Mesophyll cells of C4 plants have fewer chloroplasts than those of closely related C3 plants

The evolution of C₄ photosynthesis from C₃ ancestors eliminates ribulose bisphosphate carboxylation in the mesophyll (M) cell chloroplast while activating phosphoenolpyruvate (PEP) carboxylation in the cytosol. These changes may lead to fewer chloroplasts and different chloroplast positioning within M cells. To evaluate these possibilities, we compared chloroplast number, size and position in M cells of closely related C₃, C₃–C₄ intermediate and C₄ species from 12 lineages of C₄ evolution. All C₃ species had more chloroplasts per M cell area than their C₄ relatives in high‐light growth conditions. C₃ species also had higher chloroplast coverage of the M cell periphery than C₄ species, particularly opposite intercellular air spaces. In M cells from 10 of the 12 C₄ lineages, a greater fraction of the chloroplast envelope was pulled away from the plasmalemma in the C₄ species than their C₃ relatives. C₃–C₄ intermediate species generally exhibited similar patterns as their C₃ relatives. We interpret these results to reflect adaptive shifts that facilitate efficient C₄ function by enhancing diffusive access to the site of primary carbon fixation in the cytosol. Fewer chloroplasts in C₄ M cells would also reduce shading of the bundle sheath chloroplasts, which also generate energy required by C₄ photosynthesis.     

Drought turns a Central European Norway spruce forest soil from an N2O source to a transient N2O sink

Based on current climate scenarios, a higher frequency of summer drought periods followed by heavy rainfall events is predicted for Central Europe. It is expected that drying/rewetting events induce an increased matter cycling in soils and may contribute considerably to increased emissions of the greenhouse gas N₂O on annual scales. To investigate the influence of drying/rewetting events on N₂O emissions in a mature Norway spruce forest in the Fichtelgebirge area (NE Bavaria, Germany), a summer drought period of 46 days was induced by roof installations on triplicate plots, followed by a rewetting event of 66 mm experimental rainfall in 2 days. Three nonmanipulated plots served as controls. The experimentally induced soil drought was accompanied by a natural drought. During the drought period, the soil of both the throughfall exclusion and control plots served as an N₂O sink. This was accompanied by subambient N₂O concentrations in upper soil horizons. The sink strength of the throughfall exclusion plots was doubled compared with the control plots. We conclude that the soil water status together with the soil nitrate availability was an important driving factor for the N₂O sink strength. Rewetting quickly turned the soil into a source for atmospheric N₂O again, but it took almost 4 months to turn the cumulative soil N₂O fluxes from negative (sink) to positive (source) values. N₂O concentration and isotope analyses along soil profiles revealed that N₂O produced in the subsoil was subsequently consumed during upward diffusion along the soil profile throughout the entire experiment. Our results show that long drought periods can lead to drastic decreases of N₂O fluxes from soils to the atmosphere or may even turn forest soils temporarily to N₂O sinks. Accumulation of more field‐scale data on soil N₂O uptake as well as a better understanding of underlying mechanisms would essentially advance our knowledge of the global N₂O budget.     

Impact of altering the water table height of an acidic fen on N2O and NO fluxes and soil concentrations

The impact of experimentally intensified summer drought and precipitation on N₂O and NO turnover and fluxes was investigated in a minerotrophic fen over a 2‐year period. On three treatment plots, drought was induced for 6 and 10 weeks by means of roofs and drainage and decreased water table levels by 0.1–0.3 m compared with three nonmanipulated control plots. When averaged over the three treatment plots, both N₂O and NO emission showed only little response to the drought. On the single plot scale, however, a clear impact of the treatment on N₂O and NO fluxes could be identified. On the plot with the weakest water table reduction hardly any response could be observed, while on the plot with the greatest drainage effect, N₂O and NO fluxes increased by 530% and 270%, respectively. Rewetting reduced NO emissions to background levels (0.05–0.15 μmol m^?2 h^?1), but heavily enhanced N₂O emission (18–36 μmol m^?2 h^?1) for several days in the plots with largest water table reduction. These peaks contributed up to 40% to the cumulative N₂O fluxes and were caused by rapid N₂O production according to isotope abundance data. According to N₂O concentrations and isotope abundance analysis N₂O was mostly produced at depths between 0.3 and 0.5 m. During water table reduction net N₂O production in 0.1 m depth steadily increased in the most effectively dried plot from 2 up to 44 pmol cm^?3 day^?1. Rewetting immediately increased net N₂O production in the topsoil of the drought plots, showing rates of 18–174 pmol cm^?3 day^?1. This study demonstrates that drought and rewetting can temporarily increase N₂O emission to levels that have to date only been reported from nutrient rich and degraded fens that have been drained for agricultural purposes.     

Drought stress provokes the down‐regulation of methionine and ethylene biosynthesis pathways in Medicago truncatula roots and nodules

Symbiotic nitrogen fixation is one of the first physiological processes inhibited in legume plants under water‐deficit conditions. Despite the progress made in the last decades, the molecular mechanisms behind this regulation are not fully understood yet. Recent proteomic work carried out in the model legume Medicago truncatula provided the first indications of a possible involvement of nodule methionine (Met) biosynthesis and related pathways in response to water‐deficit conditions. To better understand this involvement, the drought‐induced changes in expression and content of enzymes involved in the biosynthesis of Met, S‐adenosyl‐L‐methionine (SAM) and ethylene in M. truncatula root and nodules were analyzed using targeted approaches. Nitrogen‐fixing plants were subjected to a progressive water deficit and a subsequent recovery period. Besides the physiological characterization of the plants, the content of total sulphur, sulphate and main S‐containing metabolites was measured. Results presented here show that S availability is not a limiting factor in the drought‐induced decline of nitrogen fixation rates in M. truncatula plants and provide evidences for a down‐regulation of the Met and ethylene biosynthesis pathways in roots and nodules in response to water‐deficit conditions.     

Lecanicillium fungicola: causal agent of dry bubble disease in white‐button mushroom

Lecanicillium fungicola causes dry bubble disease in commercially cultivated mushroom. This review summarizes current knowledge on the biology of the pathogen and the interaction between the pathogen and its most important host, the white‐button mushroom, Agaricus bisporus. The ecology of the pathogen is discussed with emphasis on host range, dispersal and primary source of infection. In addition, current knowledge on mushroom defence mechanisms is reviewed. Taxonomy: Lecanicillium fungicola (Preuss) Zare and Gams: Kingdom Fungi; Phylum Ascomycota; Subphylum Pezizomycotina; Class Sordariomycetes; Subclass Hypocreales; Order Hypocreomycetidae; Family Cordycipitaceae; genus Lecanicillium. Host range: Agaricus bisporus, Agaricus bitorquis and Pleurotus ostreatus. Although its pathogenicity for other species has not been established, it has been isolated from numerous other basidiomycetes. Disease symptoms: Disease symptoms vary from small necrotic lesions on the caps of the fruiting bodies to partially deformed fruiting bodies, called stipe blow‐out, or totally deformed and undifferentiated masses of mushroom tissue, called dry bubble. The disease symptoms and severity depend on the time point of infection. Small necrotic lesions result from late infections on the fruiting bodies, whereas stipe blow‐out and dry bubble are the result of interactions between the pathogen and the host in the casing layer. Economic importance: Lecanicillium fungicola is a devastating pathogen in the mushroom industry and causes significant losses in the commercial production of its main host, Agaricus bisporus. Annual costs for mushroom growers are estimated at 2–4% of total revenue. Reports on the disease originate mainly from North America and Europe. Although China is the main producer of white‐button mushrooms in the world, little is known in the international literature about the impact of dry bubble disease in this region. Control: The control of L. fungicola relies on strict hygiene and the use of fungicides. Few chemicals can be used for the control of dry bubble because the host is also sensitive to fungicides. Notably, the development of resistance of L. fungicola has been reported against the fungicides that are used to control dry bubble disease. In addition, some of these fungicides may be banned in the near future. Useful websites: http://www.mycobank.org ; http://www.isms.biz ; http://www.cbs.knaw.nl