Visualizing the actin cytoskeleton in living plant cells using a photo-convertible mEos::FABD-mTn fluorescent fusion protein

Using ER-Targeted Photoconvertible Fluorescent Proteins in Living Plant Cells

Photoconvertible fluorescent proteins (pcFPs) enable differential coloring of a single organelle. Several pcFP-based probes have been targeted to the endoplasmic reticulum (ER) and can serve as useful tools to study ER dynamics and interactions with other organelles. Here, we describe the procedure to conduct live-cell imaging experiments using ER-targeted pcFP-based probes. Potential problems that might occur during the experiments, their solutions, and several ways to improve the experiments are discussed.

Thermoinduced Crystallization-Driven Self-Assembly of Bioinspired Block Copolymers in Aqueous Solution

Delicate control over architectures via crystallization-driven self-assembly (CDSA) in aqueous solution, particularly combined with external stimuli, is rare and challenging. Here, we report a stepwise CDSA process thermally initiated from amphiphilic poly(N-allylglycine)-b-poly(N-octylglycine) (PNAG-b-PNOG) conjugated with thiol-terminated triethylene glycol monomethyl ethers ((PNAG-g-EG₃)-b-PNOG) in aqueous solution. The diblock copolymers show a reversible thermoresponsive behavior with nearly identical cloud points in both heating and cooling runs. In contrast, the morphology transition of the assemblies is irreversible upon a heating-cooling cycle because of the presence of a confined domain arising from crystalline PNOG, which allows for the achievement of different nanostructured assemblies by the same polymer. We demonstrated that the thermoresponsive property of PNAG-g-EG₃ initiates assembly kinetically that is subsequently promoted by crystallization of PNOG thermodynamically. The irreversible morphology transition behavior provides a convenient platform for comparing the cellular uptake efficiency of nanostructured assemblies with various morphologies that are otherwise similar.

The Photoconvertible Fluorescent Protein Dendra2 Tag as a Tool to Investigate Intracellular Protein Dynamics

Fluorescence proteins changing spectral properties after exposure to light with a specific wavelength have recently become outstanding aids in the study of intracellular protein dynamics. Herein we show using Arabidopsis SYNAPTOTAGMIN 1 as a model protein that the Dendra2 green to red photoconvertible protein tag in combination with confocal scanning laser microscopy is a useful tool to study membrane protein intracellular dynamics.

Single-molecule detection and tracking in plants

Combining optical properties with a limited choice of fluorophores turns single-molecule imaging in plants into a challenging task. This explains why the technique, despite its success in the field of animal cell biology, is far from being routinely applied in plant cell research. The same challenges, however, also apply to the application of single-molecule microscopy to any intact tissue or multicellular 3D cell culture. As recent and upcoming progress in fluorescence microscopy will permit single-molecule detection in the context of multicellular systems, plant tissue imaging will experience a huge benefit from this progress. In this review, we address every step of a single-molecule experiment, highlight the critical aspects of each and elaborate on optimizations and developments required for improvements. We relate each step to recent achievements, which have so far been conducted exclusively on the root epidermis of Arabidopsis thaliana seedlings with inclined illumination and show examples of single-molecule measurements using different cells or illumination schemes.

Phototransformable fluorescent proteins: which one for which application?

In these last two decades , fluorescent proteins (FPs) have become highly valued imaging tools for cell biology, owing to their compatibility with living samples, their low levels of invasiveness and the possibility to specifically fuse them to a variety of proteins of interest. Remarkably, the recent development of phototransformable fluorescent proteins (PTFPs) has made it possible to conceive optical imaging experiments that were unimaginable only a few years ago. For example, it is nowadays possible to monitor intra- or intercellular trafficking, to optically individualize single cells in tissues or to observe single molecules in live cells. The tagging specificity brought by these genetically encoded highlighters leads to constant progress in the engineering of increasingly powerful, versatile and non-cytotoxic FPs. This review is focused on the recent developments of PTFPs and highlights their contribution to studies within cells, tissues and even living organisms. The aspects of single-molecule localization microscopy, intracellular tracking of photoactivated molecules, applications of PTFPs in biotechnology/optobiology and complementarities between PTFPs and other microscopy techniques are particularly discussed.

Interaction between Calcium and Actin in Guard Cell and Pollen Signaling Networks

Calcium (Ca(2+)) plays important roles in plant growth, development, and signal transduction. It is a vital nutrient for plant physical design, such as cell wall and membrane, and also serves as a counter-cation for biochemical, inorganic, and organic anions, and more particularly, its concentration change in cytosol is a ubiquitous second messenger in plant physiological signaling in responses to developmental and environmental stimuli. Actin cytoskeleton is well known for its importance in cellular architecture maintenance and its significance in cytoplasmic streaming and cell division. In plant cell system, the actin dynamics is a process of polymerization and de-polymerization of globular actin and filamentous actin and that acts as an active regulator for calcium signaling by controlling calcium evoked physiological responses. The elucidation of the interaction between calcium and actin dynamics will be helpful for further investigation of plant cell signaling networks at molecular level. This review mainly focuses on the recent advances in understanding the interaction between the two aforementioned signaling components in two well-established model systems of plant, guard cell, and pollen.

PIN2 turnover in Arabidopsis root epidermal cells explored by the photoconvertible protein Dendra2

The steady state level of integral membrane proteins is dependent on a strictly controlled delivery and removal. Here we show that Dendra2, a green-to-red photoconvertible fluorescent protein, is a suitable tool to study protein turnover in plants. We characterized the fluorescence properties of Dendra2 expressed either as a free protein or as a tag in Arabidopsis thaliana roots and optimized photoconversion settings to study protein turnover. Dendra2 was fused to the PIN2 protein, an auxin transporter in the root tip, and by time-lapse imaging and assessment of red and green signal intensities in the membrane after photoconversion we quantified directly and simultaneously the rate of PIN2 delivery of the newly synthesized protein into the plasma membrane as well as the disappearance of the protein from the plasma membrane due to degradation. Additionally we have verified several factors which are expected to affect PIN2 protein turnover and therefore potentially regulate root growth.

Smaller, faster, brighter: advances in optical imaging of living plant cells

The advent of fluorescent proteins and access to modern imaging technologies have dramatically accelerated the pace of discovery in plant cell biology. Remarkable new insights into such diverse areas as plant pathogenesis, cytoskeletal dynamics, sugar transport, cell wall synthesis, secretory control, and hormone signaling have come from careful examination of living cells using advanced optical probes. New technologies, both commercially available and on the horizon, promise a continued march toward more quantitative methods for imaging and for extending the optical exploration of biological structure and activity to molecular scales. In this review, we lay out fundamental issues in imaging plant specimens and look ahead to several technological innovations in molecular tools, instrumentation, imaging methods, and specimen handling that show promise for shaping the coming era of plant cell biology.

Differential coloring reveals that plastids do not form networks for exchanging macromolecules

Stroma-filled tubules named stromules are sporadic extensions of plastids. Earlier, photobleaching was used to demonstrate fluorescent protein diffusion between already interconnected plastids and formed the basis for suggesting that all plastids are able to form networks for exchanging macromolecules. However, a critical appraisal of literature shows that this conjecture is not supported by unequivocal experimental evidence. Here, using photoconvertible mEosFP, we created color differences between similar organelles that enabled us to distinguish clearly between organelle fusion and nonfusion events. Individual plastids, despite conveying a strong impression of interactivity and fusion, maintained well-defined boundaries and did not exchange fluorescent proteins. Moreover, the high pleomorphy of etioplasts from dark-grown seedlings, leucoplasts from roots, and assorted plastids in the accumulation and replication of chloroplasts5 (arc5), arc6, and phosphoglucomutase1 mutants of Arabidopsis thaliana suggested that a single plastid unit might be easily mistaken for interconnected plastids. Our observations provide succinct evidence to refute the long-standing dogma of interplastid connectivity. The ability to create and maintain a large number of unique biochemical factories in the form of singular plastids might be a key feature underlying the versatility of green plants as it provides increased internal diversity for them to combat a wide range of environmental fluctuations and stresses.

Green-to-red photoconvertible mEosFP-aided live imaging in plants

Numerous subcellular-targeted probes have been created using a monomeric green-to-red photoconvertible Eos fluorescent protein for understanding the growth and development of plants. These probes can be used to create color-based differentiation between similar cells, differentially label organelle subpopulations, and track subcellular structures and their interactions. Both green and red fluorescent forms of mEosFP are stable and compatible with single colored FPs. Differential highlighting using mEosFP probes greatly increases spatiotemporal precision during live imaging.

Assessing the utility of photoswitchable fluorescent proteins for tracking intercellular protein movement in the Arabidopsis root

One way in which cells communicate is through the direct transfer of proteins. In plants, many of these proteins are transcription factors, which are made by one cell type and traffic into another. In order to understand how this movement occurs and its role in development, we would like to track this movement in live, intact plants in real-time. Here we examine the utility of the photoconvertible proteins, Dendra2 and (to a lesser extent) EosFP as tags for studying intracellular and intercellular protein movement in the Arabidopsis root. To this end, we made fusions between Dendra2 and six mobile transcription factors. Our results show that Dendra2 is an effective tool for studying protein movement between plant cells. Interestingly, we found that Dendra2 could not simply be swapped into existing constructs that had originally contained GFP. Most of the fusions made in this way failed to produce a fluorescent fusion. In addition we found that the optimal settings for photoconversion of Dendra2 in stably transformed roots were different from what has been published for photoconversion in transient assays in plants or in animal cells. By modifying the confocal setting, we were able to photoconvert Dendra2 in all cell layers in the root. However the efficiency of photoconversion was affected by the position of the cell layer within the root, with more internal tissues requiring more energy. By examining the Dendra2 fusions, we confirmed the mobility of the SHORT-ROOT (SHR) and CAPRICE (CPC) transcription factors between cells and we further discovered that SHR movement in stele and CPC movement in the epidermis are non-directional.

From EosFP to mIrisFP: structure-based development of advanced photoactivatable marker proteins of the GFP-family

Fluorescent proteins from the GFP family have become indispensable imaging tools in life sciences research. In recent years, a wide variety of these proteins were discovered in non-bioluminescent anthozoa. Some of them feature exciting new properties, including the possibility to change their fluorescence quantum yield and/or color by irradiating with light of specific wavelengths. These photoactivatable fluorescent proteins enable many interesting applications including pulse-chase experiments and super-resolution imaging. In this review, we discuss the development of advanced variants, using a structure-function based, molecular biophysics approach, of the photoactivatable fluorescent protein EosFP, which can be photoconverted from green to red fluorescence by ~400 nm light. A variety of applications are presented that demonstrate the versatility of these marker proteins in live-cell imaging.

Development and application of probes for labeling the actin cytoskeleton in living plant cells

The actin cytoskeleton is one of the most important components of eukaryotic cytoskeletons. It participates in numerous crucial procedures of cells and has been studied by using various methods. The development and application of appropriate probes for actin visualization is the first and foremost step for functional analysis of actin in vivo. Since the actin cytoskeleton is a highly dynamic and sensitive structure, methods previously used to visualize actin often harm cells and cannot reveal the native state of the actin cytoskeleton in living cells. The development of labeling technologies for living plant cells, especially the emergence and application of green fluorescent protein-tagged actin markers, has provided new insights into the structure and function of the actin cytoskeleton in vivo. There has been a number of probes for actin labeling in living plant cells though they each present different advantages and defects. In this review, we discuss and compare those widely used methods for actin visualization and analysis.

Fluorescence perturbation techniques to study mobility and molecular dynamics of proteins in live cells: FRAP, photoactivation, photoconversion, and FLIP

The technique of fluorescence recovery after photobleaching (FRAP) was introduced in the mid-1970s to study the diffusion of biomolecules in living cells. For several years, it was used mainly by a small number of biophysicists who had developed their own photobleaching systems. Since the mid-1990s, FRAP has gained increasing popularity because of the conjunction of two factors: First, photobleaching techniques are easily implemented on confocal laser-scanning microscopes (CLSMs), and so FRAP has become available to anyone who has access to such equipment. Second, the advent of green fluorescent protein (GFP) has allowed easy fluorescent tagging of proteins and their observation in living cells. Thanks both to the versatility of modern CLSMs, which allow control of laser intensity at any point of the image, and to the development of new fluorescent probes, additional photoperturbation techniques have emerged during the last few years. After the photoperturbation event, one observes and then analyzes how the fluorescence distribution relaxes toward the steady state. Because the photochemical perturbation of suitable fluorophores is essentially irreversible, changes of fluorescence intensity in the perturbed and unperturbed regions are due to the exchange of tagged molecules between those regions. This article first discusses the materials required for performing FRAP experiments on a CLSM and the software for data analysis. It then describes general considerations on how to perform FRAP experiments as well as the necessary controls. Finally, different possible ways to analyze the data are presented.

mEosFP-based green-to-red photoconvertible subcellular probes for plants

Photoconvertible fluorescent proteins (FPs) are recent additions to the biologists' toolbox for understanding the living cell. Like green fluorescent protein (GFP), monomeric EosFP is bright green in color but is efficiently photoconverted into a red fluorescent form using a mild violet-blue excitation. Here, we report mEosFP-based probes that localize to the cytosol, plasma membrane invaginations, endosomes, prevacuolar vesicles, vacuoles, the endoplasmic reticulum, Golgi bodies, mitochondria, peroxisomes, and the two major cytoskeletal elements, filamentous actin and cortical microtubules. The mEosFP fusion proteins are smaller than GFP/red fluorescent protein-based probes and, as demonstrated here, provide several significant advantages for imaging of living plant cells. These include an ability to differentially color label a single cell or a group of cells in a developing organ, selectively highlight a region of a cell or a subpopulation of organelles and vesicles within a cell for tracking them, and understanding spatiotemporal aspects of interactions between similar as well as different organelles. In addition, mEosFP probes introduce a milder alternative to fluorescence recovery after photobleaching, whereby instead of photobleaching, photoconversion followed by recovery of green fluorescence can be used for estimating subcellular dynamics. Most importantly, the two fluorescent forms of mEosFP furnish bright internal controls during imaging experiments and are fully compatible with cyan fluorescent protein, GFP, yellow fluorescent protein, and red fluorescent protein fluorochromes for use in simultaneous, multicolor labeling schemes. Photoconvertible mEosFP-based subcellular probes promise to usher in a much higher degree of precision to live imaging of plant cells than has been possible so far using single-colored FPs.

Exploring plant endomembrane dynamics using the photoconvertible protein Kaede

Photoactivatable and photoconvertible fluorescent proteins capable of pronounced light-induced spectral changes are a powerful addition to the fluorescent protein toolbox of the cell biologist. They permit specific tracking of one subcellular structure (organelle or cell subdomain) within a differentially labelled population. They also enable pulse-chase analysis of protein traffic. The Kaede gene codes for a tetrameric protein found in the stony coral Trachyphyllia geoffroyi, which emits green fluorescence that irreversibly shifts to red following radiation with UV or violet light. We report here the use of Kaede to explore the plant secretory pathway. Kaede versions of the Golgi marker sialyl-transferase (ST-Kaede) and of the vacuolar pathway marker cardosin A (cardA-Kaede) were engineered. Several optical devices enabling photoconversion and observation of Kaede using these two constructs were assessed to optimize Kaede-based imaging protocols. Photoconverted ST-Kaede red-labelled organelles can be followed within neighbouring populations of non-converted green Golgi stacks, by their gradual development of orange/yellow coloration from de novo synthesis of Golgi proteins (green). Results highlight some aspects on the dynamics of the plant Golgi. For plant bio-imaging, the photoconvertible Kaede offers a powerful tool to track the dynamic behaviour of designated subpopulations of Golgi within living cells, while visualizing the de novo formation of proteins and structures, such as a Golgi stack.

Transient expression in Nicotiana benthamiana fluorescent marker lines provides enhanced definition of protein localization, movement and interactions in planta

Here, we report on the construction of a novel series of Gateway-compatible plant transformation vectors containing genes encoding autofluorescent proteins, including Cerulean, Dendra2, DRONPA, TagRFP and Venus, for the expression of protein fusions in plant cells. To assist users in the selection of vectors, we have determined the relative in planta photostability and brightness of nine autofluorescent proteins (AFPs), and have compared the use of DRONPA and Dendra2 in photoactivation and photoconversion experiments. Additionally, we have generated transgenic Nicotiana benthamiana lines that express fluorescent protein markers targeted to nuclei, endoplasmic reticulum or actin filaments. We show that conducting bimolecular fluorescence complementation assays in plants that constitutively express cyan fluorescent protein fused to histone 2B provides enhanced data quality and content over assays conducted without the benefit of a subcellular marker. In addition to testing protein interactions, we demonstrate that our transgenic lines that express red fluorescent protein markers offer exceptional support in experiments aimed at defining nuclear or endomembrane localization. Taken together, the new combination of pSITE-BiFC and pSITEII vectors for studying intracellular protein interaction, localization and movement, in conjunction with our transgenic marker lines, constitute powerful tools for the plant biology community.