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Microbial plant pathogens deploy amphipathic cyclic lipopeptides to reduce surface tension in their environment. While plants can detect these molecules to activate cellular stress responses, the role of these lipopeptides or associated host responses in pathogenesis are not fully clear. The gramillin cyclic lipopeptide is produced by the Fusarium graminearum fungus and is a virulence factor and toxin in maize. Here, we show that gramillin promotes virulence and necrosis in both monocots and dicots by disrupting ion balance across membranes. Gramillin is a cation-conducting ionophore and causes plasma membrane depolarization. This disruption triggers cellular signaling, including a burst of reactive oxygen species (ROS), transcriptional reprogramming, and callose production. Gramillin-induced ROS depends on expression of host ILK1 and RBOHD genes, which promote fungal induction of virulence genes during infection and host susceptibility. We conclude that gramillin's ionophore activity targets plant membranes to coordinate attack by the F. graminearum fungus. [Display omitted] • F. graminearum produces an ion-conducting toxin, gramillin, that promotes plant host infection • Gramillin induces host stress responses that are genetically linked to disease susceptibility • Targeting ionophore-response genes is a novel approach for developing plant resistance Brauer et al. describe the ion-conducting activity of the gramillin toxin from Fusarium graminearum. Gramillin advances fungal infestation of the plant by promoting fungal secondary metabolite gene expression. Plants detect ionophores, including gramillin, to activate cellular stress responses, and the genes influencing these responses promote host susceptibility. [ABSTRACT FROM AUTHOR]

• Several classes of small peptides amplify danger pattern perception and immunity. • Small peptides and their receptors regulate immune receptor complex formation. • Growth-promoting peptides influence plant defense responses to different pathogens. • Advanced genomic and proteomic tools will uncover new classes of small peptides. Small peptides regulate the cellular coordination of growth, development, and stress tolerance in plants. In addition to direct antimicrobial activities, small secreted peptides have emerged as key signaling molecules in the plant immune response. Here, we highlight recent discoveries of several small peptides that amplify and fine-tune immune signaling. [ABSTRACT FROM AUTHOR]

Arabidopsis ( Arabidopsis thaliana ) At BIK1 and rice ( Oryza sativa ) OsRLCK176 are orthologous receptor-like cytoplasmic kinases involved in immune signaling. Recent studies indicate that proteasomal turnover of these kinases is regulated by orthologous Ca 2+ -dependent protein kinases At CPK28 and Os CPK4, revealing conserved interplay between phosphorylation and ubiquitination in immune homeostasis. [ABSTRACT FROM AUTHOR]

Mechanisms to sense and respond to calcium have evolved in all organisms. Calmodulin is a universal calcium sensor across eukaryotes that directly binds calcium and associates with many downstream signal transducers including protein kinases. All eukaryotes encode calcium-dependent and/or calmodulin-dependent kinases, however there are distinct protein families across kingdoms. Here, we compare the activation mechanisms of calmodulin-dependent protein kinases (CaMKs), calcium- and calmodulin-dependent protein kinases (CCaMKs) and calcium-dependent protein kinases (CDPKs), noting striking similarities regarding phosphorylation in a regulatory segment known as the autoinhibitory junction. We thus propose that conserved regulation by phosphorylation underlies the activation of calcium-responsive proteins from different kingdoms. [Display omitted] • Structural comparison of AIJ in CaMKs, CCaMKs, and CDPKs. • Conserved autophosphorylation in AIJ of CaMKs, CCaMKs, and CDPKs across kingdoms. • Conserved mechanism-of-action of CaMKs, CCaMKs, and CDPKs. • Phosphorylation in the AIJ may allow for the decoding of specific calcium signatures. [ABSTRACT FROM AUTHOR]

Cell surface pattern recognition receptors sense invading pathogens by binding microbial or endogenous elicitors to activate plant immunity. These responses are under tight control to avoid excessive or untimely activation of cellular responses, which may otherwise be detrimental to host cells. How this fine-tuning is accomplished is an area of active study. We previously described a suppressor screen that identified Arabidopsis thaliana mutants with regained immune signaling in the immunodeficient genetic background bak1-5, which we named modifier of bak1-5 (mob) mutants. Here, we report that bak1-5 mob7 mutant restores elicitor-induced signaling. Using a combination of map-based cloning and whole-genome resequencing, we identified MOB7 as conserved binding of eIF4E1 (CBE1), a plant-specific protein that interacts with the highly conserved eukaryotic translation initiation factor eIF4E1. Our data demonstrate that CBE1 regulates the accumulation of respiratory burst oxidase homolog D, the NADPH oxidase responsible for elicitor-induced apoplastic reactive oxygen species production. Furthermore, several mRNA decapping and translation initiation factors colocalize with CBE1 and similarly regulate immune signaling. This study thus identifies a novel regulator of immune signaling and provides new insights into reactive oxygen species regulation, potentially through translational control, during plant stress responses. [ABSTRACT FROM AUTHOR]

A key feature of innate immunity is the ability to recognize and respond to potential pathogens in a highly sensitive and specific manner. In plants, the activation of pattern recognition receptors (PRRs) by pathogen-associated molecular patterns (PAMPs) elicits a defense programme known as PAMP-triggered immunity (PTI). Although only a handful of PAMP-PRR pairs have been defined, all known PRRs are modular transmembrane proteins containing ligand-binding ectodomains. It is becoming clear that PRRs do not act alone but rather function as part of multi-protein complexes at the plasma membrane. Recent studies describing the molecular interactions and protein modifications that occur between PRRs and their regulatory proteins have provided important mechanistic insight into how plants avoid infection and achieve immunity. [Copyright &y& Elsevier]

Recent studies suggest that immune-induced alternative splice variants of the Arabidopsis thaliana Ca2+-dependent protein kinase (CDPK) At CPK28 may result in signal attenuation. We put forward the hypothesis that expression of alternative truncated variants may be a broadly conserved regulatory mechanism of CDPKs throughout the green lineage. [ABSTRACT FROM AUTHOR]

Plant calcium (Ca2+)-dependent protein kinases (CPKs) represent the primary Ca2+-dependent protein kinase activities in plant systems. CPKs are composed of a dual specificity (Ser/Thr and Tyr) kinase domain tethered to a calmodulin-like domain (CLD) via an autoinhibitory junction (J). Although regulation of CPKs by Ca2+ has been extensively studied, the contribution of autophosphorylation in controlling CPK activity is less well understood. Furthermore, whether calmodulin (CaM) contributes to CPK regulation, as is the case for Ca2+/CaM-dependent protein kinases outside the plant lineage, remains an open question. We therefore screened a subset of plant CPKs for CaM binding and found that CPK28 is a high affinity Ca2+/CaMbinding protein. Using synthetic peptides and native gel electrophoresis, we coarsely mapped the CaM-binding domain to a site within the CPK28 J domain that overlaps with the known site of intramolecular interaction between the J domain and the CLD. Peptide kinase activity of fully dephosphorylated CPK28 was Ca2+-responsive and was inhibited by Ca2+/CaM. Using in situ autophosphorylated protein, we expand on the known set of CPK28 autophosphorylation sites, and we demonstrate that, unexpectedly, autophosphorylated CPK28 had enhanced kinase activity at physiological concentrations of Ca2+ compared with the dephosphorylated protein, suggesting that autophosphorylation functions to prime CPK28 for Ca2+ activation and might also allow CPK28 to remain active when Ca2+ levels are low. Furthermore, CPK28 autophosphorylation substantially reducedsensitivity of the kinase to Ca2+/CaM inhibition. Overall, our analyses uncover new complexities in the control of CPK28 and provide mechanistic support for Ca2+ signaling specificity through Ca2+ sensor priming. [ABSTRACT FROM AUTHOR]