The stoichiometry of the two photosystems in higher plants revisited.

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes.

Modelling advection and diffusion of water isotopologues in leaves.

We described advection and diffusion of water isotopologues in leaves in the non-steady state, applied specifically to amphistomatous leaves. This explains the isotopic enrichment of leaf water from the xylem to the mesophyll, and we showed how it relates to earlier models of leaf water enrichment in non-steady state. The effective length or tortuosity factor of isotopologue movement in leaves is unknown and, therefore, is a fitted parameter in the model.

Signaling against the wind: modifying motion-signal structure in response to increased noise.

Animal signals are optimized for particular signaling environments [1-3]. While signaling, senders often choose favorable conditions that ensure reliable detection and transmission [4-8], suggesting that they are sensitive to changes in signal efficacy. Recent evidence has also shown that animals will increase the amplitude or intensity of their acoustic signals at times of increased environmental noise [9-11].

Factors that Mediate Root Initiation in Plants.

When the leaf explants of Medicago truncatula are placed on auxin medium they form root initials within a week. Our studies have shown that the cells associated with the veins are stimulated into division by the added auxin, forming what we called vein-derived cells (VDCs) that grow out into callus and it is from these cells that the root meristems are formed. The way auxin interacts with other hormones is a key factor in determining the stem cell fate.

Relative contributions of leaf area ratio and net assimilation rate to change in growth rate depend on growth temperature: comparative analysis of subantarctic and alpine grasses.

The present study shows that the relative contributions of leaf area ratio (LAR) and net assimilation rate (NAR) to variation among species in relative growth rate (RGR) depend on growth temperature. We grew three subantarctic and three alpine Poa species at daytime temperatures of 7, 12 and 17 degrees C, and analysed interspecific and temperature-related variation in RGRs by growth analysis. Variation in NAR accounted for most of the interspecific differences in RGR at low growth temperature, whereas variation in both NAR and LAR contributed strongly to interspecific differences in RGR at high growth temperature.

PathExpress: a web-based tool to identify relevant pathways in gene expression data.

PathExpress is a web-based tool developed to interpret gene expression data obtained from microarray experiments by identifying the most relevant metabolic pathways associated with a subset of genes (e.g. differentially expressed genes).

Advances in understanding the cyanobacterial CO2-concentrating-mechanism (CCM): functional components, Ci transporters, diversity, genetic regulation and prospects for engineering into plants.

Cyanobacteria have evolved a significant environmental adaptation, known as a CO(2)-concentrating-mechanism (CCM), that vastly improves photosynthetic performance and survival under limiting CO(2) concentrations. The CCM functions to transport and accumulate inorganic carbon actively (Ci; HCO(3)(-), and CO(2)) within the cell where the Ci pool is utilized to provide elevated CO(2) concentrations around the primary CO(2)-fixing enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). In cyanobacteria, Rubisco is encapsulated in unique micro-compartments known as carboxysomes.

The preferences of the honeybee (Apis mellifera) for different visual cues during the learning process.

By working with very simple images, a number of different visual cues used by the honeybee have been described over the past decades. In most of the work, the bees had no control over the choice of the images, and it was not clear whether they learned the rewarded pattern or the difference between two images. Preferences were known to exist when untrained bees selected one pattern from a variety of them, but because the preferences of the bees were ignored, it was not possible to understand how natural images displaying several cues were detected.

Relative amounts of soluble and insoluble forms of phosphorus and other elements in intraradical hyphae and arbuscules of arbuscular mycorrhizas.

Transport of phosphorus (P) into host plants and its release to root cells is an important function of arbuscular mycorrhizal fungi (AMF). However, relatively little is known about the forms and water solubilities of P compounds in specific locations in the intraradical fungal structures. We determined concentrations and solubility of P components in these structures in white clover (Trifolium repens L.

Frequency of cancer genes on the chicken z chromosome and its human homologues: implications for sex chromosome evolution.

It has been suggested that there are special evolutionary forces that act on sex chromosomes. Hemizygosity of the X chromosome in male mammals has led to selection for male-advantage genes, and against genes posing extreme risks of tumor development. A similar bias against cancer genes should also apply to the Z chromosome that is present as a single copy in female birds.

Complex cells increase their phase sensitivity at low contrasts and following adaptation.

One of the best-known dichotomies in neuroscience is the division of neurons in the mammalian primary visual cortex into simple and complex cells. Simple cells have receptive fields with separate on and off subregions and give phase-sensitive responses to moving gratings, whereas complex cells have uniform receptive fields and are phase invariant. The phase sensitivity of a cell is calculated as the ratio of the first Fourier coefficient (F1) to the mean time-average (Fo) of the response to moving sinusoidal gratings at 100% contrast.

Animal navigation: the difficulty of moving in a straight line.

In principle, there are two strategies for navigating a straight course. One is to use an external directional reference and continually reorienting with reference to it, while the other is to infer body rotations from internal sensory information only. We show here that, while the first strategy will enable an animal or mobile agent to move arbitrarily far away from its starting point, the second strategy will not do so, even after an infinite number of steps.

Expression of genomic AtCYCD2;1 in Arabidopsis induces cell division at smaller cell sizes: implications for the control of plant growth.

The Arabidopsis (Arabidopsis thaliana) CYCD2;1 gene introduced in genomic form increased cell formation in the Arabidopsis root apex and leaf, while generating full-length mRNA, raised CDK/CYCLIN enzyme activity, reduced G1-phase duration, and reduced size of cells at S phase and division. Other cell cycle genes, CDKA;1, CYCLIN B;1, and the cDNA form of CYCD2;1 that produced an aberrantly spliced mRNA, produced smaller or zero increases in CDK/CYCLIN activity and did not increase the number of cells formed. Plants with a homozygous single insert of genomic CYCD2;1 grew with normal morphology and without accelerated growth of root or shoot, not providing evidence that cell formation or CYCLIN D2 controls growth of postembryonic vegetative tissues.

A muscarinic cholinergic antagonist and a dopamine agonist rapidly increase ZENK mRNA expression in the form-deprived chicken retina.

Increases in the expression of the immediate early gene ZENK in the retina, measured by changes in the levels of mRNA and protein immunoreactivity, are amongst the most rapid responses so far measured to conditions that decrease the rate of eye growth in chickens. Our aim was to determine whether atropine, a muscarinic cholinergic antagonist, and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide, a dopamine agonist, which are known to block excessive eye growth, produce similar changes in ZENK expression. Form-deprivation resulted in significant down-regulation of the expression of retinal ZENK mRNA within 1 h of fitting the diffusers, whereas removal of the diffusers from the eyes of chickens that had developed form-deprivation myopia resulted in significant up-regulation of retinal ZENK expression within 1 h.

The importance of protein-protein interactions for optimising oxygen activity in photosystem II: reconstitution with a recombinant thioredoxin--manganese stabilising protein.

In this paper we describe how photosystem II (PSII) from higher plants, which have been depleted, of the extrinsic proteins can be reconstituted with a chimeric fusion protein comprising thioredoxin from Escherichia coli and the manganese stabilising protein from Thermosynechococcus elongatus. Surprisingly, even though E. coli thioredoxin is completely unrelated to PSII, the fusion protein restores higher rates of activity upon rebinding to PSII than either the native spinach MSP, or T.

Research note: Deposition patterns of cellulose microfibrils in flange wall ingrowths of transfer cells indicate clear parallels with those of secondary wall thickenings.

The arrangement of cellulose microfibrils and cortical microtubules in transfer cells depositing flange wall ingrowths have been determined with field emission scanning electron microscopy and immunofluorescence confocal microscopy. In xylem transfer cells of wheat (Triticum aestivum) stem nodes and transfer cells of corn (Zea mays) endosperm tissue, cellulose microfibrils were aligned in parallel bundles to form the linear wall ingrowths characteristic of flange ingrowth morphology. In both cell types, linear bundles of cellulose microfibrils were deposited over an underlying wall composed of randomly arranged microfibrils.

A quantitative assessment of the carbonic anhydrase activity in photosystem II.

Using a carbonic anhydrase assay based on membrane inlet mass spectrometry (MIMS), we have extended our earlier investigations of Photosystem II (PSII)-associated carbonic anhydrase activity in spinach PSII preparations (W. Hillier, I. McConnell, M.

Oxygen isotope enrichment of organic matter in Ricinus communis during the diel course and as affected by assimilate transport.

The oxygen isotope composition in leaf water and organic compounds in different plant tissues is useful for assessing the physiological performance of plants in their environment, but more information is needed on Delta(18)O variation during a diel course. Here, we assessed Delta(18)O of the organic matter in leaves, phloem and xylem in stem segments, and fine roots of Ricinus communis during a full diel cycle. Enrichment of newly assimilated organic matter in equilibrium with leaf water was calculated by applying a nonsteady-state evaporative enrichment model.

Eph receptor and ephrin signaling in developing and adult brain of the honeybee (Apis mellifera).

Roles for Eph receptor tyrosine kinase and ephrin signaling in vertebrate brain development are well established. Their involvement in the modulation of mammalian synaptic structure and physiology is also emerging. However, less is known of their effects on brain development and their function in adult invertebrate nervous systems.

Depth, contrast and view-based homing in outdoor scenes.

Panoramic image differences can be used for view-based homing under natural outdoor conditions, because they increase smoothly with distance from a reference location (Zeil et al., J Opt Soc Am A 20(3):450-469, 2003). The particular shape, slope and depth of such image difference functions (IDFs) recorded at any one place, however, depend on a number of factors that so far have only been qualitatively identified.

Detection without deflection? A hypothesis for direct sensing of sound pressure by hair cells.

It is widely thought that organisms detect sound by sensing the deflection of hair-like projections, the stereocilia, at the apex of hair cells. In the case of mammals, the standard interpretation is that hair cells in the cochlea respond to deflection of stereocilia induced by motion generated by a hydrodynamic travelling wave. But in the light of persistent anomalies, an alternative hypothesis seems to have some merit: that sensing cells (in particular the outer hair cells) may, at least at low intensities, be reacting to a different stimulus - the rapid pressure wave that sweeps through the cochlear fluids at the speed of sound in water.

Conduction properties of KcsA measured using brownian dynamics with flexible carbonyl groups in the selectivity filter.

In the narrow segment of an ion conducting pathway, it is likely that a permeating ion influences the positions of the nearby atoms that carry partial or full electronic charges. Here we introduce a method of incorporating the motion of charged atoms lining the pore into Brownian dynamics simulations of ion conduction. The movements of the carbonyl groups in the selectivity filter of the KcsA channel are calculated explicitly, allowing their bond lengths, bond angles, and dihedral angels to change in response to the forces acting upon them.

Thylakoid membrane landscape in the sixties: a tribute to Andrew Benson.

Prior to the 1960s, the model for the molecular structure of cell membranes consisted of a lipid bilayer held in place by a thin film of electrostatically-associated protein stretched over the bilayer surface: (the Danielli-Davson-Robertson "unit membrane" model). Andrew Benson, an expert in the lipids of chloroplast thylakoid membranes, questioned the relevance of the unit membrane model for biological membranes, especially for thylakoid membranes, instead of emphasizing evidence in favour of hydrophobic interactions of membrane lipids within complementary hydrophobic regions of membrane-spanning proteins. With Elliot Weier, Benson postulated a remarkable subunit lipoprotein monolayer model for thylakoids.

Impairment of the photorespiratory pathway accelerates photoinhibition of photosystem II by suppression of repair but not acceleration of damage processes in Arabidopsis.

Oxygenation of ribulose-1,5-bisphosphate catalyzed by Rubisco produces glycolate-2-P. The photorespiratory pathway, which consists of photorespiratory carbon and nitrogen cycles, metabolizes glycolate-2-P to the Calvin cycle intermediate glycerate-3-P and is proposed to be important for avoiding photoinhibition of photosystem II (PSII), especially in C3 plants. We show here that mutants of Arabidopsis (Arabidopsis thaliana) with impairment of ferredoxin-dependent glutamate synthase, serine hydroxymethyltransferase, glutamate/malate transporter, and glycerate kinase had accelerated photoinhibition of PSII by suppression of the repair of photodamaged PSII and not acceleration of the photodamage to PSII.