Does the Response of Perennial Ryegrass to Elevated CO2 Concentration Depend on the Form of the Supplied Nitrogen?

To test whether different nitrogen form (nitrate or ammonium) in substrate can alter the response to elevated partial pressure of CO₂ (pCO₂) plants of perennial ryegrass (Lolium perenne cv. Bastion) were grown from seeds in growth chambers under pCO₂ of either 35 Pa (ambient, CA) or 70 Pa (elevated, CE) in a hydroponic system (with nutrient and pH control) for 24 d. Nitrogen was supplied as ammonium, nitrate or an equimolar mixture of both N forms. Under CE plants grew faster than their counterparts under CA during the first 14 d but after 23 d of cultivation stimulation disappeared. Despite the strong positive effect of mixed forms of N on plant growth, the beneficial effect of CE was similar to that in the other two N treatments. However, the almost alike final growth response to CE had different underlying mechanisms in different N treatments. Plants supplied with nitrate as a sole source of nitrogen had lower leaf mass ratio but much higher specific leaf area compared to plants supplied with ammonium. The decrease in the content of leaf organic N (per unit of structural dry mass) under CE was found only in leaves of plants supplied with ammonium on day 14. Nevertheless, the available form of N evidently contributes to changes of leaf N content under CE. The high levels of N and non-structural saccharides in plants supplied with ammonium at CE suggest that the CO₂ response of these plants was controlled by factors other than amount of available carbon and nitrogen.     

Why are there about 5000 species of phytoplankton in the sea?

The relative abundances of phytoplankton taxa conform approximatelyto a finite geometric series in which there are 20–25species per decade of ranked abundance. Such series can contain160–400 species between the commonest (10²²–10²⁶cells) and the rarest (10¹⁰–10¹⁴ cells). Thus, between12 and 31 such series are needed to explain the observed diversity,5x10³ species, of marine phytoplankton. The number of seriesis similar to the number (20–25) of upper-ocean watermasses defined by dilution time scale of order 10¹–10²years. Interpretations of this coincidence are discussed.     

General Amyloid Inhibitors? A Critical Examination of the Inhibition of IAPP Amyloid Formation by Inositol Stereoisomers

Islet amyloid polypeptide (IAPP or amylin) forms amyloid deposits in the islets of Langerhans; a process that is believed to contribute to the progression of type 2 diabetes and to the failure of islet transplants. An emerging theme in amyloid research is the hypothesis that the toxic species produced during amyloid formation by different polypeptides share common features and exert their effects by common mechanisms. If correct, this suggests that inhibitors of amyloid formation by one polypeptide might be effective against other amyloidogenic sequences. IAPP and Aβ, the peptide responsible for amyloid formation in Alzheimer''s disease, are particularly interesting in this regard as they are both natively unfolded in their monomeric states and share some common characteristics. Comparatively little effort has been expended on the design of IAPP amyloid inhibitors, thus it is natural to inquire if Aβ inhibitors are effective against IAPP, especially since no IAPP inhibitors have been clinically approved. A range of compounds inhibit Aβ amyloid formation, including various stereoisomers of inositol. Myo-, scyllo-, and epi-inositol have been shown to induce conformational changes in Aβ and prevent Aβ amyloid fibril formation by stabilizing non-fibrillar β-sheet structures. We investigate the ability of inositol stereoisomers to inhibit amyloid formation by IAPP. The compounds do not induce a conformational change in IAPP and are ineffective inhibitors of IAPP amyloid formation, although some do lead to modest apparent changes in IAPP amyloid fibril morphology. Thus not all classes of Aβ inhibitors are effective against IAPP. This work provides a basis of comparison to work on polyphenol based inhibitors of IAPP amyloid formation and helps provide clues as to the features which render them effective. The study also helps provide information for further efforts in rational inhibitor design.     

What is the role of arbuscular mycorrhizal fungi in plant-to-ecosystem responses to Elevated atmospheric CO2?

 We advocate the concept of an arbuscular mycorrhiza (AM) as a temporally and spatially complex symbiosis representing a suite of hosts and fungi, as against the more traditional "dual organism" view. We use the hierarchical framework presented in Fig. 1 as a basis for organizing many unanswered questions, and several questions that have not been asked, concerning the role of AM in responses to elevated atmospheric CO₂. We include the following levels: plant host, plant population, plant community, functional group and ecosystem. Measurements of the contributions of AM fungi at the various levels require the use of different response variables. For example, hyphal nutrient translocation rates or percent AM root infection may be important measures at the individual plant level, but hyphal biomass or glomalin production and turnover are more relevant at the ecosystem level. There is a discrepancy between our knowledge of the multifaceted role of AM fungi in plant and ecosystem ecology and most of the current research aimed at elucidating the importance of this symbiosis in global-change scenarios. Our framework for more integrated and multifactorial research on mycorrhizal involvement in regulating CO₂ responses may also serve to enhance communication between researchers working at different scales on large global-change ecosystem projects. Accepted: 12 February 1999     

Why are Sun Leaves Thicker than Shade Leaves? — Consideration based on Analyses of CO2 Diffusion in the Leaf

A ) depend not only on photosynthetic biochemistry but also on mesophyll structure. Because resistance to CO₂ diffusion from the substomatal cavity to the stroma is substantial, it is likely that mesophyll structure affects A through affecting diffusion of CO₂ in the leaf. To evaluate effects of various aspects of mesophyll structure on photosynthesis, we constructed a one-dimensional model of CO₂ diffusion in the leaf. When mesophyll thickness of the leaf is changed with the Rubisco content per unit leaf area kept constant, the maximum A occurs at an almost identical mesophyll thickness irrespective of the Rubisco contents per leaf area. On the other hand, with an increase in Rubisco content per leaf area, the mesophyll thickness that realizes a given photosynthetic gain per mesophyll thickness (or per leaf cost) increases. This probably explains the strong relationship between A and mesephyll thickness. In these simulations, an increase in mesophyll thickness simultaneously means an increase in the diffusional resistance in the intercellular spaces (R _ias), an increase in the total surface area of chloroplasts facing the intercellular spaces per unit leaf area (S _c ), and an increase in construction and maintenance cost of the leaf. Leaves can increase S _c and decrease R _ias also by decreasing cell size. Leaves with smaller cells are mechanically stronger. However, actual leaves do not have very small cells. This could be because actual leaves exhibiting considerable rates of leaf area expansion, adequate heat capacitance, high efficiency of N and/or P use, etc, are favoured. Relationships between leaf longevity and mesophyll structure are also discussed. Received 20 September 2000/ Accepted in revised form 4 January 2001     

The Effect of Elevated Concentrations of Fructose 2,6-Bisphosphate on Carbon Metabolism during Deacidification in the Crassulacean Acid Metabolism Plant Kalanchöe daigremontiana

In C₃ plants, the metabolite fructose 2,6-bisphosphate (Fru 2,6-P₂) has an important role in the regulation of carbon partitioning during photosynthesis. To investigate the impact of Fru 2,6-P₂ on carbon metabolism during Crassulacean acid metabolism (CAM), we have developed an Agrobacterium tumefaciens-mediated transformation system in order to alter genetically the obligate CAM plant Kalanchöe daigremontiana. To our knowledge, this is the first report to use genetic manipulation of a CAM species to increase our understanding of this important form of plant metabolism. Transgenic plants were generated containing a modified rat liver 6-phosphofructo-2-kinase gene. In the plants analyzed the activity of 6-phosphofructo-2-kinase ranged from 175% to 198% of that observed in wild-type plants, resulting in Fru 2,6-P₂ concentrations that were 228% to 350% of wild-type plants after 2 h of illumination. A range of metabolic measurements were made on these transgenic plants to investigate the possible roles of Fru 2,6-P₂ during Suc, starch, and malic acid metabolism across the deacidification period of CAM. The results suggest that Fru 2,6-P₂ plays a major role in regulating partitioning between Suc and starch synthesis during photosynthesis. However, alterations in Fru 2,6-P₂ levels had little effect on malate mobilization during CAM fluxes.     

Modelling Plant Responses to Elevated CO2: How Important is Leaf Area Index?

BACKGROUND AND AIMS: The problem of increasing CO(2) concentration [CO(2)] and associated climate change has generated much interest in modelling effects of [CO(2)] on plants. While variation in growth and productivity is closely related to the amount of intercepted radiation, largely determined by leaf area index (LAI), effects of elevated [CO(2)] on growth are primarily via stimulation of leaf photosynthesis. Variability in LAI depends on climatic and growing conditions including [CO(2)] concentration and can be high, as is known for agricultural crops which are specifically emphasized in this report. However, modelling photosynthesis has received much attention and photosynthesis is often represented inadequately detailed in plant productivity models. Less emphasis has been placed on the modelling of leaf area dynamics, and relationships between plant growth, elevated [CO(2)] and LAI are not well understood. This Botanical Briefing aims at clarifying the relative importance of LAI for canopy assimilation and growth in biomass under conditions of rising [CO(2)] and discusses related implications for process-based modelling. MODEL: A simulation exercise performed for a wheat crop demonstrates recent experimental findings about canopy assimilation as affected by LAI and elevation of [CO(2)]. While canopy assimilation largely increases with LAI below canopy light saturation, effects on canopy assimilation of [CO(2)] elevation are less pronounced and tend to decline as LAI increases. Results from selected model-testing studies indicate that simulation of LAI is often critical and forms an important source of uncertainty in plant productivity models, particularly under conditions of limited resource supply. CONCLUSIONS: Progress in estimating plant growth and productivity under rising [CO(2)] is unlikely to be achieved without improving the modelling of LAI. This will depend on better understanding of the processes of substrate allocation, leaf area development and senescence, and the role of LAI in controlling plant adaptation to environmental changes.     

Amplectosporangium—A New Genus of Plant from the Lower Devonian of Sichuan,China

A new genus and species Amplectosporangium jiangyouense, is described from the Pingyipu Formation (Siegenian) of the Lower Devonian in Jiangyou district, Sichuan, China. The plant consists of naked dichotomizing erect axes with terminal fertile branch-systems. Sporangia are oval-shaped each with a short stalk and are arranged along the inner side of dichotomized axes in single row. By comparison with other known Devonian plants the genus is considered to represent a new morphological type of early land plants. Based on the theory of the telomic origin of the ovular integument, the plant may represent an early stage in the evolution of integument.     

Why are there so few fish in the sea?

The most dramatic gradient in global biodiversity is between marine and terrestrial environments. Terrestrial environments contain approximately 75-85% of all estimated species, but occupy only 30 per cent of the Earth's surface (and only approx. 1-10% by volume), whereas marine environments occupy a larger area and volume, but have a smaller fraction of Earth's estimated diversity. Many hypotheses have been proposed to explain this disparity, but there have been few large-scale quantitative tests. Here, we analyse patterns of diversity in actinopterygian (ray-finned) fishes, the most species-rich clade of marine vertebrates, containing 96 per cent of fish species. Despite the much greater area and productivity of marine environments, actinopterygian richness is similar in freshwater and marine habitats (15 150 versus 14 740 species). Net diversification rates (speciation-extinction) are similar in predominantly freshwater and saltwater clades. Both habitats are dominated by two hyperdiverse but relatively recent clades (Ostariophysi and Percomorpha). Remarkably, trait reconstructions (for both living and fossil taxa) suggest that all extant marine actinopterygians were derived from a freshwater ancestor, indicating a role for ancient extinction in explaining low marine richness. Finally, by analysing an entirely aquatic group, we are able to better sort among potential hypotheses for explaining the paradoxically low diversity of marine environments.     

Why are there four letters in the genetic alphabet?

We list, without thinking, the four base types that make up DNA as adenine, guanine, cytosine and thymine. But why are there four? This question is now all the more relevant as organic chemists have synthesized new base pairs that can be incorporated into nucleic acids. Here, I argue that there are theoretical, experimental and computational reasons to believe that having four base types is a frozen relic from the RNA world, when RNA was genetic as well as enzymatic material.     

Why are the genomes of endosymbiotic bacteria so stable?

The comparative analysis of three strains of the endosymbiotic bacterium Buchnera aphidicola has revealed high genome stability associated with an almost complete absence of chromosomal rearrangements and horizontal gene transfer events during the past 150 million years. The loss of genes involved in DNA uptake and recombination in the initial stages of endosymbiosis probably underlies this stability. Gene loss, which was extensive during the initial steps of Buchnera evolution, has continued in the different Buchnera lineages since their divergence.     

Why are literature data for H2O2 contents so variable? A discussion of potential difficulties in the quantitative assay of leaf extracts

Leaf metabolism produces H₂O₂ at high rates, but current conceptssuggest that the potent signalling effects of this oxidant requirethat concentrations be controlled by a battery of antioxidativeenzymes. The extent to which H₂O₂ is allowed to accumulate remainsunclear. There is little consensus on leaf H₂O₂ values in theliterature and measured concentrations in unstressed conditionsrange from 50–5000 nmol g^–1 fresh weight, a differencethat probably reflects technical inaccuracies as much as biologicalvariability. This article uses new experimental and literaturedata to examine some of the difficulties in accurately measuringH₂O₂ in leaf extracts. Potential problems relate to sensitivity,interference from other redox-active compounds, and H₂O₂ stabilityduring sample preparation. Particular attention is drawn tothe influence of tissue mass/extraction volume in the quantitativeestimation of H₂O₂ contents, and the possibility that this factorcould contribute to the variability of literature data. Key words: Ascorbate, chemiluminescence, oxidative stress, redox signalling, xylenol orange Received 4 June 2007; Revised 18 June 2007 Accepted 23 June 2007     

Critical Function of γH2A in S-Phase

Phosphorylation of histone H2AX by ATM and ATR establishes a chromatin recruitment platform for DNA damage response proteins. Phospho-H2AX (γH2AX) has been most intensively studied in the context of DNA double-strand breaks caused by exogenous clastogens, but recent studies suggest that DNA replication stress also triggers formation of γH2A (ortholog of γH2AX) in Schizosaccharomyces pombe. Here, a focused genetic screen in fission yeast reveals that γH2A is critical when there are defects in Replication Factor C (RFC), which loads proliferating cell nuclear antigen (PCNA) clamp onto duplex DNA. Surprisingly Chk1, Cds1/Chk2 and the Rad9-Hus1-Rad1 checkpoint clamp, which are crucial for surviving many genotoxins, are fully dispensable in RFC-defective cells. Immunoblot analysis confirms that Rad9-Hus1-Rad1 is not required for formation of γH2A by Rad3/ATR in S-phase. Defects in DNA polymerase epsilon, which binds PCNA in the replisome, also create an acute need for γH2A. These requirements for γH2A were traced to its role in docking with Brc1, which is a 6-BRCT-domain protein that is structurally related to budding yeast Rtt107 and mammalian PTIP. Brc1, which localizes at stalled replication forks by binding γH2A, prevents aberrant formation of Replication Protein A (RPA) foci in RFC-impaired cells, suggesting that Brc1-coated chromatin stabilizes replisomes when PCNA or DNA polymerase availability limits DNA synthesis.     

Why are Dormice rare? A case study in conservation biology

In the last 100 years, the Dormouse Muscardinus avellcmarius has disappeared from about half its geographical range in Britain. Evidence is presented which indicates that declining range and numbers are due to a complex interplay of factors which include fragmentation, deterioration and loss of specialized habitat. The Dormouse is unusual in being a relatively A-selected small mammal, with exacting ecological requirements which render it very vulnerable, particularly to habitat fragmentation. The Dormouse is also sensitive to climate, both directly and probably indirectly through the effects of weather on the timing and abundance of food (insects, flowers and fruits). Combined with low population density and low intrinsic rate of population increase, this makes the Dormouse highly vulnerable, not just to absolute climatic measures (e.g. temperature, rainfall), but especially to climatic stochasticity, particularly at the edge of its range. There are strong associations between the distribution and changing status of the Dormouse and various climatic parameters, and clear parallels with other climate-sensitive taxa, notably bats and butterflies.
The Dormouse is a very specialized species, highly sensitive to environmental change, resulting in its piecemeal, progressive extinction, particularly in northern counties. It is likely to be a very sensitive indicator species for monitoring future changing environments and an excellent model for studying the effects of habitat fragmentation, climatic shifts and climatic stochasticity.     

Why are both ends of the polypeptide chain on the outside of proteins?

Protein folding starts before the whole polypeptide has been synthesized by the ribosome. No matter how long the polypeptide is or how intricate the fold, both ends of the chain always end up on the surface. From a topological point of view, this is surprising; one would have expected to find the starting (N-terminal) end inside the core of the folded protein, just as in a ball of yarn. We suggest here that the reason for this apparent paradox is that the first amino acid of the emerging polypeptide chain is gripped during protein synthesis, perhaps by the ribosome, and is not released until the whole polypeptide has been synthesized. This binding would greatly decrease the degrees of freedom for the protein-folding process and could also explain why knots are so rare in proteins. Gripping would also guarantee that the N-terminal is accessible on the protein surface as required for binding of ubiquitin, which regulates the natural degradation of proteins and avoids buildup of protein aggregates, such as those found in Huntington's, Alzheimer's, Parkinson's, and other neurodegenerative diseases.     

A Critical Note on the Scientific Name of the Leguminous Plant“Lü Tou”

The Chinese name for the vegetable leguminous plant “LüTOU “or English,Green Gram is identified with the scientific name Phaseolus radiatus Linn.