Advances in Synthetic Fluorescent Probe Labeling for Live-Cell Imaging in Plants

Charge-regulated fluorescent anchors enable high-fidelity tracking of plasma membrane dynamics during biological events

Many biological processes generally require long-term visualization tools for time-scale dynamic changes of the plasma membrane, but there is still a lack of design rules for such imaging tools based on small-molecule fluorescent probes. Herein, we revealed the key regulatory roles of charge number and species of fluorescent dyes in the anchoring ability of the plasma membrane and found that the introduction of multi-charged units and appropriate charge species is often required for fluorescent dyes with strong plasma membrane anchoring ability by systematically investigating the structure-function relationship of cyanostyrylpyridium (CSP) dyes with different charge numbers and species and their imaging performance for the plasma membrane. The CSP-DBO dye constructed exhibits strong plasma membrane anchoring ability in staining the plasma membrane of cells, in addition to many other advantages such as excellent biocompatibility and general universality of cell types. Such a fluorescent anchor has been successfully used to monitor chemically induced plasma membrane damage and dynamically track various cellular biological events such as cell fusion and cytokinesis over a long period of time by continuously monitoring the dynamic morphological changes of the plasma membrane, providing a valuable precise visualization tool to study the physiological response to chemical stimuli and reveal the structural morphological changes and functions of the plasma membrane during these important biological events from a dynamic perspective. Furthermore, CSP-DBO exhibits excellent biocompatibility and imaging capability in vivo such as labelling the plasma membrane in vivo and monitoring the metabolic process of lipofuscin as an aging indicator.

Near-infrared imaging of phytochrome-derived autofluorescence in plant nuclei

Capturing images of the nuclear dynamics within live cells is an essential technique for comprehending the intricate biological processes inherent to plant cell nuclei. While various methods exist for imaging nuclei, including combining fluorescent proteins and dyes with microscopy, there is a dearth of commercially available dyes for live-cell imaging. In Arabidopsis thaliana, we discovered that nuclei emit autofluorescence in the near-infrared (NIR) range of the spectrum and devised a non-invasive technique for the visualization of live cell nuclei using this inherent NIR autofluorescence. Our studies demonstrated the capability of the NIR imaging technique to visualize the dynamic behavior of nuclei within primary roots, root hairs, and pollen tubes, which are tissues that harbor a limited number of other organelles displaying autofluorescence. We further demonstrated the applicability of NIR autofluorescence imaging in various other tissues by incorporating fluorescence lifetime imaging techniques. Nuclear autofluorescence was also detected across a wide range of plant species, enabling analyses without the need for transformation. The nuclear autofluorescence in the NIR wavelength range was not observed in animal or yeast cells. Genetic analysis revealed that this autofluorescence was caused by the phytochrome protein. Our studies demonstrated that nuclear autofluorescence imaging can be effectively employed not only in model plants but also for studying nuclei in non-model plant species.

Development of D-π-A organic dyes for discriminating HSA from BSA and study on dye-HSA interaction

HSA (human serum albumin), a most abundant protein in blood serum, plays a key role in maintaining human health. Abnormal HSA level is correlated with many diseases, and thus has been used as an essential biomarker for therapeutic monitoring and biomedical diagnosis. Development of small-molecule fluorescent probes allowing the selective and sensitive recognition of HSA in in vitro and in vivo is of fundamental importance in basic biological research as well as medical diagnosis. Herein, we reported a series of new synthesized fluorescent dyes containing D-π-A constitution, which exhibited different optical properties in solution and solid state. Among them, dye M-H-SO3 with a hydrophilic sulfonate group at electron-acceptor part displayed selectivity for discrimination of HSA from BSA and other enzymes. Upon binding of dye M-H-SO3 with HSA, a significant fluorescence enhancement with a turn-on ratio about 96-fold was triggered. The detection limit was estimated to be ∼ 40 nM. Studies on the interaction mechanism revealed that dye M-H-SO3 could bind to site III of HSA with a 1:1 binding stoichiometry. Furthermore, dye M-H-SO3 has been applied to determine HSA in real urine samples with good recoveries, which provided a useful method for HSA analysis in biological fluids.

Development of a water-soluble fluorescent Al3+ probe based on phenylsulfonyl-2-pyrone in biological systems

BACKGROUND

Al exists naturally in the environment and is an important component in acidic soils, which harm almost all plants. Furthermore, Al is widely used in food additives, cosmetics, and medicines, resulting in living organisms ingesting traces of Al orally or dermally every day. Accordingly, Al accumulates in the body, which can cause negative bioeffects and diseases, and this concern is gaining increasing attention. Therefore, to detect and track Al in the environment and in living organisms, the development of novel Al-selective probes that are water-soluble and exhibit fluorescence at long wavelengths is necessary.

RESULTS

In this study, an Al3+-selective fluorescent probe PSP based on a novel pyrone molecule was synthesized and characterized to detect and track Al in biological systems. PSP exhibited fluorescence enhancement at 580 nm in the presence of Al3+ in aqueous media. Binding analysis using Job's plot and structural analysis using 1H NMR showed that PSP formed a 1:1 complex with Al3+ at the two carbonyl groups of the dimethyl malonate of the pyrone ring. Upon testing in biological systems, PSP showed good cell membrane permeability, detected intracellular Al3+ in human breast cancer cells (MDA-MB-231), and successfully imaged accumulated Al3+ in Microcystis aeruginosa and the larvae of Rheocricotopus species.

SIGNIFICANCE

The novel Al3+-selective fluorescent probe PSP is highly effective and is expected to aid in elucidating the role of Al3+ in the environment and living organisms.

A sensitive coumarin fluorescence sensor designed for isoprene detection and imaging research in plants

The release of isoprene by plants is considered to be an adaptation to the environment. Herein, a highly selective coumarin fluorescent probe (DMIC) was designed for detecting isoprene. When isoprene came into contact with the maleimide of DMIC, an electrophilic addition process took place. The powerful push-pull effect of DMIC was disrupted. Simultaneously, intramolecular charge transfer was initiated. This enabled DMIC to achieve rapid detection of isoprene within 5 min. Furthermore, excellent linearity was observed in the concentration range of 1-560 ppm (R2 = 0.996). A limit of detection is 1.6 ppm. DMIC was applied to in vitro studies of plant release of liberated isoprene. By monitoring the release of isoprene from different tree species throughout the day, the dynamics of isoprene release from plants throughout the day have been successfully revealed. In addition, the release of isoprene varied considerably among different tree species. In particular, the biocompatibility of DMIC allowed for the in vivo detection of isoprene using fluorescence imaging. The results successfully revealed the dynamics of isoprene release in plants under stress. The amount of isoprene that a plant produced increased with the severity of the stress it experienced. This suggested that the level of isoprene content in plants could be used as a preliminary indicator of the physiological health status of plants. This research demonstrates great potential for clarifying signal transduction in biological systems. It provided ideas for further understanding the biology of isoprene.

Target-activated multicolor fluorescent dyes for 3D imaging of plasma membranes and tracking of apoptosis

Real-time tracking of dynamic changes in the three-dimensional morphology of the cell plasma membrane is of great importance for a deeper understanding of physiological processes related to the cell plasma membrane. However, there is a lack of imaging dyes that can specifically be used for a long term labelling of plasma membranes, especially for plant cells. Here, we have used molecular engineering strategies to develop a series of target-activated multicolour fluorescent dyes that can be used for long-term and three-dimensional imaging of plant cell plasma membranes. By combining different electron acceptors and donors, four molecular backbones with different emission colours from green to NIR have been obtained. In the designed styrene-based dyes, referred to as the SD dyes, several functional groups were introduced into the backbones to achieve the properties of target-activated fluorescence, rapid and wash-free staining, high plasma membrane targeting ability and long-term imaging function. Using onion epidermal cells as a platform, these dye molecules can provide high-quality imaging of the plasma membrane for up to 6 hours, providing a powerful tool for long-term monitoring of plasma membrane-related biological events. Calcium-mediated apoptosis of plant cells has been tracked for the first time by monitoring the morphological changes of the plasma membrane in real time using SD dyes. These dyes also exhibit excellent 3D imaging performance of the plasma membrane and were further used to track in real time the 3D morphological changes of the plasma membrane during plasmolysis of plant cells, providing a powerful imaging tool for three-dimensional (3D) biology. This work provides a set of multi-colour dye tools for long-term and three-dimensional imaging of plant cell plasma membranes, and also provides molecular design principles for guiding the transmembrane transport of small molecules.

Targeted and Long-Term Fluorescence Imaging of Plant Cytomembranes Using Main-Chain Charged Polyelectrolytes with Aggregation-Induced Emission

Fluorescent polyelectrolytes have attracted tremendous attention due to their unique properties and wide applications. However, current research objects of fluorescent polyelectrolytes mainly focus on side-chain charged polyelectrolytes, and the applications of polyelectrolytes in plant cytomembrane imaging with long time and high specificity still remain challenging. Herein, long-time and targeted fluorescence imaging of plant cytomembranes was achieved for the first time using main-chain charged polyelectrolytes (MCCPs) with aggregation-induced emission (AIE). A series of MCCPs were designed and synthesized, among which the red-emissive and AIE-active MCCP with a triphenylamine linker and a cyano group around the cationic ring-fused heterocyclic core showed the best fluorescence imaging performance of plant cells. Unlike other MCCPs and its neutral form of polymer, this cyano-substituted conjugated polyelectrolyte can specifically target the cytomembrane of plant cells within a short staining time with many advantages, including wash-free staining, high photostability and imaging integrity, excellent durability (at least 12 h), and low biotoxicity. In addition to onion epidermal cells, this AIE fluorescence probe also shows good imaging capabilities for other kinds of plant cells such as Glycine max and Vigna radiata. Such an AIE-active MCCP-based imaging system provides an effective design strategy to develop fluorescence probes with high specificity and long-term imaging ability toward plant plasma membranes.

A versatile CRISPR-based system for lineage tracing in living plants

Individual cells give rise to diverse cell lineages during the development of multicellular organisms. Understanding the contribution of these lineages to mature organisms is a central question of developmental biology. Several techniques to document cell lineages have been used, from marking single cells with mutations that express a visible marker to generating molecular bar codes by CRISPR-induced mutations and subsequent single-cell analysis. Here, we exploit the mutagenic activity of CRISPR to allow lineage tracing within living plants with a single reporter. Cas9-induced mutations are directed to correct a frameshift mutation that restores expression of a nuclear fluorescent protein, labelling the initial cell and all progenitor cells with a strong signal without modifying other phenotypes of the plants. Spatial and temporal control of Cas9 activity can be achieved using tissue-specific and/or inducible promoters. We provide proof of principle for the function of lineage tracing in two model plants. The conserved features of the components and the versatile cloning system, allowing for easy exchange of promoters, are expected to make the system widely applicable.

Long-term spatiotemporal and highly specific imaging of the plasma membrane of diverse plant cells using a near-infrared AIE probe

Fluorescent probes are valuable tools to visualize plasma membranes intuitively and clearly and their related physiological processes in a spatiotemporal manner. However, most existing probes have only realized the specific staining of the plasma membranes of animal/human cells within a very short time period, while almost no fluorescent probes have been developed for the long-term imaging of the plasma membranes of plant cells. Herein, we designed an AIE-active probe with NIR emission to achieve four-dimensional spatiotemporal imaging of the plasma membranes of plant cells based on a collaboration approach involving multiple strategies, demonstrated long-term real-time monitoring of morphological changes of plasma membranes for the first time, and further proved its wide applicability to plant cells of different types and diverse plant species. In the design concept, three effective strategies including the similarity and intermiscibility principle, antipermeability strategy and strong electrostatic interactions were combined to allow the probe to specifically target and anchor the plasma membrane for an ultralong amount of time on the premise of guaranteeing its sufficiently high aqueous solubility. The designed APMem-1 can quickly penetrate cell walls to specifically stain the plasma membranes of all plant cells in a very short time with advanced features (ultrafast staining, wash-free, and desirable biocompatibility) and the probe shows excellent plasma membrane specificity without staining other areas of the cell in comparison to commercial FM dyes. The longest imaging time of APMem-1 can be up to 10 h with comparable performance in both imaging contrast and imaging integrity. The validation experiments on different types of plant cells and diverse plants convincingly proved the universality of APMem-1. The development of plasma membrane probes with four-dimensional spatial and ultralong-term imaging ability provides a valuable tool to monitor the dynamic processes of plasma membrane-related events in an intuitive and real-time manner.

Assay of Reactive Oxygen/Nitrogen Species (ROS/RNS) in Arabidopsis Peroxisomes Through Fluorescent Protein Containing a Type 1 Peroxisomal Targeting Signal (PTS1)

Plant peroxisomes have an active nitro-oxidative metabolism. However, the assay of reactive oxygen and nitrogen species (ROS/RNS) could be a challenge since the purification of peroxisomes is technically a high time-consuming approach that needs to be optimized for each tissue/organ (root, leaf, fruit) and plant species. Arabidopsis thaliana, as a model plant for biochemical and molecular studies, has become a useful tool to study the basic metabolism, including also that of ROS/RNS. The combination of specific fluorescent probes with Arabidopsis plants expressing a fluorescent protein containing a type 1 peroxisomal targeting signal (PTS1) is a powerful tool to address the profile of ROS/RNS in peroxisomes by confocal laser scanning microscope (CLSM). This chapter provides a detailed description to detect the content and distribution of ROS and RNS in Arabidopsis peroxisomes, together with a critical analysis of their potentialities and limitations, since these approaches require appropriate controls to corroborate the obtained data.

Multi-color live-cell fluorescence imaging of primary ciliary membrane assembly and dynamics

The ciliary membrane is continuous with the plasma membrane but has distinct lipid and protein composition, which is key to defining the function of the primary cilium. Ciliary membranes dynamically assemble and disassemble in association with the cell cycle and directly transmit signals and molecules through budding membranes. Various imaging approaches have greatly advanced the understanding of the ciliary membrane function. In particular, fluorescence live-cell imaging has revealed important insights into the dynamics of ciliary membrane assembly by monitoring the changes of fluorescent-tagged ciliary proteins. Protein dynamics can be tracked simultaneously using multi-color live cell imaging by coupling ciliary-associated factors with different colored fluorescent tags. Ciliary membrane and membrane associated-proteins such as Smoothened, 5-HTr6, SSTR3, Rab8a, and Arl13b have been used to track ciliary membranes and centriole proteins like Centrin1/2, CEP164, and CEP83 are often used to mark the ciliary basal body. Here, we describe a method for studying ciliogenesis membrane dynamics using spinning disk confocal live-cell imaging.

In vivo Imaging Enables Understanding of Seamless Plant Defense Responses to Wounding and Pathogen Attack

Plants are exposed to varied biotic stresses, including sequential or simultaneous attack by insects and pathogens. To overcome these complex stresses, plants must perceive each of the stresses, then integrate and relay the information throughout the plant body and eventually activate local and systemic resistance responses. Previous molecular genetic studies identified jasmonic acid and salicylic acid as key plant hormones of wound and immune responses. These hormones, combined with their antagonistic interaction, play critical roles in the initiation and regulation of defense responses against insects and pathogens. Aside from molecular and genetic information, the latest in vivo imaging technology has revealed that plant defense responses are regulated spatially and temporally. In this review, we summarize the current knowledge of local and systemic defense responses against wounding and diseases with a focus on past and recent advances in imaging technologies. We discuss how imaging-based multiparametric analysis has improved our understanding of the spatiotemporal regulation of dynamic plant stress responses. We also emphasize the importance of compiling the knowledge generated from individual studies on plant wounding and immune responses for a more seamless understanding of plant defense responses in the natural environment.

Metal-Nano-Ink Coating for Monitoring and Quantification of Cotyledon Epidermal Cell Morphogenesis

During cotyledon growth, the pavement cells, which make up most of the epidermal layer, undergo dynamic morphological changes from simple to jigsaw puzzle-like shapes in most dicotyledonous plants. Morphological analysis of cell shapes generally involves the segmentation of cells from input images followed by the extraction of shape descriptors that can be used to assess cell shape. Traditionally, replica and fluorescent labeling methods have been used for time-lapse observation of cotyledon epidermal cell morphogenesis, but these methods require expensive microscopes and can be technically demanding. Here, we propose a silver-nano-ink coating method for time-lapse imaging and quantification of morphological changes in the epidermal cells of growing Arabidopsis thaliana cotyledons. To obtain high-resolution and wide-area cotyledon surface images, we placed the seedlings on a biaxial goniometer and adjusted the cotyledons, which were coated by dropping silver ink onto them, to be as horizontal to the focal plane as possible. The omnifocal images that had the most epidermal cell shapes in the observation area were taken at multiple points to cover the whole surface area of the cotyledon. The multi-point omnifocal images were automatically stitched, and the epidermal cells were automatically and accurately segmented by machine learning. Quantification of cell morphological features based on the segmented images demonstrated that the proposed method could quantitatively evaluate jigsaw puzzle-shaped cell growth and morphogenesis. The method was successfully applied to phenotyping of the bpp125 triple mutant, which has defective pavement cell morphogenesis. The proposed method will be useful for time-lapse non-destructive phenotyping of plant surface structures and is easier to use than the conversional methods that require fluorescent dye labeling or transformation with marker gene constructs and expensive microscopes such as the confocal laser microscope.