Single-Molecule Imaging in Living Plant Cells: A Methodological Review

C2-domain abscisic acid-related proteins regulate the dynamics of a plasma membrane H+-ATPase in response to alkali stress

Arabidopsis (Arabidopsis thaliana) H+-ATPase1 (AHA1), a plasma membrane (PM)-localized H+-ATPase, plays a key role in plant alkali stress tolerance by pumping protons from the cytoplasm to the apoplast. However, its molecular dynamics are poorly understood. We report that many C2-domain ABA-related (CAR) protein family members interact with AHA1 in Arabidopsis. Single or double mutants of CAR1, CAR6, and CAR10 had no obvious phenotype of alkali stress tolerance, while their triple mutants showed significantly higher tolerance to this stress. The disruption of AHA1 largely compromised the increased alkali stress tolerance of the car1car6car10 mutant, revealing a key role of CARs in AHA1 regulation during the plant's response to a high alkali pH. Furthermore, variable-angle total internal reflection fluorescence microscopy was used to observe AHA1-mGFP5 in intact Arabidopsis seedlings, revealing the presence of heterogeneous diffusion coefficients and oligomerization states in the AHA1 spots. In the aha1 complementation lines, alkali stress curtailed the residence time of AHA1 at the PM and increased the diffusion coefficient and particle velocity of AHA1. In contrast, the absence of CAR proteins decreased the restriction of the dynamic behavior of AHA1. Our results suggest that CARs play a negative role in plant alkali stress tolerance by interacting with AHA1 and provide a perspective to investigate the regulatory mechanism of PM H+-ATPase activity at the single-particle level.

Single-molecule insights into repetitive helicases

Helicases are ubiquitous motors involved in almost all aspects of nucleic acid metabolism; therefore, revealing their unwinding behaviors and mechanisms is fundamentally and medically essential. In recent decades, single-molecule applications have revolutionized our ability to study helicases by avoiding the averaging of bulk assays and bridging the knowledge gap between dynamics and structures. This advancement has updated our understanding of the biochemical properties of helicases, such as their rate, directionality, processivity, and step size, while also uncovering unprecedented mechanistic insights. Among these, repetitive motion, a new feature of helicases, is one of the most remarkable discoveries. However, comprehensive reviews and comparisons are still lacking. Consequently, the present review aims to summarize repetitive helicases, compare the repetitive phenomena, and discuss the underlying molecular mechanisms. This review may provide a systematic understanding of repetitive helicases and help understand their cellular functions.

Detection of exogenous siRNA inside sweet corn bundle sheath cells and the RNAi dynamics in the early stage of Maize dwarf mosaic virus infection

Maize dwarf mosaic virus (MDMV) is one of the most serious viruses of sweet corn. Utilising the process of RNA interference, the exogenous introduction of small RNA molecules mimicking virus-derived small interfering RNA (siRNA) into the plant prior to infection triggers the antiviral RNA silencing effect, thereby promoting more effective antiviral protection. Hence, a treatment with MDMV-derived small RNA was applied to sweet corn plants one day before MDMV virus inoculation. ALEXA FLUOR®488 fluorophore-bound exogenous siRNA was successfully detected inside intact sweet corn cells using confocal fluorescence microscopy. Furthermore, it was demonstrated that the exogenous siRNA treatment led to a notable upregulation of the AGO1, AGO2b, AGO10b, AGO18a, DCL1, DCL3a, DCL4, RDR1, and MOP1 genes within 24 h of the treatment. Overall, exogenous siRNA treatment resulted in better virus control of infected sweet corn plants, as indicated by the lower viral RNA and coat protein levels compared to the infected group without pre-treatment.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01500-2.

Recruitment of HAB1 and SnRK2.2 by C2-domain protein CAR1 in plasma membrane ABA signaling

Plasma membrane (PM)-associated abscisic acid (ABA) signal transduction is an important component of ABA signaling. The C2-domain ABA-related (CAR) proteins have been reported to play a crucial role in recruiting ABA receptor PYR1/PYL/RCAR (PYLs) to the PM. However, the molecular details of the involvement of CAR proteins in membrane-delimited ABA signal transduction remain unclear. For instance, where this response process takes place and whether any additional members besides PYL are taking part in this signaling process. Here, the GUS-tagged materials for all Arabidopsis CAR members were used to comprehensively visualize the extensive expression patterns of the CAR family genes. Based on the representativeness of CAR1 in response to ABA, we determined to use it as a target to study the function of CAR proteins in PM-associated ABA signaling. Single-particle tracking showed that ABA affected the spatiotemporal dynamics of CAR1. The presence of ABA prolonged the dwell time of CAR1 on the membrane and showed faster lateral mobility. Surprisingly, we verified that CAR1 could directly recruit hypersensitive to ABA1 (HAB1) and SNF1-related protein kinase 2.2 (SnRK2.2) to the PM at both the bulk and single-molecule levels. Furthermore, PM localization of CAR1 was demonstrated to be related to membrane microdomains. Collectively, our study revealed that CARs recruited the three main components of ABA signaling to the PM to respond positively to ABA. This study deepens our understanding of ABA signal transduction.

Variable-Angle Epifluorescence Microscopy for Single-Particle Tracking in the Plant ER

Single-particle tracking (SPT) of biomolecules in the plant endoplasmic reticulum has the potential to inform on the formation of protein-protein complexes, metabolons, and the transport of molecules through both the ER membrane and lumen. Plant cells are particularly challenging for observing and tracking single molecules due to their unique structure, size, and considerable autofluorescence. However, by using variable-angle or highly inclined epifluorescence microscopy (VAEM) and transient expression in tobacco, it is possible to observe single-particle dynamics in the ER. Selecting the appropriate fluorophore, and ensuring the correct fluorophore density in the ER, is essential for successful SPT. By using tuneable fluorophores, which can be photoconverted and photoactivated, it is possible to vary the density of visible fluorophores in the ER dynamically. Here we describe methods to prepare plant samples for VAEM and two methods for determining and analyzing single-particle tracks from VAEM time series.

Single-Molecule Imaging Reveals Differential AT1R Stoichiometry Change in Biased Signaling

G protein-coupled receptors (GPCRs) represent promising therapeutic targets due to their involvement in numerous physiological processes mediated by downstream G protein- and β-arrestin-mediated signal transduction cascades. Although the precise control of GPCR signaling pathways is therapeutically valuable, the molecular details for governing biased GPCR signaling remain elusive. The Angiotensin II type 1 receptor (AT1R), a prototypical class A GPCR with profound implications for cardiovascular functions, has become a focal point for biased ligand-based clinical interventions. Herein, we used single-molecule live-cell imaging techniques to evaluate the changes in stoichiometry and dynamics of AT1R with distinct biased ligand stimulations in real time. It was revealed that AT1R existed predominantly in monomers and dimers and underwent oligomerization upon ligand stimulation. Notably, β-arrestin-biased ligands induced the formation of higher-order aggregates, resulting in a slower diffusion profile for AT1R compared to G protein-biased ligands. Furthermore, we demonstrated that the augmented aggregation of AT1R, triggered by activation from each biased ligand, was completely abrogated in β-arrestin knockout cells. These findings furnish novel insights into the intricate relationship between GPCR aggregation states and biased signaling, underscoring the pivotal role of molecular behaviors in guiding the development of selective therapeutic agents.

Unravelling the Mystery inside Cells by Using Single-Molecule Fluorescence Imaging

Live-cell imaging is a powerful technique to study the dynamics and mechanics of various biological molecules like proteins, organelles, DNA, and RNA. With the rapid evolution of optical microscopy, our understanding of how these molecules are implicated in the cells' most critical physiological roles deepens. In this review, we focus on how spatiotemporal nanoscale live-cell imaging at the single molecule level allows for profound contributions towards new discoveries in life science. This review will start by summarizing how single-molecule tracking has been used to analyze membrane dynamics, receptor-ligand interactions, protein-protein interactions, inner- and extra-cellular transport, gene expression/transcription, and whole organelle tracking. We then move on to how current authors are trying to improve single-molecule tracking and overcome current limitations by offering new ways of labeling proteins of interest, multi-channel/color detection, improvements in time-lapse imaging, and new methods and programs to analyze the colocalization and movement of targets. We later discuss how single-molecule tracking can be a beneficial tool used for medical diagnosis. Finally, we wrap up with the limitations and future perspectives of single-molecule tracking and total internal reflection microscopy.

Fluorescence Microscopy Methods for the Analysis and Characterization of Lignin

Lignin is one of the most studied and analyzed materials due to its importance in cell structure and in lignocellulosic biomass. Because lignin exhibits autofluorescence, methods have been developed that allow it to be analyzed and characterized directly in plant tissue and in samples of lignocellulose fibers. Compared to destructive and costly analytical techniques, fluorescence microscopy presents suitable alternatives for the analysis of lignin autofluorescence. Therefore, this review article analyzes the different methods that exist and that have focused specifically on the study of lignin because with the revised methods, lignin is characterized efficiently and in a short time. The existing qualitative methods are Epifluorescence and Confocal Laser Scanning Microscopy; however, other semi-qualitative methods have been developed that allow fluorescence measurements and to quantify the differences in the structural composition of lignin. The methods are fluorescence lifetime spectroscopy, two-photon microscopy, Föster resonance energy transfer, fluorescence recovery after photobleaching, total internal reflection fluorescence, and stimulated emission depletion. With these methods, it is possible to analyze the transport and polymerization of lignin monomers, distribution of lignin of the syringyl or guaiacyl type in the tissues of various plant species, and changes in the degradation of wood by pulping and biopulping treatments as well as identify the purity of cellulose nanofibers though lignocellulosic biomass.

Macrophage Identification In Situ

Understanding the processes of inflammation and tissue regeneration after injury is of great importance. For a long time, macrophages have been known to play a central role during different stages of inflammation and tissue regeneration. However, the molecular and cellular mechanisms by which they exert their effects are as yet mostly unknown. While in vitro macrophages have been characterized, recent progress in macrophage biology studies revealed that macrophages in vivo exhibited distinctive features. Actually, the precise characterization of the macrophages in vivo is essential to develop new healing treatments and can be approached via in situ analyses. Nowadays, the characterization of macrophages in situ has improved significantly using antigen surface markers and cytokine secretion identification resulting in specific patterns. This review aims for a comprehensive overview of different tools used for in situ macrophage identification, reporter genes, immunolabeling and in situ hybridization, discussing their advantages and limitations.