Single cell technology

Towards single-cell ionomics: a novel micro-scaled method for multi-element analysis of nanogram-sized biological samples

BACKGROUND

To understand processes regulating nutrient homeostasis at the single-cell level there is a need for new methods that allow multi-element profiling of biological samples ultimately only available as isolated tissues or cells, typically in nanogram-sized samples. Apart from tissue isolation, the main challenges for such analyses are to obtain a complete and homogeneous digestion of each sample, to keep sample dilution at a minimum and to produce accurate and reproducible results. In particular, determining the weight of small samples becomes increasingly challenging when the sample amount decreases.

RESULTS

We developed a novel method for sampling, digestion and multi-element analysis of nanogram-sized plant tissue, along with strategies to quantify element concentrations in samples too small to be weighed. The method is based on tissue isolation by laser capture microdissection (LCM), followed by pressurized micro-digestion and ICP-MS analysis, the latter utilizing a stable µL min^-1 sample aspiration system. The method allowed for isolation, digestion and analysis of micro-dissected tissues from barley roots with an estimated sample weight of only ~ 400 ng. In the collection and analysis steps, a number of contamination sources were identified. Following elimination of these sources, several elements, including magnesium (Mg), phosphorus (P), potassium (K) and manganese (Mn), could be quantified. By measuring the exact area and thickness of each of the micro-dissected tissues, their volume was calculated. Combined with an estimated sample density, the sample weights could subsequently be calculated and the fact that these samples were too small to be weighed could thereby be circumvented. The method was further documented by analysis of Arabidopsis seeds (~ 20 µg) as well as tissue fractions of such seeds (~ 10 µg).

CONCLUSIONS

The presented method enables collection and multi-element analysis of small-sized biological samples, ranging down to the nanogram level. As such, the method paves the road for single cell and tissue-specific quantitative ionomics, which allow for future transcriptional, proteomic and metabolomic data to be correlated with ionomic profiles. Such analyses will deepen our understanding of how the elemental composition of plants is regulated, e.g. by transporter proteins and physical barriers (i.e. the Casparian strip and suberin lamellae in the root endodermis).

Spatial-Resolution Cell Type Proteome Profiling of Cancer Tissue by Fully Integrated Proteomics Technology

Increasing attention has been focused on cell type proteome profiling for understanding the heterogeneous multicellular microenvironment in tissue samples. However, current cell type proteome profiling methods need large amounts of starting materials which preclude their application to clinical tumor specimens with limited access. Here, by seamlessly combining laser capture microdissection and integrated proteomics sample preparation technology SISPROT, specific cell types in tumor samples could be precisely dissected with single cell resolution and processed for high-sensitivity proteome profiling. Sample loss and contamination due to the multiple transfer steps are significantly reduced by the full integration and noncontact design. H&E staining dyes which are necessary for cell type investigation could be selectively removed by the unique two-stage design of the spintip device. This easy-to-use proteome profiling technology achieved high sensitivity with the identification of more than 500 proteins from only 0.1 mm² and 10 μm thickness colon cancer tissue section. The first cell type proteome profiling of four cell types from one colon tumor and surrounding normal tissue, including cancer cells, enterocytes, lymphocytes, and smooth muscle cells, was obtained. 5271, 4691, 4876, and 2140 protein groups were identified, respectively, from tissue section of only 5 mm² and 10 μm thickness. Furthermore, spatially resolved proteome distribution profiles of enterocytes, lymphocytes, and smooth muscle cells on the same tissue slices and across four consecutive sections with micrometer distance were successfully achieved. This fully integrated proteomics technology, termed LCM-SISPROT, is therefore promising for spatial-resolution cell type proteome profiling of tumor microenvironment with a minute amount of clinical starting materials.

Laser Capture Micro-Dissection Coupled to RNA Sequencing: A Powerful Approach Applied to the Model Legume Medicago truncatula in Interaction with Sinorhizobium meliloti

Understanding the development of multicellular organisms requires the identification of regulators, notably transcription factors, and specific transcript populations associated with tissue differentiation. Laser capture microdissection (LCM) is one of the techniques that enable the analysis of distinct tissues or cells within an organ. Coupling this technique with RNA sequencing (RNAseq) makes it extremely powerful to obtain a genome-wide and dynamic view of gene expression. Moreover, RNA sequencing allows two or potentially more interacting organisms to be analyzed simultaneously. In this chapter, a LCM-RNAseq protocol optimized for root and symbiotic root nodule analysis is presented, using the model legume Medicago truncatula (in interaction with Sinorhizobium meliloti in the nodule samples). This includes the description of procedures for plant material fixation, embedding, and micro-dissection; it is followed by a presentation of techniques for RNA extraction and amplification, adapted for the simultaneous analysis of plant and bacterial cells in interaction or, more generally, polyadenylated and non-polyadenylated RNAs. Finally, step-by-step statistical analyses of RNAseq data are described. Those are critical for quality assessment of the whole procedure and for the identification of differentially expressed genes.

Laser-Capture Microdissection of Maize Kernel Compartments for RNA-Seq-Based Expression Analysis

Laser-capture microdissection (LCM) enables isolation of single cells or groups of cells for a variety of downstream applications including transcriptome profiling. Recently, this methodology has found a more widespread use particularly with the advent of next-generation sequencing techniques that enable deep profiling of the limited amounts of RNA obtained from fixed or frozen sections. When used with fixed tissues, a major experimental challenge is to balance the tissue integrity needed for microscopic visualization of the cell types of interest with that of the RNA quality necessary for deep profiling. Complex biological structures such as seeds or kernels pose an especially difficult case in this context as in many instances the key internal structures such as the embryo and the endosperm are relatively inaccessible. Here, we present an optimized LCM protocol for maize kernel that has been developed specifically to enable profiling of the early stages of endosperm development using RNA-Seq.

Application of single-cell technology in cancer research

In this review, we have outlined the application of single-cell technology in cancer research. Single-cell technology has made encouraging progress in recent years and now provides the means to detect rare cancer cells such as circulating tumor cells and cancer stem cells. We reveal how this technology has advanced the analysis of intratumor heterogeneity and tumor epigenetics, and guided individualized treatment strategies. The future prospects now are to bring single-cell technology into the clinical arena. We believe that the clinical application of single-cell technology will be beneficial in cancer diagnostics and treatment, and ultimately improve survival in cancer patients.

Tissue-specific metabolite profiling of benzylisoquinoline alkaloids in the root of Macleaya cordata by combining laser microdissection with ultra-high-performance liquid chromatography/tandem mass spectrometry

RATIONALE

Tissue-specific metabolite profiling helps to find trace alkaloids masked during organ analysis, which contributes to understanding the alkaloid biosynthetic pathways in vivo and evaluating the quality of medical plants by morphology. As Macleaya cordata contains diverse types of benzylisoquinoline alkaloids (BIAs), the alkaloid metabolite profiling was carried out on various tissues of the root.

METHODS

Laser microdissection with fluorescence detection was used to recognize and dissect different tissues from the root of M. cordata. Ultra-high-performance liquid chromatography/quadrupole time-of-flight mass spectrometry was applied to analyze the trace alkaloids in tissues. These detected alkaloids were elucidated using their accurate molecular weights, MS/MS data, MS fragmentation patterns and the known biosynthetic pathways of BIAs. Finally, the distribution of alkaloids in dissected tissues and whole sections was mapped.

RESULTS

Forty-nine alkaloids were identified from five microdissected tissues, and 24 of them were detected for the first time in M. cordata. Some types of alkaloids occurred specifically in dissected tissues. More alkaloids were detected in the cork and xylem vascular bundles which emit strong fluorescence under fluorescence microscopy. Some of the screened alkaloids were intermediates in sanguinarine and chelerythrine biosynthetic pathways, and others were speculated to be involved in the new branches of biosynthetic pathways.

CONCLUSIONS

The integrated method is sensitive, specific and reliable for determining trace alkaloids, which is also a powerful tool for metabolite profiling of tissue-specific BIAs in situ. The present findings should contribute to a better understanding of the biosynthesis of BIAs in M. cordata root and provide scientific evidence for its quality evaluation based on morphological characteristics. Copyright © 2016 John Wiley & Sons, Ltd.

Decipher the Molecular Response of Plant Single Cell Types to Environmental Stresses

The analysis of the molecular response of entire plants or organs to environmental stresses suffers from the cellular complexity of the samples used. Specifically, this cellular complexity masks cell-specific responses to environmental stresses and logically leads to the dilution of the molecular changes occurring in each cell type composing the tissue/organ/plant in response to the stress. Therefore, to generate a more accurate picture of these responses, scientists are focusing on plant single cell type approaches. Several cell types are now considered as models such as the pollen, the trichomes, the cotton fiber, various root cell types including the root hair cell, and the guard cell of stomata. Among them, several have been used to characterize plant response to abiotic and biotic stresses. In this review, we are describing the various -omic studies performed on these different plant single cell type models to better understand plant cell response to biotic and abiotic stresses.

Microfluidic enrichment for the single cell analysis of circulating tumor cells

Resistance to drug therapy is a major concern in cancer treatment. To probe clones resistant to chemotherapy, the current approach is to conduct pooled cell analysis. However, this can yield false negative outcomes, especially when we are analyzing a rare number of circulating tumor cells (CTCs) among an abundance of other cell types. Here, we develop a microfluidic device that is able to perform high throughput, selective picking and isolation of single CTC to 100% purity from a larger population of other cells. This microfluidic device can effectively separate the very rare CTCs from blood samples from as few as 1 in 20,000 white blood cells. We first demonstrate isolation of pure tumor cells from a mixed population and track variations of acquired T790M mutations before and after drug treatment using a model PC9 cell line. With clinical CTC samples, we then show that the isolated single CTCs are representative of dominant EGFR mutations such as T790M and L858R found in the primary tumor. With this single cell recovery device, we can potentially implement personalized treatment not only through detecting genetic aberrations at the single cell level, but also through tracking such changes during an anticancer therapy.

Laser Microdissection and Spatiotemporal Pinoresinol-Lariciresinol Reductase Gene Expression Assign the Cell Layer-Specific Accumulation of Secoisolariciresinol Diglucoside in Flaxseed Coats

The concentration of secoisolariciresinol diglucoside (SDG) found in flaxseed (Linum usitatissimum L.) is higher than that found in any other plant. It exists in flaxseed coats as an SDG-3-hydroxy-3-methylglutaric acid oligomer complex. A laser microdissection method was applied to harvest material from different cell layers of seed coats of mature and developing flaxseed to detect the cell-layer specific localization of SDG in flaxseed; NMR and HPLC were used to identify and quantify SDG in dissected cell layers after alkaline hydrolysis. The obtained results were further confirmed by a standard molecular method. The promoter of one pinoresinol-lariciresinol reductase gene of L. usitatissimum (LuPLR1), which is a key gene involved in SDG biosynthesis, was fused to a β-glucuronidase (GUS) reporter gene, and the spatio-temporal regulation of LuPLR1 gene expression in flaxseed was determined by histochemical and activity assays of GUS. The result showed that SDG was synthesized and accumulated in the parenchymatous cell layer of the outer integument of flaxseed coats.

Modern plant metabolomics: advanced natural product gene discoveries, improved technologies, and future prospects

Plant metabolomics has matured and modern plant metabolomics has accelerated gene discoveries and the elucidation of a variety of plant natural product biosynthetic pathways. This review covers the approximate period of 2000 to 2014, and highlights specific examples of the discovery and characterization of novel genes and enzymes associated with the biosynthesis of natural products such as flavonoids, glucosinolates, terpenoids, and alkaloids. Additional examples of the integration of metabolomics with genome-based functional characterizations of plant natural products that are important to modern pharmaceutical technology are also reviewed. This article also provides a substantial review of recent technical advances in mass spectrometry imaging, nuclear magnetic resonance imaging, integrated LC-MS-SPE-NMR for metabolite identifications, and X-ray crystallography of microgram quantities for structural determinations. The review closes with a discussion on the future prospects of metabolomics related to crop species and herbal medicine.

Single cell subtractive transcriptomics for identification of cell-specifically expressed candidate genes of pyrrolizidine alkaloid biosynthesis

Progress has recently been made in the elucidation of pathways of secondary metabolism. However, because of its diversity, genetic information concerning biosynthetic details is still missing for many natural products. This is also the case for the biosynthesis of pyrrolizidine alkaloids. To close this gap, we tested strategies using tissues that express this pathway in comparison to tissues in which this pathway is not expressed. As many pathways of secondary metabolism are known to be induced by jasmonates, the pyrrolizidine alkaloid-producing species Heliotropium indicum, Symphytum officinale, and Cynoglossum officinale of the Boraginales order were treated with methyl jasmonate. An effect on pyrrolizidine alkaloid levels and on transcript levels of homospermidine synthase, the first specific enzyme of pyrrolizidine alkaloid biosynthesis, was not detectable. Therefore, a method was developed by making use of the often observed cell-specific production of secondary compounds. H. indicum produces pyrrolizidine alkaloids exclusively in the shoot. Homospermidine synthase is expressed only in the cells of the lower leaf epidermis and the epidermis of the stem. Suggesting that the whole pathway of pyrrolizidine alkaloid biosynthesis might be localized in these cells, we have isolated single cells of the upper and lower epidermis by laser-capture microdissection. The resulting cDNA preparations have been used in a subtractive transcriptomic approach. Quantitative real-time polymerase chain reaction has shown that the resulting library is significantly enriched for homospermidine-synthase-coding transcripts providing a valuable source for the identification of further genes involved in pyrrolizidine alkaloid biosynthesis.

Plant single-cell and single-cell-type metabolomics

In conjunction with genomics, transcriptomics, and proteomics, plant metabolomics is providing large data sets that are paving the way towards a comprehensive and holistic understanding of plant growth, development, defense, and productivity. However, dilution effects from organ- and tissue-based sampling of metabolomes have limited our understanding of the intricate regulation of metabolic pathways and networks at the cellular level. Recent advances in metabolomics methodologies, along with the post-genomic expansion of bioinformatics knowledge and functional genomics tools, have allowed the gathering of enriched information on individual cells and single cell types. Here we review progress, current status, opportunities, and challenges presented by single cell-based metabolomics research in plants.

Laser microdissection: a sample preparation technique for plant micrometabolic profiling

INTRODUCTION

Unlike unicellular organisms, plants have evolved as complex organisms that are defined by their ability to distribute special vital functions to spatially separated organs and tissues. Current phytochemical approaches mostly ignore this fact by analysing samples that consist of different cell types and thus average the information obtained. A comprehensive metabolite analysis with high spatial resolution is essential to fully characterise the state of a certain tissue; hence, the analysis of metabolites occurring in specialised plant cells is of considerable interest in chemical ecology, plant natural product chemistry and other bioscience disciplines. Laser microdissection (LMD), including laser capture microdissection and laser microdissection and pressure catapulting, is a convenient sampling technique to harvest homogeneous cell types for the microanalysis of plant metabolites.

OBJECTIVE

The objective of this work is to provide an introduction to LMD methodology and a concise review of recent applications of LMD in the high-resolution analysis of metabolites in different plant materials.

METHODS

A step-by-step approach to LMD sampling techniques is described. How LMD can be used to sample cells or microscopic tissue pieces from different plant organs, such as leaves, stems, and seeds, is shown in detail. Finally, the future of LMD in plant metabolites analysis is discussed.

RESULTS

This review summarises studies over the past decade not only showing technical details but also indicating the wide application of this method for high-resolution plant metabolite analysis.

CONCLUSION

Laser microdissection is a powerful sampling technique for plant micrometabolic profiling and metabolomics research.

Metabolite analyses of single cells

Single-cell analysis is a promising method for understanding not only cellular physiology but also biological mechanisms of multicellular organisms. Although neighboring cells in multicellular organisms originate from the same genomic information, different circumstances around cells or epigenetic differences have different influences on each cell, leading to differing expression of genes, and thus differing levels and dynamics of metabolites, in single cells. However, single-cell analysis is a tough challenge, even with recent technologies, because of the small size of single cells. Unlike genes, metabolites cannot be amplified, and therefore metabolite analysis is another issue. To analyze such a tiny quantity of metabolites in a single cell, various techniques have been tried and developed. Especially in mass spectrometry, marked improvements in both detection sensitivity and ionization techniques have opened up the challenge for the analysis of metabolites in single cells. In this review, we discuss the method for metabolite detection at the level of single cells and recent advancements in technology.

Flow cytometry and microscopy as means of studying single cells: a short introductional overview

Flow cytometry and microscopy are perhaps the two most commonly used techniques for analyzing single cells. Both techniques are typically robust and provide a high throughput analysis of living and/or fixed cells. The techniques are often combined with fluorescent labeling of cells, using for example antibodies. This chapter is a short introductory review where some of the possible applications using flow cytometry and microscopy are discussed.

A novel procedure for the quantitative analysis of metabolites, storage products and transcripts of laser microdissected seed tissues of Brassica napus

BACKGROUND

The biology of the seed is complicated by the extensive non-homogeneity (spatial gradients) in gene expression, metabolic conversions and storage product accumulation. The detailed understanding of the mechanisms underlying seed growth and storage therefore requires the development of means to obtain tissue-specific analyses. This approach also represents an important priority in the context of seed biotechnology.

RESULTS

We provide a guideline and detailed procedures towards the quantitative analysis of laser micro-dissected (LM) tissues in oilseed rape (Brassica napus). This includes protocols for laser microdissection of the seed, and the subsequent extraction and quantitative analysis of lipids, starch and metabolites (sugars, sugar phosphates, nucleotides, amino acids, intermediates of glycolysis and citric acid cycle). We have also developed a protocol allowing the parallel analysis of the transcriptome using Brassica-specific microarrays. Some data are presented regarding the compartmentation of metabolites within the oilseed rape embryo.

CONCLUSION

The described methodology allows for the rapid, combined analysis of metabolic intermediates, major storage products and transcripts in a tissue-specific manner. The protocols are robust for oilseed rape, and should be readily adjustable for other crop species. The suite of methods applied to LM tissues represents an important step in the context of both the systems biology and the biotechnology of oilseeds.

Near-IR laser-triggered target cell collection using a carbon nanotube-based cell-cultured substrate

Unique near-IR optical properties of single-walled carbon nanotube (SWNTs) are of interest in many biological applications. Here we describe the selective cell detachment and collection from an SWNT-coated cell-culture dish triggered by near-IR pulse laser irradiation. First, HeLa cells were cultured on an SWNT-coated dish prepared by a spraying of an aqueous SWNT dispersion on a glass dish. The SWNT-coated dish was found to show a good cell adhesion behavior as well as a cellular proliferation rate similar to a conventional glass dish. We discovered, by near-IR pulse laser irradiation (at the laser power over 25 mW) to the cell under optical microscopic observation, a quick single-cell detachment from the SWNT-coated surface. Shockwave generation from the irradiated SWNTs is expected to play an important role for the cell detachment. Moreover, we have succeeded in catapulting the target single cell from the cultured medium when the depth of the medium was below 150 μm and the laser power was stronger than 40 mW. The captured cell maintained its original shape. The retention of the genetic information of the cell was confirmed by the polymerase chain reaction (PCR) technique. A target single-cell collection from a culture medium under optical microscopic observation is significant in wide fields of single-cell studies in biological areas.

Engineering of red cells of Arabidopsis thaliana and comparative genome-wide gene expression analysis of red cells versus wild-type cells

We report metabolic engineering of Arabidopsis red cells and genome-wide gene expression analysis associated with anthocyanin biosynthesis and other metabolic pathways between red cells and wild-type (WT) cells. Red cells of A. thaliana were engineered for the first time from the leaves of production of anthocyanin pigment 1-Dominant (pap1-D). These red cells produced seven anthocyanin molecules including a new one that was characterized by LC-MS analysis. Wild-type cells established as a control did not produce anthocyanins. A genome-wide microarray analysis revealed that nearly 66 and 65% of genes in the genome were expressed in the red cells and wild-type cells, respectively. In comparison with the WT cells, 3.2% of expressed genes in the red cells were differentially expressed. The expression levels of 14 genes involved in the biosynthetic pathway of anthocyanin were significantly higher in the red cells than in the WT cells. Microarray and RT-PCR analyses demonstrated that the TTG1-GL3/TT8-PAP1 complex regulated the biosynthesis of anthocyanins. Furthermore, most of the genes with significant differential expression levels in the red cells versus the WT cells were characterized with diverse biochemical functions, many of which were mapped to different metabolic pathways (e.g., ribosomal protein biosynthesis, photosynthesis, glycolysis, glyoxylate metabolism, and plant secondary metabolisms) or organelles (e.g., chloroplast). We suggest that the difference in gene expression profiles between the two cell lines likely results from cell types, the overexpression of PAP1, and the high metabolic flux toward anthocyanins.

Overview of single-cell analyses: microdevices and applications

Numerous microdevices developed for single-cell analyses have been presented in the last decades. Practical usefulness in biological and clinical settings has become an important focus during the development and implementation of new structures and assays. Single-cell analysis has been applied in intracellular research, gene- and protein content and expression, PCR, cell culture and division, clone formation, differentiation, morphology, lysis, separation, sorting, cytotoxicity and fluorescence screens, antibody secretion, etc. as discussed here along with brief descriptions of the technical devices used for the studies, e.g. well-, trap-, pattern-, and droplet-based structures. This review aims to serve as an overview of available techniques for single-cell analysis by describing the different biological single-cell assays that have been performed to date and how each individual application requires a particular device design.

Imaging immune surveillance of individual natural killer cells confined in microwell arrays

New markers are constantly emerging that identify smaller and smaller subpopulations of immune cells. However, there is a growing awareness that even within very small populations, there is a marked functional heterogeneity and that measurements at the population level only gives an average estimate of the behaviour of that pool of cells. New techniques to analyze single immune cells over time are needed to overcome this limitation. For that purpose, we have designed and evaluated microwell array systems made from two materials, polydimethylsiloxane (PDMS) and silicon, for high-resolution imaging of individual natural killer (NK) cell responses. Both materials were suitable for short-term studies (<4 hours) but only silicon wells allowed long-term studies (several days). Time-lapse imaging of NK cell cytotoxicity in these microwell arrays revealed that roughly 30% of the target cells died much more rapidly than the rest upon NK cell encounter. This unexpected heterogeneity may reflect either separate mechanisms of killing or different killing efficiency by individual NK cells. Furthermore, we show that high-resolution imaging of inhibitory synapse formation, defined by clustering of MHC class I at the interface between NK and target cells, is possible in these microwells. We conclude that live cell imaging of NK-target cell interactions in multi-well microstructures are possible. The technique enables novel types of assays and allow data collection at a level of resolution not previously obtained. Furthermore, due to the large number of wells that can be simultaneously imaged, new statistical information is obtained that will lead to a better understanding of the function and regulation of the immune system at the single cell level.

Isolation of intact sub-dermal secretory cavities from Eucalyptus

BACKGROUND

The biosynthesis of plant natural products in sub-dermal secretory cavities is poorly understood at the molecular level, largely due to the difficulty of physically isolating these structures for study. Our aim was to develop a protocol for isolating live and intact sub-dermal secretory cavities, and to do this, we used leaves from three species of Eucalyptus with cavities that are relatively large and rich in essential oils.

RESULTS

Leaves were digested using a variety of commercially available enzymes. A pectinase from Aspergillus niger was found to allow isolation of intact cavities after a relatively short incubation (12 h), with no visible artifacts from digestion and no loss of cellular integrity or cavity contents. Several measurements indicated the potential of the isolated cavities for further functional studies. First, the cavities were found to consume oxygen at a rate that is comparable to that estimated from leaf respiratory rates. Second, mRNA was extracted from cavities, and it was used to amplify a cDNA fragment with high similarity to that of a monoterpene synthase. Third, the contents of the cavity lumen were extracted, showing an unexpectedly low abundance of volatile essential oils and a sizeable amount of non-volatile material, which is contrary to the widely accepted role of secretory cavities as predominantly essential oil repositories.

CONCLUSIONS

The protocol described herein is likely to be adaptable to a range of Eucalyptus species with sub-dermal secretory cavities, and should find wide application in studies of the developmental and functional biology of these structures, and the biosynthesis of the plant natural products they contain.

Comparative physiology of elemental distributions in plants

BACKGROUND

Plants contain relatively few cell types, each contributing a specialized role in shaping plant function. With respect to plant nutrition, different cell types accumulate certain elements in varying amounts within their storage vacuole. The role and mechanisms underlying cell-specific distribution of elements in plants is poorly understood.

SCOPE

The phenomenon of cell-specific elemental accumulation has been briefly reviewed previously, but recent technological advances with the potential to probe mechanisms underlying elemental compartmentation have warranted an updated evaluation. We have taken this opportunity to catalogue many of the studies, and techniques used for, recording cell-specific compartmentation of particular elements. More importantly, we use three case-study elements (Ca, Cd and Na) to highlight the basis of such phenomena in terms of their physiological implications and underpinning mechanisms; we also link such distributions to the expression of known ion or solute transporters.

CONCLUSIONS

Element accumulation patterns are clearly defined by expression of key ion or solute transporters. Although the location of element accumulation is fairly robust, alterations in expression of certain solute transporters, through genetic modifications or by growth under stress, result in perturbations to these patterns. However, redundancy or induced pleiotropic expression effects may complicate attempts to characterize the pathways that lead to cell-specific elemental distribution. Accumulation of one element often has consequences on the accumulation of others, which seems to be driven largely to maintain vacuolar and cytoplasmic osmolarity and charge balance, and also serves as a detoxification mechanism. Altered cell-specific transcriptomics can be shown, in part, to explain some of this compensation.

Miniaturization of biological assays -- overview on microwell devices for single-cell analyses

BACKGROUND

Today, cells are commonly analyzed in ensembles, i.e. thousands of cells per sample, yielding results on the average response of the cells. However, cellular heterogeneity implies the importance of studying how individual cells respond, one by one, in order to learn more about drug targeting and cellular behavior.

SCOPE OF REVIEW

This review discusses general aspects on miniaturization of biological assays and in particular summarizes single-cell assays in microwell formats. A range of microwell-based chips are discussed with regard to their well characteristics, cell handling, choice of material etc. along with available detection systems for single-cell studies. History and trends in microsystem technology, various commonly used materials for device fabrication, and conventional methods for single-cell analysis are also discussed, before a closing section with a detailed example from our research in the field.

MAJOR CONCLUSIONS

A range of miniaturized and microwell devices have shown useful for studying individual cells.

GENERAL SIGNIFICANCE

In vitro assays offering low volume sampling and rapid analysis in a high-throughput manner are of great interest in a wide range of single-cell applications. Size compatibility between a cell and micron-sized tools has encouraged the field of micro- and nanotechnologies to move into areas such as life sciences and molecular biology. To test as many compounds as possible against a given amount of patient sample requires miniaturized tools where low volume sampling is sufficient for accurate results and on which a high number of experiments per cm(2) can be performed. This article is part of a Special Issue entitled Nanotechnologies - Emerging Applications in Biomedicine.

Metabolic profiling of Arabidopsis thaliana epidermal cells

Metabolic phenotyping at cellular resolution may be considered one of the challenges in current plant physiology. A method is described which enables the cell type-specific metabolic analysis of epidermal cell types in Arabidopsis thaliana pavement, basal, and trichome cells. To achieve the required high spatial resolution, single cell sampling using microcapillaries was combined with routine gas chromatography-time of flight-mass spectrometry (GC-TOF-MS) based metabolite profiling. The identification and relative quantification of 117 mostly primary metabolites has been demonstrated. The majority, namely 90 compounds, were accessible without analytical background correction. Analyses were performed using cell type-specific pools of 200 microsampled individual cells. Moreover, among these identified metabolites, 38 exhibited differential pool sizes in trichomes, basal or pavement cells. The application of an independent component analysis confirmed the cell type-specific metabolic phenotypes. Significant pool size changes between individual cells were detectable within several classes of metabolites, namely amino acids, fatty acids and alcohols, alkanes, lipids, N-compounds, organic acids and polyhydroxy acids, polyols, sugars, sugar conjugates and phenylpropanoids. It is demonstrated here that the combination of microsampling and GC-MS based metabolite profiling provides a method to investigate the cellular metabolism of fully differentiated plant cell types in vivo.

In situ metabolic profiling of single cells by laser ablation electrospray ionization mass spectrometry

Depending on age, phase in the cell cycle, nutrition, and environmental factors, individual cells exhibit large metabolic diversity. To explore metabolic variations in cell populations, laser ablation electrospray ionization (LAESI) mass spectrometry (MS) was used for the in situ analysis of individual cells at atmospheric pressure. Single cell ablation was achieved by delivering mid-IR laser pulses through the etched tip of a GeO(2)-based glass fiber. Metabolic analysis was performed from single cells and small cell populations of Allium cepa and Narcissus pseudonarcissus bulb epidermis, as well as single eggs of Lytechinus pictus. Of the 332 peaks detected for A. cepa, 35 were assigned to metabolites with the help of accurate ion masses and tandem MS. The metabolic profiles from single cells of the two plant species included a large variety of oligosaccharides including possibly fructans in A. cepa, and alkaloids, e.g., lycorine in N. pseudonarcissus. Analysis of adjacent individual cells with a difference in pigmentation showed that, in addition to essential metabolites found in both variants, the pigmented cells contained anthocyanidins, other flavonoids, and their glucosides. Analysis of single epidermal cells from different scale leaves in an A. cepa bulb showed metabolic differences corresponding to their age. Our results indicate the feasibility of using LAESI-MS for the in situ analysis of metabolites in single cells with potential applications in studying cell differentiation, changes due to disease states, and response to xenobiotics.

Laser microdissection-mediated isolation and in vitro transcriptional amplification of plant RNA

Protocols for laser microdissection and linear amplification of RNA from fixed, sectioned plant tissues are described. When combined with quantitative RT-PCR, microarray analysis, or RNA-sequencing, these procedures enable quantitative analyses of transcript accumulation from microscopic quantities of specific plant organs, tissues, or single cells.

Cell-specific mechanisms and systemic signalling as emerging themes in light acclimation of C3 plants

Chloroplasts perform essential signalling functions in light acclimation and various stress responses in plants. Research on chloroplast signalling has provided fundamental information concerning the diversity of cellular responses to changing environmental conditions. Evidence has also accumulated indicating that different cell types possess specialized roles in regulation of leaf development and stress acclimation when challenged by environmental cues. Leaf veins are flanked by a layer of elongated chloroplast-containing bundle sheath cells, which due to their central position hold the potential to control the flux of information inside the leaves. Indeed, a specific role for bundle sheath cells in plant acclimation to various light regimes is currently emerging. Moreover, perception of light stress initiates systemic signals that spread through the vasculature to confer stress resistance in non-exposed parts of the plant. Such long-distance signalling functions are related to unique characteristics of reactive oxygen species and their detoxification in bundle sheath cells. Novel techniques for analysis of distinct tissue types, together with Arabidopsis thaliana mutants with vasculature-specific phenotypes, have proven instrumental in dissection of structural hierarchy among regulatory processes in leaves. This review emphasizes the current knowledge concerning the role of vascular bundle sheath cells in light-dependent acclimation processes of C3 plants.

Unique and overlapping expression patterns among members of photosynthesis-associated nuclear gene families in Arabidopsis

Light provides crucial positional information in plant development, and the morphogenetic processes that are orchestrated by light signals are triggered by changes of gene expression in response to variations in light parameters. Control of expression of members of the RbcS and Lhc families of photosynthesis-associated nuclear genes by light cues is a paradigm for light-regulated gene transcription, but high-resolution expression profiles for these gene families are lacking. In this study, we have investigated expression patterns of members of the RbcS and Lhc gene families in Arabidopsis (Arabidopsis thaliana) at the cellular level during undisturbed development and upon controlled interference of the light environment. Members of the RbcS and Lhc gene families are expressed in specialized territories, including root tip, leaf adaxial, abaxial, and epidermal domains, and with distinct chronologies, identifying successive stages of leaf mesophyll ontogeny. Defined spatial and temporal overlap of gene expression fields suggest that the light-harvesting and photosynthetic apparatus may have a different polypeptide composition in different cells and that such composition could change over time even within the same cell.

Plant cell types: reporting and sampling with new technologies

Plants have relatively few cell types, but their specialized functions and their interactions are essential for physiology, development, and defense. The contributions of individual cells have been distinguished by methods including in situ reporting, cell sampling, and cell separation, thus far mostly limited to measurement of single transcripts, proteins, or metabolites. Advances in transcriptomics, proteomics, metabolomics, and activity assays with small samples and in the modeling of these data into networks of expression, regulation, interaction, and metabolism make it possible to evaluate the roles of cell types at system levels. Recent analyses include cell types of developing roots, bundle sheath and mesophyll cells of C4-type leaves, xylem and phloem cells of vascular systems, and specialized regions of embryos and shoot apices.

Protein profiling of single epidermal cell types from Arabidopsis thaliana using surface-enhanced laser desorption and ionization technology

Here, we describe a novel approach for investigating differential protein expression within three epidermal cell types. In particular, 3000 single pavement, basal, and trichome cells from leaves of Arabidopsis thaliana were harvested by glass micro-capillaries. Subsequently, these single cell samples were joined to form pools of 100 individual cells and analyzed using the ProteinChip technology; SELDI: surface-enhanced laser desorption and ionization. As a result, numerous protein signals that were differentially expressed in the three epidermal cell types could be detected. One of these proteins was characterized by tryptical digestion and subsequent identification via tandem quadrupole-time of flight (Q-TOF) mass spectrometry. Down regulation of this sequenced small subunit precursor of ribulose-1,5 bisphosphate carboxylase(C) oxygenase(O) (RuBisCo) in trichome and basal cells indicates the sink status of these cell types that are located on the surface of A. thaliana source leaves. Based on the obtained protein profiles, we suggest a close functional relationship between basal and trichome cells at the protein level.

Investigating glutamate receptor-like gene co-expression in Arabidopsis thaliana

There is increasing evidence of the important roles of glutamate receptors (GLRs) in plant development and in adaptation to stresses. However, the studies of these putative ion channels, both in planta and in Xenopus oocytes, may have been limited by our lack of knowledge of possible GLR heteromer formation in plants. We have developed a modification of the single-cell sampling technique to investigate GLR co-expression, and thus potential heteromer formation, in single cells of Arabidopsis thaliana leaves. Micro-EXpression amplification (MEX) has allowed us to amplify gene transcripts from a single cell, enabling expression of up to 100 gene transcripts to be assayed. We measured, on average, the transcripts of five to six different AtGLRs in a single cell. However, no consistent patterns of co-expression or cell-type-specific expression were detected, except that cells sampled from the same plant showed similar expression profiles. The only discernible feature was the detection of AtGLR3.7 in every cell examined, an observation supported by GUS staining patterns in plants stably expressing promoter::uidA fusions. In addition, we found AtGLR3.7 expression in oocytes induces a Ba2+-, Ca2+- and Na+-permeable plasma membrane conductance.

The Powdery Mildew Disease of Arabidopsis: A Paradigm for the Interaction between Plants and Biotrophic Fungi

The powdery mildew diseases, caused by fungal species of the Erysiphales, have an important economic impact on a variety of plant species and have driven basic and applied research efforts in the field of phytopathology for many years. Although the first taxonomic reports on the Erysiphales date back to the 1850's, advances into the molecular biology of these fungal species have been hampered by their obligate biotrophic nature and difficulties associated with their cultivation and genetic manipulation in the laboratory. The discovery in the 1990's of a few species of powdery mildew fungi that cause disease on Arabidopsis has opened a new chapter in this research field. The great advantages of working with a model plant species have translated into remarkable progress in our understanding of these complex pathogens and their interaction with the plant host. Herein we summarize advances in the study of Arabidopsis-powdery mildew interactions and discuss their implications for the general field of plant pathology. We provide an overview of the life cycle of the pathogens on Arabidopsis and describe the structural and functional changes that occur during infection in the host and fungus in compatible and incompatible interactions, with special emphasis on defense signaling, resistance pathways, and compatibility factors. Finally, we discuss the future of powdery mildew research in anticipation of the sequencing of multiple powdery mildew genomes. The cumulative body of knowledge on powdery mildews of Arabidopsis provides a valuable tool for the study and understanding of disease associated with many other obligate biotrophic pathogen species.

Principles of laser-induced separation and transport of living cells

Separation and transport of defined populations of living cells grown on a thin membrane can be achieved by laser microdissection (LMD) of the sample of interest, followed by a laser-induced forward transport process [laser pressure "catapulting" (LPC)] of the dissected cell cluster. We investigate the dynamics of LMD and LPC with focused and defocused UV-A laser pulses by means of time-resolved photography. Catapulting is driven by plasma formation when tightly focused pulses are used, and by confined thermal ablation at the bottom of the sample for defocused catapulting. With both modalities, the initial specimen velocity amounts to about 50 to 60 ms. Time-resolved photography of live cell catapulting reveals that in defocused catapulting, strong shear forces arise when the sample is accelerated out of the culture medium covering the cells. By contrast, pulses focused at the periphery of the specimen cause a fast rotational movement that minimizes the flow of culture medium parallel to the sample surface, and thus the resulting shear stresses. Therefore, the recultivation rate of catapulted cells is much higher when focused pulses are used. Compared to collateral damage by mechanical forces, side effects by heat and UV exposure of the cells play only a minor role.

Mechanisms of laser-induced dissection and transport of histologic specimens

Rapid contact- and contamination-free procurement of histologic material for proteomic and genomic analysis can be achieved by laser microdissection of the sample of interest followed by laser-induced transport (laser pressure catapulting). The dynamics of laser microdissection and laser pressure catapulting of histologic samples of 80 mum diameter was investigated by means of time-resolved photography. The working mechanism of microdissection was found to be plasma-mediated ablation initiated by linear absorption. Catapulting was driven by plasma formation when tightly focused pulses were used, and by photothermal ablation at the bottom of the sample when defocused pulses producing laser spot diameters larger than 35 microm were used. With focused pulses, driving pressures of several hundred MPa accelerated the specimen to initial velocities of 100-300 m/s before they were rapidly slowed down by air friction. When the laser spot was increased to a size comparable to or larger than the sample diameter, both driving pressure and flight velocity decreased considerably. Based on a characterization of the thermal and optical properties of the histologic specimens and supporting materials used, we calculated the evolution of the heat distribution in the sample. Selected catapulted samples were examined by scanning electron microscopy or analyzed by real-time reverse-transcriptase polymerase chain reaction. We found that catapulting of dissected samples results in little collateral damage when the laser pulses are either tightly focused or when the laser spot size is comparable to the specimen size. By contrast, moderate defocusing with spot sizes up to one-third of the specimen diameter may involve significant heat and ultraviolet exposure. Potential side effects are maximal when samples are catapulted directly from a glass slide without a supporting polymer foil.

Principles of laser microdissection and catapulting of histologic specimens and live cells

Rapid contact- and contamination-free procurement of specific samples of histologic material for proteomic and genomic analysis as well as separation and transport of living cells can be achieved by laser microdissection (LMD) of the sample of interest followed by a laser-induced forward transport process [laser pressure "catapulting," (LPC)] of the dissected material. We investigated the dynamics of LMD and LPC with focused and defocused laser pulses by means of time-resolved photography. The working mechanism of microdissection was found to be plasma-mediated ablation. Catapulting is driven by plasma formation, when tightly focused pulses are used, and by ablation at the bottom of the sample for moderate and strong defocusing. Driving pressures of several hundred megapascals accelerate the specimen to initial velocities of 100-300 m/s before it is rapidly slowed down by air friction. With strong defocusing, driving pressure and initial flight velocity decrease considerably. On the basis of a characterization of the thermal and optical properties of the histologic specimens and supporting materials used, we calculated the temporal evolution of the heat distribution in the sample. After laser microdissection and laser pressure catapulting (LMPC), the samples were inspected by scanning electron microscopy. Catapulting with tightly focused or strongly defocused pulses results in very little collateral damage, while slight defocusing involves significant heat and UV exposure of up to about 10% of the specimen volume, especially if samples are catapulted directly from a glass slide. Time-resolved photography of live-cell catapulting revealed that in defocused catapulting strong shear forces originate from the flow of the thin layer of culture medium covering the cells. By contrast, pulses focused at the periphery of the specimen cause a fast rotational movement that makes the specimen wind its way out of the culture medium, thereby undergoing much less shear stresses. Therefore, the recultivation rate of catapulted cells was much higher when focused pulses were used.

Laser microdissection and cryogenic nuclear magnetic resonance spectroscopy: an alliance for cell type-specific metabolite profiling

Laser microdissection was used as a tool to harvest secretory cavities (SC) from leaves of Dilatris pillansii Barker (Haemodoraceae) and from leaves and flowers of herbarium specimens of Dilatris corymbosa Berg. and Dilatris viscosa L. Cryogenic (1)H NMR spectroscopy and HPLC analysis of microdissected samples indicated specific accumulation of methoxyphenylphenalenones in the SC. The structures of two novel and a known natural product in the secretory tissue were confirmed by comparison with authentic compounds isolated from rhizomes and roots from which further phenylphenalenones and phenylphenalenone glucosides were isolated and identified by spectroscopic methods. How it will be possible to use the LMD technique to localize natural products in specific plant cell populations is also discussed.

Noncontact laser microdissection and catapulting for pure sample capture

The understanding of the molecular mechanisms of cellular function, growth, and proliferation is based on the accurate identification, isolation, and finally characterization of a specific single cell or a population of cells and its subsets of biomolecules. For the simultaneous analysis of thousands of molecular parameters within one single experiment as realized by DNA, RNA, and protein microarray technologies, a defined number of homogeneous cells derived from a distinct morphological origin are required. Sample preparation is therefore a very crucial step preceding the functional characterization of specific cell populations. Laser microdissection and laser pressure catapulting (LMPC) enables pure and homogeneous sample preparation resulting in an increased specificity of molecular analyses. With LMPC, the force of focused laser light is utilized to excise selected cells or large tissue areas from object slides down to individual single cells and subcellular components like organelles or chromosomes. After microdissection, the sample is directly catapulted into an appropriate collection vial. As this process works entirely without mechanical contact, it enables pure sample retrieval from morphologically defined origin without cross-contamination. LMPC has been successfully applied to isolate and catapult cells from, for example, histological tissue sections, from forensic evidence material, and also from tough plant matter, supporting biomedical research, forensic science, and plant physiology studies. Even delicate living cells like stem cells have been captured for recultivation without affecting their viability or stem cell character, an important feature influencing stem cell research, regenerative medicine, and drug development. The combination of LMPC with microinjection to inject drugs or genetic material into individual cells and to capture them for molecular analyses bears great potential for efficient patient-tailored medication.

Cell type-specific gene expression profiling in plants by using a combination of laser microdissection and high-throughput technologies

Laser microdissection (LM) allows for the isolation of specific cells of interest from heterogeneous tissues under direct microscopic visualization with the assistance of a laser beam. By permitting global analyses of gene expression and metabolites in the selected cells, it is a powerful tool for understanding the biological processes in individual cell types during development or in response to various stimuli. Recently, LM technology has been successfully applied to the separation of individual plant cell types. Here, we provide an overview of applications of LM combined with high-throughput technologies including transcript analyses [microarrays, serial analysis of gene expression (SAGE) and 454-sequencing], proteomic analyses and metabolomic profiling, for cell type-specific gene expression analyses in plants.

Common plantain. A collection of expressed sequence tags from vascular tissue and a simple and efficient transformation method

The vascular tissue of higher plants consists of specialized cells that differ from all other cells with respect to their shape and size, their organellar composition, their extracellular matrix, the type of their plasmodesmata, and their physiological functions. Intact and pure vascular tissue can be isolated easily and rapidly from leaf blades of common plantain (Plantago major), a plant that has been used repeatedly for molecular studies of phloem transport. Here, we present a transcriptome analysis based on 5,900 expressed sequence tags (ESTs) and 3,247 independent mRNAs from the Plantago vasculature. The vascular specificity of these ESTs was confirmed by the identification of well-known phloem or xylem marker genes. Moreover, reverse transcription-polymerase chain reaction, macroarray, and northern analyses revealed genes and metabolic pathways that had previously not been described to be vascular specific. Moreover, common plantain transformation was established and used to confirm the vascular specificity of a Plantago promoter-beta-glucuronidase construct in transgenic Plantago plants. Eventually, the applicability and usefulness of the obtained data were also demonstrated for other plant species. Reporter gene constructs generated with promoters from Arabidopsis (Arabidopsis thaliana) homologs of newly identified Plantago vascular ESTs revealed vascular specificity of these genes in Arabidopsis as well. The presented vascular ESTs and the newly developed transformation system represent an important tool for future studies of functional genomics in the common plantain vasculature.

Transcriptional snapshots provide insights into the molecular basis of arbuscular mycorrhiza in the model legume Medicago truncatula

The arbuscular mycorrhizal (AM) association between terrestrial plants and soil fungi of the phylum Glomeromycota is the most widespread beneficial plant-microbe interaction on earth. In the course of the symbiosis, fungal hyphae colonise plant roots and supply limiting nutrients, in particular phosphorus, in exchange for carbon compounds. Owing to the obligate biotrophy of mycorrhizal fungi and the lack of genetic systems to study them, targeted molecular studies on AM symbioses proved to be difficult. With the emergence of plant genomics and the selection of suitable models, an application of untargeted expression profiling experiments became possible. In the model legume Medicago truncatula, high-throughput expressed sequence tag (EST)-sequencing in conjunction with in silico and experimental transcriptome profiling provided transcriptional snapshots that together defined the global genetic program activated during AM. Owing to an asynchronous development of the symbiosis, several hundred genes found to be activated during the symbiosis cannot be easily correlated with symbiotic structures, but the expression of selected genes has been extended to the cellular level to correlate gene expression with specific stages of AM development. These approaches identified marker genes for the AM symbiosis and provided the first insights into the molecular basis of gene expression regulation during AM.

Laser microdissection of plant tissue: what you see is what you get

Laser microdissection (LM) utilizes a cutting or harvesting laser to isolate specific cells from histological sections; the process is guided by microscopy. This provides a means of removing selected cells from complex tissues, based only on their identification by microscopic appearance, location, or staining properties (e.g., immunohistochemistry, reporter gene expression, etc.). Cells isolated by LM can be a source of cell-specific DNA, RNA, protein or metabolites for subsequent evaluation of DNA modifications, transcript/protein/metabolite profiling, or other cell-specific properties that would be averaged with those of neighboring cell types during analysis of undissected complex tissues. Plants are particularly amenable to the application of LM; the highly regular tissue organization and stable cell walls of plants facilitate the visual identification of most cell types even in unstained tissue sections. Plant cells isolated by LM have been the starting point for a variety of genomic and metabolite studies of specific cell types.

Tête à tête inside a plant cell: establishing compatibility between plants and biotrophic fungi and oomycetes

'Compatibility' describes the complementary relationship between a plant species and an adapted pathogen species that underlies susceptibility and which ultimately results in disease. Owing to elaborate surveillance systems and defence mechanisms on the plant side and a common lack of adaptation of many microbial pathogens, resistance is the rule and compatibility the exception for most plant-microbe combinations. While there has been major scientific interest in 'resistance' in the past decade, which has revealed many of its underlying molecular components, the analysis of 'compatibility', although intimately intertwined with 'resistance', has not been pursued with a similar intensity. Various recent studies, however, provide a first glimpse of the pivotal players and potential molecular mechanisms essential for compatibility in both the plant and parasite partners. In this review we highlight these findings with a particular emphasis on obligate biotrophic and hemibiotrophic fungal and oomycete pathogens and discuss novel strategies that might help to uncover further the molecular principles underlying compatibility to these highly specialized pathogens.

Regulation of oil accumulation in single glands of Eucalyptus polybractea

The accumulation of terpenoid oil was examined in the leaves of Eucalyptus polybractea at scales ranging from individual oil glands to the whole plant. Variations in oil composition and concentration of oil were measured and related to both morphological and physiological parameters. Within a plant, all glands produced oil of broadly similar composition that was not regulated by leaf age or the position of the gland within the leaf. There were, however, distinct differences between plants, suggesting that composition is controlled primarily at the whole-plant level. Oil concentration, too, was regulated primarily at the whole-plant level and was limited by gland capacity. Gland capacity was linked to leaf area and thickness, the final products of leaf expansion. Leaf and plant oil composition is determined not by a mosaic of glands specializing in producing a single or a small group of compounds, but rather by glands with remarkably similar capacities for terpenoid biosynthesis, although oil concentration, limited by gland capacity, may be linked to leaf expansion rather than biosynthetic capacity.

Global studies of cell type-specific gene expression in plants

Technological advances in expression profiling and in the ability to collect minute quantities of tissues have come together to allow a growing number of global transcriptional studies at the cell level in plants. Microarray technology, with a choice of cDNA or oligo-based slides, is now well established, with commercial full-genome platforms for rice and Arabidopsis and extensive expressed sequence tag (EST)-based designs for many other species. Microdissection and cell sorting are two established methodologies that have been used in conjunction with microarrays to provide an early glimpse of the transcriptional landscape at the level of individual cell types. The results indicate that much of the transcriptome is compartmentalized. A minor but consistent percentage of transcripts appear to be unique to specific cell types. Functional analyses of cell-specific patterns of gene expression are providing important clues to cell-specific functions. The spatial dissection of the transcriptome has also yielded insights into the localized mediators of hormone inputs and promises to provide detail on cell-specific effects of microRNAs.

Localization of tabersonine 16-hydroxylase and 16-OH tabersonine-16-O-methyltransferase to leaf epidermal cells defines them as a major site of precursor biosynthesis in the vindoline pathway in Catharanthus roseus

The Madagascar periwinkle (Catharanthus roseus) produces the well known and remarkably complex anticancer dimeric alkaloids vinblastine and vincristine, which are derived by the coupling of vindoline and catharanthine monomers. Recent data from in situ RNA hybridization and immunolocalization suggest that combinatorial cell factories within the leaf are involved in vindoline biosynthesis. In this study, the cell types responsible for vindoline biosynthesis were identified by laser-capture microdissection/RNA isolation/RT-PCR to show that geraniol hydroxylase, secologanin synthase, tryptophan decarboxylase, strictosidine synthase, strictosidine ss-glucosidase and tabersonine 16-hydroxylase can be detected preferentially in epidermal cells. A new and complementary application of the carborundum abrasion (CA) technique was developed to obtain epidermis-enriched leaf extracts that can be used to measure alkaloid metabolite levels, enzyme activities and gene expression. The CA technique showed that tabersonine and 16-methoxytabersonine, together with 16-hydroxytabersonine-16-O-methyltransferase, are found predominantly in Catharanthus leaf epidermis, in contrast to vindoline, catharanthine and later enzymatic steps in vindoline biosynthesis. The results show that leaf epidermal cells are biosynthetically competent to produce tryptamine and secologanin precursors that are converted via many enzymatic transformations to make 16-methoxytabersonine. This alkaloid or its 2,3 dihydro-derivative is then transported to cells (mesophyll/idioblast/laticifer) within Catharanthus leaves to complete the last three or four enzymatic transformations to make vindoline.