Laboratory Experiments on Arched Magnetized Plasmas

The arched plasma structures found throughout the Sun’s atmosphere can significantly impact the heliosphere and Earth through eruptive events like solar flares and coronal mass ejections. Coronal loops and solar prominences are examples of arched plasmas on the Sun. In this dissertation, we study arched plasma structures in the laboratory setting relevant to those found in the Sun. The current understanding of mechanisms leading to eruptions and general dynamics of arched solar plasmas remains limited. This is primarily due to a lack of extensive in situ diagnostics and, thus, a heavy reliance on remote imaging. Through proper scaling of relative plasma parameters, we can investigate the dynamics of arched plasma phenomena in a laboratory to better understand what is happening in the Sun. This research aims to contribute knowledge to the field and allow for the formulation of reliable predictive models.This experiment was designed with the primary goal of producing and studying arched mag- netized, current-carrying plasma. It is achieved with a hot lanthanum hexaboride (LaB6) cathode and cold copper anode plasma source operated in a vacuum vessel filled with a neutral Helium gas (up to 9 mtorr). Two magnetic fields are produced in this experiment, of which direction and magnitude can be tailored to experimental goals. The first is an arched guiding magnetic field con- necting the footpoints and guiding the plasma along the arch. The second is a horizontal overlying (ambient) magnetic field of uniform magnitude, directed perpendicular to the plane of the arch. A three-dimensional probe drive has been constructed for this work, allowing for automated in situ measurements of plasma parameters with high spatial resolution along the customizable grid. The probe drive control software was written in Python and integrated with the main data acquisition LabView software. The appropriate coordinate transformations between the probe tip position and the probe drive’s motors were determined and integrated into the control software. We have built four diagnostic in situ probes (magnetic field, 2- and 3-tip Langmuir, Mach), resistant up to 700◦C temperatures. This high temperature-resistant construction allows for measurements very close to the hot cathode source (up to 5 cm away). The probes were used along with two fast cam- eras to better understand phenomena in experiments presented here. We have carried out multiple arched plasma source maintenance and upgrade routines, improving its duty cycle and stability over the course of this work. A power supply control system was developed for both background and arched plasma source heaters. This system allowed us to gradually raise the heaters’ current in an automated and remote fashion. All data analyses were conducted in Python using the Jupyter Notebook environment. An extensive library of data analysis functions and procedures resulting from this work is tailored to this setup. One focus of this dissertation is the effects of a nearly horizontal overlying (strapping) magnetic field on the evolution and morphology of the arched magnetized plasma. The electric current in the arched plasma was kept low enough to keep it kink stable. The experimental results show that the sigmoid plasma structures are naturally produced in a sheared magnetic field configuration. The magnitude and handedness of the writhe of the arched plasma strongly depend on the structure of the guiding magnetic field. We found that orienting the guiding magnetic field nearly parallel to the electric current in the arched plasma results in a reverse-S shape. For an antiparallel orientation, the arched plasma assumed a forward-S shape. Moreover, the magnitude of the writhe and twist was correlated with the strength of the shear in the guiding field (strength of strapping magnetic field applied). These results are significant in light of the distribution of arched plasma structures on the Sun. Namely, the reverse-S shaped structures are more common in the Northern Solar Hemisphere, while forward-S shaped structures are characteristic of the Southern Hemisphere. The presence of strong shear in the magnetic field was observed to cause an eruption of a tran- sient jet structure out of the arched laboratory plasma. The detailed study of this phenomenon constitutes the second focus of this dissertation. Jet-like structures are commonly formed from arched plasma structures in the lower solar atmosphere (e.g., anemone jets and spicules). Due to our experiment’s relatively high density of neutrals, we can simulate the highly collisional con- ditions of the photosphere and lower chromosphere of the Sun. This capability is very unique to our laboratory setup. The ion-neutral collisions significantly impact the dynamics of lower atmo- sphere solar structures and our laboratory plasma structures. We employed the diagnostics of the magnetic field, density, temperature, and ion flow to characterize the jet structure formed in these experiments. We found that in its early stages, the laboratory jet has a supersonic (around Alfv�n speed) ion flow away from the arch, driven primarily by a large gradient in the magnetic field. On the Sun, structures like spicules are also found to carry an ion flow at velocities around the Alfv�n speed. The jet under study carries the electric current, which returns to the arch gradually with distance through an ion-neutral charge transfer collision mechanism. The electron current returns to the anode via a path crossing the weakest magnetic field lines, making a sharp turn near the magnetic null. The work presented here has contributed to our knowledge of the dynamics of arched magne- tized plasmas relevant to similar structures in the lower solar atmosphere. The arched plasma’s electric current in our experiments was naturally low enough to keep it kink-stable. This unique feature of our experimental setup allowed us to study the arched plasma’s sigmoid shape mor- phology and the plasma jet eruption dynamics purely in terms of the pre-existing magnetic field configuration. Our studies show that strong shear in the vacuum magnetic field not only impacts the morphology of the arched plasma but can also drive a formation of a jet structure. The signifi- cant presence of neutrals in our plasma is yet another unique aspect of this experiment. At around 2% ionization level, the plasma studied here is relevant to solar plasma found in prominences, chromosphere, and photosphere. With the new insights and all hardware and software developed during this dissertation, we have established a platform for further research on these topics. We hope this dissertation is but a building block of a future predictive model of eruptive arched plasma structures on the Sun

A Kinase-Phosphatase Signaling Module with BSK8 and BSL2 Involved in Regulation of Sucrose-Phosphate Synthase

External supply of sucrose to carbon-starved <i>Arabidopsis</i> seedlings induced changes in phosphorylation of Brassinosteroid Signaling Kinase 8 (BSK8) at two different sites. Serine S²⁰ lies within a phosphorylation hotspot at the N-terminal region of the protein, while S²¹³ is located within the kinase domain of BSK8. Upon sucrose supply phosphorylation of BSK8^S20 and BSK8^S213 showed opposite behavior with increasing phosphorylation of S²¹³ and decreased phosphorylation of S²⁰ at 5 min after sucrose supply. Here we aim to systematically analyze the effects of BSK8 mutations on downstream cellular regulatory events and characterize molecular functions of BSK8 and its phosphorylation. Comparative phosphoproteomic profiling of a <i>bsk8</i> knockout mutant and wild type revealed potential targets in sucrose metabolism. Activity of sucrose-phosphate synthase (SPS) was decreased by phosphorylation at S¹⁵², and SPS phosphorylation inversely correlated with sucrose-induced BSK8 activity. Furthermore, BSK8 was found to interact with BSL2, a Kelch-type phosphatase. On the basis of a combination of kinase activity measurements, SPS activity assays, and phosphorylation site mutations in BSK8 at S²⁰ and S²¹³, we conclude that regulation of SPS by BSK8 occurs through activation of a phosphatase that in turn may dephosphorylate SPS and thus activates the enzyme

A Kinase-Phosphatase Signaling Module with BSK8 and BSL2 Involved in Regulation of Sucrose-Phosphate Synthase

External supply of sucrose to carbon-starved <i>Arabidopsis</i> seedlings induced changes in phosphorylation of Brassinosteroid Signaling Kinase 8 (BSK8) at two different sites. Serine S²⁰ lies within a phosphorylation hotspot at the N-terminal region of the protein, while S²¹³ is located within the kinase domain of BSK8. Upon sucrose supply phosphorylation of BSK8^S20 and BSK8^S213 showed opposite behavior with increasing phosphorylation of S²¹³ and decreased phosphorylation of S²⁰ at 5 min after sucrose supply. Here we aim to systematically analyze the effects of BSK8 mutations on downstream cellular regulatory events and characterize molecular functions of BSK8 and its phosphorylation. Comparative phosphoproteomic profiling of a <i>bsk8</i> knockout mutant and wild type revealed potential targets in sucrose metabolism. Activity of sucrose-phosphate synthase (SPS) was decreased by phosphorylation at S¹⁵², and SPS phosphorylation inversely correlated with sucrose-induced BSK8 activity. Furthermore, BSK8 was found to interact with BSL2, a Kelch-type phosphatase. On the basis of a combination of kinase activity measurements, SPS activity assays, and phosphorylation site mutations in BSK8 at S²⁰ and S²¹³, we conclude that regulation of SPS by BSK8 occurs through activation of a phosphatase that in turn may dephosphorylate SPS and thus activates the enzyme

The receptor-like pseudokinase MRH1 interacts with the voltage-gated potassium channel AKT2

The potassium channel AKT2 plays important roles in phloem loading and unloading. It can operate as inward-rectifying channel that allows H -ATPase-energized K uptake. Moreover, through reversible post-translational modifications it can also function as an open, K -selective channel, which taps a 'potassium battery', providing additional energy for transmembrane transport processes. Knowledge about proteins involved in the regulation of the operational mode of AKT2 is very limited. Here, we employed a large-scale yeast two-hybrid screen in combination with fluorescence tagging and null-allele mutant phenotype analysis and identified the plasma membrane localized receptor-like kinase MRH1/MDIS2 (AT4G18640) as interaction partner of AKT2. The phenotype of the mrh1-1 knockout plant mirrors that of akt2 knockout plants in energy limiting conditions. Electrophysiological analyses showed that MRH1/MDIS2 failed to exert any functional regulation on AKT2. Using structural protein modeling approaches, we instead gathered evidence that the putative kinase domain of MRH1/MDIS2 lacks essential sites that are indispensable for a functional kinase suggesting that MRH1/MDIS2 is a pseudokinase. We propose that MRH1/MDIS2 and AKT2 are likely parts of a bigger protein complex. MRH1 might help to recruit other, so far unknown partners, which post-translationally regulate AKT2. Additionally, MRH1 might be involved in the recognition of chemical signals.This work was supported by grants from the German Science Foundation (DFG; DR 430/9-1), the Spanish Ministerio de Economía y Competitividad (BFU2011-28815 and BFU2014-55575-R) and a Marie Curie Career Integration Grant to Ingo Dreyer (FP7-PEOPLE- 2011-CIG No. 303674 – Regopoc), grants of the Comisión Nacional Científica y Tecnológica of Chile to Julio Caballero (FONDECYT N° 1130141), Gonzalo Riadi (FONDECYT N° 11140869) and Janin Riedelsberger (FONDECYT N° 3150173), and a doctoral fellowship from the Max-Planck Research School “Primary Metabolism and Plant Growth” for Kamil Sklodowski.Peer reviewe

A primary cell wall cellulose-dependent defense mechanism against vascular pathogens revealed by time-resolved dual transcriptomics

[Background]: Cell walls (CWs) are protein-rich polysaccharide matrices essential for plant growth and environmental acclimation. The CW constitutes the first physical barrier as well as a primary source of nutrients for microbes interacting with plants, such as the vascular pathogen Fusarium oxysporum (Fo). Fo colonizes roots, advancing through the plant primary CWs towards the vasculature, where it grows causing devastation in many crops. The pathogenicity of Fo and other vascular microbes relies on their capacity to reach and colonize the xylem. However, little is known about the root-microbe interaction before the pathogen reaches the vasculature and the role of the plant CW during this process.[Results]: Using the pathosystem Arabidopsis-Fo5176, we show dynamic transcriptional changes in both fungus and root during their interaction. One of the earliest plant responses to Fo5176 was the downregulation of primary CW synthesis genes. We observed enhanced resistance to Fo5176 in Arabidopsis mutants impaired in primary CW cellulose synthesis. We confirmed that Arabidopsis roots deposit lignin in response to Fo5176 infection, but we show that lignin-deficient mutants were as susceptible as wildtype plants to Fo5176. Genetic impairment of jasmonic acid biosynthesis and signaling did not alter Arabidopsis response to Fo5176, whereas impairment of ethylene signaling did increase vasculature colonization by Fo5176. Abolishing ethylene signaling attenuated the observed resistance while maintaining the dwarfism observed in primary CW cellulose-deficient mutants.[Conclusions]: Our study provides significant insights on the dynamic root-vascular pathogen interaction at the transcriptome level and the vital role of primary CW cellulose during defense response to these pathogens. These findings represent an essential resource for the generation of plant resistance to Fo that can be transferred to other vascular pathosystems.The work described in this paper was supported by the Swiss National foundation to CSR (SNF 31003A_163065/1 to AM and GS, and SNF 310030_184769 to SD and GS), the Ministry of Science and Innovation and Innovation State Research Agency to NSC (PID2019-108595RB-I00/AEI/10.13039/501100011033), the German Research Foundation to DG (DFG grant GA2419/2-1), and a Marie Skłodowska-Curie postdoctoral fellowship to M.K.M.Peer reviewe

Sucrose-induced Receptor Kinase 1 is Modulated by an Interacting Kinase with Short Extracellular Domain

Sucrose as a product of photosynthesis is the major carbohydrate translocated from photosynthetic leaves to growing nonphotosynthetic organs such as roots and seeds. These growing tissues, besides carbohydrate supply, require uptake of water through aquaporins to enhance cell expansion during growth. Previous work revealed Sucrose Induced Receptor Kinase, SIRK1, to control aquaporin activity via phosphorylation in response to external sucrose stimulation. Here, we present the regulatory role of AT3G02880 (QSK1), a receptor kinase with a short external domain, in modulation of SIRK1 activity. Our results suggest that SIRK1 autophosphorylates at Ser-744 after sucrose treatment. Autophosphorylated SIRK1 then interacts with and transphosphorylates QSK1 and QSK2. Upon interaction with QSK1, SIRK1 phosphorylates aquaporins at their regulatory C-terminal phosphorylation sites. Consequently, in root protoplast swelling assays, the qsk1qsk2 mutant showed reduced water influx rates under iso-osmotic sucrose stimulation, confirming an involvement in the same signaling pathway as the receptor kinase SIRK1. Large-scale phosphoproteomics comparing single mutant sirk1, qsk1, and double mutant sirk1 qsk1 revealed that aquaporins were regulated by phosphorylation depending on an activated receptor kinase complex of SIRK1, as well as QSK1. QSK1 thereby acts as a coreceptor stabilizing and enhancing SIRK1 activity and recruiting substrate proteins, such as aquaporins.(VLID)439433

The cellulose synthases are cargo of the TPLATE adaptor complex

This work was supported by the Max-Planck Gesellschaft, the Deutsche Forschungsgemeinschaft, the National Natural Science Foundation of China (Grants 31530051), the Swiss Federal Institute of Technology of Zurich (ETH-Z), the Swiss National Foundation (SNF 2-77212-15), the University of Melbourne, the Australian Research Council (CE1101007, FT160100218, DP110100410), the Ministry of Education, Culture, Sports, Science, and Technology of Japan (24114003, 15H04382, and 17K19412), the European Research Council (ERC grant 682436), the IRRTF-RNC (no. 501892) and an USA National Science Foundation CAREER Award