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.

Reactive oxygen species (ROS) are potent signal molecules rapidly generated in response to stress. Detection of pathogenassociated molecular patterns induces a transient apoplastic ROS through the function of the NADPH respiratory burst oxidase homologs D (RbohD). However, little is known about the regulation of pathogen-associated molecular pattern-elicited ROS or its role in plant immunity. We investigated ROS production triggered by bacterial flagellin (flg22) in Arabidopsis (Arabidopsis thaliana). The oxidative burst was diminished in ethylene-insensitive mutants. Flagellin Sensitive2 (FLS2) accumulation was reduced in etr1 and ein2, indicating a requirement of ethylene signaling for FLS2 expression. Multiplication of virulent bacteria was enhanced in Arabidopsis lines displaying altered ROS production at early but not late stages of infection, suggesting an impairment of preinvasive immunity. Stomatal closure, a mechanism used to reduce bacterial entry into plant tissues, was abolished in etr1, ein2, and rbohD mutants. These results point to the importance of flg22-triggered ROS at an early stage of the plant immune response.

BackgroundPlant Receptor-like/Pelle kinases (RLK) are a group of conserved signalling components that regulate developmental programs and responses to biotic and abiotic stresses. One of the largest RLK groups is formed by the Domain of Unknown Function 26 (DUF26) RLKs, also called Cysteine-rich Receptor-like Kinases (CRKs), which have been suggested to play important roles in the regulation of pathogen defence and programmed cell death. Despite the vast number of RLKs present in plants, however, only a few of them have been functionally characterized.ResultsWe examined the transcriptional regulation of all Arabidopsis CRKs by ozone (O3), high light and pathogen/elicitor treatment - conditions known to induce the production of reactive oxygen species (ROS) in various subcellular compartments. Several CRKs were transcriptionally induced by exposure to O3 but not by light stress. O3 induces an extracellular oxidative burst, whilst light stress leads to ROS production in chloroplasts. Analysis of publicly available microarray data revealed that the transcriptional responses of the CRKs to O3 were very similar to responses to microbes or pathogen-associated molecular patterns (PAMPs). Several mutants altered in hormone biosynthesis or signalling showed changes in basal and O3-induced transcriptional responses.ConclusionsCombining expression analysis from multiple treatments with mutants altered in hormone biosynthesis or signalling suggest a model in which O3 and salicylic acid (SA) activate separate signaling pathways that exhibit negative crosstalk. Although O3 is classified as an abiotic stress to plants, transcriptional profiling of CRKs showed strong similarities between the O3 and biotic stress responses.

Guard cells regulate plant gas exchange and transpiration by modulation of stomatal aperture upon integrating external cues like photosynthetic effective illumination, CO2 levels and water availability and internal signals like abscisic acid (ABA). Being pores, stomata constitute a natural entry site for potentially harmful microbes. To prevent microbial invasion, stomata close upon perception of microbe-associated molecular patterns (MAMPs), and this represents an important layer of active immunity at the preinvasive level. The signaling pathways leading to stomatal closure triggered by biotic and abiotic stresses employ several common components, such as reactive oxygen species, calcium, kinases, and hormones, suggesting considerable intersection between MAMP- and ABA-induced stomatal closures, which we will discuss in this review.

Ü b e r s i c h t s a rt i k e lZusammenfassung Pflanzen erkennen Mikroorganismen anhand konservierter Strukturen, was eine aktive Immunabwehr auslöst. Pathogene müssen dies umgehen, doch Pflanzen können ihr Immunsystem anpassen, so kommt es zu einem Wettrüsten zwischen Pflanze und Erreger.Pflanzen müssen sich durch ihre sessile Lebensform an unterschiedliche Bedingungen wie Sonneneinstrahlung, Temperatur, Trockenheit oder Mikroorganismen anpassen. Sie schützen sich vor Krankheiten durch eine frühzeitige Erkennung von potenziellen Erregern und eine effiziente Immunabwehr. Auf einer ersten Ebene kann das pflanzliche Immunsystem Mikroben global erkennen, wodurch es aktiviert wird (Bittel und Robatzek 2007). Auf einer zweiten Ebene erkennen bestimmte Pflanzenkultivare spezifisch einzelne mikrobielle Stämme - ein Phänomen, das auch als "Gen-für-Gen-Resistenz" bezeichnet wird (Jones und Dangl 2006). Die erste Ebene des Immunsystems läuft schnell ab und führt zu einer aktiven Abwehr, die in der Regel ohne Schaden der pflanzlichen Zellen abläuft. Die zweite Ebene des pflanzlichen Immunsystems bildet sich nach einigen Tagen aus und umfasst einen lokalen Zelltod, der die Erreger von einem weiteren Eindringen in das Gewebe abhält. Neben diesen Zell-autonomen Abwehrsystemen haben Pflanzen auch Strategien zur systemischen Immunität entwickelt.Schlüsselwörter Immunabwehr · Flagellin · Mikroorganismen · Pathogene Self or Non-Self: The Receptors of the Plant Immune System A Continuous Adaptation Leads to an Arms Race Between Plants and PathogensAbstract Plants recognize conserved molecular structures from microorganisms, which triggers active immune responses. Successful pathogens have to overcome this level of immunity; however, plants in turn can adapt their immune system, thus plants and pathogens are in an evolutionary arms race.As being sessile organisms, plants need to integrate and adapt to changing environmental conditions such as light, temperature, drought, or microorganisms. Plants protect themselves against diseases through sensitive recognition of potential pathogens and effective defense systems. The first level of the plant immune system provides recognition of a broad spectrum of microorganisms leading to defense activation (Bittel and Robatzek 2007). The second level of the plant immunity allows certain plant cultivars to detect of specific pathogen strains-a phenomenon also referred to as "gene-for-gene resistance" (Jones and Dangl 2006). The first level of immunity occurs rapidly and triggers active defenses normally without harm to the plant cell. The second level of plant immunity develops over days and deploys a local cell death, which prevents pathogens from further spread into tissues. In addition to these cell-autonomous defense systems, plants have also evolved strategies of systemic immunity.