What limits meiotic crossovers?

Comment on: Crismani W, et al. Science 2012; 336:1588-90.     

Discrepancy between nitrate reduction rates in intact leaves and nitrate reductase activity in leaf extracts: What limits nitrate reduction in situ?

Nitrate reductase (NR) activity in spinach leaf extracts prepared in the presence of a protein phosphatase inhibitor (50 μM cantharidine) was measured in the presence of Mg²⁺ (NRact) or EDTA (NRmax), under substrate saturation. These in-vitro activities were compared with nitrate reduction rates in leaves from nitrate-sufficient plants. Spinach leaves containing up to 60 μmol nitrate per g fresh weight were illuminated in air with their petiole in water. Their nitrate content decreased with time, permitting an estimation of nitrate reduction in situ. The initial rates (1–2 h) of nitrate consumption were usually lower than NRact, and with longer illumination time (4 h) the discrepancy grew even larger. When leaves were fed through their petiole with 30 mM nitrate, initial in-situ reduction rates calculated from nitrate uptake and consumption were still lower than NRact. However, nitrate feeding through the petiole maintained the in situ-nitrate reduction rate for a longer time. Initial rates of nitrate reduction in situ only matched NRact when leaves were illuminated in 5% CO₂. In CO₂-free air or in the dark, both NRact and in-situ nitrate reduction decreased, but NRact still exceeded in-situ reduction. More extremely, under anoxia or after feeding 5-amino-4-imidazole carboxyamide ribonucleoside in the dark, NR was activated to the high light level; yet in spite of that, nitrate reduction in the leaf remained very low. It was examined whether the standard assay for NRact would overestimate the in-situ rates due to a dissociation of the inactive phospho-NR-14-3-3 complex after extraction and dilution, but no evidence for that was found. In-situ NR obviously operates below substrate saturation, except in the light at high ambient CO₂. It is suggested that in the short term (2 h), nitrate reduction in situ is mainly limited by cytosolic NADH, and cytosolic nitrate becomes limiting only after the vacuolar nitrate pool has been partially emptied. Received: 19 June 1999 / Accepted: 12 October 1999     

What proportion of household dust is derived from outdoor soil?

This report estimated the amount of outdoor soil in indoor dust in the Calabrese et al. (1989) children soil ingestion study via the use of statistical modelling. The estimate used data on outdoor soil and indoor dust in the homes of 60 children with eight tracer elements (Al, Ba, Mn, Si, Ti, V, Y, and Zr). The model estimated that 31.3% of indoor dust had an origin of outdoor soil. Based on a previous report (Stanek and Calabrese, 1992) on differential soil from dust ingestion in the Calabrese et al. (1989) study and the data of the present analysis, the median outdoor soil ingestion of the Calabrese et al. (1989) study should be revised downward by 35%. For the three most reliable tracers, the median soil ingestion estimates would be reduced from 29 to 19 mg/d for Al, 55 to 36 mg/d for Ti, and 16 to 10 mg/d for, Zr.     

What limits the Serengeti zebra population?

The populations of the ecologically dominant ungulates in the Serengeti ecosystem (zebra, wildebeest and buffalo) have shown markedly different trends since the 1960s: the two ruminants both irrupted after the elimination of rinderpest in 1960, while the zebras have remained stable. The ruminants are resource limited (though parts of the buffalo population have been limited by poaching since the 1980s). The zebras resource acquisition tactics should allow them to outcompete the ruminants, but their greater spatial dispersion makes them more available to predators, and it has been suggested that this population is limited by predation. To investigate the mechanisms involved in the population dynamics of Serengeti zebra, we compared population dynamics among the three species using demographic models based on age-class-specific survival and fecundity. The only major difference between zebra and the two ruminants occurred in the first-year survival. We show that wildebeest have a higher reproductive potential than zebra (younger age at first breeding and shorter generation time). Nevertheless, these differences in reproduction cannot account for the observed differences in the population trends between the zebra and the ruminants. On the other hand, among-species differences in first-year survival are great enough to account for the constancy of zebra population size. We conclude that the very low first-year survival of zebra limits this population. We provide new data on predation in the Serengeti and show that, as in other ecosystems, predation rates on zebras are high, so predation could hold the population in a predator pit. However, lion and hyena feed principally on adult zebras, and further work is required to discover the process involved in the high mortality of foals.     

Abiotic nitrate incorporation in soil: is it real?

In acid forest soils nitrate (NO₃⁻) from anthropogenic nitrogen deposition is retained at levels beyond what can be explained by known biological mechanisms. A number of researchers have hypothesized that abiotic NO₃⁻ incorporation into soil organic matter might be responsible for this phenomenon, however studies have been limited to a few temperate forest sites. The goal of this study was to determine if abiotic NO₃⁻ incorporation is important across a wide range of soil types. We collected 44 soils from a number of different ecosystem types in North and South America and measured the extent of abiotic NO₃⁻ incorporation. Significant abiotic nitrate incorporation did not occur in any of the soils examined. We show that the apparent abiotic incorporation observed in previous studies is likely the result of iron interference with NO₃⁻ measurements. Our results suggest that abiotic NO₃⁻ incorporation is not a likely explanation for the high rates of NO₃⁻ retention observed in some ecosystems.     

What limits the evolutionary emergence of pathogens?

The ability of a pathogen to cause an epidemic when introduced in a new host population often relies on its ability to adapt to this new environment. Here, we give a brief overview of recent theoretical and empirical studies of such evolutionary emergence of pathogens. We discuss the effects of several ecological and genetic factors that may affect the likelihood of emergence: migration, life history of the infectious agent, host heterogeneity, and the rate and effects of mutations. We contrast different modelling approaches and indicate how details in the way we model each step of a life cycle can have important consequences on the predicted probability of evolutionary emergence. These different theoretical perspectives yield important insights into optimal surveillance and intervention strategies, which should aim for a reduction in the emergence (and re-emergence) of infectious diseases.     

What in earth? Analysing soil bacterial communities

Walk into any field and you will at once see dozens of plant and animal species. However, below your feet and largely unnoticed, lies a population of unseen millions. Can the introduction of powerful molecular biological techniques increase our understanding of how these hidden bacterial communities function in their natural environment?     

Increased plant density of winter wheat can enhance nitrogen–uptake from deep soil

Background and Aims

Increased plant density improves grain yield and nitrogen (N)–use efficiency in winter wheat (Triticum aestivum L.) by increasing the root length density (RLD) in the soil and aboveground N–uptake (AGN) at maturity. However, how the root distribution and N–uptake at different soil depths is affected by plant density is largely unknown.

Methods

A 2–year field study using the winter wheat cultivar Tainong 18 was conducted by injecting ¹⁵?N–labeled urea into soil at depths of 0.2, 0.6, and 1.0 m under four plant densities of 135 m^?2, 270 m^?2,405 m^?2, and 540 m^?2.

Results

We observed significant RLD and ¹⁵?N–uptake increases at each soil depth as the plant density increased from 135 to 405 m^?2. ¹⁵?N–uptake increased with plant density as the soil depth increased, although the corresponding RLD value fell with depth. The ¹⁵?N–uptake at each soil depth was positively related to the RLD at the same depth. The total AGN was positively related to RLD in deep soil, especially at 0.8–1.2 m.

Conclusions

Increasing the plant density from 135 m^?2 to the optimum increases AGN primarily by increasing the RLD in deep soil and therefore increasing the plant density of winter wheat can be used to efficiently recover N leached to deep soil. Moreover, the total root numbers per unit area and RLD still increased at supraoptimal density while shoot number and N uptake stagnated.     

What distinguishes tenascin from fibronectin?

Tenascin and fibronectin are two major extracellular matrix glycoproteins. They both consist of large disulfide-linked subunits composed of multiple structural domains. More than half of each molecule consists of so-called fibronectin type III repeats, but the other domains differ. Fibronectin is a dimer, whereas tenascin is a hexamer. Often fibronectin and tenascin are colocalized in tissues, but the occurrence of tenascin is much more restricted when compared with fibronectin. Tenascin is transiently expressed in many developing organs such as connective tissues, the mesenchyme of epithelial organs, and also the central and peripheral nervous systems, and it reappears in the stroma of many tumors. The distinctive and highly regulated expression of tenascin has provoked interest in trying to identify possible functions of tenascin in cell-cell and cell-substratum adhesion, cell migration, growth, and cell differentiation during morphogenesis.     

What information is conveyed by an ABA signal from maize roots in drying field soil?

During two seasons, ABA concentrations were monitored in roots, leaves and xylem sap of field-grown maize. The water status of soil and plant was also measured. Plants were grown on plots with compacted or non-compacted soil, which were irrigated or remained unwatered. ABA concentration in the xylem sap before dawn and in the roots increases 25-fold and five-fold, respectively, as the soil dried, with a close correlation with the soil water status, but with no clear effect of the soil structure. In contrast to the results of several laboratory experiments, no appreciable increase in xylem [ABA] and reduction in stomatal conductance were observed with dehydration of the part of the root system located in soil upper layers. These responses only occurred when the water reserve of the whole soil profile was close to depletion and the transpiration declined. Xylem [ABA] measured during the day was appreciably higher in the compacted treatment than in non-compacted treatment, unlike that measured before dawn. Since a mechanical message is unlikely to undergo such day-night alterations, we suggest that this was due to a faster decrease in root water potential and water flux in the compacted treatment, linked to the root spatial arrangement. These results raise the possibility that ABA concentration in the xylem sap could be controlled by two coexisting mechanisms: (1) the rate of ABA synthesis in the roots linked to the soil or root water status, as shown in laboratory experiments; (2) the dilution of ABA in the water flow from roots, which could be an overriding mechanism in field conditions. This second mechanism would allow the plant to sense the water flux through the root system.     

What limits herb biomass in grasslands: competition or herbivory?

Competition and herbivory are two of the main forces shaping plant communities. Although several studies have investigated their impact on plant populations separately, few investigations have examined how they might interact. With the purpose of clarifying the combined roles of competition and herbivory on herb biomass in a grassland, we assessed the effects of different herbivores with reduced grass competition. We conducted a field experiment in 2000-2001 in a British acid grassland (Oak Mead), where we experimentally reduced grass biomass and excluded rabbits, insects and molluscs in a factorial design. Removing the grasses from Oak Mead dramatically increased herb biomass and total above-ground biomass. Herbivore exclusions (i.e. rabbits, insects and molluscs) did not affect total above-ground biomass, but they altered relative abundance of several species. Grass removal and rabbit exclusion had positive interactions on biomass of several herb species, and there were some subtle interactions between different herbivore groups: monocots benefitted when both rabbits and molluscs were excluded, and some herb species had greater biomass when insects and rabbits were absent. We then compared the results with a 10-year experiment that manipulated similar variables in neighbouring grassland (Nashs Field). The comparison between Oak Mead and Nashs Field showed that cessation of herbicide application returns the system to its previous state of grass dominance after 3 years. Therefore, even when herbs were more abundant, they could not prevent reinvasion of the grasses once external factors were removed.     

What limits the velocity of fast-skeletal muscle contraction in mammals?

In rat skeletal muscle the unloaded shortening velocity (Vo) is defined by the myosin isoform expressed in the muscle fibre. In 2001 we suggested that ADP release from actomyosin in solution (controlled by k(-AD)) was of the right size to limit Vo. However, to compare mechanical and solution kinetic data required a series of corrections to compensate for the differences in experimental conditions (0.5 M KCl, 22 degrees C for kinetic assays of myosin, 200 mM ionic strength, 12 degrees C to measure Vo). Here, a method was developed to prepare heavy meromyosin (HMM) from pure myosin isoforms isolated from single muscle fibres and to study k(-AD) (determined from the affinity of the acto-myosin complex for ADP, KAD) and the rate of ATP-induced acto-HMM dissociation (controlled by K1k+2) under the same experimental condition used to measure Vo). In fast-muscle myosin isolated from a wide range of mammalian muscles, k(-AD) was found to be too fast to limit Vo, whereas K1k+2 was of the right magnitude for ATP-induced dissociation of the cross-bridge to limit shortening velocity. The result was unexpected and prompted further experiments using the stopped-flow approach on myosin subfragment-1 (S1) and HMM obtained from bulk preparations of rabbit and rat muscle. These confirmed that the rate of cross-bridge dissociation by ATP limits the velocity of contraction for fast myosin II isoforms at 12 degrees C, while k(-AD) limits the velocity of slow myosin II isoforms. Extrapolating our data to 37 degrees C suggests that at physiological temperature the rate of ADP dissociation may limit Vo for both isoforms.     

What stabilizes biological soil crusts in the Negev Desert?

Aims

Biological soil crusts (biocrusts) are widespread in many drylands, where plant growth is limited due to water scarcity. One of their most important functions is the stabilization of the topsoil, particularly in regions with sandy soils prone to desertification. Since the mechanisms playing a role in soil stabilization are poorly understood, this study aims to shed light on the connection between crust stability and different cementing agents.

Methods

We measured the penetration resistance and the concentrations of different cementing agents of biocrusts in the Israeli Negev Desert. Structural equation modelling was performed to examine the direct and indirect effects of the variables analyzed and identify variables that are best able to explain the observed patterns of penetration resistance.

Results

All observed variables showed a high variability within and between sites. Structural equation modelling revealed that the main parameters explaining penetration resistance are the content of fines and the electrical conductivity, while carbonates and organic carbon only have an indirect effect.

Conclusions

Our results suggest that adding silt and clay to (natural or induced) biocrusts is very likely to produce stronger, more stable crusts, which will be more effective in combating desertification and improve their ability to survive trampling by livestock.

     

What have we learnt from SARS?

With outbreaks of infectious disease emerging from animal sources, we have learnt to expect the unexpected. We were, and are, expecting a new influenza A pandemic, but no one predicted the emergence of an unknown coronavirus (CoV) as a deadly human pathogen. Thanks to the preparedness of the international network of influenza researchers and laboratories, the cause of severe acute respiratory syndrome (SARS) was rapidly identified, but there is no complacency over the global or local management of the epidemic in terms of public health logistics. The human population was lucky that only a small proportion of infected persons proved to be highly infectious to others, and that they did not become so before they felt ill. These were the features that helped to make the outbreak containable. The next outbreak of another kind of transmissible disease may well be quite different.     

What limits production of unusual monoenoic fatty acids in transgenic plants?

Unusual monounsaturated fatty acids are major constituents (greater than 80%) in seeds of Coriandrum sativum L. (coriander) and Thunbergia alata Bojer, as well as in glandular trichomes (greater than 80% derived products) of Pelargonium x hortorum (geranium). These diverged fatty acid structures are produced via distinct plastidial acyl-acyl carrier protein (ACP) desaturases. When expressed in Arabidopsis thaliana (L.) Heynh. under strong seed-specific promoters the unusual acyl-ACP desaturases resulted in accumulation of unusual monoene fatty acids at 1-15% of seed fatty acid mass. In this study, we have examined several factors that potentially limit higher production of unusual monoenes in transgenic oilseeds. (i) Immunoblots indicated that the introduced desaturases were expressed at levels equivalent to or higher than the endogenous delta9 18:0-ACP desaturase. However, the level of unusual fatty acid produced in transgenic plants was not correlated with the level of desaturase expression. (ii) The unusual desaturases were expressed in several backgrounds, including antisense 18:0-ACP desaturase plants, in fab1 mutants, and co-expressed with specialized ACP or ferredoxin isoforms. None of these experiments led to high production of expected products. (iii) No evidence was found for degradation of the unusual fatty acids during seed development. (iv) Petroselinic acid added to developing seeds was incorporated into triacylglycerol as readily as oleic acid, suggesting no major barriers to its metabolism by enzymes of glycerolipid assembly. (v) In vitro and in situ assay of acyl-ACP desaturases revealed a large discrepancy of activity when comparing unusual acyl-ACP desaturases with the endogenous delta9 18:0-ACP desaturase. The combined results, coupled with the sensitivity of acyl-ACP desaturase activity to centrifugation and low salt or detergent suggests low production of unusual monoenes in transgenic plants may be due to the lack of, or incorrect assemble of, a necessary multi-component enzyme association.