The view of plant-cell cytokinesis commonly depicted in textbooks is of a symmetrical process, with the phragmoplast initiating in the center of the cell and growing outward to the parental cell membrane. In contrast to this picture, we observe that cell-plate development in Arabidopsis shoot cells is highly polarized along the plane of division. Three-dimensional live-cell imaging reveals that the mitotic spindle and phragmoplast are laterally displaced, and that the growing cell plate anchors on one side of the cell at an early stage of cytokinesis. Growth of phragmoplast across the cell creates a new partition in its wake, giving the visual effect of a curtain being pulled across the cell. Throughout this process, the advancing front of the phragmoplast is in intimate contact with the parental wall, suggesting that short-range interactions between the phragmoplast and plasma membrane may play important roles in guiding the cell plate throughout much of its development. Polarized cytokinesis was observed in a wide variety of vacuolate shoot cells and in some small root cells, implying that it is not solely a function of cell size. This mode of cytokinesis may provide a mechanically robust mechanism for cell-plate formation in large cells and suggests a simple explanation for the occurrence of cell wall stubs observed upon drug treatment or in cytokinetic mutants.

Plant cells create highly structured microtubule arrays at the cell cortex without a central organizing center to anchor the microtubule ends. In vivo imaging of individual microtubules in Arabidopsis plants revealed that new microtubules are initiated at the cell cortex and exhibit dynamics at both ends. Polymerization-biased dynamic instability at one end and slow depolymerization at the other end result in sustained microtubule migration across the cell cortex by a hybrid treadmilling mechanism. This motility causes widespread microtubule repositioning and contributes to changes in array organization through microtubule reorientation and bundling.

Fluorescent proteins are generating fresh insight into plant cell function by providing new opportunities to visualize structure and dynamic events in live cells. Novel and transient structures, such as discreet locations in the nucleus where activated photoreceptor proteins accumulate, have recently been identified with fluorescent protein tags. Fluorescent proteins have also enabled the discovery of new dynamic molecular behaviors, such as the repositioning of cortical microtubules by polymer treadmilling. The early potential of fluorescent proteins to reveal protein interactions in living cells is being realized, as demonstrated in recent studies of transcription factors and signal transduction proteins. A promising new approach to the creation of fluorescent-protein-based biosensors has produced an exciting family of tools for visualizing small sugars, and perhaps will produce a wide variety of other small molecules in the future.

We developed a high-throughput methodology, termed fluorescent tagging of full-length proteins (FTFLP), to analyze expression patterns and subcellular localization of Arabidopsis gene products in planta. Determination of these parameters is a logical first step in functional characterization of the approximately one-third of all known Arabidopsis genes that encode novel proteins of unknown function. Our FTFLP-based approach offers two significant advantages: first, it produces internally-tagged full-length proteins that are likely to exhibit native intracellular localization, and second, it yields information about the tissue specificity of gene expression by the use of native promoters. To demonstrate how FTFLP may be used for characterization of the Arabidopsis proteome, we tagged a series of known proteins with diverse subcellular targeting patterns as well as several proteins with unknown function and unassigned subcellular localization.

The sku6-1 mutant of Arabidopsis thaliana exhibits altered patterns of root and organ growth. sku6 roots, etiolated hypocotyls, and leaf petioles exhibit right-handed axial twisting, and root growth on inclined agar media is strongly right skewed. The touch-dependent sku6 root skewing phenotype is suppressed by the antimicrotubule drugs propyzamide and oryzalin, and right skewing is exacerbated by cold treatment. Cloning revealed that sku6-1 is allelic to spiral1-1 (spr1-1). However, modifiers in the Columbia (Col) and Landsberg erecta (Ler) ecotype backgrounds mask noncomplementation in sku6-1 (Col)/spr1-1 (Ler) F1 plants. The SPR1 gene encodes a plant-specific 12-kD protein that is ubiquitously expressed and belongs to a six-member gene family in Arabidopsis. An SPR1:green fluorescent protein (GFP) fusion expressed in transgenic seedlings localized to microtubules within the cortical array, preprophase band, phragmoplast, and mitotic spindle. SPR1:GFP was concentrated at the growing ends of cortical microtubules and was dependent on polymer growth state; the microtubule-related fluorescence dissipated upon polymer shortening. The protein has a repeated motif at both ends, separated by a predicted rod-like domain, suggesting that it may act as an intermolecular linker. These observations suggest that SPR1 is involved in microtubule polymerization dynamics and/or guidance, which in turn influences touch-induced directional cell expansion and axial twisting.

By 2010, it is expected that biochemical functions will be assigned to many of the products of the approximately 30 000 Arabidopsis genes. Moreover, systematic analysis of mutants will provide insight into the biological function of the gene products. Metabolomic technologies complement these approaches by testing for changes in cellular ion and metabolite patterns, providing essential information for the construction of cellular and whole-plant models of metabolism. However, one important set of information that is especially relevant for multicellular organisms is still lacking, that is, knowledge of the cellular and subcellular variation in metabolite levels. The recent development of protein-based nanosensors for metabolites will help to close this gap by providing a set of tools that can be used to determine cytosolic and subcellular metabolite levels in real time using fluorescence-based microscopy. A major challenge for the future is the application of these nanosensors to quantify metabolite levels in plant cells and tissues.

Precise knowledge of spatial and temporal patterns of cell division, including number and orientation of divisions, and knowledge of cell expansion, is central to understanding morphogenesis. Our current knowledge of cell division patterns during plant and animal morphogenesis is largely deduced from analysis of clonal shapes and sizes. But such an analysis can reveal only the number, not the orientation or exact rate, of cell divisions. In this study, we have analyzed growth in real time by monitoring individual cell divisions in the shoot apical meristems (SAMs) of Arabidopsis thaliana. The live imaging technique has led to the development of a spatial and temporal map of cell division patterns. We have integrated cell behavior over time to visualize growth. Our analysis reveals temporal variation in mitotic activity and the cell division is coordinated across clonally distinct layers of cells. Temporal variation in mitotic activity is not correlated to the estimated plastochron length and diurnal rhythms. Cell division rates vary across the SAM surface. Cells in the peripheral zone (PZ) divide at a faster rate than in the central zone (CZ). Cell division rates in the CZ are relatively heterogeneous when compared with PZ cells. We have analyzed the cell behavior associated with flower primordium development starting from a stage at which the future flower comprises four cells in the L1 epidermal layer. Primordium development is a sequential process linked to distinct cellular behavior. Oriented cell divisions, in primordial progenitors and in cells located proximal to them, are associated with initial primordial outgrowth. The oriented cell divisions are followed by a rapid burst of cell expansion and cell division, which transforms a flower primordium into a three-dimensional flower bud. Distinct lack of cell expansion is seen in a narrow band of cells, which forms the boundary region between developing flower bud and the SAM. We discuss these results in the context of SAM morphogenesis.

Measuring subcellular glucose levels deep in tissues can provide new insights into compartmentalization and specialization of glucose metabolism among different cells. As shown previously, a FRET-based glucose-sensor consisting of two GFP-variants and the Escherichia coli periplasmic glucose/galactose binding protein was successfully expressed in the cytosol of COS7-cells and used to determine cytosolic glucose levels. Recording cytosolic fluorescence intensities in cells located in deeper layers of tissues is often difficult due to loss of signal intensity caused by effects of other cell layers on excitation and emission light. These interfering effects may be reduced by restricting fluorophores to occupy only a fraction of the assayed tissue volume. This can be accomplished by confining fluorophores to a sub-compartment of each cell in the tissue, such as the nucleus. The glucose-sensor was targeted to nuclei of COS7-cells. To determine, whether nuclear glucose levels can be used to track cytosolic changes, nuclear glucose concentrations were quantified as the cells were challenged with external glucose over a range of 0.5 to 10 mM and compared to cytosolic levels. Internal glucose concentrations in both compartments were similar, corresponding to similar to50% of the external concentration. Taken together, these results indicate that nuclear glucose levels can be used to determine cytosolic levels indirectly, permitting more reliable quantification of fluorescence intensities and providing a tool for measurements not only in cell cultures but also in tissues.

Attack by the host powdery mildew Erysiphe cichoracearum usually results in successful penetration and rapid proliferation of the fungus on Arabidopsis. By contrast, the nonhost barley powdery mildew Blumeria graminis f. sp. hordei (Bgh) typically fails to penetrate Arabidopsis epidermal cells. In both instances the plant secretes cell wall appositions, or papillae beneath the penetration peg of the fungus. Genetic screens for mutations that result in increased penetration of Bgh on Arabidopsis have recently identified the PEN1 syntaxin. Here we examine the role of PEN1 and of its closest homologue, SYP122, identified as a syntaxin whose expression is responsive to infection. pen1 syp122 double mutants are both dwarfed and necrotic, suggesting that the two syntaxins have overlapping functions. Although syp122-1 and the cell wall mur mutants have considerably more pronounced primary cell wall defects than pen1 mutants, these have relatively subtle or no effects on penetration resistance. Upon fungal attack, PEN1 appears to be actively recruited to papillae, and there is a 2-h delay in papillae formation in the pen1-1 mutant. We conclude that SYP122 may have a general function in secretion, including a role in cell wall deposition. By contrast, PEN1 appears to have a basal function in secretion and a specialized defense-related function, being required for the polarized secretion events that give rise to papilla formation.