Characterization of E3 Ubiquitin Ligase 1 as a novel regulator of PTI-associated resistance in Arabidopsis thaliana
E3 Ubiqutin Ligase 1 (E3L1), a plasma membrane-localized E3 Ligase in Arabidopsis thaliana, has previously been shown as being rapidly phosphorylated during pattern-triggered immunity (PTI)-associated defense responses. In this work, our characterization of T-DNA insertional mutants (e3l1-1, e3l1-2) implicate E3L1 as a positive regulator of PTI. The mutant plants are compromised in their ability to manifest PTI-induced resistance following pretreat with flg22. However, several other typical defense related phenotypes, including defense transcript accumulation, ROS production, and ligand-induced endocytosis of FLS2, were unaltered, indicating that E3L1 functions downstream or independently of these responses. In a previous study, we demonstrated that loss of MAP Kinase Phosphatase 1 (MKP1) results in increased resistance against Psuedomonas syringae pv tomato DC3000 (DC3000). This resistance was shown to be caused by a decrease in secretion of extracellular T3SS-inducing metabolites, indicating alterations in metabolite transporter activity or accumulation in mkp1. Due to the predicted function of E3L1 targeting proteins for degradation or endocytosis at the plasma membrane, we hypothesized a genetic link between E3L1 and MKP1. Infection of a double mutant, e3l1 mkp1, shows a suppression of mkp1-related resistance, genetically placing E3L1 in the MKP1-dependent pathway.
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Characterizing a TIR-only/CC-NBS-LRR Dual Receptor System for Plant Pathogen Recognition
RBA1 is an Arabidopsis TIR-only immune receptor responsible for the recognition of the Pseudomonas syringae type III effector, HopBA1. RBA1 is unusual, lacking C-terminal domains that regulate canonical TIR-NBS-LRR (TNL) immune receptors. In the Arabidopsis accession Ag-0, recognition of HopBA1 by RBA1 triggers cell death. In other Arabidopsis accessions (e.g. Col-0), HopBA1 is instead recognized by ZAR1, a CC-NBS-NLR (CNL), to trigger disease resistance without cell death. It is currently unclear if RBA1 and ZAR1 immune responses are independent or if one regulates the other. To determine the genetic requirements for cell death and disease resistance in Ag-0 we have generated rba1, zar1, and rba1/zar1 Cas9 mutants. These mutants reveal the hypersensitive response to HopBA1 in Ag-0 previously attributed to RBA1 also requires ZAR1. ZAR1 is known to be activated upon recognition of modified PBL kinases. We hypothesize that HopBA1 targets one or more PBL kinases, leading to both ZAR1 and RBA1 activation. To identity putative HopBA1 virulence targets, we have screened the Arabidopsis PBL kinase protein family for physical interactions with HopBA1. The biological relevance of these interactions is being assessed using reverse genetics in Arabidopsis. The HopBA1/RBA1/ZAR1 system will help explain how structurally diverse receptors can work together to form an immune system that appropriately responds to pathogens to trigger disease resistance and, in some cases, cell death.
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A cell wall degrading enzyme and its putative receptor/co-receptor activate rice and Arabidopsis immune responses via different signaling pathways
Plant pathogens secrete various cell wall degrading enzymes (CWDE) to degrade different components of the plant cell wall. Plants can perceive damage caused by these CWDE and mount immune responses (IRs). But how do plants perceive it and mount immunity is not well explored. Lipase/esterase (LipA) is a CWDE that is secreted by Xanthomonas oryzae pv. oryzae (Xoo), the bacterial blight pathogen of rice. Treatment of rice or Arabidopsis tissue with LipA or LipA treated cell wall extract induces IRs. Using transcriptome analysis, we identified a rice receptor kinase OsWAKL21 which is upregulated following treatment of rice leaves with LipA or Xoo but not after treatment of LipA mutant of Xoo. Downregulation of OsWAKL21 in rice attenuated LipA induced IRs. Overexpression of OsWAKL21 in rice mimics LipA treatment in terms of IRs. Ectopic expression of OsWAKL21 in Arabidopsis also induces similar IRs. OsWAKL21 is a moonlighting kinase that has kinase and guanylate cyclase (GC) activities. Interestingly, kinase activity of OsWAKL21 is required for induction of rice IRs while in Arabidopsis it needs its GC activity. The expression profile of defence genes and experiments with mutant Arabidopsis lines indicate LipA treatment or overexpression of OsWAKL21 induces JA pathway in rice while it acts via SA pathway in Arabidopsis. Thus, our results indicate that LipA and its putative receptor/co-receptor OsWAKL21 activate rice and Arabidopsis immunity via two different signaling pathways.
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