Maize streak virus: an old and complex 'emerging' pathogen

Maize streak virus (MSV; Genus Mastrevirus, Family Geminiviridae) occurs throughout Africa, where it causes what is probably the most serious viral crop disease on the continent. It is obligately transmitted by as many as six leafhopper species in the Genus Cicadulina, but mainly by C. mbila Naudé and C. storeyi. In addition to maize, it can infect over 80 other species in the Family Poaceae. Whereas 11 strains of MSV are currently known, only the MSV‐A strain is known to cause economically significant streak disease in maize. Severe maize streak disease (MSD) manifests as pronounced, continuous parallel chlorotic streaks on leaves, with severe stunting of the affected plant and, usuallly, a failure to produce complete cobs or seed. Natural resistance to MSV in maize, and/or maize infections caused by non‐maize‐adapted MSV strains, can result in narrow, interrupted streaks and no obvious yield losses. MSV epidemiology is primarily governed by environmental influences on its vector species, resulting in erratic epidemics every 3–10 years. Even in epidemic years, disease incidences can vary from a few infected plants per field, with little associated yield loss, to 100% infection rates and complete yield loss. Taxonomy: The only virus species known to cause MSD is MSV, the type member of the Genus Mastrevirus in the Family Geminiviridae. In addition to the MSV‐A strain, which causes the most severe form of streak disease in maize, 10 other MSV strains (MSV‐B to MSV‐K) are known to infect barley, wheat, oats, rye, sugarcane, millet and many wild, mostly annual, grass species. Seven other mastrevirus species, many with host and geographical ranges partially overlapping those of MSV, appear to infect primarily perennial grasses. Physical properties: MSV and all related grass mastreviruses have single‐component, circular, single‐stranded DNA genomes of approximately 2700 bases, encapsidated in 22 × 38‐nm geminate particles comprising two incomplete T = 1 icosahedra, with 22 pentameric capsomers composed of a single 32‐kDa capsid protein. Particles are generally stable in buffers of pH 4–8. Disease symptoms: In infected maize plants, streak disease initially manifests as minute, pale, circular spots on the lowest exposed portion of the youngest leaves. The only leaves that develop symptoms are those formed after infection, with older leaves remaining healthy. As the disease progresses, newer leaves emerge containing streaks up to several millimetres in length along the leaf veins, with primary veins being less affected than secondary or tertiary veins. The streaks are often fused laterally, appearing as narrow, broken, chlorotic stripes, which may extend over the entire length of severely affected leaves. Lesion colour generally varies from white to yellow, with some virus strains causing red pigmentation on maize leaves and abnormal shoot and flower bunching in grasses. Reduced photosynthesis and increased respiration usually lead to a reduction in leaf length and plant height; thus, maize plants infected at an early stage become severely stunted, producing undersized, misshapen cobs or giving no yield at all. Yield loss in susceptible maize is directly related to the time of infection: infected seedlings produce no yield or are killed, whereas plants infected at later times are proportionately less affected. Disease control: Disease avoidance can be practised by only planting maize during the early season when viral inoculum loads are lowest. Leafhopper vectors can also be controlled with insecticides such as carbofuran. However, the development and use of streak‐resistant cultivars is probably the most effective and economically viable means of preventing streak epidemics. Naturally occurring tolerance to MSV (meaning that, although plants become systemically infected, they do not suffer serious yield losses) has been found, which has primarily been attributed to a single gene, msv‐1. However, other MSV resistance genes also exist and improved resistance has been achieved by concentrating these within individual maize genotypes. Whereas true MSV immunity (meaning that plants cannot be symptomatically infected by the virus) has been achieved in lines that include multiple small‐effect resistance genes together with msv‐1, it has proven difficult to transfer this immunity into commercial maize genotypes. An alternative resistance strategy using genetic engineering is currently being investigated in South Africa. Useful websites: 〈 http://www.mcb.uct.ac.za/MSV/mastrevirus.htm 〉; 〈 http://www.danforthcenter.org/iltab/geminiviridae/geminiaccess/mastrevirus/Mastrevirus.htm 〉.     

Reorganization of membrane lipids during fast and slow cold hardening in Drosophila melanogaster

Abstract Rapid cold hardening is a naturally occurring phenomenon in insects that is thought to be responsible for increased cold tolerance during diurnal variations in temperature. The underlying physiological mechanisms are still not fully resolved but, in Drosophila melanogaster (Meigen 1830), rapid cold hardening is accompanied by specific changes in the membrane lipid composition. To further understand the link between rapid cold hardening and adjustments in the membrane lipid composition, the present study investigates how different rates of cooling affect thermotolerance and the composition of phospholipid fatty acids. Female Drosophila are cooled gradually from 25 to 0 °C at 0.01, 0.05, 0.1 or 0.5 °C min^?1, respectively, and, subsequently, phospholipid fatty acid composition and survival after a 1‐h cold shock at ?5 °C is measured. The rapid cold hardening treatments all influence cold tolerance differently so that short and intermediate rapid cold hardening treatments (0.05, 0.1 or 0.5 °C min^?1 cooling rates) increase cold shock survival, whereas the slow cooling treatment (0.01 °C min^?1) decreases survival relative to an untreated control. The intermediate rapid cold hardening treatments (0.05 or 0.1 °C min^?1) induce a similar type of response characterized by an increase in the molar percentage of linoleic acid, 18:2(n‐6), at the expense of 16:0 and 18:1(n‐9), which leads to an increase in the degree of unsaturation. The slowest cooling treatment (0.01 °C min^?1) results in a large increase in cis‐16:1(n‐7) and significant reductions in the saturated phospholipid fatty acids 16:0, 18:0 and the unsaturated 16:1(n‐9) and 18:2(n‐6) fatty acids. These changes cause a slight decrease in the average length of the phospholipid fatty acids and an increase in the overall ratio of unsaturated vs. saturated fatty acids. These findings demonstrate that the rate of cooling is important for both the reorganization of membrane lipids, and for the degree of acquired cold tolerance during rapid cold hardening, and they suggest an important role for rapid cold hardening during diurnal rather than seasonal temperature changes.     

Cryobanking of viable biomaterials: implementation of new strategies for conservation purposes

Cryobanking, the freezing of biological specimens to maintain their integrity for a variety of anticipated and unanticipated uses, offers unique opportunities to advance the basic knowledge of biological systems and their evolution. Notably, cryobanking provides a crucial opportunity to support conservation efforts for endangered species. Historically, cryobanking has been developed mostly in response to human economic and medical needs — these needs must now be extended to biodiversity conservation. Reproduction technologies utilizing cryobanked gametes, embryos and somatic cells are already vital components of endangered species recovery efforts. Advances in modern biological research (e.g. stem cell research, genomics and proteomics) are already drawing heavily on cryobanked specimens, and future needs are anticipated to be immense. The challenges of developing and applying cryobanking for a broader diversity of species were addressed at an international conference held at Trier University (Germany) in June 2008. However, the magnitude of the potential benefits of cryobanking stood in stark contrast to the lack of substantial resources available for this area of strategic interest for biological science — and society at large. The meeting at Trier established a foundation for a strong global incentive to cryobank threatened species. The establishment of an Amphibian Ark cryobanking programme offers the first opportunity for global cooperation to achieve the cryobanking of the threatened species from an entire vertebrate class.     

Habitat selection of the globally threatened Aquatic Warbler Acrocephalus paludicola at the western margin of its breeding range and implications for management

The globally threatened Aquatic Warbler Acrocephalus paludicola is an umbrella species for fen mires and is at risk of extinction in its westernmost breeding population due to severe habitat loss. We used boosted regression trees to model Aquatic Warbler habitat selection in order to make recommendations for effective management of the last remnant habitats. Habitat data were collected in the years 2004–2006 in all remaining breeding sites in Pomerania (eastern Germany and western Poland) as well as in recently abandoned sites. Models were validated using data from similar Aquatic Warbler habitats in Lithuania. The probability of occurrence of Aquatic Warblers in late May/early June was positively associated with low isolation from other occupied sites, less eutrophic conditions, a high proportion of area mown early in the preceding year, high availability of vegetation 60–70 cm high, high prey abundance and high habitat heterogeneity. Early summer land management is needed in the more productive sites to prevent habitat deterioration by succession to higher and denser vegetation. As this also poses a serious threat to broods, management that creates a mosaic of early and late used patches is recommended to preserve and restore productive Aquatic Warbler sites. In less productive sites, winter mowing can maintain suitable habitat conditions. Aquatic Warbler‐friendly land use supports a variety of other threatened plant and animal species typical of fens and sedge meadows and can meet the economic interests of local land users.     

Interannual variation in soil CO2 efflux and the response of root respiration to climate and canopy gas exchange in mature ponderosa pine

We examined a 6‐year record of automated chamber‐based soil CO₂ efflux (F_s) and the underlying processes in relation to climate and canopy gas exchange at an AmeriFlux site in a seasonally drought‐stressed pine forest. Interannual variability of F_s was large (CV=17%) with a range of 427 g C m^?2 yr^?1 around a mean annual F_s of 811 g C m^?2 yr^?1. On average, 76% of the variation of daily mean F_s could be quantified using an empirical model with year‐specific basal respiration rate that was a linear function of tree basal area increment (BAI) and modulated by a common response to soil temperature and moisture. Interannual variability in F_s could be attributed almost equally to interannual variability in BAI (a proxy for above‐ground productivity) and interannual variability in soil climate. Seasonal total F_s was twice as sensitive to soil moisture variability during the summer months compared with temperature variability during the same period and almost insensitive to the natural range of interannual variability in spring temperatures. A strong seasonality in both root respiration (R_r) and heterotrophic respiration (R_h) was observed with the fraction attributed to R_r steadily increasing from 18% in mid‐March to 50% in early June through early July before dropping rapidly to 10% of F_s by mid‐August. The seasonal pattern in R_r (10‐day averages) was strongly linearly correlated with tree transpiration (r²=0.90, P<0.01) as measured using sap flux techniques and gross ecosystem productivity (GEP, r²=0.83, P<0.01) measured by the eddy‐covariance approach. R_r increased by 0.43 g C m^?2 day^?1 for every 1 g C m^?2 day^?1 increase in GEP. The strong linear correlation of R_r to seasonal changes in GEP and transpiration combined with longer‐term interannual variability in the base rate of F_s, as a linear function of BAI (r²=0.64, P=0.06), provides compelling justification for including canopy processes in future models of F_s.     

Metabolites of the Free-Living Amoeba Phreatamoeba balamuthi Analyzed by 13C- and 31P-NMR Spectroscopy: Occurrence of Phosphoinositol Diphosphates

ABSTRACT. Phreatamoeba balamuthi is a free-living heterotrophic amoeba that lacks mitochondria. Metabolites of axenically-grown cells were characterized by natural-abundance ¹³C-NMR and ³¹P-NMR spectroscopy on acellular perchloric acid extracts. The amoebae were found to contain glycogen and trehalose as storage carbohydrates, together with putrescine and several amino acids, most prominently proline; we propose that proline and trehalose may serve in osmoregulation. Glycerophosphocholine and glycerophosphoethanolamine were present with their phosphomonoester derivatives, phosphocholine and phosphoethanolamine. Along with inorganic phosphate, inorganic pyrophosphate, nucleoside diphosphates, nucleoside triphosphates and NAD, P. balamuthi amoebae also contained unusual phosphoinositol diphosphates in large quantities (0.5 μmol/g wet cells).     

Potential long-term impacts of livestock introduction on carbon and nitrogen cycling in grasslands of Southern South America

Empirical evidence based on grazing exclusion at the scale of years to decades shows that grazing modifies carbon (C) and nitrogen (N) cycling. However, long‐term effects at the scale of centuries are less known, yet highly relevant to understand local and global impacts of grazing. Additionally, most studies have focused on the isolated response of C and N, with little understanding of their interactions. Using CENTURY, a process‐based biogeochemical model, we analyzed the impacts of 370 years of livestock grazing (i.e. long term, from early European colonization to present) in 11 sites across the Río de la Plata grasslands and compared them with those resulting from two decades of grazing (i.e. mid‐term, typical exclosure experiment). In the long term, livestock grazing primarily altered the N cycle through faster N returns to the soil via urine and dung, which were offset by uninterrupted N outputs by volatilization and leaching. As a result, soil organic N decreased by ?880 kg ha^?1 or ?19%. Higher N outputs (mainly as NH₃) opened the N cycle, potentially decreasing N₂O and NO_x emissions and increasing N depositions over the region. These greater outputs of N constrained C accumulation in soils, reducing soil organic C by ?21 200 kg ha^?1 (?22%, a reduction of ?1.5 Pg of C for the whole region) and net primary production by ?2192 kg ha^?1 yr^?1 (?24%). Mid‐term simulations showed that the effects of livestock introduction in a decadal time scale were substantially different both in magnitude and direction from long‐term responses. Long‐term results were not substantially affected when atmospheric CO₂ content, species composition and fire regime were changed within plausible ranges, but highlighted fire‐grazing interactions as a major constraint of long‐term C and N dynamics in these grasslands.     

Region-specific assessment of greenhouse gas mitigation with different manure management strategies in four agroecological zones

Livestock farming systems are major sources of trace gases contributing to emissions of the greenhouse gases (GHG) nitrous oxide (N₂O) and methane (CH₄), i.e. N₂O accounts for 10% and CH₄ for 30% of the anthropogenic contributions to net global warming. This paper presents scenario assessments of whole-system effects of technologies for reducing GHG emissions from livestock model farms using slurry-based manure management. Changes in housing and storage practice, mechanical separation, and incineration of the solid fraction derived from separation were evaluated in scenarios for Sweden, Denmark, France, and Italy. The results demonstrated that changes in manure management can induce significant changes in CH₄ and N₂O emissions and carbon sequestration, and that the effect of introducing environmental technologies may vary significantly with livestock farming practice and interact with climatic conditions. Shortening the in-house manure storage time reduced GHG emissions by 0–40%. The largest GHG reductions of 49 to, in one case, 82% were obtained with a combination of slurry separation and incineration, the latter process contributing to a positive GHG balance of the system by substituting fossil fuels. The amount and composition of volatile solids (VS) and nitrogen pools were main drivers in the calculations performed, and requirements to improve the assessment of VS composition and turnover during storage and in the field were identified. Nevertheless, the results clearly showed that GHG emission estimates will be unrealistic, if the assumed manure management or climatic conditions do not properly represent a given country or region. The results also showed that the mitigation potential of specific manure management strategies and technologies varied depending on current management and climatic conditions.