NOSTOCACEAN MACRO‐MORPHOGENESIS

A poorly understood feature of nostocacean growth and development is the formation of ordered macroscopic structures from microscopic cells, trichomes, and filaments. Using macro‐photography, time‐lapse micro‐cinematography, light and electron microscopy of Nostoc species in pure culture, it has been possible to demonstrate how motility, adhesion and aggregation of photo‐induced hormogonia result in macro‐morphogenesis of dendroid forms. Red‐light induced hormogonia from synchronized cultures aggregate rapidly on agar as tight flowing streams, in patterns responsive to the direction and quality of incident light. Unlike the even textured cell surfaces of heterocystous filaments, the cell walls of swarming hormogonia are covered with a striate mucoid layer containing pili attached to cells of adjacent hormogonia. During differentiation to an aseriate phase, cell wall fusions occur and a gelatinous matrix forms around the enlarging sub‐globose cells. Liquid suspensions of hormogonia aggregate in a solid mass following the net centripetal movement of interlaced loops of curved hormogonia attached by adhesive pili. In darkness or dim white light, compressed hormogonial aggregates form erect tree‐like (dendroid) macro‐structures by photo‐tactic reversal of streaming motility. Hormogonia within the aggregates re‐organize into streams that push upward into the light, forming structured, positively phototropic protuberances, several millimeters in length. Under weak illumination, the structures become branched with crowns of waving hormogonia. The dendroid morphology is stabilized by deposit of gelatinous material derived from successive cycles of cell‐filament development, liberation of heterocysts and formation of dormant cells and trichomes.     

Development of motility in cultures of the cyanobacterium Mastigocladus laminosus

Abstract The time course for the development of motility in cultures of the cyanobacterium Mastigocladus laminosus was established quantitatively using a slicer tool as described here. The slicer tool produces samples of trichomes from centrifuged pellets that, under identical conditions, shed comparable numbers of hormogonia. The number of hormogonia formed in liquid culture rises steeply between 24 and 31 h of incubation, returning to essentially zero in the next 24 h. The initial lag may be devoted to the cell divisions needed to form the cells of the hormogonium. The drop in motility could be due to one or more heat-stable substance(s) accumulated in the medium, since used media inhibited motility and the effect resisted autoclaving. The fact that the inoculum needed to be ground in order for motility to occur suggests that the structure of the clump inhibits the shedding of hormogonia. Some ecological implications are proposed, assuming that the clump structure interferes with light and mass transfer.     

Short-Range Guiding Can Result in the Formation of Circular Aggregates in Myxobacteria Populations

Myxobacteria are social bacteria that upon starvation form multicellular fruiting bodies whose shape in different species can range from simple mounds to elaborate tree-like structures. The formation of fruiting bodies is a result of collective cell movement on a solid surface. In the course of development, groups of flexible rod-shaped cells form streams and move in circular or spiral patterns to form aggregation centers that can become sites of fruiting body formation. The mechanisms of such cell movement patterns are not well understood. It has been suggested that myxobacterial development depends on short-range contact-mediated interactions between individual cells, i.e. cell aggregation does not require long-range signaling in the population. In this study, by means of a computational mass-spring model, we investigate what types of short-range interactions between cells can result in the formation of streams and circular aggregates during myxobacterial development. We consider short-range head-to-tail guiding between individual cells, whereby movement direction of the head of one cell is affected by the nearby presence of the tail of another cell. We demonstrate that stable streams and circular aggregates can arise only when the trailing cell, in addition to being steered by the tail of the leading cell, is able to speed up to catch up with it. It is suggested that necessary head-to-tail interactions between cells can arise from physical adhesion, response to a diffusible substance or slime extruded by cells, or pulling by motility engine pili. Finally, we consider a case of long-range guiding between cells and show that circular aggregates are able to form without cells increasing speed. These findings present a possibility to discriminate between short-range and long-range guiding mechanisms in myxobacteria by experimentally measuring distribution of cell speeds in circular aggregates.     

Motility in cyanobacteria: polysaccharide tracks and Type IV pilus motors

Motility in cyanobacteria is useful for purposes that range from seeking out favourable light environments to establishing symbioses with plants and fungi. No known cyanobacterium is equipped with flagella, but a diverse range of species is able to ‘glide’ or ‘twitch’ across surfaces. Cyanobacteria with this capacity range from unicellular species to complex filamentous forms, including species such as Nostoc punctiforme, which can generate specialised motile filaments called hormogonia. Recent work on the model unicellular cyanobacterium Synechocystis sp. PCC 6803 has shown that its means of propulsion has much in common with the twitching motility of heterotrophs such as Pseudomonas and Myxococcus. Movement depends on Type IV pili, which are extended, adhere to the substrate and then retract to pull the cell across the surface. Previous work on filamentous cyanobacteria suggested a very different mechanism, with movement powered by the directional extrusion of polysaccharide from pores close to the cell junctions. Now a new report by Khayatan and colleagues in this issue of Molecular Microbiology suggests that the motility of Nostoc hormogonia has much more in common with Synechocystis than was previously thought. In both cases, polysaccharide secretion is important for preparing the surface, but the directional motive force comes from Type IV pili.     

Hormogonia in two nostocalean cyanophytes (cyanobacteria) from the genera Hapalosiphon and Fischerella

The formation of hormogonia in the nostocalean cyanophytes/cyanobacteria Hapalosiphon fontinalis (C. Agardh) Bornet and Fischerella sp. was studied in natural populations collected from the Klin peatbog, northern Slovakia. Hormogonia were produced terminally in lateral branches of filaments (both species), or also directly on the main branches (Fischerella sp.). In contrast to vegetative filaments, hormogonia were not ramified, lacked heterocytes, were embedded in mucilaginous envelopes, were able to move, and their cells contained aerotopes. They were released by gliding through an opening in the sheath at the end of lateral branches of filaments. Released hormogonia of H. fontinalis were solitary or agglomerated into common fascicles morphologically resembling planktic colonies of Aphanizomenon flos-aquae (L.) Ralfs ex Bornet et Flahault or Dolichospermum affine (Lemmermann) Wacklin, Hoffmann et Komárek (syn. Anabaena affinis Lemmermann). Occasionally, lateral or sessile Nostochopsis-like heterocytes and apical spherical monocytes were formed on the main filaments. Hormogonia of Fischerella sp. were formed not only in apical part of lateral trichomes, but also directly on the main trichomes. Their cells were markedly larger than the vegetative cells and possessed well-developed aerotopes. Released hormogonia remained solitary, and were not agglomerated into fascicles. Apical hormogonia were released by gliding through an opening in the sheath at the end of lateral branches of filaments, and basal hormogonia were released by breaking off the main axis. In contrast to filaments of H. fontinalis which were very common and represented the dominant species of the cyanophyte communities in the locality, filaments of Fischerella sp. were observed only in one sample and for a limited period. This is the first record of a representative of the genus Fischerella in Slovakia.     

丝状体蓝藻藻殖段的分化及其调节机制

本文介绍了丝状体蓝藻(亦称蓝细菌) 的藻殖段的分化及其调节机制。藻殖段与正常藻丝体的区别在于细胞形状、细胞内存有气囊和可移动的短而直的藻丝链等。本文对许多环境因子包括光和营养因素等促进或抑制藻殖段的分化进行了讨论;还介绍了念珠藻(Nostoc) ,单歧藻(Tolypothrix) 和眉藻(Calothrix)所具有复杂的细胞发育过程,即具气囊又可移动的藻殖段分化,异形胞分化以及营养细胞的补偿性色适应。这三种细胞类型的适应形成取决于两种不同的光受体系统。藻殖段和异形胞两者的分化可能取决于光合电子传递链;而营养细胞的补偿性色适应则受光敏色素的调节。此外,谷酰胺合成酶合成和活性调节的PII蛋白,在协同藻殖段分化、异形胞分化及营养细胞的补偿色适应中起重要作用。由于蓝藻藻殖段分化及其调节机制是一个新的研究领域,关于它的知识尚不完整,亟待人们加强研究。     

丝状体蓝藻藻殖段的分化及其调节机制

本文介绍了丝状体蓝藻（亦称蓝细菌）的藻殖段的分化及其调节机制。藻殖段与正常藻丝体的区别在于细胞开状、细胞内存有气囊和可移动的短而真的藻丝链等。本文对许多环境因子包括光和营养因素等促进或抑制藻殖段的分化进行一讨论;还介绍了含球藻（Ｎｏｓｔｏｃ）,单歧藻（Ｔｏｌｙｐｏｔｈｒｉｘ）和眉藻（Ｃａｌｏｔｈｒｉｘ）所具有复杂的细胞发育过程,即具气囊又可移动的藻殖段分化,异形胞分化以及营养细胞的被偿性色适应。这     

Branching and intercellular communication in the Section V cyanobacterium Mastigocladus laminosus,a complex multicellular prokaryote

The filamentous Section V cyanobacterium Mastigocladus laminosus is one of the most morphologically complex prokaryotes. It exhibits cellular division in multiple planes, resulting in the formation of true branches, and cell differentiation into heterocysts, hormogonia and necridia. Here, we investigate branch formation and intercellular communication in M. laminosus. Monitoring of membrane rearrangement suggests that branch formation results from a randomized direction of cell growth. Transmission electron microscopy reveals cell junction structures likely to be involved in intercellular communication. We identify a sepJ gene, coding for a potential key protein in intercellular communication, and show that SepJ is localized at the septa. To directly investigate intercellular communication, we loaded the fluorescent tracer 5‐carboxyfluorescein diacetate into the cytoplasm, and quantified its intercellular exchange by fluorescence recovery after photobleaching. Results demonstrate connectivity of the main trichome and branches, enabling molecular exchange throughout the filament network. Necridia formation inhibits further molecular exchange, determining the fate of a branch likely to become a hormogonium. Cells in young, narrow trichomes and hormogonia exhibited faster exchange rates than cells in older, wider trichomes. Signal transduction to co‐ordinate movement of hormogonia might be accelerated by reducing cell volume.     

Cyanobacteria-bryophyte symbioses

Cyanobacteria are a large group of photosynthetic prokaryotesof enormous environmental importance, being responsible fora large proportion of global CO₂ and N₂ fixation. They formsymbiotic associations with a wide range of eukaryotic hostsincluding plants, fungi, sponges, and protists. The cyanobacterialsymbionts are often filamentous and fix N₂ in specialized cellsknown as heterocysts, enabling them to provide the host withfixed nitrogen and, in the case of non-photosynthetic hosts,with fixed carbon. The best studied cyanobacterial symbiosesare those with plants, in which the cyanobacteria can infectthe roots, stems, leaves, and, in the case of the liverwortsand hornworts, the subject of this review, the thallus. Thesymbionts are usually Nostoc spp. that gain entry to the hostby means of specialized motile filaments known as hormogonia.The host plant releases chemical signals that stimulate hormogoniaformation and, by chemoattraction, guide the hormogonia to thepoint of entry into the plant tissue. Inside the symbiotic cavity,host signals inhibit further hormogonia formation and stimulateheterocyst development and dinitrogen fixation. The cyanobiontsundergo morphological and physiological changes, including reducedgrowth rate and CO₂ fixation, and enhanced N₂ fixation, andrelease to the plant much of the dinitrogen fixed. This shortreview summarizes knowledge of the cyanobacterial symbioseswith liverworts and hornworts, with particular emphasis on theimportance of pili and gliding motility for the symbiotic competenceof hormogonia. Key words: Bryophyte, cyanobacteria, gliding motility, pili, symbiosis Received 1 August 2007; Revised 23 December 2007 Accepted 7 January 2008     

Swapping genes to survive - a new role for archaeal type IV pili

Type IV pili are filamentous structures that are found on the surface of many bacterial and archaeal cells, they are involved in cell motility and surface adhesion. In the crenarchaeon Sulfolobus solfataricus, type IV pili formation is strongly induced by UV irradiation and leads to cellular aggregation. The study by Ajon et al. (2011) published in this issue of Molecular Microbiology shows that UV-induced cellular aggregation greatly stimulates the exchange of chromosomal markers among irradiated cells, and that this strategy helps with cell survival. Sulfolobus knockout strains that are incapable of forming pili proved to be deficient in aggregation, and also showed decreased cellular survival after UV irradiation. The UV-induced pili of three different Sulfolobus species had distinct morphologies, and correspondingly these three species were able to aggregate only with their own kind. This work has defined a new role for type IV pili in both the transfer of genes within species and the recovery from UV-induced DNA damage.     

发状念珠藻藻殖段的分化及其光合特性的研究

Hormogonia of Nostoc flagelliforme is one of the developmental stages in the life cycle of cyanobacterium. High yields of pure hormogonia were obtained by weak light (the filaments were covered by sterilized sand for blocking light), red light, white light plus DCMU (3, 4-dichlorophenyl-1, 1-dimethylurea) in the culture. These pure fractions of hormogonia allowed the study of physiological measurements in comparison to vegetative filaments. The photosynthesis in the hormogonia and the vegetative filaments was characterized by fluorescence emission spectra at 77 K, absorption spectrum and oxygen evolution. Absorption spectrum of the hormogoia and vegetative filaments did not reveal difference. The data indicated the similarity of pigment contents between hormogonia and vegetative filaments. Some differences were observed in oxygen evolution of vegetative filaments and hormogonia in the temperature range of 15 ℃ to 45 ℃ and light intensity around 110 μmol·m-2·s-1 to 1200 μmol·m-2·s-1. The fluorescence emission spectra showed that energy distribution between the two photosystems in mature colonies was more balance than in hormogonia. The absorption of light energy in phycobilisomes and the transfer to the two photosystems in the hormogonia were more effective than in the mature colonies. It may be concluded that the formation of hormogonia affected on the structure and function of phytosynthesis.     

Uncovering the Mechanism of Trapping and Cell Orientation during Neisseria gonorrhoeae Twitching Motility

Neisseria gonorrheae bacteria are the causative agent of the second most common sexually transmitted infection in the world. The bacteria move on a surface by means of twitching motility. Their movement is mediated by multiple long and flexible filaments, called type IV pili, that extend from the cell body, attach to the surface, and retract, thus generating a pulling force. Moving cells also use pili to aggregate and form microcolonies. However, the mechanism by which the pili surrounding the cell body work together to propel bacteria remains unclear. Understanding this process will help describe the motility of N. gonorrheae bacteria, and thus the dissemination of the disease which they cause. In this article we track individual twitching cells and observe that their trajectories consist of alternating moving and pausing intervals, while the cell body is preferably oriented with its wide side toward the direction of motion. Based on these data, we propose a model for the collective pili operation of N. gonorrheae bacteria that explains the experimentally observed behavior. Individual pili function independently but can lead to coordinated motion or pausing via the force balance. The geometry of the cell defines its orientation during motion. We show that by changing pili substrate interactions, the motility pattern can be altered in a predictable way. Although the model proposed is tangibly simple, it still has sufficient robustness to incorporate further advanced pili features and various cell geometries to describe other bacteria that employ pili to move on surfaces.     

Uncovering the Mechanism of Trapping and Cell Orientation during Neisseria gonorrhoeae Twitching Motility

Neisseria gonorrheae bacteria are the causative agent of the second most common sexually transmitted infection in the world. The bacteria move on a surface by means of twitching motility. Their movement is mediated by multiple long and flexible filaments, called type IV pili, that extend from the cell body, attach to the surface, and retract, thus generating a pulling force. Moving cells also use pili to aggregate and form microcolonies. However, the mechanism by which the pili surrounding the cell body work together to propel bacteria remains unclear. Understanding this process will help describe the motility of N. gonorrheae bacteria, and thus the dissemination of the disease which they cause. In this article we track individual twitching cells and observe that their trajectories consist of alternating moving and pausing intervals, while the cell body is preferably oriented with its wide side toward the direction of motion. Based on these data, we propose a model for the collective pili operation of N. gonorrheae bacteria that explains the experimentally observed behavior. Individual pili function independently but can lead to coordinated motion or pausing via the force balance. The geometry of the cell defines its orientation during motion. We show that by changing pili substrate interactions, the motility pattern can be altered in a predictable way. Although the model proposed is tangibly simple, it still has sufficient robustness to incorporate further advanced pili features and various cell geometries to describe other bacteria that employ pili to move on surfaces.     

A puddle: an ombrophilic cyano-bacterial community

Sequential stages of formation of an ombrophilic cyano-bacterial community on clay were determined in a laboratory model of a puddle community. In a suspension of washed clay obtained from loamy soil, with montmorillonite as the predominant phase, a bacterial neustonic film is initially formed; it acts as a support for cyanobacterial hormogonia. At the next stage, the upper layer of precipitated clay (about 1 mm) is reinforced by a cyano-bacterial structure of Phormidium sp. trichomes and develops a tissue-like structure. The hormogonia and sheathless cyanobacteria remain free from mineral particles. Subsequently, gas formation results in a separation of a dense cyano-bacterial film from the underlying loose suspension and in formation of gas swellings. The mineral component of the film is differentiated: mineral particles of quartz and feldspar grains are attached to Phormidium sp. trichomes, which act as a factor of mineral selection.     

Motility of endodermal epithelial cells plays a major role in reorganizing the two epithelial layers in Hydra

Cell-cell interactions and cell rearrangements play important roles during development. Aggregates of Hydra cells reorganize into the two epithelial layers and subsequently form a normal animal. Examination of the formation of the two layers under various situations, indicates that the motility of endodermal epithelial cells, but not the differential adhesive forces of the two types of epithelial cells, plays the critical role in setting up the two epithelial layers. (1) When aggregates of ectodermal cells and of endodermal cells were placed in direct contact, the endodermal cells migrated into the interior of the ectodermal aggregate. This process was completely inhibited by cytochalasin B although initial firm attachment between the two aggregates was not blocked. (2) A single endodermal epithelial cell placed in contact with an ectodermal aggregate, actively extended pseudopod-like structures and migrated toward the center of the ectodermal aggregate. In contrast, an ectodermal epithelial cell remained in contact with an endodermal aggregate and never exhibited migratory behavior. Cytochalasin treatment of only endodermal epithelial cells abolished the migration. (3) One to 4 endodermal epithelial cells and/or ectodermal epithelial cells were placed in contact with one another forming up to 4-cell aggregates. Endodermal epithelial cells exhibited high motility that can be attributed to the migratory movement described above. Finally, formation of actin bundles, as visualized with rhodamine-phalloidin, was always correlated with pseudopod formation in endodermal epithelial cells during early and mid stages of aggregate formation.     

Role of streams in myxobacteria aggregate formation

Cell contact, movement and directionality are important factors in biological development (morphogenesis), and myxobacteria are a model system for studying cell-cell interaction and cell organization preceding differentiation. When starved, thousands of myxobacteria cells align, stream and form aggregates which later develop into round, non-motile spores. Canonically, cell aggregation has been attributed to attractive chemotaxis, a long range interaction, but there is growing evidence that myxobacteria organization depends on contact-mediated cell-cell communication. We present a discrete stochastic model based on contact-mediated signaling that suggests an explanation for the initialization of early aggregates, aggregation dynamics and final aggregate distribution. Our model qualitatively reproduces the unique structures of myxobacteria aggregates and detailed stages which occur during myxobacteria aggregation: first, aggregates initialize in random positions and cells join aggregates by random walk; second, cells redistribute by moving within transient streams connecting aggregates. Streams play a critical role in final aggregate size distribution by redistributing cells among fewer, larger aggregates. The mechanism by which streams redistribute cells depends on aggregate sizes and is enhanced by noise. Our model predicts that with increased internal noise, more streams would form and streams would last longer. Simulation results suggest a series of new experiments.     

Trichome micromorphology of the Chinese-Himalayan genus <Emphasis Type="Italic">Colquhounia</Emphasis> (Lamiaceae), with emphasis on taxonomic implications

Trichome micromorphology of leaves and young stems of nine taxa (including four varieties) of Colquhounia were examined using light and scanning microscopy. Two basic types of trichomes were recognized: eglandular and glandular. Eglandular trichomes are subdivided into simple and branched trichomes. Based on the number of cells and trichome configuration, simple eglandular trichomes are further divided into four forms: unicellular, two-celled, three-celled and more than three-celled trichomes. Based on branching configuration, the branched eglandular trichomes can be separated into three forms: biramous, stellate and dendroid. Glandular trichomes can be divided into two subtypes: capitate and peltate glandular trichomes. Results from this study of morphological diversity of trichomes within Colquhounia lend insight into infrageneric classification and species relationships. Based on the presence of branched trichomes in C. elegans, this species should be transferred from Colquhounia sect. Simplicipili to sect. Colquhounia. We provide a taxonomic key to species of Chinese Colquhounia based on trichome morphology and other important morphological traits.     

Dynamics and control of biofilms of the oligotrophic bacterium Caulobacter crescentus

Caulobacter crescentus is an oligotrophic alpha-proteobacterium with a complex cell cycle involving sessile-stalked and piliated, flagellated swarmer cells. Because the natural lifestyle of C. crescentus intrinsically involves a surface-associated, sessile state, we investigated the dynamics and control of C. crescentus biofilms developing on glass surfaces in a hydrodynamic system. In contrast to biofilms of the well-studied Pseudomonas aeruginosa, Escherichia coli, and Vibrio cholerae, C. crescentus CB15 cells form biphasic biofilms, consisting predominantly of a cell monolayer biofilm and a biofilm containing densely packed, mushroom-shaped structures. Based on comparisons between the C. crescentus strain CB15 wild type and its holdfast (hfsA; DeltaCC0095), pili (DeltapilA-cpaF::Omegaaac3), motility (motA), flagellum (flgH) mutants, and a double mutant lacking holdfast and flagellum (hfsA; flgH), a model for biofilm formation in C. crescentus is proposed. For both biofilm forms, the holdfast structure at the tip of a stalked cell is crucial for mediating the initial attachment. Swimming motility by means of the single polar flagellum enhances initial attachment and enables progeny swarmer cells to escape from the monolayer biofilm. The flagellum structure also contributes to maintaining the mushroom structure. Type IV pili enhance but are not absolutely required for the initial adhesion phase. However, pili are essential for forming and maintaining the well-defined three-dimensional mushroom-shaped biofilm. The involvement of pili in mushroom architecture is a novel function for type IV pili in C. crescentus. These unique biofilm features demonstrate a spatial diversification of the C. crescentus population into a sessile, "stem cell"-like subpopulation (monolayer biofilm), which generates progeny cells capable of exploring the aqueous, oligotrophic environment by swimming motility and a subpopulation accumulating in large mushroom structures.     

FINE STRUCTURE AND SIZE DISTRIBUTION OF FREE THICK FILAMENTS IN EARLY FIBRILLOGENESIS OF ASCIDIAN TADPOLE

The development and the size distribution of free thick filaments which accumulate in the early stages of myofibril formation in somitic myoblasts of the ascidian tadpole were studied by electron microscopy. Such filaments appeared in the cell cortex but, rather dominantly, the aggregates of these thick filaments and filamentous structures were observed in the interior of the cell. The aggregate consisted of some of the following elements: filamentous structures (20–60 A in diameter); free thick filaments (60–220 A); dense Z-band precursor materials; bundles of thick (140–160 A) and thin (60–70 A) filaments; and ribosomal clusters. The free thick filaments were variable in diameter and showed long lateral projections (300–600 A) and tapered ends.
The variation curve in diameter of the free thick filaments indicates a continuous size distribution, suggesting the continuous growth of these filaments by polymerization of myosin molecules. Free thick filaments thicker than myosin filaments which were found within myofibrils were present; their significance is discussed in relation to myosin filament formation.