A Multi-level Mechanical Study of Hypocotyl Growth in the Dark

Dark-grown hypocotyls of Arabidopsis thaliana exhibit a striking wave of cell elongation which moves acropetally from the base to the tip of the organ over time. The combined effect of this wave of cell elongation is a pattern of organ elongation. It was aimed to understand the coordination of cellular growth and growth mechanisms through multi-level studies including modelling approaches, transcriptome analysis and hormone analysis. The first aim of this thesis was to describe the hypocotyl growth at a cellular level, including cell length and diameter measures, over a 72-hour period after germination. It was observed that the ‘wave of growth’ is exhibited only in the cell length dimension, not in diameter. The precise position and magnitude of the elongation wave was quantified as a foundation for subsequent aims. Microtubules were quantified and co-ordinated transverse alignments at inner walls was found to be associated with cell growth rate. In the second aim of this thesis a dynamic, intrinsic, model was built based on the physical factors controlling cell elongation, using a bottom-up approach. Using a chain of cell units, each modelled with a modified Lockhart model, hypocotyl elongation in silico mirrored experimental data. The model successfully simulated behaviours of cell size, turgor pressure and yield stress over time and it responded to simulated parameter changes (representing physical factors) reasonably well. The model performance was compared to experimental manipulations. A concept of ‘bond energy distribution’ was introduced to the model in relation to yield stress, and it suggested that a change in the cell wall structure, or bond distribution, can be an efficient way of controlling cell growth in comparison to change in physical factors. The model also implied that a quantitative ‘chemical signal’ may exist and can be involved in the initiation of cell growth and thus the acropetal wave. For the next aim, an empirical model of hypocotyl cell growth was built based on data of cell sizes up to 72 hours post germination. Using data fitting and parameter extrapolation, the model predicted the progression of cell sizes until 196 hours post germination, when the hypocotyl elongation was terminating. The prediction fitted well with the measured hypocotyl growth at organ level. The Probit function had the best performance among the three sigmoidal functions selected for fitting and parameter extrapolation, which is a model describing the transition from cellular to organ level growth. The fourth aim of this thesis was to investigate which genes, related to physical growth parameters, were related to cell growth. An RNAseq experiment was conducted and analysed which yielded differentially expressed genes from slow and fast-elongating regions of the hypocotyl (non-wave vs 4 wave) at three time points (24/36/48HPG). Several groups of genes that were involved in cell wall modification and hormones were studied in detail and those with differential expression within the wave were identified. The final aim of this thesis involved investigating a hormonal signal for the acropetal wave and cell growth initiation. The intrinsic cell model in Aim 2 indicated the need for a growth signal to start cell elongation. Hormones are strong candidates for growth signals and the transcriptome analysis in Aim 4 indicated that gibberellin may be a good candidate. GA levels were examined in the hypocotyl using the nlsGPS1 GA-biosensor. It was found that the level of sensor emission ratio qualitatively correlated well with the cellular growth rates, indicating the existence of global control on cell growth through chemical signals.Cambridge Trus

Gate defined quantum dot realized in a single crystalline InSb nanosheet

Single crystalline InSb nanosheet is an emerging planar semiconductor material with potential applications in electronics, infrared optoelectronics, spintronics and topological quantum computing. Here we report on realization of a quantum dot device from a single crystalline InSb nanosheet grown by molecular-beam epitaxy. The device is fabricated from the nanosheet on a Si/SiO2 substrate and the quantum dot confinement is achieved by top gate technique. Transport measurements show a series of Coulomb diamonds, demonstrating that the quantum dot is well defined and highly tunable. Tunable, gate-defined, planar InSb quantum dots offer a renewed platform for developing semiconductor-based quantum computation technology.Comment: 12 pages, 4 figure

Effective Room-Temperature Ammonia-Sensitive Composite Sensor Based on Graphene Nanoplates and PANI

The graphene nanoplate (GN)-polyaniline (PANI) composite was developed via in-situ polymerization method and simultaneously assembled on interdigital electrodes (IDEs) at low temperature for ammonia (NH3) detection. The assembled composite sensor showed excellent sensing performance toward different concentrations of NH3, 1.5 of response value and 123 s/204 s for the response/recovery time to 15 ppm NH3. Meanwhile, an interesting supersaturation phenomenon was observed at high concentration of NH3. A reasonable speculation was proposed for this special sensing behavior and the mechanism for enhanced sensing properties was also analyzed

Reconstruction of Cardiac Cine MRI under Free-breathing using Motion-guided Deformable Alignment and Multi-resolution Fusion

Objective: Cardiac cine magnetic resonance imaging (MRI) is one of the important means to assess cardiac functions and vascular abnormalities. However, due to cardiac beat, blood flow, or the patient's involuntary movement during the long acquisition, the reconstructed images are prone to motion artifacts that affect the clinical diagnosis. Therefore, accelerated cardiac cine MRI acquisition to achieve high-quality images is necessary for clinical practice. Approach: A novel end-to-end deep learning network is developed to improve cardiac cine MRI reconstruction under free breathing conditions. First, a U-Net is adopted to obtain the initial reconstructed images in k-space. Further to remove the motion artifacts, the Motion-Guided Deformable Alignment (MGDA) method with second-order bidirectional propagation is introduced to align the adjacent cine MRI frames by maximizing spatial-temporal information to alleviate motion artifacts. Finally, the Multi-Resolution Fusion (MRF) module is designed to correct the blur and artifacts generated from alignment operation and obtain the last high-quality reconstructed cardiac images. Main results: At an 8$\times$ acceleration rate, the numerical measurements on the ACDC dataset are SSIM of 78.40%$\pm$4.57%, PSNR of 30.46$\pm$1.22 dB, and NMSE of 0.0468$\pm$0.0075. On the ACMRI dataset, the results are SSIM of 87.65%$\pm$4.20%, PSNR of 30.04$\pm$1.18 dB, and NMSE of 0.0473$\pm$0.0072. Significance: The proposed method exhibits high-quality results with richer details and fewer artifacts for cardiac cine MRI reconstruction on different accelerations under free breathing conditions.Comment: 28 pages, 5 tables, 11 figure

Optical trapping with structured light : a review

Funding: This work was supported by the National Natural Science Foundation of China (11874102 and 61975047), the Sichuan Province Science and Technology Support Program (2020JDRC0006), and the Fundamental Research Funds for the Central Universities (ZYGX2019J102). M.C. and Y.A. thank the UK Engineering and Physical Sciences Research Council for funding.Optical trapping describes the interaction between light and matter to manipulate micro-objects through momentum transfer. In the case of 3D trapping with a single beam, this is termed optical tweezers. Optical tweezers are a powerful and noninvasive tool for manipulating small objects, and have become indispensable in many fields, including physics, biology, soft condensed matter, among others. In the early days, optical trapping was typically accomplished with a single Gaussian beam. In recent years, we have witnessed rapid progress in the use of structured light beams with customized phase, amplitude, and polarization in optical trapping. Unusual beam properties, such as phase singularities on-axis and propagation invariant nature, have opened up novel capabilities to the study of micromanipulation in liquid, air, and vacuum. We summarize the recent advances in the field of optical trapping using structured light beams.Publisher PDFPeer reviewe

Anisotropic growth is achieved through the additive mechanical effect of material anisotropy and elastic asymmetry.

Fast directional growth is a necessity for the young seedling; after germination, it needs to quickly penetrate the soil to begin its autotrophic life. In most dicot plants, this rapid escape is due to the anisotropic elongation of the hypocotyl, the columnar organ between the root and the shoot meristems. Anisotropic growth is common in plant organs and is canonically attributed to cell wall anisotropy produced by oriented cellulose fibers. Recently, a mechanism based on asymmetric pectin-based cell wall elasticity has been proposed. Here we present a harmonizing model for anisotropic growth control in the dark-grown Arabidopsis thaliana hypocotyl: basic anisotropic information is provided by cellulose orientation) and additive anisotropic information is provided by pectin-based elastic asymmetry in the epidermis. We quantitatively show that hypocotyl elongation is anisotropic starting at germination. We present experimental evidence for pectin biochemical differences and wall mechanics providing important growth regulation in the hypocotyl. Lastly, our in silico modelling experiments indicate an additive collaboration between pectin biochemistry and cellulose orientation in promoting anisotropic growth