Interaction of <i>Acaena elongata</i> L. with Arbuscular Mycorrhizal Fungi under Phosphorus Limitation Conditions in a Temperate Forest

The aim of this study was to analyze the performance of Acaena elongata colonized by arbuscular mycorrhizal fungi (AMF) to different phosphorus (P) concentrations, as a measure of AMF dependency. A. elongata, is a species from soils where P availability is limited, such as temperate forests. Our research questions were: 1) How do different P concentrations affect the AMF association in Acaena elongata, and 2) How does the AMF association influence A. elongata’s growth under different P concentrations? A. elongata’s growth, P content in plant tissue, AMF colonization and dependency were measured under four P concentrations: control (0 g P kg⁻¹ ), low (0.05 g P kg⁻¹ ), intermediate (0.2 g P kg⁻¹ ) and high (2 g P kg⁻¹ ) in different harvests. A complete randomized block design was applied. A. elongata’s growth was higher under -AMF in intermediate and high P concentrations, and the lowest growth corresponded to +AMF in the low and intermediate P concentration. We observed a negative effect on the root biomass under +AMF in intermediate P concentration, while the P concentration had a positive effect on the leaf area ratio. The AMF colonization in A. elongata decreased in the highest P concentration and it was favored under intermediate P concentration; while the low and the high concentrations generated a cost-benefit imbalance. Our results suggest that the performance of some plant species in soils with low P availability may not be favored by their association with AMF, but a synergy between AMF and intermediate P concentrations might drive A. elongata’s growth.     

The viability and germination characteristics of exhumed Solanum mauritianum seeds buried for different periods of time

The site, depth and duration of burial significantly influenced the viability and state of dormancy of Solanum mauritianum seeds. Burial at a depth of 15 cm was most effective in reducing the level of conditional dormancy. Secondary dormancy was not induced at any of the environmental (burial) sites when seeds were maintained at 15 cm, where light and temperature fluctuations were minimal. When buried at 4 cm or maintained on the soil surface secondary dormancy was induced, particularly at the inland sites where environmental conditions such as temperature and moisture were more extreme. Conditional dormancy could generally be overcome by incubating seeds at 15/30 °C in the light, even after prolonged burial at unfavourable germination conditions. Gibberellic acid (500 mg l^–1) was very effective in breaking secondary dormancy of seeds induced by storage under unfavourable conditions after burial. These results have important implications for the control of this week in commercial forests.     

An analysis of the Impact of <Emphasis Type="Italic">NRG1</Emphasis> Overexpression on the <Emphasis Type="Italic">Candida albicans</Emphasis> Response to Specific Environmental Stimuli

The ability of the opportunistic fungal pathogen Candida albicans to form filaments has been strongly linked to its capacity to cause disease in humans. We previously described the construction of a strain in which filamentation can be modulated both in vitro and in vivo by placing one copy of the NRG1 gene under the control of a tetracycline-regulatable promoter. To further characterize the role of NRG1 in controlling filamentous growth, and in an attempt to determine whether NRG1 downregulation is a requirement for filamentation per se, or is only necessary under certain environmental conditions, we have conducted an analysis of the growth of the tet-NRG1 strain under a variety of in vitro conditions. Through overexpression of NRG1, we were able to block filamentation of C. albicans in both liquid media and on solid media. Filamentation in response to the low-oxygen environment of embedded growth was also inhibited. In all of these conditions, normal filamentation could be restored by down regulating expression from the tet-NRG1 allele. Interestingly, although elevated NRG1 levels were able to inhibit the formation of true hyphae in response to a wide range of environmental stimuli, elevated NRG1 expression did not affect the formation of pseudohyphae on nitrogen-limiting synthetic low ammonia dextrose (SLAD) medium. This work further illustrates the key role played by NRG1 in the control of filamentation and suggests that, although NRG1 repression plays a key role in regulating true hyphal growth, it apparently does not regulate pseudohyphal growth in the same fashion.     

Root exudates as mediators of mineral acquisition in low-nutrient environments

Plant developmental processes are controlled by internal signals that depend on the adequate supply of mineral nutrients by soil to roots. Thus, the availability of nutrient elements can be a major constraint to plant growth in many environments of the world, especially the tropics where soils are extremely low in nutrients. Plants take up most mineral nutrients through the rhizosphere where micro-organisms interact with plant products in root exudates. Plant root exudates consist of a complex mixture of organic acid anions, phytosiderophores, sugars, vitamins, amino acids, purines, nucleosides, inorganic ions (e.g. HCO₃ ^–, OH^–, H⁺), gaseous molecules (CO₂, H₂), enzymes and root border cells which have major direct or indirect effects on the acquisition of mineral nutrients required for plant growth. Phenolics and aldonic acids exuded directly by roots of N₂-fixing legumes serve as major signals to Rhizobiaceae bacteria which form root nodules where N₂ is reduced to ammonia. Some of the same compounds affect development of mycorrhizal fungi that are crucial for phosphate uptake. Plants growing in low-nutrient environments also employ root exudates in ways other than as symbiotic signals to soil microbes involved in nutrient procurement. Extracellular enzymes release P from organic compounds, and several types of molecules increase iron availability through chelation. Organic acids from root exudates can solubilize unavailable soil Ca, Fe and Al phosphates. Plants growing on nitrate generally maintain electronic neutrality by releasing an excess of anions, including hydroxyl ions. Legumes, which can grow well without nitrate through the benefits of N₂ reduction in the root nodules, must release a net excess of protons. These protons can markedly lower rhizosphere pH and decrease the availability of some mineral nutrients as well as the effective functioning of some soil bacteria, such as the rhizobial bacteria themselves. Thus, environments which are naturally very acidic can pose a challenge to nutrient acquisition by plant roots, and threaten the survival of many beneficial microbes including the roots themselves. A few plants such as Rooibos tea (Aspalathus linearis L.) actively modify their rhizosphere pH by extruding OH^– and HCO₃ ^– to facilitate growth in low pH soils (pH 3 – 5). Our current understanding of how plants use root exudates to modify rhizosphere pH and the potential benefits associated with such processes are assessed in this review.     

Morphological plasticity as a bacterial survival strategy

Bacteria have evolved complex systems to maintain consistent cell morphologies. Nevertheless, in certain circumstances, bacteria alter this highly regulated process to transform into filamentous organisms. Accumulating evidence attributes important biological roles to filamentation in stressful environments, including, but not limited to, sites of interaction between pathogenic bacteria and their hosts. Filamentation could represent an intended response to specific environmental cues that promote survival amidst the threats of consumption and killing.     

Characterization of industrial strains of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> exhibiting filamentous growth induced by alcohols and nutrient deprivation

Summary The use of microorganisms in biotechnology is an important economic area of interest in Brazil, especially the use of Saccharomyces cerevisiae in the baking and alcohol fermentation industries. Dimorphism in S. cerevisiae (cell morphology alterations from budding cells to filamentous structures) has been observed in conditions of nitrogen and carbon deprivation and in the presence of fusel alcohols. This can be described as a defense mechanism that allows the yeast to forage for nutrients through cell elongation, hyphal formation and invasive growth. In this work fifteen industrial strains of S. cerevisiae (including haploid and diploid strains) isolated from the fermentative process for alcohol production were characterized for filamentation on solid culture media under growth conditions of carbon- and nitrogen-deprivation and in the presence of fusel alcohols. The majority of strains showed filamentation induced by isoamyl alcohol, butanol, isopropanol and isobutanol, but not by methanol. In rich medium (YEPD), both haploid and diploid strains showed invasive growth, although this kind of filamentous growth was more common in haploid strains. Similar results were observed when fructose or mannose was used as the sole carbon source. In nitrogen-deficient medium (SLAD) the strains did not filament. The results obtained indicate that the filamentation induced by higher alcohols and carbon deprivation (specially carbon) is a common process in industrial strains of S. cerevisiae contributing towards their maintenance/survival in adverse conditions.     

A Candida albicans cell wall‐linked protein promotes invasive filamentation into semi‐solid medium

Growth of cells in contact with an abiotic or biological surface profoundly affects cellular physiology. In the opportunistic human pathogen, Candida albicans, growth on a semi‐solid matrix such as agar results in invasive filamentation, a process in which cells change their morphology to highly elongated filamentous hyphae that grow into the matrix. We hypothesized that a plasma membrane receptor‐type protein would sense the presence of matrix and activate a signal transduction cascade, thus promoting invasive filamentation. In this communication, we demonstrate that during growth in contact with a semi‐solid surface, activation of a MAP kinase, Cek1p, is promoted, in part, by a plasma membrane protein termed Dfi1p and results in invasive filamentation. A C. albicans mutant lacking Dfi1p showed reduced virulence in a murine model of disseminated candidiasis. Dfi1p is a relatively small, integral membrane protein that localizes to the plasma membrane. Some Dfi1p molecules become cross‐linked to the carbohydrate polymers of the cell wall. Thus, Dfi1p is capable of linking the cell wall to the plasma membrane and cytoplasm.     

<Emphasis Type="Italic">MPK1</Emphasis> gene is required for filamentous growth induced by isoamyl alcohol in <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> strains from the alcoholic fermentation

The aim of this study was to evaluate the MPK1 (SLT2) gene deletion upon filamentous growth induced by isoamyl alcohol (IAA) in two haploid industrial strains of Saccharomyces cerevisiae using oligonucleotides especially designed for a laboratory S. cerevisiae strain. The gene deletion was performed by replacing part of the open reading frames from the target gene with the KanMX gene. The recombinant strains were selected by their resistance to G418, and after deletion confirmation by polymerase chain reaction, they were cultivated in a yeast extract peptone dextrose medium + 0.5% IAA to evaluate the filamentous growth in comparison to wild strains. Mpk1 derivatives were obtained for both industrial yeasts showing the feasibility of the oligonucleotides especially designed for a laboratory strain (Σ1278b) by Martinez-Anaya et al. (In yeast, the pseudohyphal phenotype induced by isoamyl alcohol results from the operation of the morphogenesis checkpoint. J Cell Sci 116:3423–3431, 2003). The filamentation rate in these derivatives was significantly lower for both strains, as induced by IAA. This drastic reduction in the filamentation ability in the deleted strains suggests that the gene MPK1 is required for IAA-induced filamentation response. The growth curves of wild and derivative strains did not differ substantially. It is not known yet whether the switch to filamentous growth affects the fermentative characteristics of the yeast or other physiological traits. A genetically modified strain for nonfilamentous growth would be useful for these studies, and the gene MPK1 could be a target gene. The feasibility of designed oligonucleotides for this deletion in industrial yeast strains is shown.     

Filamentous growth ofPseudomonas aeruginosa

Summary The growth of two strains ofPseudomonas aeruginosa in stirred batch cultures was monitored by optical density, DNA concentration, and acridine orange direct cell count measurements. Growth of adherent bacteria in pure culture was also observed on suspended glass discs by light and scanning electron microscopy. Strain MUCOID produced significant numbers of filamentous cells in broth culture and in the adherent population, while strain PAO 381 did not produce elongated cells. Filamentous growth of MUCOID could be prevented by the addition of 5 × 10^–2 M Mg²⁺. However, the addition of 0.66 mM EDTA caused an increased proportion of the population (>50%) of MUCOID cells to become filamentous in broth culture. The results are discussed and related to theories regarding bacterial plasticity, and filamentation of normally bacillary cells.     

Nitrogen fixation associated with non-legumes in agriculture

Summary This review examines the nitrogen cycle in upland agricultural situations where nonlegume N₂-fixation is likely to be important for crop growth. Evidence for associative fixation is adduced from accumulation of N in the top 15 cm soil under grasses, from N balances for crop production obtained from both pot and field experiments, in tropical and temperate environments, measurements of nitrogen (C₂H₂ reduction) activity, uptake of¹⁵N₂ by plants and¹⁵N isotope dilution. Factors influencing the activity such as the provision of carbon substrate by the plant and the efficiency of its utilisation by the bacteria, plant cultivar, soil moisture and N levels, and inoculation with N₂-fixing bacteria are discussed. Crop responses to inoculation withAzospirillum are detailed. The breakdown of crop residues, particularly straw, can support large levels of N₂-fixation. Cyanobacteria as crusts on the soil surface also fix nitrogen actively in many environments. Fixation by the nodulated, non-legume treesCasuarina andParasponia has beneficial effects in some cropping systems in Asia. I conclude that nonlegume N₂-fixation makes a significant contribution to the production of some major cereal crops in both temperate and tropical environments.     

Dimorphism and virulence in Candida albicans

Two regulatory pathways govern filamentation in the pathogenic fungus Candida albicans. Recent virulence studies of filamentation regulatory mutants argue that both yeast and filamentous forms have roles in infection. Filamentation control pathways seem closely related in C. albicans and in Saccharomyces cerevisiae, thus permitting speculation about C. albicans filamentation genes not yet discovered.     

Secondary production,longevity and resource consumption rates of freshwater shrimps in two tropical streams with contrasting geomorphology and food web structure

1. Freshwater shrimps often dominate the biomass of tropical island streams and are known to have strong effects on stream ecosystem structure and function, but little effort has been dedicated toward quantifying basic energetic and life history attributes such as growth, production and longevity. Such information is critical for understanding both the role of shrimps in ecosystem dynamics and the gravity of threats to shrimp populations posed by human activities such as shrimp harvesting, dam construction and water withdrawal. 2. We quantified growth rates and secondary production of dominant freshwater shrimps for 3 years in two Puerto Rican headwater streams that differ in food web structure because of the presence or absence of predatory fishes that are excluded from reaches above waterfalls. Using growth data, we constructed a minimum longevity model to explore the likely minimum life spans of the two dominant taxa (Atya spp. and Xiphocaris elongata). Finally, we used a bioenergetics model to quantify annual consumption rates of major basal resources by the two taxa. 3. Daily growth rates ranged from ?0.001 to 0.011 day^?1, were inversely related to body size, and were higher for small individuals of X. elongata than Atya spp. Mean annual shrimp biomass and secondary production were an order of magnitude higher in the stream that lacked predatory fishes (biomass: 4.34 g AFDM m^?2; production: 0.89 g AFDM m^?2 year^?1) than in the stream with predatory fishes (biomass: 0.12 g AFDM m^?2; production: 0.02 g AFDM m^?2 year^?1). Production : biomass ratios ranged from 0.01 to 0.38. 4. Our longevity model predicted a minimum life span of 8 years for Atya spp. and 5 years for X. elongata in the stream lacking predatory fishes. In contrast, due to a larger average size of X. elongata in the stream with predatory fishes, our model predicted a minimum life span of 11 years. Actual life spans of these taxa are likely to be much longer based on long‐term observations of marked individuals. 5. Estimated consumption rates from the bioenergetics model indicated that Atya spp. and X. elongata are important processors of organic matter resources in streams where they occur at high densities. Atya spp. and X. elongata appeared capable of consuming a large proportion of algal and insect production and the proportion of direct leaf litter inputs consumed was also appreciable (c. 40–60%). However, the consumption of suspended fine particulate organic matter (SFPOM) by Atya spp. is probably only a minor proportion of total SFPOM flux in these streams. 6. Our study suggests that geomorphic features such as waterfalls may play an important role in controlling the distribution and production of freshwater shrimps through their effects on predatory fish movement. Spatial differences in shrimp densities result in landscape‐scale variation in the significance to ecosystem processes of these long‐lived organisms, particularly as processors of major organic matter resources.     

N-acetylglucosamine (GlcNAc) Triggers a Rapid,Temperature-Responsive Morphogenetic Program in Thermally Dimorphic Fungi

The monosaccharide N-acetylglucosamine (GlcNAc) is a major component of microbial cell walls and is ubiquitous in the environment. GlcNAc stimulates developmental pathways in the fungal pathogen Candida albicans, which is a commensal organism that colonizes the mammalian gut and causes disease in the setting of host immunodeficiency. Here we investigate GlcNAc signaling in thermally dimorphic human fungal pathogens, a group of fungi that are highly evolutionarily diverged from C. albicans and cause disease even in healthy individuals. These soil organisms grow as polarized, multicellular hyphal filaments that transition into a unicellular, pathogenic yeast form when inhaled by a human host. Temperature is the primary environmental cue that promotes reversible cellular differentiation into either yeast or filaments; however, a shift to a lower temperature in vitro induces filamentous growth in an inefficient and asynchronous manner. We found GlcNAc to be a potent and specific inducer of the yeast-to-filament transition in two thermally dimorphic fungi, Histoplasma capsulatum and Blastomyces dermatitidis. In addition to increasing the rate of filamentous growth, micromolar concentrations of GlcNAc induced a robust morphological transition of H. capsulatum after temperature shift that was independent of GlcNAc catabolism, indicating that fungal cells sense GlcNAc to promote filamentation. Whole-genome expression profiling to identify candidate genes involved in establishing the filamentous growth program uncovered two genes encoding GlcNAc transporters, NGT1 and NGT2, that were necessary for H. capsulatum cells to robustly filament in response to GlcNAc. Unexpectedly, NGT1 and NGT2 were important for efficient H. capsulatum yeast-to-filament conversion in standard glucose medium, suggesting that Ngt1 and Ngt2 monitor endogenous levels of GlcNAc to control multicellular filamentous growth in response to temperature. Overall, our work indicates that GlcNAc functions as a highly conserved cue of morphogenesis in fungi, which further enhances the significance of this ubiquitous sugar in cellular signaling in eukaryotes.     

Impact of epistasis and QTL×environment interaction on the developmental behavior of plant height in rice (Oryza sativa L.)

QTLs with epistatic effects and environmental interaction effects for the developmental behavior of plant height in rice were studied by conventional and conditional methods for quantitative trait loci (QTLs) by mapping with a doubled-haploid population of 123 lines from IR64/Azucena in three environments. The results showed that epistatic effects were important and most epistasis could be detected only by conditional QTL mapping, while most non–epistatic QTLs could be detected by both conventional and conditional methods. Many modificative QTLs showed only epistatic effects without their own additive effects at some stages. QTL×environment (QE) interaction effects were detected more often than QTL main effects for plant-height behavior, which might indicate that gene expression could be greatly affected by the environment. No QTLs had effects during the whole of ontogeny. Conditional QTL mapping might be a valid way to reveal dynamic gene expression for the development of quantitative traits, especially for epistatic effects. Received: 19 May 2000 / Accepted: 27 October 2000