New papers from the Dresselhaus, Ott and Parniske labs.
External application of gametophyte-specific ZmPMEI1 induces pollen tube burst in maize.
Mayada Woriedh, Sebastian Wolf, Mihaela L. Márton, Axel Hinze, Manfred Gahrtz, Dirk Becker and Thomas Dresselhaus
Regulated demethylesterification of homogalacturonan, a major component of plant cell walls, by the activity of pectin methylesterases (PMEs), plays a critical role for cell wall stability and integrity. Especially fast growing plant cells such as pollen tubes secrete large amounts of PMEs toward their apoplasmic space. PME activity itself is tightly regulated by its inhibitor named as PME inhibitor and is thought to be required especially at the very pollen tube tip. We report here the identification and functional characterization of PMEI1 from maize (ZmPMEI1). We could show that the protein acts as an inhibitor of PME but not of invertases and found that its gene is strongly expressed in both gametophytes (pollen grain and embryo sac). Promoter reporter studies showed gene activity also during pollen tube growth toward and inside the transmitting tract. All embryo sac cells except the central cell displayed strong expression. Weaker signals were visible at sporophytic cells of the micropylar region. ZmPMEI1–EGFP fusion protein is transported within granules inside the tube and accumulates at the pollen tube tip as well as at sites where pollen tubes bend and/or change growth directions. The female gametophyte putatively influences pollen tube growth behavior by exposing it to ZmPMEI1. We therefore simulated this effect by applying recombinant protein at different concentrations on growing pollen tubes. ZmPMEI1 did not arrest growth, but destabilized the cell wall inducing burst. Compared with female gametophyte secreted defensin-like ZmES4, which induces burst at the very pollen tube tip, ZmPMEI1-induced burst occurs at the subapical region. These findings indicate that ZmPMEI1 secreted by the embryo sac likely destabilizes the pollen tube wall during perception and together with other proteins such as ZmES4 leads to burst and thus sperm release.
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Intrinsic Disorder in Pathogen Effectors: Protein Flexibility as an Evolutionary Hallmark in a Molecular Arms Race[W].
Macarena Marína, Vladimir N. Uversky and Thomas Ott
Effector proteins represent a refined mechanism of bacterial pathogens to overcome plants’ innate immune systems. These modular proteins often manipulate host physiology by directly interfering with immune signaling of plant cells. Even if host cells have developed efficient strategies to perceive the presence of pathogenic microbes and to recognize intracellular effector activity, it remains an open question why only few effectors are recognized directly by plant resistance proteins. Based on in-silico genome-wide surveys and a reevaluation of published structural data, we estimated that bacterial effectors of phytopathogens are highly enriched in long-disordered regions (>50 residues). These structurally flexible segments have no secondary structure under physiological conditions but can fold in a stimulus-dependent manner (e.g., during protein–protein interactions). The high abundance of intrinsic disorder in effectors strongly suggests positive evolutionary selection of this structural feature and highlights the dynamic nature of these proteins. We postulate that such structural flexibility may be essential for (1) effector translocation, (2) evasion of the innate immune system, and (3) host function mimicry. The study of these dynamical regions will greatly complement current structural approaches to understand the molecular mechanisms of these proteins and may help in the prediction of new effectors.
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Cell and developmental biology of arbuscular mycorrhiza symbiosis.
Caroline Gutjahr and Martin Parniske
Annual Reviews of Cell and Developmental Biology
The default mineral nutrient acquisition strategy of land plants is the sym-biosis with arbuscular mycorrhiza (AM) fungi. Research into the cell anddevelopmental biology of AM revealed fascinating insights into the plas-ticity of plant cell development and of interorganismic communication. Itis driven by the prospect of increased exploitation of AM benefits for sus-tainable agriculture. The plant cell developmental program for intracellularaccommodation of AM fungi is activated by a genetically defined signal-ing pathway involving calcium spiking in the nucleus as second messenger.Calcium spiking is triggered by chitooligosaccharides released by AM fungithat are probably perceived via LysM domain receptor kinases. Fungal in-fection and calcium spiking are spatiotemporally coordinated, and only cellscommitted to accommodating the fungus undergo high-frequency spiking.Delivery of mineral nutrients by AM fungi occurs at tree-shaped hyphalstructures, the arbuscules, in plant cortical cells. Nutrients are taken up ata plant-derived periarbuscular membrane, which surrounds fungal hyphaeand carries a specific transporter composition that is of direct importance forsymbiotic efficiency. An elegant study has unveiled a new and unexpectedmechanism for specific protein localization to the periarbuscular membrane,which relies on the timing of gene expression to synchronize protein biosyn-thesis with a redirection of secretion. The control of AM development byphytohormones is currently subject to active investigation and has led to therediscovery of strigolactones. Nearly all tested phytohormones regulate AMdevelopment, and major insights into the mechanisms of this regulation areexpected in the near future.
