Biophysical Dynamics in Disorderly Environments

The cell wall is an essential cellular component of bacteria and the target of many antibiotics. However, how bacteria regulate the rate of cell wall biosynthesis as growth rates change remains unresolved. In E. coli, cell wall growth was thought to proceed independently from turgor pressure1, the osmotic pressure that the cytoplasm exerts on the cell wall. Here, we uncover a striking increase of turgor pressure with growth rate. Modulating turgor pressure and measuring cell wall biosynthesis, we find that turgor pressure is directly controls the rate of cell wall biosynthesis. The picture that emerges is that turgor pressure is largely generated by counterions of negatively charged cellular biomass. The increase in turgor pressure with growth rates results from more ribosomes and therefore higher concentrations of negatively charged ribosomal RNA. Elegantly, the coupling between biomass composition, turgor pressure and cell wall biosynthesis simultaneously explains how bacteria achieve homeostasis of cytoplasmic crowding and how they regulate the rate of cell wall biosynthesis across growth rates.

SummaryBecause of their large sizes and simple shapes, giant‐celled algae have been used to study how the structural and mechanical properties of cell walls influence cell growth. Here we review known relationships between cell wall and cell growth properties that are characteristic of three representative taxa of giant‐celled algae, namely, Valonia ventricosa, internodal cells of characean algae, and Vaucheria frigida. Tip‐growing cells of the genus Vaucheria differ from cells undergoing diffuse growth in V. ventricosa and characean algae in terms of their basic architectures (non‐lamellate vs. multilamellate) and their dependence upon pH and Ca2+ for cell wall extensibility. To further understand the mechanisms controlling cell growth by cell walls, comparative analyses of cell wall structures and/or associated growth modes will be useful. The giant‐celled algae potentially serve as good models for such investigations because of their wide variety of developmental processes and cell shapes exhibited.

Numerous studies deal with composition and molecular processes involved in primary cell wall formation and alteration in Arabidopsis. However, it still remains difficult to assess the relation between physiological properties and mechanical function at the cell wall level. The thin and fragile structure of primary cell walls and their large biological variability, partly related to structural changes during growth, make mechanical experiments challenging. Since, to the best of our knowledge, there is no reliable data in the literature about how the properties of the fully elongated zone of hypocotyls change with age. We studied in a series of experiments on two different seed batches the tensile properties the region below the growth zone of 4 to 7 day old etiolated Arabidopsis hypocotyls. Additionally, we analysed geometrical parameters, hypocotyl density and cellulose content as individual traits and their relation to tissue mechanics. No significant differences of the mechanical parameters of the non-growing region between 5–7 day old plants could be found whereas in 4 day old plants both tensile stiffness and ultimate tensile stress were significantly lower than in the older plants. Furthermore hypocotyl diameters and densities remain almost the same for 5, 6 and 7 day old seedlings. Naturally, hypocotyl lengths increase with age. The evaluation whether the choice–age or length—influences the mechanical properties showed that both are equally applicable sampling parameters. Additionally, our detailed study allows for the estimation of biological variability, connections between mechanics and hypocotyl age could be established and complement the knowledge on biochemistry and genetics affecting primary plant cell wall growth. The application of two different micromechanical devices for testing living Arabidopsis hypocotyls allows for emphasizing and discussing experimental limitations and for presenting a wide range of possibilities to address current and future questions related to plant cell wall mechanics, synthesis and growth in combination with molecular biology methodologies.

Herbivore impacts on macrophyte growth vary with the identity of the herbivores and macrophytes, as well as under different abiotic conditions. This interaction is further complicated by anthropogenic alterations to the environment, such as eutrophication. In this study, we utilized in situ herbivore exclusion experiments and mesocosm feeding preference assays to examine the impacts of different herbivores on the growth of two morphologically similar, co-occurring macroalgal bloom Ulva species in a nutrient-rich environment. We found that herbivory had a measurable impact on Ulva biomass, though the rate of consumption rarely surpassed growth for either Ulva species. We determined that the primary herbivores within the blooms were amphipods and mud crabs, and that their effects varied among study sites and months. Our results also confirmed that, even with a diverse suite of consumers, Ulva blooms are capable of escaping herbivore control, particularly early in the growing season when growth rates peak and herbivore activity is limited. Furthermore, our experiments revealed species-specific feeding preferences among herbivores, as well as differences in growth rates and chemistry between the two Ulva species, which likely influence bloom dynamics.

A model for predicting growth responses in plants to changes in external water potential: Zea mays primary roots

Scum actinomycetes in sewage treatment plants—Part 1: Growth kinetics of Nocardia amarae in chemostat culture

Study of the cell wall growth in Bacillus megaterium by pulse-labeling a DAP- Lys- mutant with tritiated diaminopimelic acid (DAP) had revealed the presence of intracytoplasmic radioactivity. The nature of this radioactivity was studied on one hand by autoradiographic analysis of bacteria treated in different ways and on the other hand by chromatography of the radioactive compounds extracted with boiling water. It is shown that cytoplasmic radioactivity corresponds neither to free DAP nor to DAP metabolized into lysine, but to murein precursors. Autoradiographic analysis of bacteria in which all murein precursors were removed gives exactly the same cell wall growth pattern as the one previously obtained for untreated bacteria. It can be concluded that, in B. megaterium, cell wall elongation occurs by diffuse intercalation of newly synthesized murein along the cylindrical part of the cell wall and that only cross wall formation occurs in a precise growth zone.

Irreversible cell enlargement begins with cell wall loosening which induces wall stress relaxation, leading to cell water uptake and cell enlargement. Growing cell walls consist of a cohesive network of cellulose microfibrils embedded in hydrophilic pectins and cellulose‐binding hemicelluloses. Models of the growing cell wall are provisional hypotheses about how these wall polymers interact to make a strong yet extensible wall. Recent results clarify how wall strength, plasticity and elasticity depend mostly on the stretching, straightening and sliding of cellulose microfibril networks in the wall. Passive sliding of cellulose microfibrils is facilitated by nonenzymic proteins named expansins while various enzymes modify pectins and hemicelluloses, altering their interactions with each other and with cellulose. Growth cessation is correlated with reduced expression of genes that promote wall loosening as well as changes in the direction of cellulose deposition and the structure of matrix polysaccharides, leading to a less extensible cell wall. Key Concepts Irreversible cell enlargement is an essential aspect of plant growth and morphogenesis. Surface enlargement of the cell wall may be highly localised, as in tip‐growing cells, or more evenly distributed over the cell wall surface, a ‘diffuse growth’ pattern common to most cells in the plant body. Cells typically enlarge substantially after they leave the meristem and before they differentiate into mature cells. Cell growth entails simultaneous uptake of water into the cell and irreversible expansion of the cell wall. In physical terms, growth begins with molecular loosening of the cell wall which leads to wall stress relaxation ; this creates the water potential difference needed for water uptake by the cell, resulting in physical enlargement of the cell. The growing cell wall is composed of a strong noncovalent network of cellulose microfibrils embedded in highly hydrated pectins and hemicelluloses that comprise the soft wall matrix. The tensile (in‐plane) mechanics of growing walls is largely determined by the behaviour of the cellulose microfibril network while pectins influence the indentation (out‐of‐plane) mechanics. Growing plant cell walls typically enlarge more rapidly at low pH (called ‘acid growth’), a process that is mediated by nonenzymatic proteins named α‐expansins; α‐expansins facilitate the passive sliding of a network of cellulose microfibrils in response to tensile wall stresses originating from cell turgor pressure. Deposition of new polymers to the wall is usually coordinated with surface expansion, but these are separable processes. Several classes of enzymes modify the structure of pectins and hemicelluloses in the cell wall, potentially affecting cell wall structure and mechanics. Cell wall mechanical properties are complex, encompassing a number of different physical behaviours that often are not well correlated with each other or with cell wall growth. Cessation of cell enlargement likely involves multiple processes, including changes in the direction of cellulose deposition, stiffening of cellulose and matrix networks, and reduced expression of wall loosening proteins.

Actin homologues of the MreB family have an important role in specifying the morphology of many non-spherical eubacteria. The mreC and mreD genes have been implicated in control of cell morphology but their precise functions are unknown. In Bacillus subtilis the MreB homologue Mbl directs helical insertion of new cell wall material in the cylindrical part of the rod-shaped cell. Depletion of either MreC or MreD abolishes the control of cell shape. In the presence of high concentrations of magnesium cells depleted of MreC or MreD can be propagated indefinitely, although they have a spheroidal shape. We show that growth of the spheroidal mutants is based on insertion of new wall material at cell division sites and that this localized growth is dependent on cell division. Under some conditions the MreC and MreD proteins localize in a helical configuration. This localization pattern resembles that of the helical cables of Mbl protein. These results suggest that MreC and MreD act in a morphogenic pathway that couples the helical cytosolic Mbl cables to the extracellular cell wall synthetic machinery, which is critical for cylindrical elongation of the rod-shaped cells.

3D Microscopy of Rod-Shaped Bacteria Reveals Roles of MreB in Diameter Control and Center-Line Curvature

Formation, growth and regeneration of incomplete cell wall in spheroplasts of the blue-green algaAnacystis nidulans in liquid media

Cellulose and xyloglucan (XG) assemble to form the cellulose/XG network, which is considered to be the dominant load-bearing structure in the growing cell walls of non-graminaceous land plants. We have extended the most commonly accepted model for the macromolecular organization of XG in this network, based on the structural and quantitative analysis of three distinct XG fractions that can be differentially extracted from the cell walls isolated from etiolated pea stems. Approximately 8% of the dry weight of these cell walls consists of XG that can be solubilized by treatment of the walls with a XG-specific endoglucanase (XEG). This material corresponds to an enzyme-susceptible XG domain, proposed to form the cross-links between cellulose microfibrils. Another 10% of the cell wall consists of XG that can be solubilized by concentrated KOH after XEG treatment. This material constitutes another XG domain, proposed to be closely associated with the surface of the cellulose microfibrils. An additional 3% of the cell wall consists of XG that can be solubilized only when the XEG- and KOH-treated cell walls are treated with cellulase. This material constitutes a third XG domain, proposed to be entrapped within or between cellulose microfibrils. Analysis of the three fractions indicates that metabolism is essentially limited to the enzyme-susceptible domain. These results support the hypothesis that enzyme-catalyzed modification of XG cross-links in the cellulose/XG network is required for the growth and development of the primary plant cell wall, and demonstrate that the structural consequences of these metabolic events can be analyzed in detail.

In the preparation for missions to Mars, basic knowledge of the mechanisms of growth and development of living plants under microgravity (micro-g) conditions is essential. Focus has centered on the g-effects on rigidity, including mechanisms of signal perception, transduction, and response in gravity resistance. These components of gravity resistance are linked to the evolution and acquisition of responses to various mechanical stresses. An overview is given both on the basic effect of hypergravity as well as of micro-g conditions in the cell wall changes. The review includes plant experiments in the US Space Shuttle and the effect of short space stays (8–14 days) on single cells (plant protoplasts). Regeneration of protoplasts is dependent on cortical microtubules to orient the nascent cellulose microfibrils in the cell wall. The space protoplast experiments demonstrated that the regeneration capacity of protoplasts was retarded. Two critical factors are the basis for longer space experiments: a. the effects of gravity on the molecular mechanisms for cell wall development, b. the availability of facilities and hardware for performing cell wall experiments in space and return of RNA/DNA back to the Earth. Linked to these aspects is a description of existing hardware functioning on the International Space Station.

Elasticity, plasticity and fine structure of plant cell walls

Analyses are presented of the structure and the development of several biological entities, ranging in level of organization from the molecular to the organismal. Classical methods of analysis which have been used in the physical sciences have proved appropriate, showing the applicability of physical principles to biological structure and morphogenesis. At the ultrastructural level, bacterial flagella, microtubules and similar objects exhibit regularities of packing of their constituent protein subunits, which can be analyzed in crystallographic terms. The growth of a plant cell wall is shown to depend on the physical properties of the wall itself, and its susceptibility to strain hardening and metabolic softening. In multicellular tissues and organs, the growth deformations which lead to mature structures can be described and analyzed appropriately using concepts and principles drawn from fluid dynamics. In some cases growth is steady, that is the growth pattern is invariant with time over some period. A structure such as a meristem, may maintain its form, while the elements or cells of which it consists are continually changing. It is suggested that plants exploit the physical properties of their constituent materials, and physical forces such as turgor stress, in their morphogenesis.