Application of controlled mesocosms for understanding mercury air-soil-plant exchange.

Whole system elemental mercury (Hg0) flux was measured for approximately 1.5 years using two large gas exchange mesocosms containing approximately 100 two-year old aspen trees (Populus tremuloides) planted in soil with elevated mercury concentrations (12.3 microg/g). We hypothesized that during leafout, whole mesocosm Hg0 flux would increase due to movement of Hg0 in the transpiration stream from the soil to the air. This hypothesis was not supported; plants were found to assimilate Hg0 from the contaminated air, and whole system Hg0 emissions were reduced as plants leafed-out due to shading of the soil. Surface disturbance, watering, and increases in soil moisture, light, and temperature were all found to increase whole system Hg0 flux, with light being a more significant factor. Although surface soils were maintained at 15-20% moisture, daily watering caused pulses of Hg0 to be released from the soil throughout the experiment. Data developed in this experiment suggested that those processes acting on the soil surface are the primary influence on Hg emissions and that the presence of vegetation, which shields soil surfaces from incident light, reduces Hg emissions from enriched soils.

Responsiveness to a pandemic alert: use of reverse genetics for rapid development of influenza vaccines.

BACKGROUND: In response to the emergence of severe infection capable of rapid global spread, WHO will issue a pandemic alert. Such alerts are rare; however, on Feb 19, 2003, a pandemic alert was issued in response to human infections caused by an avian H5N1 influenza virus, A/Hong Kong/213/03. H5N1 had been noted once before in human beings in 1997 and killed a third (6/18) of infected people. The 2003 variant seemed to have been transmitted directly from birds to human beings and caused fatal pneumonia in one of two infected individuals. Candidate vaccines were sought, but no avirulent viruses antigenically similar to the pathogen were available, and the isolate killed embryonated chicken eggs. Since traditional strategies of vaccine production were not viable, we sought to produce a candidate reference virus using reverse genetics. METHODS: We removed the polybasic aminoacids that are associated with high virulence from the haemagglutinin cleavage site of A/Hong Kong/213/03 using influenza reverse genetics techniques. A reference vaccine virus was then produced on an A/Puerto Rico/8/34 (PR8) backbone on WHO-approved Vero cells. We assessed this reference virus for pathogenicity in in-vivo and in-vitro assays. FINDINGS: A reference vaccine virus was produced in Good Manufacturing Practice (GMP)-grade facilities in less than 4 weeks from the time of virus isolation. This virus proved to be non-pathogenic in chickens and ferrets and was shown to be stable after multiple passages in embryonated chicken eggs. INTERPRETATION: The ability to produce a candidate reference virus in such a short period of time sets a new standard for rapid response to emerging infectious disease threats and clearly shows the usefulness of reverse genetics for influenza vaccine development. The same technologies and procedures are currently being used to create reference vaccine viruses against the 2004 H5N1 viruses circulating in Asia.

Molecular, physiological, and growth responses to sodium stress in C4 grasses from a soil salinity gradient in the Serengeti ecosystem.

The concentration of soil sodium (Na) is an important factor that influences species distribution in the Serengeti short-grass plains, Tanzania. Experiments were conducted to characterize physiological (growth, photosynthetic, nutrients, and water relations) and molecular (heat shock proteins and organic solutes) responses to high soil sodium in four Serengeti C(4) grasses. The species tested were Andropogon greenwayi and three species of Sporobulus, S. ioclados, S. kentrophyllus and S. spicatus. Andropogon greenwayi occurs on locations with low soil Na concentrations, S. ioclados on low to moderate, S. kentrophyllus moderate to high, and S. spicatus on soils with high Na concentration.Among all four species, short-term physiological and molecular responses to Na treatments (0, 100, and 400 mmol/L Na) were correlated with their field soil Na concentrations. Sporobulus kentrophyllus and S. spicatus exhibited rapid molecular induction of heat shock proteins in response to experimental soil Na treatments within 24 h and had increased levels of proline within 96 h in contrast to A. greenwayi and S. ioclados. Photosynthetic rates and water relations were positively correlated with field soil Na concentrations and Hsp induction was clearly associated with photosynthetic tolerance. Long-term (6 wk) responses of the four species to Na treatment were consistent with the short-term responses to Na. Species that occur on low Na soils in the field did not survive past week 1 when treated with 400 mmol/L Na and exhibited significant reductions in biomass when treated with 100 mmol/L Na. Reduced biomass was associated with increased shoot tissue Na concentrations, and thus Na tolerance correlated with the Na concentrations of field leaf tissue. The results demonstrate that the community distribution of these species reflects their Na tolerance and that the observed physiological and molecular responses in tolerant species may have adaptive significance.

Heat-shock proteins are induced in unstressed leaves of Nicotiana attenuata (Solanaceae) when distant leaves are stressed.

In Nicotiana attenuata, systemic induction of heat-shock proteins (Hsps) was detected in response to the treatment of single leaves by either heat shock, mechanical damage, or exogenous application of methyl jasmonate (MJ). All treatments increased the abundance of members of the 70-kD Hsp (Hsp70) family and induced synthesis of one or more of the small Hsps (sHsp) (16-23 kD) in both treated and untreated leaves. These results provide the first evidence that Hsps can be systemically induced in plants and suggest that systemic induction of Hsps may be important in pre-adapting leaves to stress.

Elevated CO2 increases productivity and invasive species success in an arid ecosystem.

Arid ecosystems, which occupy about 20% of the earth's terrestrial surface area, have been predicted to be one of the most responsive ecosystem types to elevated atmospheric CO2 and associated global climate change. Here we show, using free-air CO2 enrichment (FACE) technology in an intact Mojave Desert ecosystem, that new shoot production of a dominant perennial shrub is doubled by a 50% increase in atmospheric CO2 concentration in a high rainfall year. However, elevated CO2 does not enhance production in a drought year. We also found that above-ground production and seed rain of an invasive annual grass increases more at elevated CO2 than in several species of native annuals. Consequently, elevated CO2 might enhance the long-term success and dominance of exotic annual grasses in the region. This shift in species composition in favour of exotic annual grasses, driven by global change, has the potential to accelerate the fire cycle, reduce biodiversity and alter ecosystem function in the deserts of western North America.

The small, methionine-rich chloroplast heat-shock protein protects photosystem II electron transport during heat stress.

Evidence suggests that the small chloroplast heat-shock protein (Hsp) is involved in plant thermotolerance but its site of action is unknown. Functional disruption of this Hsp using anti-Hsp antibodies or addition of purified Hsp to chloroplasts indicated that (a) this Hsp protects thermolabile photosystem II and, consequently, whole-chain electron transport during heat stress; and (b) this Hsp completely accounted for heat acclimation of electron transport in pre-heat-stressed plants. Therefore, this Hsp is a major adaptation to acute heat stress in plants.

Evidence that abscisic acid does not regulate a centralized whole-plant response to low soil-resource availability.

It has been suggested that abscisic acid (ABA) regulates a centralized response of plants to low soil resource availability that is characterized by decreased shoot growth relative to root growth, decreased photosynthesis and stomatal conductance, and decreased plant growth rate. The hypothesis was tested that an ABA-deficient mutant of tomato (flacca; flc) would not exhibit the same pattern of down-regulation of photosynthesis, conductance, leaf area and growth, as well as increased root/shoot partitioning, as its near isogenic wild-type in response to nitrogen or water deficiency, or at least not exhibit these responses to the same degree. Plants were grown from seed in acid-washed sand and exposed to control, nutrient stress, or water stress treatments. Additionally, exogenous ABA was sprayed onto the leaves of a separate group of flc individuals in each treatment. Growth analysis, based on data from frequent harvests of a few individuals, was used to assess the growth and partitioning responses of plants, and gas exchange characteristics were measured on plants throughout the experiment to examine the response of photosynthesis and stomatal conductance. Differences in growth, partitioning and gas exchange variables were found between flc and wild-type individuals, and both nutrient and water treatments caused significant reductions in relative growth rate (RGR) and changes in biomass partitioning. Only the nutrient treatment caused significant reductions in photosynthetic rates. However, flc and wild-type plants responded identically to nutrient and water stress for all but one of the variables measured. The exception was that flc showed a greater decrease in the relative change in leaf area per unit increase of plant biomass (an estimate of the dynamics of leaf area ratio) in response to nutrient stress-a result that is opposite to that predicted by the centralized stress response model. Furthermore, addition of exogenous ABA to flc did not significantly alter any of the responses to nutrient and water stress that we examined. Although it was clear that ABA regulated short-term stomatal responses, we found no evidence to support a pivotal role for ABA, at least absolute amounts of ABA, in regulating a centralized whole-plant response to low soil resource availability.

Why it matters where on a leaf a folivore feeds.

Because the tip of many dicot leaves matures and ceases expansion well before the base, we predicted that the removal of a given amount of leaf tissue from the base of an expanding leaf would result in greater reductions in final leaf area and overall plant performance than removal of the same amount of tissue from the tip of an expanding leaf or from either the base or tip of mature, fully expanded leaves. We tested this notion by removing a circular 3.9 cm2 hole from either the base or tip of rapidly expanding leaves (20-30% expanded, two nodes from the apex) or nearly fully expanded, mature leaves (85-100% expanded, five nodes from the apex) of tobacco (Nicotiana tabaccum) and measuring the final area of the hole, the final area of the fully expanded damaged leaf, and the number and mass of fruits produced by a plant. A given amount of area removed from the base of an expanding leaf resulted in almost 4 times the amount of visible damage than occurred when the same amount of damage was applied to the tip or base of a mature leaf and over twice the amount of visible damage than occurred on the tip of an expanding leaf. Furthermore, damage to the base of an expanding leaf resulted in nearly a 40% reduction in the final area that the leaf would have achieved without damage and a 35% reduction in the number and mass of fruits produced. These results not only suggest that where on a leaf a folivore feeds has consequences to the ultimate area that a leaf can reach and to overall plant performance, but they also have strong implications for a number of research areas in plant-herbivore interactions. For example, these data show that a lack of consideration of leaf developmental patterns can result in gross overestimates of consumption by folivores and severe under-estimates of the effect of folivory on leaf area display.

Interpreting phenotypic variation in plants.

Plant ecologists and evolutionary biologists frequently examine patterns of phenotypic variation across variable environments or genetic identities. Too often, we ignore the fact that most phenotypic traits change throughout growth and development of individual plants, and that rates of growth and development are highly variable. Plants growing in different environments are likely to grow at different rates, and will be of different sizes and stages of development at a particular age. When we compare plants as a function of plant size or developmental stage, as well as a function of age, we broaden our understanding of phenotypic variation between plants.

Elevated CO2 and plant nitrogen-use: is reduced tissue nitrogen concentration size-dependent?

Plants often respond to elevated atmospheric CO2 levels with reduced tissue nitrogen concentrations relative to ambient CO2-grown plants when comparisons are made at a common time. Another common response to enriched CO2 atmospheres is an acceleration in plant growth rates. Because plant nitrogen concentrations are often highest in seedlings and subsequently decrease during growth, comparisons between ambient and elevated CO2-grown plants made at a common time may not demonstrate CO2-induced reductions in plant nitrogen concentration per se. Rather, this comparison may be highlighting differences in nitrogen concentration between bigger, more developed plants and smaller, less developed plants. In this study, we directly examined whether elevated CO2 environments reduce plant nitrogen concentrations independent of changes in plant growth rates. We grew two annual plant species. Abutilon theophrasti (C3 photosynthetic pathway) and Amaranthus retroflexus (C4 photosynthetic pathway), from seed in glass-sided growth chambers with atmospheric CO2 levels of 350 µmol·mol-1 or 700 µmol·mol-1 and with high or low fertilizer applications. Individual plants were harvested every 2 days starting 3 days after germination to determine plant biomass and nitrogen concentration. We found: 1. High CO2-grown plants had reduced nitrogen concentrations and increased biomass relative to ambient CO2-grown plants when compared at a common time; 2. Tissue nitrogen concentrations did not vary as a function of CO2 level when plants were compared at a common size; and 3. The rate of biomass accumulation per rate of increase in plant nitrogen was unaffected by CO2 availability, but was altered by nutrient availability. These results indicate that a CO2-induced reduction in plant nitrogen concentration may not be due to physiological changes in plant nitrogen use efficiency, but is probably a size-dependent phenomenon resulting from accelerated plant growth.

Monitoring the AIDS epidemic in the United States: a network approach.

Respondents in the 1988 General Social Survey (GSS) were asked to scan their acquaintance networks to identify all those who had been a victim of a homicide or had acquired immunodeficiency syndrome (AIDS). Estimates of the sex, race, age, and regional breakdowns for homicides in the last year and for people with AIDS were compared with official statistics. The GSS estimates for the distribution of homicide victims replicate the official statistics quite well. The GSS estimates for AIDS cases suggest that the data provided to the Centers for Disease Control may underestimate by a substantial margin the prevalence of AIDS in the white population of higher socioeconomic status, overstate the relative prevalence of the disease in the minority populations, underestimate the prevalence of the disease in the Midwest, and overstate it for the East.

Regulation of acetyl-CoA synthetase of Saccharomyces cerevisiae.

Acetyl-coenzyme A synthetase (EC 6.2.1.1) activity of Saccharomyces cerevisiae was determined by a radioactive assay procedure. The activity in vitro was inhibited significantly by NADPH, NADH, or AMP and to a lesser extent by NADP, NAD, or ADP. Glutamic acid and alpha-ketoglutaric acid were not inhibitory. The enzyme level was repressed when the cells were grown in a complex nutrient medium as opposed to the minimal medium. However, a glutamic acid auxotroph glul, when grown in excess glutamic acid, demonstrated a fivefold increase of acetyl-CoA synthetase.

Regulation of citrate synthase activity of Saccharomyces cerevisiae.

Citrate synthase activity of Saccharomyces cerevisiae was determined by a radioactive assay procedure and the reaction product, 14C-citric acid, was identified by chromatographic techniques. ATP, d-ATP, GTP and NADH were most inhibitory to the citrate synthase invitro. The activity was inhibited to a lesser extent by ADP, UTP, and NADP whereas, AMP and CTP were much less inhibitory. NADH, like NAD, glutamic acid, glutamine, arginine, ornithine, proline, aspartic acid and alpha-ketoglutarate exhibited no inhibition. These results have been discussed in the light of the role of citrate synthase for the energy metabolism and glutamic acid biosynthesis.