[show abstract][hide abstract] ABSTRACT: Guard cells regulate plant gas exchange and transpiration by modulation of stomatal aperture upon integrating external cues like photosynthetic effective illumination, CO(2) 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 pre-invasive level. The signaling pathways leading to stomatal closure triggered by biotic and abiotic stresses employ several common components like reactive oxygen species (ROS), calcium, kinases and hormones, suggesting considerable intersection between MAMP- and ABA-induced stomatal closures, which we will discuss in this review.
[show abstract][hide abstract] ABSTRACT: Zusammenfassung 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.
[show abstract][hide abstract] ABSTRACT: Reactive oxygen species (ROS) are potent signal molecules rapidly generated in response to stress. Detection of pathogen-associated 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.
[show abstract][hide abstract] ABSTRACT: Plant 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.
We 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.
Combining 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.
[show abstract][hide abstract] ABSTRACT: 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.
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.
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.
Current biology: CB 01/2009; 18(23):1824-32. · 10.99 Impact Factor