Trifluoromethyloxadiazoles: inhibitors of histone deacetylases for control of Asian soybean rust.

BACKGROUND: Trifluoromethyloxadiazoles (TFMOs) are selective inhibitors of class II histone deacetylases (HDACs). To date, class II HDACs have not been addressed as target enzymes by commercial fungicides. RESULTS: Antifungal testing of a broad variety of TFMOs against several important plant pathogens showed activity against only rusts, and especially Phakopsora pachyrhizi, the cause of Asian soybean rust. A structure-activity relationship was established, leading to highly active fungicides that inhibit fungal class II and HOS3-type HDACs of Aspergillus nidulans. Studies of the enzyme-inhibitor binding mode using protein structural information based on the crystal structure of human HDAC4 argue that TFMOs inhibit these enzymes only after undergoing hydration. CONCLUSION: Fungal class II HDACs are potential target enzymes for the control of at least some biotrophic crop diseases, in particular Asian soybean rust. As with any novel mode-of-action, class II HDAC fungicides would offer the potential to control fungal isolates that show reduced sensitivity toward existing commercial fungicides.

Innovative selection approach for a new antifungal agent mefentrifluconazole (Revysol®) and the impact upon its toxicity profile.

Mefentrifluconazole (trade name: Revysol®) is an agrochemical active ingredient from the new sub-class of isopropanol-triazole fungicides, with high selective fungicide activity. A full program of toxicity testing conducted according to OECD guidelines has shown mefentrifluconazole (MFZ) to be non-genotoxic and non-carcinogenic. Repeated dose studies in rats, mice and dogs identified the liver as the main target organ. Prenatal developmental toxicity studies in rats and rabbits did not indicate treatment-related embryofetal toxicity or teratogenicity up to the highest dose levels tested. In a two-generation dietary study in rats, the high dose level resulted in reduced food consumption and body weight gain throughout the dosing-period. Mating performance and fertility, estrous cycles, gestation length and pre-and post-natal survival of offspring were essentially unaffected and there was no evidence of masculinization of female pups or feminization of male pups. The screening strategy that led to the selection of MFZ was aimed to identify candidates with both high fungicidal activity and minimal likelihood of adverse side effects thought to arise from aromatase inhibition. The success of the selection strategy has been illustrated for MFZ by the absence in toxicity studies of effects that would indicate an endocrine disrupting potential.

The cytochrome bc1 complex inhibitor Ametoctradin has an unusual binding mode.

Ametoctradin is an agricultural fungicide that selectively inhibits the cytochrome bc1 complex of oomycetes. Previous spectrophotometric studies using the purified cytochrome bc1 complex from Pythium sp. showed that Ametoctradin binds to the Qo-site of the enzyme. However, as modeling studies suggested a binding mode like that of the substrate ubiquinol, the possibility for a dual Qo- and Qi-site binding mode was left open. In this work, binding studies and enzyme assays with mitochondrial membrane preparations from Pythium sp. and an S. cerevisiae strain with a modified Qi-site were used to investigate further the binding mode of Ametoctradin. The results obtained argue that the compound could bind to both the Qo- and Qi-sites of the cytochrome bc1 complex and that its position or binding pose in the Qi-site differs from that of Cyazofamid and Amisulbrom, the two Qi-site-targeting, anti-oomycetes compounds. Furthermore, the data support the argument that Ametoctradin prefers binding to the reduced cytochrome bc1 complex. Thus, Ametoctradin has an unusual binding mode and further studies with this compound may offer the opportunity to better understand the catalytic cycle of the cytochrome bc1 complex.

An RNAi-Based Control of Fusarium graminearum Infections Through Spraying of Long dsRNAs Involves a Plant Passage and Is Controlled by the Fungal Silencing Machinery.

Meeting the increasing food and energy demands of a growing population will require the development of ground-breaking strategies that promote sustainable plant production. Host-induced gene silencing has shown great potential for controlling pest and diseases in crop plants. However, while delivery of inhibitory noncoding double-stranded (ds)RNA by transgenic expression is a promising concept, it requires the generation of transgenic crop plants which may cause substantial delay for application strategies depending on the transformability and genetic stability of the crop plant species. Using the agronomically important barley-Fusarium graminearum pathosystem, we alternatively demonstrate that a spray application of a long noncoding dsRNA (791 nt CYP3-dsRNA), which targets the three fungal cytochrome P450 lanosterol C-14α-demethylases, required for biosynthesis of fungal ergosterol, inhibits fungal growth in the directly sprayed (local) as well as the non-sprayed (distal) parts of detached leaves. Unexpectedly, efficient spray-induced control of fungal infections in the distal tissue involved passage of CYP3-dsRNA via the plant vascular system and processing into small interfering (si)RNAs by fungal DICER-LIKE 1 (FgDCL-1) after uptake by the pathogen. We discuss important consequences of this new finding on future RNA-based disease control strategies. Given the ease of design, high specificity, and applicability to diverse pathogens, the use of target-specific dsRNA as an anti-fungal agent offers unprecedented potential as a new plant protection strategy.

Improving protein and proteome coverage through data-independent multiplexed peptide fragmentation.

Performance differences in protein and proteome characterization achieved by data-independent acquisition (DIA) LC/MS(E) and data-dependent acquisition (DDA) LC/MS/MS approaches were investigated. LC/MS(E) is a novel mode of generating product ion data for all coeluting precursors in parallel as opposed to LC/MS/MS where coeluting precursors must be serially fragmented one at a time. During LC/MS(E) analysis, alternating MS scans of "normal" and "elevated" collision energy are collected at regular intervals, providing nearly a 100% duty cycle for precursor detection and fragmentation because all precursors are fragmented across their full chromatographic elution profile. This is in contrast to DDA-based MS/MS where serial selection of precursor ions is biased toward interrogation and detection of the highest abundance sample components by virtue of the intensity-driven interrogation scheme employed. Both modes of acquisition were applied to a simple four-protein standard mixture with a 16-fold dynamic range in concentration, an in-gel digest of the Arabidopsis thaliana protein FLS2 purified by immunoprecipitation, and a solution-digested tomato leaf proteome sample. Dramatic improvement for individual protein sequence coverage was obtained for all three samples analyzed by the DIA approach, particularly for the lowest abundance sample components. In many instances, precursors readily detected and identified during DIA were either interrogated by MS/MS during DDA at inopportune points in their chromatographic elution profiles resulting in poor quality product ion spectra or not interrogated at all. Detailed evaluation of both DDA and DIA raw data and timing of the MS-to-MS/MS switching events clearly revealed the fundamental limitations of serial MS/MS interrogation and the advantages of parallel fragmentation by DIA for more comprehensive protein identification and characterization which holds promise for enhanced isoform and post-translational modification analysis.

Perception of the Arabidopsis danger signal peptide 1 involves the pattern recognition receptor AtPEPR1 and its close homologue AtPEPR2.

Plasma membrane-borne pattern recognition receptors, which recognize microbe-associated molecular patterns and endogenous damage-associated molecular patterns, provide the first line of defense in innate immunity. In plants, leucine-rich repeat receptor kinases fulfill this role, as exemplified by FLS2 and EFR, the receptors for the microbe-associated molecular patterns flagellin and elongation factor Tu. Here we examined the perception of the damage-associated molecular pattern peptide 1 (AtPep1), an endogenous peptide of Arabidopsis identified earlier and shown to be perceived by the leucine-rich repeat protein kinase PEPR1. Using seedling growth inhibition, elicitation of an oxidative burst and induction of ethylene biosynthesis, we show that wild type plants and the pepr1 and pepr2 mutants, affected in PEPR1 and in its homologue PEPR2, are sensitive to AtPep1, but that the double mutant pepr1/pepr2 is completely insensitive. As a central body of our study, we provide electrophysiological evidence that at the level of the plasma membrane, AtPep1 triggers a receptor-dependent transient depolarization through activation of plasma membrane anion channels, and that this effect is absent in the double mutant pepr1/pepr2. The double mutant also fails to respond to AtPep2 and AtPep3, two distant homologues of AtPep1 on the basis of homology screening, implying that the PEPR1 and PEPR2 are responsible for their perception too. Our findings provide a basic framework to study the biological role of AtPep1-related danger signals and their cognate receptors.

Rapid heteromerization and phosphorylation of ligand-activated plant transmembrane receptors and their associated kinase BAK1.

In plants leucine-rich repeat receptor kinases (LRR-RKs) located at the plasma membrane play a pivotal role in the perception of extracellular signals. For two of these LRR-RKs, the brassinosteroid receptor BRI1 and the flagellin receptor FLS2, interaction with the LRR receptor-like kinase BAK1 (BRI1-associated receptor kinase 1) was shown to be required for signal transduction. Here we report that FLS2.BAK1 heteromerization occurs almost instantaneously after perception of the ligand, the flagellin-derived peptide flg22. Flg22 can induce formation of a stable FLS2.BAK1 complex in microsomal membrane preparations in vitro, and the kinase inhibitor K-252a does not prevent complex formation. A kinase dead version of BAK1 associates with FLS2 in a flg22-dependent manner but does not restore responsiveness to flg22 in cells of bak1 plants, demonstrating that kinase activity of BAK1 is essential for FLS2 signaling. Furthermore, using in vivo phospholabeling, we are able to detect de novo phosphorylation of both FLS2 and BAK1 within 15 s of stimulation with flg22. Similarly, brassinolide induces BAK1 phosphorylation within seconds. Other triggers of plant defense, such as bacterial EF-Tu and the endogenous AtPep1 likewise induce rapid formation of heterocomplexes consisting of de novo phosphorylated BAK1 and proteins representing the ligand-specific binding receptors EF-Tu receptor and Pep1 receptor 1, respectively. Thus, we propose that several LRR-RKs form tight complexes with BAK1 almost instantaneously after ligand binding and that the subsequent phosphorylation events are key initial steps in signal transduction.

Uncoupling of sustained MAMP receptor signaling from early outputs in an Arabidopsis endoplasmic reticulum glucosidase II allele.

Recognition of microbe-associated molecular patterns (MAMPs), conserved structures typical of a microbial class, triggers immune responses in eukaryotes. This is accompanied by a diverse set of physiological responses that are thought to enhance defense activity in plants. However, the extent and mechanisms by which MAMP-induced events contribute to host immunity are poorly understood. Here we reveal Arabidopsis priority in sweet life4 (psl4) and psl5 mutants that are insensitive to the bacterial elongation factor (EF)-Tu epitope elf18 but responsive to flagellin epitope flg22. PSL4 and PSL5, respectively, identify beta- and alpha-subunits of endoplasmic reticulum-resident glucosidase II, which is essential for stable accumulation and quality control of the elf18 receptor EFR but not the flg22 receptor FLS2. We notice that EFR signaling is partially and differentially impaired without a significant decrease of the receptor steady-state levels in 2 weakly dysfunctional gIIalpha alleles, designated psl5-1 and rsw3. Remarkably, rsw3 plants exhibit marked supersusceptibility against a virulent bacterial phytopathogen despite nearly intact coactivation of MAPKs, reactive oxygen species, ethylene biosynthesis, and callose deposition in response to elf18, demonstrating that these signaling outputs alone are insufficient to mount effective immunity. However, rsw3 plants fail to maintain high transcript levels of defense-promoting WRKY, PR1, and PR2 genes at late time points (4 to 24 h) after elf18 elicitation. This points to an unexpected separation between initial and sustained activation of EFR-mediated signaling in the absence of proper glucosidase II-mediated endoplasmic reticulum quality control. Our findings strongly suggest the importance of sustained MAMP receptor signaling as a key step in the establishment of robust immunity.

Plant pattern-recognition receptor FLS2 is directed for degradation by the bacterial ubiquitin ligase AvrPtoB.

BACKGROUND: An important layer of active defense in plant immunity is the detection of pathogen-associated molecular patterns (PAMPs) mediated by cell-surface receptors. For the establishment of disease, pathogens depend on the ability to overcome PAMP perception and disable plant signaling pathways activated in response to PAMPs. Pattern recognition receptors (PRRs) are therefore prime targets for pathogen effectors. FLS2, its coreceptor BAK1, and EFR encode receptor-like kinases that play a role in immunity against bacterial pathogens. RESULTS: Here, we report that virulence of Pseudomonas syringae pv tomato DC3000 (PtoDC3000) in Arabidopsis is enhanced through the action of its effector AvrPtoB, which promotes degradation of FLS2. We show that AvrPtoB, through its N terminus, associates with FLS2 and BAK1, of which interaction with FLS2 is enhanced by flg22 activation. In vitro, AvrPtoB is active as an E3 ligase to catalyze polyubiquitination of the kinase domain of FLS2, a process confirmed in planta. Full enhancement of PtoDC3000 virulence appears to require the E3 ligase activity of AvrPtoB. CONCLUSIONS: AvrPtoB, initially identified through its activation of hypersensitive resistance in tomato cultivars expressing the Pto kinase, is composed of at least two functional domains: the N terminus is responsible for interaction with Pto, and the C terminus carries an E3 ligase activity. Based on our findings, we propose that both domains of AvrPtoB act together to support the virulence of PtoDC3000 in Arabidopsis through their ability to eliminate FLS2 from the cell periphery, and probably also other PAMP sensors that are constitutively expressed or induced after pathogen challenge.