responses. The HopZ3 effector ofP.syringaepv. syringae B728a (PsyB728a) is an acetyltransferase that modifies not only components of plant immune complexes, but also thePsyeffectors that activate these complexes. In Arabidopsis, HopZ3 acetylates the host RPM1 complex and thePsyeffectors AvrRpm1 and AvrB3. This study focuses on the role of HopZ3 during tomato infection. InPsy-resistant tomato, the main immune complex includes PRF and PTO, a RIPK-family kinase that recognizes the AvrPto effector. HopZ3 acts as a virulence factor on tomato by suppressing AvrPto1Psy-triggered immunity. HopZ3 acetylates AvrPto1Psyand the host proteins PTO, SlRIPK and SlRIN4s. Biochemical reconstruction and site-directed mutagenesis experiments suggest that acetylation acts in multiple ways to suppress immune signaling in tomato. First, acetylation disrupts the critical AvrPto1Psy-PTO interaction needed to initiate the immune response. Unmodified residues at the binding interface of both proteins and at other residues needed for binding are acetylated. Second, acetylation occurs at residues important for AvrPto1Psyfunction but not for binding to PTO. Finally, acetylation reduces specific phosphorylations needed for promoting the immune-inducing activity of HopZ3’s targets such as AvrPto1Psyand PTO. In some cases, acetylation competes with phosphorylation. HopZ3-mediated acetylation suppresses the kinase activity of SlRIPK and the phosphorylation of its SlRIN4 substrate previously implicated in PTO-signaling. Thus, HopZ3 disrupts the functions of multiple immune components and the effectors that trigger them, leading to increased susceptibility to infection. Finally, mass spectrometry used to map specific acetylated residues confirmed HopZ3’s unusual capacity to modify histidine in addition to serine, threonine and lysine residues.

    more » « less
  2. Summary

    Effective and durable disease resistance for bacterial blight (BB) of rice is a continuous challenge due to the evolution and adaptation of the pathogen,Xanthomonas oryzaepv.oryzae(Xoo), on cultivated rice varieties. Fundamental to this pathogens’ virulence is transcription activator‐like (TAL) effectors that activate transcription of host genes and contribute differently to pathogen virulence, fitness or both. Host plant resistance is predicted to be more durable if directed at strategic virulence factors that impact both pathogen virulence and fitness. We characterized Tal7b, a minor‐effect virulence factor that contributes incrementally to pathogen virulence in rice, is a fitness factor to the pathogen and is widely present in geographically diverse strains ofXoo. To identify sources of resistance to this conserved effector, we used a highly virulent strain carrying a plasmid borne copy of Tal7b to screen an indica multi‐parent advanced generation inter‐cross (MAGIC) population. Of 18 QTL revealed by genome‐wide association studies and interval mapping analysis, six were specific to Tal7b (qBB‐tal7b). Overall, 150 predicted Tal7b gene targets overlapped with qBB‐tal7bQTL. Of these, 21 showed polymorphisms in the predicted effector binding element (EBE) site and 23 lost the EBE sequence altogether. Inoculation and bioinformatics studies suggest that the Tal7b target in one of the Tal7b‐specific QTL, qBB‐tal7b‐8, is a disease susceptibility gene and that the resistance mechanism for this locus may be through loss of susceptibility. Our work demonstrates that minor‐effect virulence factors significantly contribute to disease and provide a potential new approach to identify effective disease resistance.

    more » « less
  3. Summary

    GacS/GacA is a conserved two‐component system that functions as a master regulator of virulence‐associated traits in many bacterial pathogens, includingPseudomonasspp., that collectively infect both plant and animal hosts. Among many GacS/GacA‐regulated traits, type III secretion of effector proteins into host cells plays a critical role in bacterial virulence. In the opportunistic plant and animal pathogenPseudomonas aeruginosa, GacS/GacA negatively regulates the expression of type III secretion system (T3SS)‐encoding genes. However, in the plant pathogenic bacteriumPseudomonas syringae, strain‐to‐strain variation exists in the requirement of GacS/GacA for T3SS deployment, and this variability has limited the development of predictive models of how GacS/GacA functions in this species. In this work we re‐evaluated the function of GacA inP. syringaepv.tomatoDC3000. Contrary to previous reports, we discovered that GacA negatively regulates the expression of T3SS genes in DC3000, and that GacA is not required for DC3000 virulence insideArabidopsisleaf tissue. However, our results show that GacA is required for full virulence of leaf surface‐inoculated bacteria. These data significantly revise current understanding of GacS/GacA in regulatingP. syringaevirulence.

    more » « less
  4. Abstract Background

    Xanthomonas oryzaepv.oryzae(Xoo) causes bacterial leaf blight, a devastating disease of rice. Among the type-3 effectors secreted byXooto support pathogen virulence, the Transcription Activator-Like Effector (TALE) family plays a critical role. Some TALEs are major virulence factors that activate susceptibility (S) genes, overexpression of which contributes to disease development. Host incompatibility can result from TALE-induced expression of so-called executor (E) genes leading to a strong and rapid resistance response that blocks disease development. In that context, the TALE functions as an avirulence (Avr) factor. To date no such avirulence factors have been identified in African strains ofXoo.


    With respect to the importance of TALEs in the Rice-Xoopathosystem, we aimed at identifying those that may act as Avr factor within AfricanXoo. We screened 86 rice accessions, and identified 12 that were resistant to two African strains while being susceptible to a well-studied Asian strain. In a gain of function approach based on the introduction of each of the ninetalgenes of the avirulent African strain MAI1 into the virulent Asian strain PXO99A, four were found to trigger resistance on specific rice accessions. Loss-of-function mutational analysis further demonstrated theavractivity of two of them,talDandtalI,on the rice varieties IR64 and CT13432 respectively. Further analysis of TalI demonstrated the requirement of its activation domain for triggering resistance in CT13432. Resistance in 9 of the 12 rice accessions that were resistant against AfricanXoospecifically, including CT13432, could be suppressed or largely suppressed by trans-expression of the truncTALEtal2h, similarly to resistance conferred by theXa1gene which recognizes TALEs generally independently of their activation domain.


    We identified and characterized TalD and TalI as two AfricanXooTALEs with avirulence activity on IR64 and CT13432 respectively. Resistance of CT13432 against AfricanXooresults from the combination of two mechanisms, one relying on the TalI-mediated induction of an unknown executor gene and the other on anXa1-like gene or allele.

    more » « less
  5. Intracellular pathogens are responsible for an enormous amount of worldwide morbidity and mortality, and each has evolved specialized strategies to establish and maintain their replicative niche.Listeria monocytogenesis a facultative intracellular pathogen that secretes a pore-forming cytolysin called listeriolysin O (LLO), which disrupts the phagosomal membrane and, thereby, allows the bacteria access to their replicative niche in the cytosol. Nonsynonymous and synonymous mutations in a PEST-like domain near the LLO N terminus cause enhanced LLO translation during intracellular growth, leading to host cell death and loss of virulence. Here, we explore the mechanism of translational control and show that there is extensive codon restriction within the PEST-encoding region of the LLO messenger RNA (mRNA) (hly). This region has considerable complementarity with the 5′ UTR and is predicted to form an extensive secondary structure that overlaps the ribosome binding site. Analysis of both 5′ UTR and synonymous mutations in the PEST-like domain that are predicted to disrupt the secondary structure resulted in up to a 10,000-fold drop in virulence during mouse infection, while compensatory double mutants restored virulence to WT levels. We showed by dynamic protein radiolabeling that LLO synthesis was growth phase-dependent. These data provide a mechanism to explain how the bacteria regulate translation of LLO to promote translation during starvation in a phagosome while repressing it during growth in the cytosol. These studies also provide a molecular explanation for codon bias at the 5′ end of this essential determinant of pathogenesis.

    more » « less