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1. Pseudomonas syringae effector HopZ3 suppresses the bacterial AvrPto1– tomato PTO immune complex via acetylation

   Jeleńska, J. ; Lee, J. ; Manning, A.J. ; Wolfgeher, D.J. ; Ahn, Y. ; Walters-Marrah, G. ; Lopez, I.E. ; Garcia, L. ; Sheri A. McClerklin, S.A. ; Michelmore, R.W. ; et al ( January 2021 , PLOS pathogens)

   null (Ed.)

   The plant pathogen Pseudomonas syringae secretes 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 of P. syringae pv. syringae B728a (PsyB728a) is an acetyltransferase that modifies not only components of plant immune complexes, but also the Psy effectors that activate these complexes. In Arabidopsis, HopZ3 acetylates the host RPM1 complex and the Psy effectors AvrRpm1 and AvrB3. This study focuses on the role of HopZ3 during tomato infection. In Psy-resistant tomato, the main immune complex includes PRF and PTO, a RIPK-family kinase that recognizes the AvrPto effector. HopZ3 acts as avirulence factor on tomato by suppressing AvrPto1Psy-triggered immunity. HopZ3acetylates AvrPto1Psy and 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 AvrPto1Psy function but not for binding to PTO. Finally, acetylation reduces specific phosphorylations needed for promoting the immune-inducing activity of HopZ3’s targets such as AvrPto1Psy and 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 44 spectrometry used to map specific acetylated residues confirmed HopZ3’s unusual capacity to modify histidine in addition to serine, threonine and lysine residues. 

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2. Integrative network-centric approach reveals signaling pathways associated with plant resistance and susceptibility to Pseudomonas syringae

   Brauer, Elizabeth K ; Popescu, George V. ; Singh, Dharmendra K. ; Calviño, Mauricio ; Gupta, Kamala ; Gupta, Bhaskar ; Chakravarthy, Suma ; Popescu, Sorina C. ( December 2018 , PLoS biology)

   Kamoun, Sophien (Ed.)

   Plant protein kinases form redundant signaling pathways to perceive microbial pathogens and activate immunity. Bacterial pathogens repress cellular immune responses by secreting effectors, some of which bind and inhibit multiple host kinases. To understand how broadly bacterial effectors may bind protein kinases and the function of these kinase interactors, we first tested kinase–effector (K-E) interactions using the Pseudomonas syringae pv. tomato–tomato pathosystem. We tested interactions between five individual effectors (HopAI1, AvrPto, HopA1, HopM1, and HopAF1) and 279 tomato kinases in tomato cells. Over half of the tested kinases interacted with at least one effector, and 48% of these kinases interacted with more than three effectors, suggesting a role in the defense. Next, we characterized the role of select multi-effector–interacting kinases and revealed their roles in basal resistance, effector-triggered immunity (ETI), or programmed cell death (PCD). The immune function of several of these kinases was only detectable in the presence of effectors, suggesting that these kinases are critical when particular cell functions are perturbed or that their role is typically masked. To visualize the kinase networks underlying the cellular responses, we derived signal-specific networks. A comparison of the networks revealed a limited overlap between ETI and basal immunity networks. In addition, the basal immune network complexity increased when exposed to some of the effectors. The networks were used to successfully predict the role of a new set of kinases in basal immunity. Our work indicates the complexity of the larger kinase-based defense network and demonstrates how virulence- and avirulence-associated bacterial effectors alter sectors of the defense network. 

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3. Calmodulin‐binding transcription activator AtSR1 / CAMTA3 fine‐tunes plant immune response by transcriptional regulation of the salicylate receptor NPR1

   https://doi.org/10.1111/pce.14123  

   Yuan, Peiguo ; Tanaka, Kiwamu ; Poovaiah, B. W. ( June 2021 , Plant, Cell & Environment)

   Calcium (Ca²⁺) signalling regulates salicylic acid (SA)‐mediated immune response through calmodulin‐meditated transcriptional activators, AtSRs/CAMTAs, but its mechanism is not fully understood. Here, we report an AtSR1/CAMTA3‐mediated regulatory mechanism involving the expression of the SA receptor, NPR1. Results indicate that the transcriptional expression ofNPR1was regulated by AtSR1 binding to a CGCG box in theNPR1promotor. Theatsr1mutant exhibited resistance to the virulent strain ofPseudomonas syringaepv.tomato(Pst), however, was susceptible to an avirulentPststrain carryingavrRpt2, due to the failure of the induction of hypersensitive responses. These resistant/susceptible phenotypes in theatsr1mutant were reversed in thenpr1mutant background, suggesting that AtSR1 regulates NPR1 as a downstream target during plant immune response. The virulentPststrain triggered a transient elevation in intracellular Ca²⁺concentration, whereas the avirulentPststrain triggered a prolonged change. The distinct Ca²⁺signatures were decoded into the regulation of NPR1 expression through AtSR1's IQ motif binding with Ca²⁺‐free‐CaM2, while AtSR1's calmodulin‐binding domain with Ca²⁺‐bound‐CaM2. These observations reveal a role for AtSR1 as a Ca²⁺‐mediated transcription regulator in controlling the NPR1‐mediated plant immune response.

    

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4. An important role of l ‐fucose biosynthesis and protein fucosylation genes in Arabidopsis immunity

   https://doi.org/10.1111/nph.15639  

   Zhang, Li ; Paasch, Bradley C. ; Chen, Jin ; Day, Brad ; He, Sheng Yang ( January 2019 , New Phytologist)

   Plants mount coordinated immune responses to defend themselves against pathogens. However, the cellular components required for plant immunity are not fully understood. The jasmonate‐mimicking coronatine (COR) toxin produced byPseudomonas syringaepv.tomato(Pst)DC3000 functions to overcome plant immunity. We previously isolated eight Arabidopsis (scord) mutants that exhibit increased susceptibility to aCOR‐deficient mutant ofPstDC3000. Among them, thescord6mutant exhibits defects both in stomatal closure response and in restricting bacterial multiplication inside the apoplast. However, the identity ofSCORD6remained elusive.

   In this study, we aim to identify theSCORD6gene.

   We identifiedSCORD6via next‐generation sequencing and found it to beMURUS1(MUR1), which is involved in the biosynthesis ofGDP‐l‐fucose.

   Discovery ofSCORD6asMUR1led to a series of experiments that revealed a multi‐faceted role ofl‐fucose biosynthesis in stomatal and apoplastic defenses as well as in pattern‐triggered immunity and effector‐triggered immunity, including glycosylation of pattern‐recognition receptors. Furthermore, compromised stomatal and/or apoplastic defenses were observed in mutants of several fucosyltransferases with specific substrates (e.g.O‐glycan,N‐glycan or theDELLAtranscriptional repressors). Collectively, these results uncover a novel and broad role ofl‐fucose and protein fucosylation in plant immunity.

    

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5. Involvement of Arabidopsis Acyl Carrier Protein 1 in PAMP-triggered immunity

   https://doi.org/10.1094/MPMI-02-22-0049-R  

   Zhao, Zhenzhen ; Fan, Jiangbo ; Yang, Piao ; Wang, Zonghua ; Stephen, Opiyo ; Mackey, David ; Ye, Xia ( January 2022 , Molecular Plant-Microbe Interactions®)

   Plant fatty acids (FAs) and lipids are essential in storing energy and act as structural components for cell membranes and signaling molecules for plant growth and stress responses. Acyl Carrier Proteins (ACPs) are small acidic proteins that covalently bind the fatty acyl intermediates during the elongation of FAs. The Arabidopsis thaliana ACP family has eight members. Through reverse genetic, molecular, and biochemical approaches, we have discovered that ACP1 localizes to the chloroplast and limits the magnitude of pattern-triggered immunity (PTI) against the bacterial pathogen Pseudomonas syringae pathovar tomato (Pto). The mutant acp1 plants have reduced levels of linolenic acid (18:3), which is the primary precursor for the biosynthesis of the phytohormone jasmonic acid (JA), and a corresponding decrease in the abundance of JA. Consistent with the known antagonistic relationship between JA and salicylic acid (SA), acp1 mutant plants also accumulate higher level of SA and display the corresponding shifts in JA- and SA-regulated transcriptional outputs. Moreover, the methyl JA and linolenic acid treatments cause an apparently enhanced decrease of resistance against Pto in acp1 mutants than that in wild-type plants. The ability of ACP1 to prevent this hormone imbalance likely underlies its negative impact on PTI in plant defense. Thus, ACP1 links FA metabolism to stress hormone homeostasis to be negatively involved in PTI in Arabidopsis plant defense. 

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