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 FLS2⅐BAK1 heteromerization occurs almost instantaneously after perception of the ligand, the flagellin-derived peptide flg22. Flg22 can induce formation of a stable FLS2⅐BAK1 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.One of the central themes in cell biology is the sensing of extracellular chemical signals through cell surface receptors: How does the event of receptor-ligand interaction on the outside of the cell activate a signal transduction chain in the inside of the cell? In higher plants, the most prominent class of membrane receptors is formed by proteins with intracellular serine/ threonine-type protein kinases. These receptors account for ϳ2.5-4% of all proteins encoded by the genome of a plant (1). Despite their importance, there is still little experimental evidence on the molecular activation mechanisms of plant transmembrane receptor kinases. Current models are based on the precedent of animal receptor tyrosine kinases where ligand binding causes receptor tyrosine kinases to form homo-or hetero-oligomers, followed by transphosphorylation (2). In the case of the epidermal growth factor receptor, these phosphorylation events occur within 60 s of receptor activation (3).The best studied plant transmembrane receptor kinase is BRI1, the receptor for the brassinosteroid growth hormones (4). BRI1 is one of the 224 members of LRR-RKs 2 in Arabidopsis (5). Upon ligand binding BRI1 interacts with a second LRR receptor-like kinase named BAK1 (6, 7). Two further well characterized plant LRR-RKs are the flagellin receptor FLS2 (flagellin sensing 2) (8) and the EF-Tu receptor (EFR) (9). FLS2 perceives a generally conser...

Plant cells can be sensitized toward a subsequent pathogen attack by avirulent pathogens or by chemicals such as b-aminobutyric acid (BABA). This process is called priming. Using a reverse genetic approach in Arabidopsis thaliana, we demonstrate that the BABA-responsive L-type lectin receptor kinase-VI.2 (LecRK-VI.2) contributes to disease resistance against the hemibiotrophic Pseudomonas syringae and the necrotrophic Pectobacterium carotovorum bacteria. Accordingly, LecRK-VI.2 mRNA levels increased after bacterial inoculation or treatments with microbe-associated molecular patterns (MAMPs). We also show that LecRK-VI.2 is required for full activation of pattern-triggered immunity (PTI); notably, lecrk-VI.2-1 mutants show reduced upregulation of PTI marker genes, impaired callose deposition, and defective stomatal closure. Overexpression studies combined with genome-wide microarray analyses indicate that LecRK-VI.2 positively regulates the PTI response. LecRK-VI.2 is demonstrated to act upstream of mitogen-activated protein kinase signaling, but independently of reactive oxygen production and BOTRYTIS-INDUCED KINASE1 phosphorylation. In addition, complex formation between the MAMP receptor FLAGELLIN SENSING2 and its signaling partner BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 is observed in flg22-treated lecrk-VI.2-1 mutants. LecRK-VI.2 is also required for full BABA-induced resistance and priming of PTI. Our work identifies LecRK-VI.2 as a novel mediator of the Arabidopsis PTI response and provides insight into molecular mechanisms governing priming.

In higher plants, biotic stress (e.g., herbivore or pathogen attack) as well as abiotic stress (in particular heavy metals) often induce the synthesis and accumulation of the same defense-related secondary metabolites. This well-known finding still awaits an explanation regarding the common features of both stress types. In this study, a mechanism is proposed that links reactive oxygen species (ROS) generation with lipid oxidation processes, ultimately resulting in the formation of similar, highly active signalling compounds. The generation of ROS is a common event in both heavy metal treatment and biotic stress although it can depend on quite different, enzymatic and non-enzymatic reactions. Regardless, ROS are involved in the oxidation of unsaturated fatty acids which initiate the formation of oxylipins, a highly variable class of lipid-derived compounds in plants. Oxylipins represent new endogenous signals involved in biotic-and abiotic-induced stress responses.

Plant volatiles emitted by Medicago truncatula in response to feeding larvae of Spodoptera exigua are composed of a complex blend of terpenoids. The cDNAs of three terpene synthases (TPSs), which contribute to the blend of terpenoids, were cloned from M. truncatula. Their functional characterization proved MtTPS1 to be a -caryophyllene synthase and MtTPS5 to be a multi-product sesquiterpene synthase. MtTPS3 encodes a bifunctional enzyme producing (E)-nerolidol and geranyllinalool (precursors of C 11 and C 16 homoterpenes) from diVerent prenyl diphosphates serving as substrates. The addition of jasmonic acid (JA) induced expression of the TPS genes, but terpenoid emission was higher from plants treated with JA and the ethylene precursor 1-amino-cyclopropyl-1-carboxylic acid. Compared to infested wild-type M. truncatula plants, lower amounts of various sesquiterpenes and a C 11 -homoterpene were released from an ethylene-insensitive mutant skl. This diVerence coincided with lower transcript levels of MtTPS5 and of 1-deoxy-D-xylulose-5-phosphate synthase (MtDXS2) in the damaged skl leaves. Moreover, ethephon, an ethylene-releasing compound, modiWed the extent and mode of the herbivore-stimulated Ca 2+ variations in the cytoplasm that is necessary for both JA and terpene biosynthesis. Thus, ethylene contributes to the herbivory-induced terpenoid biosynthesis at least twice: by modulating both early signaling events such as cytoplasmic Ca 2+ -inXux and the downstream JA-dependent biosynthesis of terpenoids.

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The endogenous Arabidopsis (Arabidopsis thaliana) peptides, AtPeps, elicit an innate immune response reminiscent of patterntriggered immunity. Detection of various danger signals, including microbe-associated molecular patterns (MAMPs), leads to elevated transcription of PROPEPs, the AtPep precursors, and PEPRs, the AtPep receptors. It has been hypothesized that AtPeps are involved in enhancing pattern-triggered immunity. Following this idea, we analyzed the relationship between MAMP-and AtPep-elicited signaling. We found that the perception of MAMPs enhanced a subsequent AtPep-triggered production of reactive oxygen species (ROS). Intriguingly, other components of AtPep-triggered immunity like Ca 2+ influx, mitogen-activated protein kinase phosphorylation, ethylene production, and expression of early defense genes, as well as ROS-activated genes, remained unchanged. By contrast, treatment with methyl jasmonate promoted an increase of all analyzed AtPep-triggered responses. We positively correlated the intensities of generic AtPep-triggered responses with the abundance of the two AtPep receptors by generating constitutively expressing PEPR1 and PEPR2 transgenic lines and by analyzing pepr1 and pepr2 mutants. Further, we show that enhanced, as well as basal, ROS production triggered by AtPeps is absent in the double mutant of the respiratory burst oxidase homologs D and F (rbohD rbohF). We present evidence that the enhancement of AtPep-triggered ROS is not based on changes in the ROS detoxification machinery and is independent of mitogen-activated protein kinase and Ca 2+ signaling pathways. Taken together, these results indicate an additional level of regulation besides receptor abundance for the RbohD/RbohF-dependent production of AtPep-elicited ROS, which is specifically operated by MAMP-triggered pathways.

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In the literature it is often hypothesised that contracts and vertical integration are of paramount importance for the future competitiveness of meat supply chains. The continued coexistence of two different types of pork production chains throughout the world - less integrated systems in, for instance, many Western European countries and stricter coordinated systems based on contracts and vertical integration in countries such as the U.S.A. and Denmark - is rarely discussed in the agribusines literature. Some authors expect a trend towards more intense vertical coordination and consider the loose relationships in some European markets as an obsolescent model. Based on an overview of theoretical approaches towards the organisation of meat supply chains, we confront the contract and integration hypothesis with current developments at the slaughterhouse level and in quality assurance systems as well as results from a recent survey among German pig producers concerning their attitudes towards contracts. A binary logit regression is carried out to test for the determinants of contracting decisions. Cluster analysis reveals four groups of farmers with very different attitudes, from inveterate antagonists to contract supporters. In conclusion, reflections on TCE indicate the long-term viability of slaughter pig markets with a low degree of vertical coordination, which may have efficiency advantages over more integrated meat supply chains. This argument is supported by the results of a large-scale survey among German pig producers, who strongly reject the notion of engaging in contracts with slaughterhouses. In this situation, trust management can be considered a suitable instrument for gaining some of the advantages usually attributed to contracts. Trust-building instruments should be integrated into a broader approach of supplier relationship management.