Primary cilia are sensory organelles that translate extracellular chemical and mechanical cues into cellular responses. Bone is an exquisitely mechanosensitive organ, and its homeostasis depends on the ability of bone cells to sense and respond to mechanical stimuli. One such stimulus is dynamic fluid flow, which triggers biochemical and transcriptional changes in bone cells by an unknown mechanism. Here we report that bone cells possess primary cilia that project from the cell surface and deflect during fluid flow and that these primary cilia are required for osteogenic and bone resorptive responses to dynamic fluid flow. We also show that, unlike in kidney cells, primary cilia in bone translate fluid flow into cellular responses in bone cells independently of Ca 2+ flux and stretch-activated ion channels. These results suggest that primary cilia might regulate homeostasis in diverse tissues by allowing mechanical signals to alter cellular activity via tissue-specific pathways. Our identification of a mechanism for mechanotransduction in bone could lead to therapeutic approaches for combating bone loss due to osteoporosis and disuse.

Cellulose forms the major load-bearing network of the plant cell wall, which simultaneously protects the cell and directs its growth. Although the process of cellulose synthesis has been observed, little is known about the behavior of cellulose in the wall after synthesis. Using Pontamine Fast Scarlet 4B, a dye that fluoresces preferentially in the presence of cellulose and has excitation and emission wavelengths suitable for confocal microscopy, we imaged the architecture and dynamics of cellulose in the cell walls of expanding root cells. We found that cellulose exists in Arabidopsis (Arabidopsis thaliana) cell walls in large fibrillar bundles that vary in orientation. During anisotropic wall expansion in wild-type plants, we observed that these cellulose bundles rotate in a transverse to longitudinal direction. We also found that cellulose organization is significantly altered in mutants lacking either a cellulose synthase subunit or two xyloglucan xylosyltransferase isoforms. Our results support a model in which cellulose is deposited transversely to accommodate longitudinal cell expansion and reoriented during expansion to generate a cell wall that is fortified against strain from any direction.

Pectins are acidic carbohydrates that comprise a significant fraction of the primary walls of eudicotyledonous plant cells. They influence wall porosity and extensibility, thus controlling cell and organ growth during plant development. The regulated degradation of pectins is required for many cell separation events in plants, but the role of pectin degradation in cell expansion is poorly defined. Using an activation tag screen designed to isolate genes involved in wall expansion, we identified a gene encoding a putative polygalacturonase that, when overexpressed, resulted in enhanced hypocotyl elongation in etiolated Arabidopsis thaliana seedlings. We named this gene POLYGALACTURONASE INVOLVED IN EXPANSION1 (PGX1). Plants lacking PGX1 display reduced hypocotyl elongation that is complemented by transgenic PGX1 expression. PGX1 is expressed in expanding tissues throughout development, including seedlings, roots, leaves, and flowers. PGX1-GFP (green fluorescent protein) localizes to the apoplast, and heterologously expressed PGX1 displays in vitro polygalacturonase activity, supporting a function for this protein in apoplastic pectin degradation. Plants either overexpressing or lacking PGX1 display alterations in total polygalacturonase activity, pectin molecular mass, and wall composition and also display higher proportions of flowers with extra petals, suggesting PGX1's involvement in floral organ patterning. These results reveal new roles for polygalacturonases in plant development.

Clathrin-mediated endocytosis (CME) is the best-characterized type of endocytosis in eukaryotic cells. Plants appear to possess all of the molecular components necessary to carry out CME; however, functional characterization of the components is still in its infancy. A yeast two-hybrid screen identified m2 as a putative interaction partner of CELLULOSE SYNTHASE6 (CESA6). Arabidopsis (Arabidopsis thaliana) m2 is homologous to the medium subunit 2 of the mammalian ADAPTOR PROTEIN COMPLEX2 (AP2). In mammals, the AP2 complex acts as the central hub of CME by docking to the plasma membrane while concomitantly recruiting cargo proteins, clathrin triskelia, and accessory proteins to the sites of endocytosis. We confirmed that m2 interacts with multiple CESA proteins through the m-homology domain of m2, which is involved in specific interactions with endocytic cargo proteins in mammals. Consistent with its role in mediating the endocytosis of cargos at the plasma membrane, m2-YELLOW FLUORESCENT PROTEIN localized to transient foci at the plasma membrane, and loss of m2 resulted in defects in bulk endocytosis. Furthermore, loss of m2 led to increased accumulation of YELLOW FLUORESCENT PROTEIN-CESA6 particles at the plasma membrane. Our results suggest that CESA represents a new class of CME cargo proteins and that plant cells might regulate cellulose synthesis by controlling the abundance of active CESA complexes at the plasma membrane through CME.

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Cadherin–catenin complexes, localized to adherens junctions, are essential for cell–cell adhesion. One means of regulating adhesion is through the juxtamembrane domain of the cadherin cytoplasmic tail. This region is the binding site for p120, leading to the hypothesis that p120 is a key regulator of cell adhesion. p120 has also been suggested to regulate the GTPase Rho and to regulate transcription via its binding partner Kaiso. To test these hypothesized functions, we turned to Drosophila, which has only a single p120 family member. It localizes to adherens junctions and binds the juxtamembrane region of DE-cadherin (DE-cad). We generated null alleles of p120 and found that mutants are viable and fertile and have no substantial changes in junction structure or function. However, p120 mutations strongly enhance mutations in the genes encoding DE-cadherin or Armadillo, the β-catenin homologue. Finally, we examined the localization of p120 during embryogenesis. p120 localizes to adherens junctions, but its localization there is less universal than that of core adherens junction proteins. Together, these data suggest that p120 is an important positive modulator of adhesion but that it is not an essential core component of adherens junctions.

Summary Primary cilia are microtubule-based sensory organelles that are present in most mammalian tissues and play important roles in development and disease [1]. They are required for the Sonic hedgehog (Shh) [2-4] and PDGF [5] signalling pathways. Primary cilia grow from the older of the two centrioles of the centrosome, referred to as the mother centriole. In cycling cells the cilium typically grows in G1 and is lost before mitosis, but the regulation of its growth is poorly understood. Centriole duplication at G1/S results in two centrosomes, one with an older mother centriole and one with a new mother centriole, that are segregated in mitosis. We report that primary cilia grow asynchronously in sister cells resulting from a mitotic division, and that the sister cell receiving the older mother centriole usually grows a primary cilium first. We also show that the signaling proteins inversin [6] and PDGFRα localize asynchronously to sister cell primary cilia, and that sister cells respond asymmetrically to Sonic hedgehog. These results suggest that the segregation of differently aged mother centrioles, an asymmetry inherent to every animal cell division, can influence the ability of sister cells to respond to environmental signals, potentially altering the behavior or fate of one or both sister cells.

Polysaccharide-rich cell walls are a defining feature of plants that influence cell division and growth, but many details of cell-wall organization and dynamics are unknown because of a lack of suitable chemical probes. Metabolic labeling using sugar analogs compatible with click chemistry has the potential to provide new insights into cell-wall structure and dynamics. Using this approach, we found that an alkynylated fucose analog (FucAl) is metabolically incorporated into the cell walls of Arabidopsis thaliana roots and that a significant fraction of the incorporated FucAl is present in pectic rhamnogalacturonan-I (RG-I). Time-course experiments revealed that FucAl-containing RG-I first localizes in cell walls as uniformly distributed punctae that likely mark the sites of vesiclemediated delivery of new polysaccharides to growing cell walls. In addition, we found that the pattern of incorporated FucAl differs markedly along the developmental gradient of the root. Using pulse-chase experiments, we also discovered that the pectin network is reoriented in elongating root epidermal cells. These results reveal previously undescribed details of polysaccharide delivery, organization, and dynamics in cell walls.fluorescence | secretion | small molecule T he primary cell walls of plants consist of a complex polysaccharide-rich network containing cellulose, hemicellulose, pectin, and structural proteins. Cellulose, which is the primary load-bearing component of the wall, is hydrogen-bonded to hemicelluloses, and this composite is embedded in a pectin matrix (1, 2). The cell wall must simultaneously maintain the structural integrity of the plant cell by resisting the osmotic pressure necessary for turgor-mediated cell growth, and remain sufficiently dynamic to expand along with the growing cell. These requirements are satisfied by the controlled synthesis, deposition, and alteration of cell-wall components. Cellulose is synthesized at the plasma membrane (3), whereas hemicelluloses and pectins are synthesized intracellularly and secreted into the wall via an incompletely characterized vesicle-mediated trafficking pathway (2).In contrast to proteins, for which genetically encoded fluorescent tags have provided many insights into localization and function, detailed characterization of cell-wall polysaccharide structure and dynamics in intact plants has proven difficult. Chemical (4, 5) and biochemical (6-8) techniques can provide primary structural information for extracted cell-wall polymers. Imaging approaches, including transmission electron microscopy (9, 10), Raman microspectroscopy (11), and studies using a growing array of lectins, carbohydrate-binding modules, and carbohydrate-specific antibodies (12) have provided information regarding the location or abundance of cell-wall polysaccharides in various cell types (13). However, these imaging techniques suffer from limitations, including long sample-preparation times, fixation artifacts, lack of temporal information, lack of polymer specificity (12), and masking of a...