JAZ8 lacks a canonical degron and has an EAR motif that mediates transcriptional repression of jasmonate responses in Arabidopsis.

The lipid-derived hormone jasmonoyl-L-Ile (JA-Ile) initiates large-scale changes in gene expression by stabilizing the interaction of JASMONATE ZIM domain (JAZ) repressors with the F-box protein CORONATINE INSENSITIVE1 (COI1), which results in JAZ degradation by the ubiquitin-proteasome pathway. Recent structural studies show that the JAZ1 degradation signal (degron) includes a short conserved LPIAR motif that seals JA-Ile in its binding pocket at the COI1-JAZ interface. Here, we show that Arabidopsis thaliana JAZ8 lacks this motif and thus is unable to associate strongly with COI1 in the presence of JA-Ile. As a consequence, JAZ8 is stabilized against jasmonate (JA)-mediated degradation and, when ectopically expressed in Arabidopsis, represses JA-regulated growth and defense responses. These findings indicate that sequence variation in a hypervariable region of the degron affects JAZ stability and JA-regulated physiological responses. We also show that JAZ8-mediated repression depends on an LxLxL-type EAR (for ERF-associated amphiphilic repression) motif at the JAZ8 N terminus that binds the corepressor TOPLESS and represses transcriptional activation. JAZ8-mediated repression does not require the ZIM domain, which, in other JAZ proteins, recruits TOPLESS through the EAR motif-containing adaptor protein NINJA. These findings show that EAR repression domains in a subgroup of JAZ proteins repress gene expression through direct recruitment of corepressors to cognate transcription factors.

JAZ repressor proteins are targets of the SCF(COI1) complex during jasmonate signalling.

Jasmonate and related signalling compounds have a crucial role in both host immunity and development in plants, but the molecular details of the signalling mechanism are poorly understood. Here we identify members of the jasmonate ZIM-domain (JAZ) protein family as key regulators of jasmonate signalling. JAZ1 protein acts to repress transcription of jasmonate-responsive genes. Jasmonate treatment causes JAZ1 degradation and this degradation is dependent on activities of the SCF(COI1) ubiquitin ligase and the 26S proteasome. Furthermore, the jasmonoyl-isoleucine (JA-Ile) conjugate, but not other jasmonate-derivatives such as jasmonate, 12-oxo-phytodienoic acid, or methyl-jasmonate, promotes physical interaction between COI1 and JAZ1 proteins in the absence of other plant proteins. Our results suggest a model in which jasmonate ligands promote the binding of the SCF(COI1) ubiquitin ligase to and subsequent degradation of the JAZ1 repressor protein, and implicate the SCF(COI1)-JAZ1 protein complex as a site of perception of the plant hormone JA-Ile.

Bacterial outer membrane vesicles induce a transcriptional shift in arabidopsis towards immune system activation leading to suppression of pathogen growth in planta.

Gram-negative bacteria form spherical blebs on their cell periphery, which later dissociate from the bacterial cell wall to form extracellular vesicles. These nano scale structures, known as outer membrane vesicles (OMVs), have been shown to promote infection and disease and can induce typical immune outputs in both mammal and plant hosts. To better understand the broad transcriptional change plants undergo following exposure to OMVs, we treated Arabidopsis thaliana (Arabidopsis) seedlings with OMVs purified from the Gram-negative plant pathogenic bacterium Xanthomonas campestris pv. campestris and performed RNA-seq analysis on OMV- and mock-treated plants at 2, 6 and 24 h post challenge. The most pronounced transcriptional shift occurred at the first two time points tested, as reflected by the number of differentially expressed genes and the average fold change. OMVs induce a major transcriptional shift towards immune system activation, upregulating a multitude of immune-related pathways including a variety of immune receptors. Comparing the response of Arabidopsis to OMVs and to purified elicitors, revealed that OMVs induce a similar suite of genes and pathways as single elicitors, however, pathways activated by OMVs and not by other elicitors were detected. Pretreating Arabidopsis plants with OMVs and subsequently infecting with a bacterial pathogen led to a significant reduction in pathogen growth. Mutations in the plant elongation factor receptor (EFR), flagellin receptor (FLS2), or the brassinosteroid-insensitive 1-associated kinase (BAK1) co-receptor, did not significantly affect the immune priming effect of OMVs. All together these results show that OMVs induce a broad transcriptional shift in Arabidopsis leading to upregulation of multiple immune pathways, and that this transcriptional change may facilitate resistance to bacterial infection.

COI1 is a critical component of a receptor for jasmonate and the bacterial virulence factor coronatine.

Jasmonate (JA) is a lipid-derived hormone that regulates diverse aspects of plant immunity and development. An amino acid-conjugated form of JA, jasmonoyl-isoleucine (JA-Ile), stimulates binding of the F-box protein coronatine-insensitive 1 (COI1) to, and subsequent ubiquitin-dependent degradation of, jasmonate ZIM domain (JAZ) proteins that repress transcription of JA-responsive genes. The virulence factor coronatine (COR), which is produced by plant pathogenic strains of Pseudomonas syringae, suppresses host defense responses by activating JA signaling in a COI1-dependent manner. Although previous data indicate that COR acts as a molecular mimic of JA-Ile, the mechanism by which JA-Ile and COR are perceived by plant cells remains unknown. Here, we show that interaction of tomato COI1 with divergent members of the JAZ family is highly specific for JA-Ile and structurally related JA conjugates and that COR is approximately 1,000-fold more active than JA-Ile in promoting this interaction in vitro. JA-Ile competes for binding of COR to COI1-JAZ complexes, demonstrating that COR and JA-Ile are recognized by the same receptor. Binding of COR to the COI1-JAZ complex requires COI1 and is severely impaired by a point mutation in the putative ligand-binding pocket of COI1. Finally, we show that the C-terminal region of JAZ3 containing the highly conserved Jas motif is necessary and sufficient for hormone-induced COI1-JAZ interaction. These findings demonstrate that COI1 is a critical component of the JA receptor and that COR exerts its virulence effects by functioning as a potent agonist of this receptor system.

Jasmonate signaling: a conserved mechanism of hormone sensing.

The lipid-derived hormone jasmonate (JA) regulates diverse aspects of plant immunity and development. Among the central components of the JA signaling cascade are the E3 ubiquitin ligase SCFCOI1 and Jasmonate ZIM-domain (JAZ) proteins that repress transcription of JA-responsive genes. Recent studies provide evidence that amino acid-conjugated forms of JA initiate signal transduction upon formation of a coronatine-insensitive1 (COI1)-JA-JAZ ternary complex in which JAZs are ubiquitinated and subsequently degraded. Coronatine, a virulence factor produced by the plant pathogen Pseudomonas syringae, is a potent agonist of this hormone receptor system. Coronatine-induced targeting of JAZs to COI1 obstructs host immune responses to P. syrinage, providing a striking example of how pathogens exploit hormone signaling pathways in the host to promote disease. These findings, together with homology between COI1 and the auxin receptor, TIR1, extend the paradigm of F-box proteins as intracellular sensors of small molecules, and suggest a common evolutionary origin of the auxin and JA response pathways.

A critical role of two positively charged amino acids in the Jas motif of Arabidopsis JAZ proteins in mediating coronatine- and jasmonoyl isoleucine-dependent interactions with the COI1 F-box protein.

SUMMARY: Coronatine is an important virulence factor produced by several pathovars of the bacterial pathogen Pseudomonas syringae. The structure of coronatine is similar to that of a class of plant hormones called jasmonates (JAs). An important step in JA signaling is the SCF(COI1) E3 ubiquitin ligase-dependent degradation of JAZ repressor proteins. We have recently shown that jasmonoyl isoleucine (JA-Ile) promotes physical interaction between Arabidopsis JAZ1 and COI1 (the F-box component of SCF(COI1)) proteins, and that the JA-Ile-dependent COI1-JAZ1 interaction could be reconstituted in yeast cells (i.e. in the absence of other plant proteins). Here we show that coronatine, but not its two biosynthetic precursors, also promotes interaction between Arabidopsis COI1 and multiple JAZ proteins. The C-terminal Jas motif, but not the N-terminal (NT) domain or central ZIM domain of JAZ proteins, is critical for JA-Ile/coronatine-dependent interaction with COI1. Two positively charged amino acid residues in the Jas domain were identified as essential for coronatine-dependent COI1-JAZ interactions. Mutations of these two residues did not affect the ability of JAZ1 and JAZ9 to interact with the transcription factor AtMYC2. Importantly, transgenic Arabidopsis plants expressing JAZ1 carrying these two mutations exhibited JA-insensitive phenotypes, including male sterility and enhanced resistance to P. syringae infection. These results not only suggest that coronatine and JA-Ile target the physical interaction between COI1 and the Jas domain of JAZ repressors, but also illustrate the critical role of positively charged amino acids in the Jas domain in mediating the JA-Ile/coronatine-dependent JAZ interaction with COI1.

Genome Analysis of Haplotype D of Candidatus Liberibacter Solanacearum.

Candidatus Liberibacter solanacearum (Lso) haplotype D (LsoD) is a suspected bacterial pathogen, spread by the phloem-feeding psyllid Bactericera trigonica Hodkinson and found to infect carrot plants throughout the Mediterranean. Haplotype D is one of six haplotypes of Lso that each have specific and overlapping host preferences, disease symptoms, and psyllid vectors. Genotyping of rRNA genes has allowed for tracking the haplotype diversity of Lso and genome sequencing of several haplotypes has been performed to advance a comprehensive understanding of Lso diseases and of the phylogenetic relationships among the haplotypes. To further pursue that aim we have sequenced the genome of LsoD from its psyllid vector and report here its draft genome. Genome-based single nucleotide polymorphism analysis indicates LsoD is most closely related to the A haplotype. Genomic features and the metabolic potential of LsoD are assessed in relation to Lso haplotypes A, B, and C, as well as the facultative strain Liberibacter crescens. We identify genes unique to haplotype D as well as putative secreted effectors that may play a role in disease characteristics specific to this haplotype of Lso.

Genome Sequence of "Candidatus Carsonella ruddii" Strain BT from the Psyllid Bactericera trigonica.

The genome of "Candidatus Carsonella ruddii" strain BT from Bactericera trigonica in Israel was sequenced. The full-length genome is 173,904 bp long and has a G+C content of 14.6%, with 224 predicted open reading frames (ORFs) and 30 RNAs.

Peptide signaling in plant development.

Cell-to-cell communication is integral to the evolution of multicellularity. In plant development, peptide signals relay information coordinating cell proliferation and differentiation. These peptides are often encoded by gene families and bind to corresponding families of receptors. The precise spatiotemporal expression of signals and their cognate receptors underlies developmental patterning, and expressional and biochemical changes over evolutionary time have likely contributed to the refinement and complexity of developmental programs. Here, we discuss two major plant peptide families which have central roles in plant development: the CLAVATA3/ENDOSPERM SURROUNDING REGION (CLE) peptide family and the EPIDERMAL PATTERNING FACTOR (EPF) family. We discuss how specialization has enabled the CLE peptides to modulate stem cell differentiation in various tissue types, and how differing activities of EPF peptides precisely regulate the stomatal developmental program, and we examine the contributions of these peptide families to plant development from an evolutionary perspective.

Role of radical formation at tyrosine 193 in the allene oxide synthase domain of a lipoxygenase-AOS fusion protein from coral.

Coral allene oxide synthase (cAOS), a fusion protein with 8R-lipoxygenase in Plexaura homomalla, is a hemoprotein with sequence similarity to catalases. cAOS reacts rapidly with the oxidant peracetic acid to form heme compound I and intermediate II. Concomitantly, an electron paramagnetic resonance (EPR) signal with tyrosyl radical-like features, centered at a g-value of 2.004-2.005, is formed. The radical is identified as tyrosyl by changes in EPR spectra when deuterated tyrosine is incorporated in cAOS. The radical location in cAOS is determined by mutagenesis of Y193 and Y209. Upon oxidation, native cAOS and mutant Y209F exhibit the same radical spectrum, but no significant tyrosine radical forms in mutant Y193H, implicating Y193 as the radical site in native cAOS. Estimates of the side chain torsion angles for the radical at Y193, based on the beta-proton isotropic EPR hyperfine splitting, A(iso), are theta(1) = 21 to 30 degrees and theta(2) = -99 to -90 degrees. The results show that cAOS can cleave nonsubstrate hydroperoxides by a heterolytic path, although a homolytic course is likely taken in converting the normal substrate, 8R-hydroperoxyeicosatetraenoic acid (8R-HpETE), to product. Coral AOS achieves specificity for the allene oxide formed by selection of the homolytic pathway normally, while it inactivates by the heterolytic path with nonoptimal substrates. Accordingly, with the nonoptimal substrate, 13R-hydroperoxyoctadecadienoic acid (13R-HpODE), mutant Y193H is inactivated after turning over significantly fewer substrate molecules than required to inactivate native cAOS or the Y209F mutant because it cannot absorb oxidizing equivalents by forming a radical at Y193.