Identification and preliminary characterization of a new chemical affecting glucosyltransferase activities involved in plant cell wall biosynthesis

Inhibition of a conserved bacterial dual-specificity phosphatase confers plant tolerance to Candidatus Liberibacter spp

"Candidatus Liberibacter spp." are insect-vectored, fastidious, and vascular-limited phytopathogens. They are the presumptive causal agents of potato zebra chip, tomato vein clearing, and the devastating citrus greening disease worldwide. There is an urgent need to develop new strategies to control them. In this study, we characterized a dual-specificity serine/tyrosine phosphatase (STP) that is well conserved among thirty-three geographically diverse "Candidatus Liberibacter spp." and strains that infect multiple Solanaceaea and citrus spp. The STP is expressed in infected plant tissues, localized at the plant cytosol and plasma membrane, and interferes with plant cell death responses. We employed an in silico target-based molecular modeling and ligand screen to identify two small molecules with high binding affinity to STP. Efficacy studies demonstrated that the two molecules can inhibit "Candidatus Liberibacter spp." but not unrelated pathogens and confer plant disease tolerance. The inhibitors and strategies are promising means to control "Candidatus Liberibacter spp."

Post-Synthetic Reduction of Pectin Methylesterification Causes Morphological Abnormalities and Alterations to Stress Response in Arabidopsis thaliana

Pectin is a critical component of the plant cell wall, supporting wall biomechanics and contributing to cell wall signaling in response to stress. The plant cell carefully regulates pectin methylesterification with endogenous pectin methylesterases (PMEs) and their inhibitors (PMEIs) to promote growth and protect against pathogens. We expressed Aspergillus nidulans pectin methylesterase (AnPME) in Arabidopsis thaliana plants to determine the impacts of methylesterification status on pectin function. Plants expressing AnPME had a roughly 50% reduction in methylester content compared with control plants. AnPME plants displayed a severe dwarf phenotype, including small, bushy rosettes and shorter roots. This phenotype was caused by a reduction in cell elongation. Cell wall composition was altered in AnPME plants, with significantly more arabinose and significantly less galacturonic acid, suggesting that plants actively monitor and compensate for altered pectin content. Cell walls of AnPME plants were more readily degraded by polygalacturonase (PG) alone but were less susceptible to treatment with a mixture of PG and PME. AnPME plants were insensitive to osmotic stress, and their susceptibility to Botrytis cinerea was comparable to wild type plants despite their compromised cell walls. This is likely due to upregulated expression of defense response genes observed in AnPME plants. These results demonstrate the importance of pectin in both normal growth and development, and in response to biotic and abiotic stresses.

Spatiotemporal cytoskeleton organizations determine morphogenesis of multicellular trichomes in tomato

Plant trichomes originate from epidermal cell, forming protective structure from abiotic and biotic stresses. Different from the unicellular trichome in Arabidopsis, tomato trichomes are multicellular structure and can be classified into seven different types based on cell number, shape and the presence of glandular cells. Despite the importance of tomato trichomes in insect resistance, our understanding of the tomato trichome morphogenesis remains elusive. In this study, we quantitatively analyzed morphological traits of trichomes in tomato and further performed live imaging of cytoskeletons in stably transformed lines with actin and microtubule markers. At different developmental stages, two types of cytoskeletons exhibited distinct patterns in different trichome cells, ranging from transverse, spiral to longitudinal. This gradual transition of actin filament angle from basal to top cells could correlate with the spatial expansion mode in different cells. Further genetic screen for aberrant trichome morphology led to the discovery of a number of independent mutations in SCAR/WAVE and ARP2/3 complex, which resulted in actin bundling and distorted trichomes. Disruption of microtubules caused isotropic expansion while abolished actin filaments entirely inhibited axial extension of trichomes, indicating that microtubules and actin filaments may control distinct aspects of trichome cell expansion. Our results shed light on the roles of cytoskeletons in the formation of multicellular structure of tomato trichomes.

Re-targeting of a plant defense protease by a cyst nematode effector

Plants mount defense responses during pathogen attacks, and robust host defense suppression by pathogen effector proteins is essential for infection success. 4E02 is an effector of the sugar beet cyst nematode Heterodera schachtii. Arabidopsis thaliana lines expressing the effector-coding sequence showed altered expression levels of defense response genes, as well as higher susceptibility to both the biotroph H. schachtii and the necrotroph Botrytis cinerea, indicating a potential suppression of defenses by 4E02. Yeast two-hybrid analyses showed that 4E02 targets A. thaliana vacuolar papain-like cysteine protease (PLCP) 'Responsive to Dehydration 21A' (RD21A), which has been shown to function in the plant defense response. Activity-based protein profiling analyses documented that the in planta presence of 4E02 does not impede enzymatic activity of RD21A. Instead, 4E02 mediates a re-localization of this protease from the vacuole to the nucleus and cytoplasm, which is likely to prevent the protease from performing its defense function and at the same time, brings it in contact with novel substrates. Yeast two-hybrid analyses showed that RD21A interacts with multiple host proteins including enzymes involved in defense responses as well as carbohydrate metabolism. In support of a role in carbohydrate metabolism of RD21A after its effector-mediated re-localization, we observed cell wall compositional changes in 4E02 expressing A. thaliana lines. Collectively, our study shows that 4E02 removes RD21A from its defense-inducing pathway and repurposes this enzyme by targeting the active protease to different cell compartments.

Cell growth and homeostasis are disrupted in arabidopsis rns2-2 mutants missing the main vacuolar RNase activity

BACKGROUND AND AIMS

Enzymes belonging to the RNase T2 family are essential for normal rRNA turnover in eukaryotes. In Arabidopsis thaliana, this function is performed by RNS2. The null mutant rns2-2 has increased rRNA half-life and constitutive autophagy. The aim of this work was to determine the molecular changes that take place in the rns2-2 mutant that may lead to altered cellular homeostasis, manifested by the observed cellular phenotype.

METHODS

To determine the effect of defective rRNA turnover on cellular homeostasis, comparative transcriptome and metabolome analyses of 10-day-old wild-type and rns2-2 seedlings were used to identify molecular processes affected in the mutant. Bioinformatics analyses suggested additional phenotypes that were confirmed through direct plant size measurements and microscopy.

KEY RESULTS

Few genes were differentially expressed in the rns2-2 mutant, indicating that control of autophagy in this genotype is mainly achieved at the post-transcriptional level. Among differentially expressed genes, transcripts related to carbon flux processes, particularly the pentose phosphate pathway (PPP), were identified. Metabolite analyses confirmed changes in the levels of PPP intermediates. Genes related to cell wall loosening were also differentially expressed in the mutant, and a decrease in monosaccharide components of cell wall hemicellulose were found. As a potential effect of weaker cell walls, rns2-2 plants are larger than wild-type controls, due to larger cells and increased water content. Elevated levels of reactive oxygen species (ROS) were also measured in rns2-2, and the constitutive autophagy phenotype was blocked by preventing ROS production via NADPH oxidase.

CONCLUSIONS

Lack of rRNA recycling in rns2-2 cells triggers a change in carbon flux, which is redirected through the PPP to produce ribose-5-phosphate for de novo nucleoside synthesis. rRNA or ribosome turnover is thus essential for cellular homeostasis, probably through maintenance of nucleoside levels as part of the salvage pathway.

Modification of plant cell wall structure accompanied by enhancement of saccharification efficiency using a chemical, lasalocid sodium

The cell wall is one major determinant of plant cell morphology, and is an attractive bioresource. Here, we report a novel strategy to modify plant cell wall property by small molecules. Lasalocid sodium (LS) was isolated by chemical screening to identify molecules that affect the cell morphology of tobacco BY-2 cells. LS treatment led to an increase in cell wall thickness, whilst the quantity and sugar composition of the cell wall remained unchanged in BY-2 cells. The chemical also disordered the cellular arrangement of hypocotyls of Arabidopsis plants, resulting in a decrease in hypocotyl length. LS treatment enhanced enzymatic saccharification efficiency in both BY-2 cells and Arabidopsis plants. Microarray analysis on Arabidopsis showed that exposure to LS upregulated type III peroxidase genes, of which some are involved in lignin biogenesis, and jasmonic acid response genes, and phloroglucinol staining supported the activation of lignification by the LS treatment. As jasmonic acid-mediated lignification is a typical reaction to cell wall damage, it is possible that LS induces cell wall loosening, which can trigger cell wall damage response. Thus, LS is a unique chemical for modification of cell wall and morphology through changes in cell wall architecture.

Post-Synthetic Defucosylation of AGP by Aspergillus nidulans α-1,2-Fucosidase Expressed in Arabidopsis Apoplast Induces Compensatory Upregulation of α-1,2-Fucosyltransferases

Cell walls are essential components of plant cells which perform a variety of important functions for the different cell types, tissues and organs of a plant. Besides mechanical function providing cell shape, cell walls participate in intercellular communication, defense during plant-microbe interactions, and plant growth. The plant cell wall consists predominantly of polysaccharides with the addition of structural glycoproteins, phenolic esters, minerals, lignin, and associated enzymes. Alterations in the cell wall composition created through either changes in biosynthesis of specific constituents or their post-synthetic modifications in the apoplast compromise cell wall integrity and frequently induce plant compensatory responses as a result of these alterations. Here we report that post-synthetic removal of fucose residues specifically from arabinogalactan proteins in the Arabidopsis plant cell wall induces differential expression of fucosyltransferases and leads to the root and hypocotyl elongation changes. These results demonstrate that the post-synthetic modification of cell wall components presents a valuable approach to investigate the potential signaling pathways induced during plant responses to such modifications that usually occur during plant development and stress responses.

Decreased Polysaccharide Feruloylation Compromises Plant Cell Wall Integrity and Increases Susceptibility to Necrotrophic Fungal Pathogens

The complexity of cell wall composition and structure determines the strength, flexibility, and function of the primary cell wall in plants. However, the contribution of the various components to cell wall integrity (CWI) and function remains unclear. Modifications of cell wall composition can induce plant responses known as CWI control. In this study, we used transgenic expression of the fungal feruloyl esterase AnFAE to examine the effect of post-synthetic modification of Arabidopsis and Brachypodium cell walls. Transgenic Arabidopsis plants expressing AnFAE showed a significant reduction of monomeric ferulic acid, decreased amounts of wall-associated extensins, and increased susceptibility to Botrytis cinerea, compared with wild type. Transgenic Brachypodium showed reductions in monomeric and dimeric ferulic acids and increased susceptibility to Bipolaris sorokiniana. Upon infection, transgenic Arabidopsis and Brachypodium plants also showed increased expression of several defense-related genes compared with wild type. These results demonstrate a role, in both monocot and dicot plants, of polysaccharide feruloylation in plant CWI, which contributes to plant resistance to necrotrophic pathogens.

Characterizing visible and invisible cell wall mutant phenotypes

About 10% of a plant's genome is devoted to generating the protein machinery to synthesize, remodel, and deconstruct the cell wall. High-throughput genome sequencing technologies have enabled a reasonably complete inventory of wall-related genes that can be assembled into families of common evolutionary origin. Assigning function to each gene family member has been aided immensely by identification of mutants with visible phenotypes or by chemical and spectroscopic analysis of mutants with 'invisible' phenotypes of modified cell wall composition and architecture that do not otherwise affect plant growth or development. This review connects the inference of gene function on the basis of deviation from the wild type in genetic functional analyses to insights provided by modern analytical techniques that have brought us ever closer to elucidating the sequence structures of the major polysaccharide components of the plant cell wall.

Techniques for assessing the effects of pharmacological inhibitors on intercellular protein movement

Intercellular protein movement is an important mechanism in plant development. Here we present an integrated protocol that utilizes an inducible system to block plasmodesmata-dependent movement and assessment of fluorescent recovery after photobleaching (FRAP) to identify compounds that influence intercellular protein movement.

Considerations for designing chemical screening strategies in plant biology

Traditionally, biologists regularly used classical genetic approaches to characterize and dissect plant processes. However, this strategy is often impaired by redundancy, lethality or pleiotropy of gene functions, which prevent the isolation of viable mutants. The chemical genetic approach has been recognized as an alternative experimental strategy, which has the potential to circumvent these problems. It relies on the capacity of small molecules to modify biological processes by specific binding to protein target(s), thereby conditionally modifying protein function(s), which phenotypically resemble mutation(s) of the encoding gene(s). A successful chemical screening campaign comprises three equally important elements: (1) a reliable, robust, and quantitative bioassay, which allows to distinguish between potent and less potent compounds, (2) a rigorous validation process for candidate compounds to establish their selectivity, and (3) an experimental strategy for elucidating a compound's mode of action and molecular target. In this review we will discuss details of this general strategy and additional aspects that deserve consideration in order to take full advantage of the power provided by the chemical approach to plant biology. In addition, we will highlight some success stories of recent chemical screenings in plant systems, which may serve as teaching examples for the implementation of future chemical biology projects.

Using chemical genomics to study cell wall formation and cell growth in Arabidopsis thaliana and Penium margaritaceum

The cell wall is directly involved in cell growth, and its ability to loosen and rearrange allows for cell expansion through the existing turgor pressure. Thus, information on cell wall deposition and rearrangement can provide insights into the overall plant growth. This chapter describes two methods that can be used to evaluate cell expansion (1) in the model plant Arabidopsis thaliana and (2) the model alga Penium margaritaceum. These methods are further used to screen for small molecules that induce cell growth phenotypic changes affecting cell wall. Identification of such small molecules is beneficial due to their posttranslational mechanism of action that can be controlled in a temporal and spatial manner. Chemical genomics has the ability to overcome issues of genetic redundancy and lethality, which can hinder traditional genetic methods. The identification of small molecules in these screens will provide useful information on plant cell wall biology and overall plant growth.

Arabidopsis and Brachypodium distachyon transgenic plants expressing Aspergillus nidulans acetylesterases have decreased degree of polysaccharide acetylation and increased resistance to pathogens

The plant cell wall has many significant structural and physiological roles, but the contributions of the various components to these roles remain unclear. Modification of cell wall properties can affect key agronomic traits such as disease resistance and plant growth. The plant cell wall is composed of diverse polysaccharides often decorated with methyl, acetyl, and feruloyl groups linked to the sugar subunits. In this study, we examined the effect of perturbing cell wall acetylation by making transgenic Arabidopsis (Arabidopsis thaliana) and Brachypodium (Brachypodium distachyon) plants expressing hemicellulose- and pectin-specific fungal acetylesterases. All transgenic plants carried highly expressed active Aspergillus nidulans acetylesterases localized to the apoplast and had significant reduction of cell wall acetylation compared with wild-type plants. Partial deacetylation of polysaccharides caused compensatory up-regulation of three known acetyltransferases and increased polysaccharide accessibility to glycosyl hydrolases. Transgenic plants showed increased resistance to the fungal pathogens Botrytis cinerea and Bipolaris sorokiniana but not to the bacterial pathogens Pseudomonas syringae and Xanthomonas oryzae. These results demonstrate a role, in both monocot and dicot plants, of hemicellulose and pectin acetylation in plant defense against fungal pathogens.

Radiometric and spectrophotometric in vitro assays of glycosyltransferases involved in plant cell wall carbohydrate biosynthesis

Most of the glycosyltransferases (GTs) that catalyze the formation of plant cell wall carbohydrates remain to be biochemically characterized. This can be achieved only if specific assays are available for these enzymes. Here we present a protocol for in vitro assays of processive and nonprocessive membrane-bound GTs. The assays are either based on the use of radioactive nucleotide sugars (NDP sugars; e.g., UDP-[U-(14)C]glucose) and the quantification of the radiolabeled monosaccharides incorporated into soluble or insoluble carbohydrates, or on the coupling of the GT reaction with that of pyruvate kinase (PK) and the oxidation of NADH by lactate dehydrogenase (LDH). The radiometric assays are more suitable for exploratory work on poorly characterized enzymes, whereas the spectrophotometric assays require the availability of highly enriched GTs. Both assays can be performed within 1 d, depending on the number of fractions to be assayed or reaction mixtures to be tested.

Multidimensional solid-state NMR studies of the structure and dynamics of pectic polysaccharides in uniformly 13C-labeled Arabidopsis primary cell walls

Plant cell wall (CW) polysaccharides are responsible for the mechanical strength and growth of plant cells; however, the high-resolution structure and dynamics of the CW polysaccharides are still poorly understood because of the insoluble nature of these molecules. Here, we use 2D and 3D magic-angle-spinning (MAS) solid-state NMR (SSNMR) to investigate the structural role of pectins in the plant CW. Intact and partially depectinated primary CWs of Arabidopsis thaliana were uniformly labeled with (13)C and their NMR spectra were compared. Recent (13)C resonance assignment of the major polysaccharides in Arabidopsis thaliana CWs allowed us to determine the effects of depectination on the intermolecular packing and dynamics of the remaining wall polysaccharides. 2D and 3D correlation spectra show the suppression of pectin signals, confirming partial pectin removal by chelating agents and sodium carbonate. Importantly, higher cross peaks are observed in 2D and 3D (13)C spectra of the depectinated CW, suggesting higher rigidity and denser packing of the remaining wall polysaccharides compared with the intact CW. (13)C spin-lattice relaxation times and (1)H rotating-frame spin-lattice relaxation times indicate that the polysaccharides are more rigid on both the nanosecond and microsecond timescales in the depectinated CW. Taken together, these results indicate that pectic polysaccharides are highly dynamic and endow the polysaccharide network of the primary CW with mobility and flexibility, which may be important for pectin functions. This study demonstrates the capability of multidimensional SSNMR to determine the intermolecular interactions and dynamic structures of complex plant materials under near-native conditions.

An efficient method for transient gene expression in monocots applied to modify the Brachypodium distachyon cell wall

BACKGROUND

Agrobacterium-mediated transformation is widely used to produce insertions into plant genomes. There are a number of well-developed Agrobacterium-mediated transformation methods for dicotyledonous plants, but there are few for monocotyledonous plants.

METHODS

Three hydrolase genes were transiently expressed in Brachypodium distachyon plants using specially designed vectors that express the gene product of interest and target it to the plant cell wall. Expression of functional hydrolases in genotyped plants was confirmed using western blotting, activity assays, cell wall compositional analysis and digestibility tests.

KEY RESULTS

An efficient, new, Agrobacterium-mediated approach was developed for transient gene expression in the grass B. distachyon, using co-cultivation of mature seeds with bacterial cells. This method allows transformed tissues to be obtained rapidly, within 3-4 weeks after co-cultivation. Also, the plants carried transgenic tissue and maintained transgenic protein expression throughout plant maturation. The efficiency of transformation was estimated at around 5 % of initially co-cultivated seeds. Application of this approach to express three Aspergillus nidulans hydrolases in the Brachypodium cell wall successfully confirmed its utility and resulted in the expected expression of active microbial proteins and alterations of cell wall composition. Cell wall modifications caused by expression of A. nidulans α-arabinofuranosidase and α-galactosidase increased the biodegradability of plant biomass.

CONCLUSIONS

This newly developed approach is a quick and efficient technique for expressing genes of interest in Brachypodium plants, which express the gene product throughout development. In the future, this could be used for broad functional genomics studies of monocots and for biotechnological applications, such as plant biomass modification for biofuel production.

Characterization of European forage maize lines for stover composition and associations with polymorphisms within O-methyltransferase genes

Cell wall components, such as lignin, cellulose, and hemicelluloses, play an important role in the conversion efficiency of corn stover into ethanol. Understanding the molecular basis of cell wall formation is fundamental for marker assisted selection to develop lines more suitable for ethanol production. In this study, we evaluated a set of 40 European forage maize lines for cellulose, lignin, total hemicellulose, glucuronoarabinoxylan (GAX), and monosaccharides, such as arabinose (ara), xylose (xyl), and glucuronic acid (GlcA). The most significant correlations were observed between hemicelluloses and GAX (0.9), and hemicelluloses and cellulose (-0.81). Cell wall digestibility (CWD, estimated by digestible neutral detergent fiber, DNDF) was negatively correlated with Xyl (-0.34). The association analysis between the evaluated traits and polymorphisms within ten "lignin" genes revealed significant associations between polymorphisms within CCoAOMT1, CCoAOMT2, 4CL2 and C4H, and cellulose/xyl, cellulose, cellulose, and GclA, respectively. None of the QTPs identified in this study corresponded to previously reported CWD QTPs.

Post-synthetic modification of plant cell walls by expression of microbial hydrolases in the apoplast

The systematic creation of defined cell wall modifications in the model plant Arabidopsis thaliana by expression of microbial hydrolases with known specific activities is a promising approach to examine the impacts of cell wall composition and structure on both plant fitness and cell wall recalcitrance. Moreover, this approach allows the direct evaluation in living plants of hydrolase specificity, which can differ from in vitro specificity. To express genes encoding microbial hydrolases in A. thaliana, and target the hydrolases to the apoplast compartment, we constructed an expression cassette composed of the Cauliflower Mosaic Virus 35S RNA promoter, the A. thaliana β-expansin signal peptide, and the fluorescent marker protein YFP. Using this construct we successfully introduced into Colombia-0 plants three Aspergillus nidulans hydrolases, β-xylosidase/α-arabinosidase, feruloyl esterase, acetylxylan esterase, and a Xanthomonas oryzae putative a-L: -arabinofuranosidase. Fusion with YFP permitted quick and easy screening of transformants, detection of apoplastic localization, and protein size confirmation. Compared to wild-type Col-0, all transgenic lines showed a significant increase in the corresponding hydrolytic activity in the apoplast and changes in cell wall composition. Examination of hydrolytic activity in the transgenic plants also showed, for the first time, that the X. oryzae gene indeed encoded an enzyme with α-L: -arabinofuranosidase activity. None of the transgenic plants showed a visible phenotype; however, the induced compositional changes increased the degradability of biomass from plants expressing feruloyl esterase and β-xylosidase/α-arabinosidase. Our results demonstrate the viability of creating a set of transgenic A. thaliana plants with modified cell walls to use as a toolset for investigation of how cell wall composition contributes to recalcitrance and affects plant fitness.

Advances in dissecting endomembrane trafficking with small molecules

Chemical genomics is relatively new to plant biology in academia; however, the ability of this approach to present novel discoveries is being demonstrated clearly. One particularly suitable application of this approach is plant endomembrane trafficking. The rapid and dynamic nature of vesicular trafficking plus genetic redundancy has hampered effective study of this complex network. The ability of small molecules to act quickly to inhibit or arrest vesicular trafficking should permit the association of specific vesicles, especially endosome compartments, with their cargoes, particularly those destined for the plasma membrane. This approach and the large target space presented by the endomembrane trafficking network require the discovery of many new bioactive molecules. Advances in high-throughput chemical screening in plants are making this a reality. However, successful chemical genomic approaches in plants must be coupled with improvements in automated microscopy, image analysis, and target identification. In addition, the ability to correlate specific molecules with complex phenotypic data will be crucial. The data obtained from these experiments will be composed of a matrix of intracellular markers displaying complex chemically induced phenotypes as well as whole plant and perhaps data generated by genomics, proteomics, and metabolomics. In this manner, it should be possible to view endomembrane trafficking and its interactions as a systems-based network.

The CELLULOSE-SYNTHASE LIKE C (CSLC) family of barley includes members that are integral membrane proteins targeted to the plasma membrane

The CELLULOSE SYNTHASE-LIKE C (CSLC) family is an ancient lineage within the CELLULOSE SYNTHASE/CELLULOSE SYNTHASE-LIKE (CESA/CSL) polysaccharide synthase superfamily that is thought to have arisen before the divergence of mosses and vascular plants. As studies in the flowering plant Arabidopsis have suggested synthesis of the (1,4)-beta-glucan backbone of xyloglucan (XyG), a wall polysaccharide that tethers adjacent cellulose microfibrils to each other, as a probable function for the CSLCs, CSLC function was investigated in barley (Hordeum vulgare L.), a species with low amounts of XyG in its walls. Four barley CSLC genes were identified (designated HvCSLC1-4). Phylogenetic analysis reveals three well supported clades of CSLCs in flowering plants, with barley having representatives in two of these clades. The four barley CSLCs were expressed in various tissues, with in situ PCR detecting transcripts in all cell types of the coleoptile and root, including cells with primary and secondary cell walls. Co-expression analysis showed that HvCSLC3 was coordinately expressed with putative XyG xylosyltransferase genes. Both immuno-EM and membrane fractionation showed that HvCSLC2 was located in the plasma membrane of barley suspension-cultured cells and was not in internal membranes such as endoplasmic reticulum or Golgi apparatus. Based on our current knowledge of the sub-cellular locations of polysaccharide synthesis, we conclude that the CSLC family probably contains more than one type of polysaccharide synthase.

Chemical dissection of endosomal pathways

Membrane trafficking and associated signal transduction pathways are critical for plant development and responses to environment. These transduction pathways, including those for brassinosteroids and auxins, require endocytosis to endosomes and recycling back to the plasma membrane. A major challenge toward understanding these processes and their biological roles has been the highly dynamic nature of endomembrane trafficking. To effectively study endocytosis and recycling, which occur in a time frame of minutes, bioactive chemicals provide a powerful and exacting tool. Pharmacological inhibitors such as Brefeldin A (BFA) and the newly identified Endosidin 1 (ES1) have been used to define endosome compartments. ES1 is a clear example of the ability of chemicals to dissect even distinct subpopulations of endosomes involved in trafficking and signal transduction. The ability to characterize and dissect such highly dynamic pathways in a temporal and spatial manner is possible only using pharmacological reagents which can act rapidly and reversibly.