Aggregation and collapse in a mechanical model of fungal tip growth

 Interconnected hyphal tubes form the mycelia of a fungal colony. The growth of the colony results from the elongation and branching of these single hyphae. The material being incorporated into the extending hyphal wall is supplied by vesicles which are formed further back in the hyphal tip. Such wall-destined vesicles appear conspicuously concentrated in the interior of the hypha, just before the hyphal apex, in the form of an apical body or Spitzenk?rper. The cytoskeleton of the hyphal tube has been implicated in the organisation of the Spitzenk?rper and the transport of vesicles, but as yet there is no postulated mechanism for this. We propose a mechanism by which forces generated by the cytoskeleton are responsible for biasing the movement of vesicles. A mathematical model is derived where the cytoskeleton is described as a viscoelastic fluid. Viscoelastic forces are coupled to the conservation equation governing the vesicle dynamics, by weighting the diffusion of vesicles via the strain tensor. The model displays collapse and aggregation patterns in one and two dimensions. These are interpreted in terms of the formation of the Spitzenk?rper and the initiation of apical branching. Received: 16 September 1996 / Revised version: 20 July 1998     

Comparison of temperature effects on soil respiration and bacterial and fungal growth rates

Temperature is an important factor regulating microbial activity and shaping the soil microbial community. Little is known, however, on how temperature affects the most important groups of the soil microorganisms, the bacteria and the fungi, in situ. We have therefore measured the instantaneous total activity (respiration rate), bacterial activity (growth rate as thymidine incorporation rate) and fungal activity (growth rate as acetate-in-ergosterol incorporation rate) in soil at different temperatures (0-45 degrees C). Two soils were compared: one was an agricultural soil low in organic matter and with high pH, and the other was a forest humus soil with high organic matter content and low pH. Fungal and bacterial growth rates had optimum temperatures around 25-30 degrees C, while at higher temperatures lower values were found. This decrease was more drastic for fungi than for bacteria, resulting in an increase in the ratio of bacterial to fungal growth rate at higher temperatures. A tendency towards the opposite effect was observed at low temperatures, indicating that fungi were more adapted to low-temperature conditions than bacteria. The temperature dependence of all three activities was well modelled by the square root (Ratkowsky) model below the optimum temperature for fungal and bacterial growth. The respiration rate increased over almost the whole temperature range, showing the highest value at around 45 degrees C. Thus, at temperatures above 30 degrees C there was an uncoupling between the instantaneous respiration rate and bacterial and fungal activity. At these high temperatures, the respiration rate closely followed the Arrhenius temperature relationship.     

Modelling the combined effect of temperature,pH and aw on the growth rate of Monascus ruber,a heat-resistant fungus isolated from green table olives

AIMS: Growth modes predicting the effect of pH (3.5-5.0), NaCl (2-10%), i.e. aw (0.937-0.970) and temperature (20-40 degrees C) on the colony growth rate of Monascus ruber, a fungus isolated from thermally-processed olives of the Conservolea variety, were developed on a solid culture medium. METHODS AND RESULTS: Fungal growth was measured as colony diameter on a daily basis. The primary predictive model of Baranyi was used to fit the growth data and estimate the maximum specific growth rates. Combined secondary predictive models were developed and comparatively evaluated based on polynomial, Davey, gamma concept and Rosso equations. The data-set was fitted successfully in all models. However, models with biological interpretable parameters (gamma concept and Rosso equation) were highly rated compared with the polynomial equation and Davey model and gave realistic cardinal pHs, temperatures and aw. CONCLUSIONS: The combined effect of temperature, pH and aw on growth responses of M. ruber could be satisfactorily predicted under the current experimental conditions, and the models examined could serve as tools for this purpose. SIGNIFICANCE AND IMPACT OF THE STUDY: The results can be successfully employed by the industry to predict the extent of fungal growth on table olives.     

Cellular automata simulations of fungal growth on solid substrates

Growth of filamentous fungi on the surface of cereal grains is a critical aspect of solid substrate fermentation (SSF). Numerous mathematical models have been developed to describe various aspects of fungal growth in SSF. These models consider hyphal geometry and nutrient availability as determinants of colony morphology and fungal physiological state. This work describes the use of cellular automata (CA) as an alternative method of modeling fungal growth. CA models reliant on a very limited set of rules or "knowledge base" display a rich array of behaviors that mimic fungal growth. By incorporating probablistic growth rules into CA models, colony characteristics such as biomass accumulation rate, colony radial growth rate, mycelial density and fungal differentiation are readily generated.     

Evaluation of the effects of chemical versus biological control on Botrytis cinerea agent of gray mould disease of strawberry

This study investigates on effects of four fungicide and six isolate from Trichoderma and Gliocladium on Botrytis cinerea agent grey mold of strawberry under library and greenhouse condition. The effect of four fungicides i.e. benomyl, dichlofluanid, captan and triadimenol on B. cinerea was studied in the laboratory condition by method mixed poison to culture medium. It was shown that the fungicide including benomyl, triadimenol, dichlofluanid and captan were able to inhibit mycelial growth of B. cinerea on PDA plate with EC50 of 0.16, 1.42, 3.40 and 7.73 ppm respectively. These fungicides delayed myceliogenic germination of sclerotia at 1000 ppm, while exhibiting no fungicidal effect. Moreover, the antagonistic effects of six fungi including Trichoderma koningii (T21), T. viride (T4), T. harzionum (T5), T. viride (T2), G. virens (G2), G. virens (G8) on B. cinerea were assessed. This assessment was done under library condition and its results as follows: The antagonistic mechanism occurred through branching at the end of B. cinerea hyphae, hyphal contact, coiling, vacuolization and lyses. Volatile metabolites of T. koningii (T21) and non-volatile metabolites of G. virens (G2 and G8) and T. koningii (T21) caused maximum inhibition of the fungal growth. Trichoderma spp and G. virens were able to colonize and sporulate on sclerotia and caused their lysis within 7-21 days. In greenhouse, a completely randomized design with 11 treatments (4 chemical and 6 biological and one untreated control) each replicated five times were used for the comparison. Greenhouse studies revealed that application of fungicides i.e. captan, dichlofluanid, triadimenol and benomyl reduces disease severity by 42, 45, 48 and 52% respectively. The fungal antagonists reduce the grey mold disease severity between 5-42%. All treatments caused a decline in post harvest disease, as the most effective treatment of chemical control was benomyl with 68.33% and for the biological treatment this was T. koningii (T21) with 56%.     

Functional analysis of tvsp1, a serine protease-encoding gene in the biocontrol agent Trichoderma virens

Serine proteases are highly conserved among fungi and considered to play a key role in different aspects of fungal biology. These proteases can be involved in development and have been related to pathogenesis or biocontrol processes. A gene (tvsp1) encoding an extracellular serine protease was cloned from Trichoderma virens, a biocontrol agent effective against soilborne fungal pathogens. The gene was expressed in Escherichia coli and a polyclonal antibody was raised against the recombinant protein. The expression pattern of tvsp1 was determined and its physiological role was addressed by mutational analysis. Strains of T. virens in which tvsp1 was deleted (PKO) or constitutively overexpressed (POE) were not affected in growth rate, conidiation, extracellular protein accumulation, antibiotic profiles nor in their ability to induce phytoalexins in cotton seedlings. Tvsp1 overexpression, however, significantly increased the ability of some strains to protect cotton seedlings against Rhizoctonia solani. Our data show that Tvsp1 is not necessary for the normal growth or development of T. virens, but plays a role in the biocontrol process.     

A proteinaceous elicitor Sm1 from the beneficial fungus Trichoderma virens is required for induced systemic resistance in maize

We have previously shown that the beneficial filamentous fungus Trichoderma virens secretes the highly effective hydrophobin-like elicitor Sm1 that induces systemic disease resistance in the dicot cotton (Gossypium hirsutum). In this study we tested whether colonization of roots by T. virens can induce systemic protection against a foliar pathogen in the monocot maize (Zea mays), and we further demonstrated the importance of Sm1 during maize-fungal interactions using a functional genomics approach. Maize seedlings were inoculated with T. virens Gv29-8 wild type and transformants in which SM1 was disrupted or constitutively overexpressed in a hydroponic system or in soil-grown maize seedlings challenged with the pathogen Colletotrichum graminicola. We show that similar to dicot plants, colonization of maize roots by T. virens induces systemic protection of the leaves inoculated with C. graminicola. This protection was associated with notable induction of jasmonic acid- and green leaf volatile-biosynthetic genes. Neither deletion nor overexpression of SM1 affected normal growth or development of T. virens, conidial germination, production of gliotoxin, hyphal coiling, hydrophobicity, or the ability to colonize maize roots. Plant bioassays showed that maize grown with SM1-deletion strains exhibited the same levels of systemic protection as non-Trichoderma-treated plants. Moreover, deletion and overexpression of SM1 resulted in significantly reduced and enhanced levels of disease protection, respectively, compared to the wild type. These data together indicate that T. virens is able to effectively activate systemic disease protection in maize and that the functional Sm1 elicitor is required for this activity.     

Kinetics and modeling of temperature effects on batch xanthan gum fermentation

Batch fermentation kinetics of xanthan gum production from glucose by Xanthomonas campestris at temperatures between 22 degrees C and 35 degrees C were studied to evaluate temperature effects on cell growth and xanthan formation. These batch xanthan fermentations were modeled by the logistic equation for cell growth, the Luedeking-Piret equation for xanthan production, and a modified Luedeking-Piret equation for glucose consumption. Temperature dependence of the parameters in this model was evaluated. Growth-associated rate constants increased to a maximum at approximately 30 degrees C and then decreased to zero at approximately 35 degrees C. This temperature effect can be modeled using a square-root model. On the contrary, non-growth-associated rate constants increased with increasing temperature, following the Arrhenius relationship, in the entire temperature range studied. The model developed in this work fits the experimental data very well and can be used in a simulation study. However, due to the empirical nature of the model, the parameter values need to be reevaluated if the model is to be applied to different growth conditions.     

The Arrhenius Equation as a Model for Explaining Plant Responses to Temperature and Water Stresses

The Arrhenius equation describes the response of biologicalprocesses to temperature. This study was conducted to examinethe applicability of the Arrhenius equation to whole plant processesand to explore the application of the Arrhenius equation asa basis for characterizing plant responses to water stress.Rates of growth of leaf area and shoot dry mass of spring wheatseedlings were measured at combinations of five soil water potentials(–0.03, –0.06, –0.10, –0.17 and –0.25MPa) and seven root temperatures (12, 14, 17, 22, 27, 29 and32 C). A non-linear least square procedure was used to fitthe modified Arrhenius equation to experimental observations.Adequate distribution of experimental observations with respectto temperature reduces the uncertainties in parameter evaluations.The standard error of the estimate of optimum temperature forleaf area growth increased from 1.4 C to 6.3 C when one ofthe data points was omitted. The optimum temperature and theenthalpy of denaturalization of enzyme systems were independentof soil water potential. A linear relation was found betweenthe rate constant and the activation energy: The Arrhenius equation was modified using this linear relation,leaving the activation energy as the only parameter affectedby water stress. The activation energy increased linearly assoil water potential decreased, with slopes of –27.18 10₃ and –28.09 10₂ K MPa^–1 for the rates ofgrowth of leaf area and shoot dry mass, respectively. Theseslopes could be used as indicators of the sensitivity of plantprocesses to water stress. Temperature, water, plants, Arrhenius equation     

Study of temperature-growth interactions of entomopathogenic fungi with potential for control of Varroa destructor (Acari: Mesostigmata) using a nonlinear model of poikilotherm development

AIMS: To investigate the thermal biology of entomopathogenic fungi being examined as potential microbial control agents of Varroa destructor, an ectoparasite of the European honey bee Apis mellifera. METHODS AND RESULTS: Colony extension rates were measured at three temperatures (20, 30 and 35 degrees C) for 41 isolates of entomopathogenic fungi. All of the isolates grew at 20 and 30 degrees C but only 11 isolates grew at 35 degrees C. Twenty-two isolates were then selected on the basis of appreciable growth at 30-35 degrees C (the temperature range found within honey bee colonies) and/or infectivity to V. destructor, and their colony extension rates were measured at 10 temperatures (12.5-35 degrees C). This data were then fitted to Schoolfield et al. [J Theor Biol (1981)88:719-731] re-formulation of the Sharpe and DeMichele [J Theor Biol (1977)64:649-670] model of poikilotherm development. Overall, this model accounted for 87.6-93.9% of the data variance. Eleven isolates exhibited growth above 35 degrees C. The optimum temperatures for extension rate ranged from 22.9 to 31.2 degrees C. Only three isolates exhibited temperature optima above 30 degrees C. The super-optimum temperatures (temperature above the optimum at which the colony extension rate was 10% of the maximum rate) ranged from 31.9 to 43.2 degrees C. CONCLUSIONS: The thermal requirements of the isolates examined against V. destructor are well matched to the temperatures in the broodless areas of honey bee colonies, and a proportion of isolates, should also be able to function within drone brood areas. SIGNIFICANCE AND IMPACT OF THE STUDY: Potential exists for the control of V. destructor with entomopathogenic fungi in honey bee colonies. The methods employed in this study could be utilized in the selection of isolates for microbial control prior to screening for infectivity and could help in predicting the activity of a fungal control agent of V. destructor under fluctuating temperature conditions.     

The impact of temperature on the production and fitness of microsclerotia of the fungal bioherbicide Mycoleptodiscus terrestris

The impact of growth temperature was evaluated for the fungal plant pathogen Mycoleptodiscus terrestris over a range of temperatures (20–36°C). The effect of temperature on biomass accumulation, colony forming units (cfu), and microsclerotia production was determined. Culture temperatures of 24–30°C produced significantly higher biomass accumulations and 20–24°C resulted in a significantly higher cfu. The growth of M. terrestris was greatly reduced at temperatures above 30°C and was absent at 36°C. The highest microsclerotia concentrations were produced over a wide range of temperatures (20–30°C). These data suggest that a growth temperature of 24°C would optimize the parameters evaluated in this study. In addition to growth parameters, we also evaluated the desiccation tolerance and storage stability of air-dried microsclerotial preparations from these cultures during storage at 4°C. During 5 months storage, there was no significant difference in viability for air-dried microsclerotial preparations from cultures grown at 20–30°C (>72% hyphal germination) or in conidia production (sporogenic germination) for air-dried preparations from cultures grown at 20–32°C. When the effect of temperature on germination by air-dried microsclerotial preparations was evaluated, data showed that temperatures of 22–30°C were optimal for hyphal and sporogenic germination. Air-dried microsclerotial preparations did not germinate hyphally at 36°C or sporogenically at 20, 32, 34, or 36°C. These data show that temperature does impact the growth and germination of M. terrestris and suggest that water temperature may be a critical environmental consideration for the application of air-dried M. terrestris preparations for use in controlling hydrilla.     

Modelling the effect of temperature and water activity on growth of Aspergillus niger strains and applications for food spoilage moulds

AIMS: To develop a model for the combined effect of water activity (a(w)) and temperature on growth of strains of Aspergillus niger, and comparison with data on food spoilage moulds in the literature. METHODS AND RESULTS: An extended combined model describing the growth of two strains of A. niger, as a function of temperature (25-30 degrees C) and a(w) (0.90-0.99) was developed. The growth rate (micro) was expressed as the increase in colony radial growth per unit of time. This extends the previous square root model showing the relationship between temperature and bacterial growth rate developed by Ratkowsky et al. (1983) and the parabolic relationship between the logarithm of the growth rate and a(w) developed by Gibson et al. (1994). A good correlation between the experimental data and the model predictions was obtained, with regression coefficients (r(2)) > 0.99. In addition, the use of this model allowed predictions of the cardinal a(w) levels: a(w(min)), and a(w(opt)). The estimation of the minimum a(w) levels (a(w(min))) was in accordance with data in the literature for similar and a range of other Aspergillus and related species, regardless of the solutes used for a(w) modification. The estimation of the optimal a(w) (a(w(opt))) and the optimal growth rate (micro(opt)) were in good agreement with the experimental results and data from the literature. CONCLUSIONS: This approach enables accurate prediction of the combined effects of environmental factors on growth of spoilage fungi for rapid prediction of cardinal limits using surface response curves. SIGNIFICANCE AND IMPACT OF THE STUDY: This approach is a rapid method for predicting optimal and marginal conditions for growth of a wide range of spoilage micro-organisms in relation to interacting environmental conditions and will have applications for improving shelf-life of intermediate moisture foods.     

Microbial Life and Temperature: A Semi Empirical Approach

Many groups have examined the effect of temperature on the survival of microorganisms, resulting in the development of several models. Some of these models are based on the Arrhenius equation and the others are based on multidimensional response surface equations. We argue that the former are inadequate and the latter lack biological meaning. We show that an equation (the GLE equation) deduced from the Theory of Rate Processes is more accurate than the Arrhenius equation. The excellent standard deviation values of the apparent free energy of activation obtained with the GLE equation for microbial growth, embryogenic and other processes show that this equation is more suitable than the Arrhenius equation. The GLE equation shows how temperature affects survival. Thus, organisms survive longer at low temperatures than at normal temperatures. The recent discovery of microorganisms in Siberian permafrost samples that are several million years old, in deep oil fields, mines and other extreme habitats appears to be consistent with the GLE equation. Another example, the enhanced resistance of spores at extreme temperatures can be easily explained by their high apparent free energy of activation We also examined the implications of the GLE equation on food sterilization practices and on exobiology.     

Determination of the shelf life of sliced cooked ham based on the growth of lactic acid bacteria in different steps of the chain

Aims: Development of a predictive model for the determination of the shelf life of modified atmosphere‐packed (MAP) cooked sliced ham in each step of the cold chain. Methods and Results: The growth of lactic acid bacteria (LAB), as well as the development of the total viable count and changes of sensory and pH value parameters in MAP cooked sliced ham, stored under different constant temperature conditions from 2 to 15°C was investigated. As a result of the measurements, the end of the shelf life could be considered as the time when LAB reach more than 7 log₁₀ CFU g^?1. Different primary and secondary models were tested and analysed to find the best way to calculate the shelf life. For primary modelling, the modified Gompertz Function and the modified Logistic Function were compared. There was no substantial difference between either model. The effect of temperature on the growth rate was modelled by using the Arrhenius and the Square root model, whereas the Arrhenius equation gave a better result. A combination of the primary and secondary model was used for shelf‐life prediction under dynamic conditions. This combination showed the best prediction of microbial counts using the modified Logistic model and the Arrhenius equation. Conclusions: With the developed model, it is possible to predict the shelf life of MAP cooked sliced ham based on the growth of LAB under different temperature conditions. Significance and Impact of the Study: The developed model can be used to calculate the remaining shelf life in different steps of the chain. Thus, it can deliver an important contribution to improve food quality by optimizing the storage management.     

High CO2 levels reduce ethylene production in kiwifruit

We examined the roles of turgor potential and osmotic adjustment in plant growth by comparing the growth of spring wheat ( Triticum aestivum cv. Siete cerrors) and sudangrass ( Sorghum vulgare var. Piper) seedlings in response to soil water and temperature stresses. The rates of leaf area expansion, leaf water potential and osmotic potential were measured at combinations of 5 soil water potentials ranging from −0.03 to −0.25 MPa and 6 soil temperatures ranging from 14 to 36°C. Spring wheat exhibited little osmotic adjustment while sudangrass exhibited a high degree of osmotic adjustment. However, the rate of leaf area growth for sudangrass was more sensitive to water stress than that of spring wheat. These results were used to evaluate the relationship between growth and turgor potential. The modified Arrhenius equation based on thermodynamic considerations of the growth process was evaluated. This equation obtains growth rate as a function of activation energy, enthalpy difference between active and inactive states of enzymes, base growth rate and optimum temperature. Analyses indicate that the modified Arrhenius equation is consistent with the Lockhart equation with a metabolically controlled cell wall extensibility.     

Functional analysis of Ras in Colletotrichum trifolii

Ras is a small monomeric GTP binding protein that transduces signals for growth and differentiation of eukaryotic organisms. Previously, a unique ras gene, designated Ct-ras, was cloned from the alfalfa fungal phytopathogen, Colletotrichum trifolii. Expression of Ct-Ras in mouse fibroblast cells (NIH3T3) demonstrated that Ct-ras is functionally similar to the mammalian ras genes since activating mutations of Ct-ras caused oncogenic phenotypes in nu/nu mice, including tumors. In C. trifolii, activated 'oncogenic' Ras (Val2) induced abnormal hyphal proliferation, defects in polarized growth and significantly reduced differentiation such as conidiation and appressorium formation in a nutrient dependent manner. Gene disruption of ct-ras was lethal. To further evaluate the function of Ct-Ras in C. trifolii, three different approaches were used: overexpression of cytosolic Ras by CAAX box deletion; expression of dominant negative Ct-RasT22N; and antisense ct-ras expression. Results showed that suppression of Ct-Ras activity significantly decreases fungal germination frequencies and hyphal growth rates. Taken together, these data suggest involvement of Ct-Ras in regulation of fungal cell growth and differentiation.