A Poly(A) Ribonuclease Controls the Cellotriose-Based Interaction between Piriformospora indica and Its Host Arabidopsis.

Piriformospora indica, an endophytic root-colonizing fungus, efficiently promotes plant growth and induces resistance to abiotic stress and biotic diseases. P. indica fungal cell wall extract induces cytoplasmic calcium elevation in host plant roots. Here, we show that cellotriose (CT) is an elicitor-active cell wall moiety released by P. indica into the medium. CT induces a mild defense-like response, including the production of reactive oxygen species, changes in membrane potential, and the expression of genes involved in growth regulation and root development. CT-based cytoplasmic calcium elevation in Arabidopsis (Arabidopsis thaliana) roots does not require the BAK1 coreceptor or the putative Ca2+ channels TPC1, GLR3.3, GLR2.4, and GLR2.5 and operates synergistically with the elicitor chitin. We identified an ethyl methanesulfonate-induced mutant (cytoplasmiccalcium elevation mutant) impaired in the response to CT and various other cellooligomers (n = 2-7), but not to chitooligomers (n = 4-8), in roots. The mutant contains a single nucleotide exchange in the gene encoding a poly(A) ribonuclease (AtPARN; At1g55870) that degrades the poly(A) tails of specific mRNAs. The wild-type PARN cDNA, expressed under the control of a 35S promoter, complements the mutant phenotype. Our identification of cellotriose as a novel chemical mediator casts light on the complex P. indica-plant mutualistic relationship.

Nucleoporin NUP88/MOS7 is required for manifestation of phenotypes associated with the Arabidopsis CHITIN ELICITOR RECEPTOR KINASE1 mutant cerk1-4.

Arabidopsis nucleoporin MOS7/NUP88 was identified in a forward-genetic screen for components that contribute to auto-immunity of the deregulated Resistance (R) gene mutant snc1, and is required for immunity to biotrophic and hemi-biotrophic pathogens. In a recent study, we showed that MOS7 is also essential to mount a full defense response against the necrotrophic fungal pathogen Botrytis cinerea, suggesting that MOS7 modulates plant defense responses to different types of pathogenic microbes. Here, we extend our analyses of MOS7-dependent plant immune responses and report the genetic requirement of MOS7 for manifestation of phenotypes associated with the CHITIN ELICITOR RECEPTOR KINASE1 (CERK1) mutant cerk1-4.

A novel Arabidopsis CHITIN ELICITOR RECEPTOR KINASE 1 (CERK1) mutant with enhanced pathogen-induced cell death and altered receptor processing.

Plants detect pathogens by sensing microbe-associated molecular patterns (MAMPs) through pattern recognition receptors. Pattern recognition receptor complexes also have roles in cell death control, but the underlying mechanisms are poorly understood. Here, we report isolation of cerk1-4, a novel mutant allele of the Arabidopsis chitin receptor CERK1 with enhanced defense responses. We identified cerk1-4 in a forward genetic screen with barley powdery mildew and consequently characterized it by pathogen assays, mutant crosses and analysis of defense pathways. CERK1 and CERK1-4 proteins were analyzed biochemically. The cerk1-4 mutation causes an amino acid exchange in the CERK1 ectodomain. Mutant plants maintain chitin signaling capacity but exhibit hyper-inducible salicylic acid concentrations and deregulated cell death upon pathogen challenge. In contrast to chitin signaling, the cerk1-4 phenotype does not require kinase activity and is conferred by the N-terminal part of the receptor. CERK1 undergoes ectodomain shedding, a well-known process in animal cell surface proteins. Wild-type plants contain the full-length CERK1 receptor protein as well as a soluble form of the CERK1 ectodomain, whereas cerk1-4 plants lack the N-terminal shedding product. Our work suggests that CERK1 may have a chitin-independent role in cell death control and is the first report of ectodomain shedding in plants.

The lysin motif receptor-like kinase (LysM-RLK) CERK1 is a major chitin-binding protein in Arabidopsis thaliana and subject to chitin-induced phosphorylation.

Plants detect potential pathogens by sensing microbe-associated molecular patterns via pattern recognition receptors. In the dicot model plant Arabidopsis, the lysin motif (LysM)-containing chitin elicitor receptor kinase 1 (CERK1) has been shown to be essential for perception of the fungal cell wall component chitin and for resistance to fungal pathogens. Recent in vitro studies with CERK1 protein expressed heterologously in yeast suggested direct chitin binding activity. Here we show in an affinity purification approach that CERK1 is a major chitin-binding protein of Arabidopsis cells, along with several known and putative chitinases. The ectodomain of CERK1 harbors three distinct LysM domains with potential ligand binding capacity. We demonstrate that the CERK1 ectodomain binds chitin and partially deacetylated chitosan directly without any requirement for interacting proteins and that all three LysM domains are necessary for chitin binding. Ligand-induced phosphorylation events are a general feature of animal and plant signal transduction pathways. Our studies show that chitin, chitin oligomers, and chitosan rapidly induce in vivo phosphorylation of CERK1 at multiple residues in the juxtamembrane and kinase domain. Functional analyses with a kinase dead variant provide evidence that kinase activity of CERK1 is required for its chitin-dependent in vivo phosphorylation, as well as for early defense responses and downstream signaling. Collectively, our data suggest that in Arabidopsis, CERK1 is a major chitin, chitosan, and chito-oligomer binding component and that chitin signaling depends on CERK1 post-translational modification and kinase activity.

Isolation and characterization of pHW15, a small cryptic plasmid from Rahnella genomospecies 2.

A small cryptic plasmid designated pHW15 was isolated from Rahnella genomospecies 2 WMR15 and its complete nucleotide sequence was determined. The plasmid contained 3002 bp with a G+C content of 47.4%. The origin of replication was identified by deletion analysis as a region of about 600 bp. This region had an identity of 70% to the replication origin of the ColE1 plasmid at the nucleotide level. Sequence analysis revealed the typical elements: RNA I, RNA II and their corresponding promoters, a sequence allowing hybridisation of RNA II to the DNA and favouring processing by RNaseH, a single-strand initiation determinant (ssi) that allows initiation of lagging-strand synthesis, and a terH sequence required for termination of lagging-strand synthesis. The plasmid contained three expressed open reading frames, one of which showed homology to a ColE1 plasmid-encoded protein. Furthermore, a multimer resolution site was identified by sequence analysis. Its deletion resulted in formation of plasmid multimers during growth leading to an increased plasmid loss rate.