The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Shared genetic risk between eating disorder- and substance-use-related phenotypes:Evidence from genome-wide association studies

First published: 16 February 202

Biochemische und molekularbiologische Untersuchungen zur Beteiligung reaktiver Sauerstoffspezies (ROS) am Wirkungsmechanismus der systemisch erworbenen Resistenz (SAR) in Arabidopsis thaliana (L.) Heynh

Systemic Acquired Resistance (SAR) can be conditioned in Arabidopsis by chemical and biological activators. This acquired resistance to a subsequent infection manifests itself as a phenocopy of the Hypersensitive Reaction (HR) and is correlated with an increased expression of so-called PR genes, which code for "pathogenesis related" proteins. SAR is also accompanied by a local and systemic accumulation of salicylic acid (SA). Treatment of plants with SA can also induce SAR against a broad spectrum of pathogens. SA serves as an important local signal substance which is essential for the conditioning of the SAR state. Further factors which might contribute to signalling in establishing the SAR state are either unknown or their role is controversial. The latter applies particular to the role of reactive oxygen species (ROS) in local and systemic signals. The work described in this thesis addresses the question of the role that various ROS play in establishing biologically and chemically induced SAR. In the first phase various test systems for the induction of SAR in Arabidopsis were established. SA accumulation as well as PR gene expression and the course of challenge inoculation were used as markers for the SAR state. If ROS accumulate either locally or systemically during SAR induction it could be possible to detect them. Therefore, various ROS were tested for. H2O2 accumulated locally but not systemically after inoculation with PmsDC3000/avrRpm1. O2.- was only detected locally after wounding. If ROS produced at the site of inoculation are involved in conditioning SAR, then treatment with ROS or ROS-generating systems might lead to the induction of resistance in systemic leaves, and / or the accumulation of molecular markers for SAR (PR mRNA, SA). To test this possibility, leaves were treated with H2O2, paraquat (in light -O2.-), H2O2-generating, O2.-generating and NO-generating enzyme systems. However, none of the treatments gave results comparable to biological or chemically induced SAR in the controls. If ROS produced at the site of inoculation are involved in conditioning SAR, then the application of inhibitors or treatments which affect ROS metabolism might lead to measurable changes in the SAR response. Candidates for ROS synthesis are the plasmamembrane NAD(P)H oxidase, cell wall peroxidases or xanthine oxidase (XO). If O2.- produced by NAD(P)H oxidase is involved in the SAR, its inhibition by DPI might be expected to lead to a reduction in SAR or its molecular markers. Similar, if O2.- is produced via XO activity, allopurinol (a XO inhibitor) treatment might be expected to lead to a reduction SAR marker accumulation and / or SAR itself. If there is H2O2 involved in conditioning SAR, then its destruction via infiltrated catalase might be expected to lead to a reduction in SAR markers. Additionally, the effects of the NO scavenger Carboxy-PTIO-potassium on SAR induction was investigated. In summary it can be said that, neither DPI nor catalase, allopurinol and Carboxy-PTIO-potassium led to a significant reduction in SA or PR transcript accumulation in either the inoculated or the systemic leaves. No significant effect on the conditioning of SAR was detected as the histological level either (challenge inoculation). The possible role of intracellular O2.- production was investigated using transgenic plants over- or under expressing (sense / antisense) a cytosolic superoxide dismutase (SOD). However, no SAR relevant phenotypes were observed. Independent of the nature of the systemic signal which leads to the conditioning of SAR throughout the plant, the pathway of signal transport was investigated. To address this aspect the pattern of assimilate transport out of an inoculated leaf into the rest of the plant was compared to the pattern induction of SAR and SAR molecular markers. The results are discussed in terms of the possible role of ROS in local and systemic signalling in Arabidopsis during the establishment of SAR

Biochemische und molekularbiologische Untersuchungen zur Beteiligung reaktiver Sauerstoffspezies (ROS) am Wirkungsmechanismus der systemisch erworbenen Resistenz (SAR) in Arabidopsis thaliana (L.) Heynh

Systemic Acquired Resistance (SAR) can be conditioned in Arabidopsis by chemical and biological activators. This acquired resistance to a subsequent infection manifests itself as a phenocopy of the Hypersensitive Reaction (HR) and is correlated with an increased expression of so-called PR genes, which code for "pathogenesis related" proteins. SAR is also accompanied by a local and systemic accumulation of salicylic acid (SA). Treatment of plants with SA can also induce SAR against a broad spectrum of pathogens. SA serves as an important local signal substance which is essential for the conditioning of the SAR state. Further factors which might contribute to signalling in establishing the SAR state are either unknown or their role is controversial. The latter applies particular to the role of reactive oxygen species (ROS) in local and systemic signals. The work described in this thesis addresses the question of the role that various ROS play in establishing biologically and chemically induced SAR. In the first phase various test systems for the induction of SAR in Arabidopsis were established. SA accumulation as well as PR gene expression and the course of challenge inoculation were used as markers for the SAR state. If ROS accumulate either locally or systemically during SAR induction it could be possible to detect them. Therefore, various ROS were tested for. H2O2 accumulated locally but not systemically after inoculation with PmsDC3000/avrRpm1. O2.- was only detected locally after wounding. If ROS produced at the site of inoculation are involved in conditioning SAR, then treatment with ROS or ROS-generating systems might lead to the induction of resistance in systemic leaves, and / or the accumulation of molecular markers for SAR (PR mRNA, SA). To test this possibility, leaves were treated with H2O2, paraquat (in light -O2.-), H2O2-generating, O2.-generating and NO-generating enzyme systems. However, none of the treatments gave results comparable to biological or chemically induced SAR in the controls. If ROS produced at the site of inoculation are involved in conditioning SAR, then the application of inhibitors or treatments which affect ROS metabolism might lead to measurable changes in the SAR response. Candidates for ROS synthesis are the plasmamembrane NAD(P)H oxidase, cell wall peroxidases or xanthine oxidase (XO). If O2.- produced by NAD(P)H oxidase is involved in the SAR, its inhibition by DPI might be expected to lead to a reduction in SAR or its molecular markers. Similar, if O2.- is produced via XO activity, allopurinol (a XO inhibitor) treatment might be expected to lead to a reduction SAR marker accumulation and / or SAR itself. If there is H2O2 involved in conditioning SAR, then its destruction via infiltrated catalase might be expected to lead to a reduction in SAR markers. Additionally, the effects of the NO scavenger Carboxy-PTIO-potassium on SAR induction was investigated. In summary it can be said that, neither DPI nor catalase, allopurinol and Carboxy-PTIO-potassium led to a significant reduction in SA or PR transcript accumulation in either the inoculated or the systemic leaves. No significant effect on the conditioning of SAR was detected as the histological level either (challenge inoculation). The possible role of intracellular O2.- production was investigated using transgenic plants over- or under expressing (sense / antisense) a cytosolic superoxide dismutase (SOD). However, no SAR relevant phenotypes were observed. Independent of the nature of the systemic signal which leads to the conditioning of SAR throughout the plant, the pathway of signal transport was investigated. To address this aspect the pattern of assimilate transport out of an inoculated leaf into the rest of the plant was compared to the pattern induction of SAR and SAR molecular markers. The results are discussed in terms of the possible role of ROS in local and systemic signalling in Arabidopsis during the establishment of SAR

The Pattern of Systemic Acquired Resistance Induction within the Arabidopsis Rosette in Relation to the Pattern of Translocation

Local leaf infections by a necrogenic pathogen can lead to systemic acquired resistance (SAR) in untreated leaves. We reasoned that, whatever the nature of the long-distance signal, if it is transported in the phloem, the pattern of SAR induced within the plant by treatment of a single leaf should match the pattern of translocation out of that leaf. The source-sink relationships (orthostichies) in the Arabidopsis rosette were established with [(14)C]Suc or phloem-mobile 3-aminotriazole at herbicidal concentrations. SAR was activated by infiltrating a single Columbia leaf with Pseudomonas syringae pv maculicola DC3000/avrRPM1, which causes a hypersensitive response. The pattern of SAR in the rosette was monitored by assessing the growth of wild-type DC3000 and by measuring the SAR markers salicylic acid and PR1 transcripts. Although the orthostichy of a single leaf was clearly limited to a row of vertically aligned leaves, SAR and SAR markers were also found outside the orthostichy. This indicates that, whatever the nature of the long-distance signal from the treated leaf to the upper responding leaves, its transport is either not limited exclusively to the phloem or the minor proportion of translocate that is not confined to the orthostichy contains enough of the SAR systemic signal to set in motion events leading to the establishment of the SAR state in the upper leaves