Phosphorus effects on metabolic processes in monoxenic arbuscular mycorrhiza cultures

The influence of external phosphorus (P) on carbon (C) allocation and metabolism as well as processes related to P metabolism was studied in monoxenic arbuscular mycorrhiza cultures of carrot (Daucus carota). Fungal hyphae of Glomus intraradices proliferated from the solid minimal medium containing the colonized roots into C-free liquid minimal medium with different P treatments. The fungus formed around three times higher biomass in P-free liquid medium than in medium with 2.5 mM inorganic P (high-P). Mycelium in the second experiment was harvested at an earlier growth stage to study metabolic processes when the mycelium was actively growing. P treatment influenced the root P content and [(13)C]glucose administered to the roots 7 d before harvest gave a negative correlation between root P content and (13)C enrichment in arbuscular mycorrhiza fungal storage lipids in the extraradical hyphae. Eighteen percent of the enriched (13)C in extraradical hyphae was recovered in the fatty acid 16:1omega5 from neutral lipids. Polyphosphate accumulated in hyphae even in P-free medium. No influence of P treatment on fungal acid phosphatase activity was observed, whereas the proportion of alkaline-phosphatase-active hyphae was highest in high-P medium. We demonstrated the presence of a motile tubular vacuolar system in G. intraradices. This system was rarely seen in hyphae subjected to the highest P treatment. We concluded that the direct responses of the extraradical hyphae to the P concentration in the medium are limited. The effects found in hyphae seemed instead to be related to increased availability of P to the host root.     

Pure cultures and characterization of yak Sertoli cells

The culture of primary Sertoli cells has become an important resource in the study of their function. However, their use is limited because of contamination of isolated cells with other testicular cells, mainly germ cells. The aim was to establish technique to obtain pure yak Sertoli cells as well as to study the growth kinetics and biological characteristics of Sertoli cells in vitro. Two-step enzyme digestion was used to separate and culture yak Sertoli cells. Cultured using starvation method and the hypotonic treatment were also invented to get pure yak Sertoli cells. Furthermore, the purification of Yak Sertoli cells were identified according to their characteristics, such as bipolar corpuscular around the nucleus and expression of Fasl, in addition to their morphology. The average viability of the Sertoli cells was 97% before freezing and 94.5% after thawing, indicating that cryopreservation in liquid nitrogen had little influence on the viability of Sertoli cells. The growth tendency of yak Sertoli cells was similar to an S-shaped growth curve. Purified yak Sertoli cells frequently exhibited bipolar corpuscula in nucleus after Feulgen staining, and did have a positive reaction of Fasl by the immunocytochemical identification. After recovery chromosomal analysis of Sertoli cells had a normal chromosomal number of 60, comprising 29 pairs of autosomes and one pair of sex chromosomes. Assays for bacteria, fungi and mycoplasmas were negative. In conclusion, yak Sertoli cells have been successfully purified and cultured in vitro, and maintain stable biological characteristics after thawing. Therefore, it will not only preserve the genetic resources of yaks at the cellular level, but also provide valuable materials for transgenic research and feeder layer and nuclear donor cells in yak somatic cell cloning technology.     

Bacterial breakdown of benomyl. I. Pure cultures

With different soil and water samples as inoculum and the benzimidazole fungicides benomyl (either as Benlate or as the pure compound) and thiabendazole as selective agents, a large number of, mainly fluorescent, Pseudomonas strains were isolated which nearly all were able to grow in a mineral medium with benomyl as the sole source of carbon. However, no growth occurred with any of a series of other benzimidazole compounds, viz. benzimidazole, 2-aminobenzimidazole (2-AB), thiabendazole and fuberidazole. Although benomyl—or rather its non-enzymatic breakdown product methyl benzimidazol-2-yl carbamate (MBC)—was partially degraded to 2-AB, most probably n-butylamine, which arises after splitting off of the butylcarbamoyl side chain, was the actual carbon source for the Pseudomonas isolates.When incorporated in a lactate medium, 2-AB markedly inhibited the growth of Pseudomonas spp. at a concentration of 250 g/ml, with complete inhibition being attained at 500 g/ml. For Bacillus spp. grown in liquid peptone media benzimidazole compounds were inhibitory at concentrations of 500–1000 g/ml, with a toxicity increasing in the order: benzimidazole 相似文献   

The mycorrhiza helper bacteria revisited

In natural conditions, mycorrhizal fungi are surrounded by complex microbial communities, which modulate the mycorrhizal symbiosis. Here, the focus is on the so-called mycorrhiza helper bacteria (MHB). This concept is revisited, and the distinction is made between the helper bacteria, which assist mycorrhiza formation, and those that interact positively with the functioning of the symbiosis. After considering some examples of MHB from the literature, the ecological and evolutionary implications of the relationships of MHB with mycorrhizal fungi are discussed. The question of the specificity of the MHB effect is addressed, and an assessment is made of progress in understanding the mechanisms of the MHB effect, which has been made possible through the development of genomics. Finally, clear evidence is presented suggesting that some MHB promote the functioning of the mycorrhizal symbiosis. This is illustrated for three critical functions of practical significance: nutrient mobilization from soil minerals, fixation of atmospheric nitrogen, and protection of plants against root pathogens. The review concludes with discussion of future research priorities regarding the potentially very fruitful concept of MHB.     

The dark side of the mycorrhiza

Plant association with arbuscular mycorrhizal (AM) fungi is usually regarded as mutualistic. However, this positive effect could disappear if the benefit of the fungal-plant association changes with colonization density. In order to test the conditionality of this interaction, we evaluated plant performance and tolerance to defoliation across five levels of commercial AM fungal inoculum concentrations. Additionally, we evaluated if plant performance and tolerance were similarly affected by a whole soil community collected under a native congener. Along the gradient of inoculation, plant performance exhibited a peak at intermediate inoculum concentration, indicating the presence of an optimum level of AM fungal concentration that maximized AM fungal benefit. Root colonization by fungal hyphae increased linearly across the experimental inoculation gradient. Paralleling root colonization, plant tolerance to defoliation decreased linearly along the inoculum gradient. Plant performance was similar under the whole soil and commercial treatments. Our results show a negative correlation between tolerance to defoliation and AM fungal inoculum concentration, indicating that AM fungi colonization could constrain the evolution of plant tolerance to herbivory.Key words: compensation, defences, ecological interactions, herbivory, multitrophic interactions, mycorrhizal fungi, toleranceArbuscular mycorrhizal (AM) fungi occur in all ecosystems of the world and associate with the roots of about 70% of all vascular plants.¹ This association is typically regarded as mutualistic, because there is a bidirectional transfer of nutrients between the host plant and its fungal partners. Carbon compounds are passed from the plant to the fungus and, in return, there is a transfer of mineral nutrients, principally nitrate and phosphate.² However, this association also entails costs. The amount of carbon allocated to AM fungi is estimated to range from 4% to 20% of a plant''s total carbon budget.² Throughout the literature, there are examples of the conditionality of this relationship exemplified by a continuum of the effects of AM fungal colonization on hosts from positive, through null to negative.^3–5 Moreover, it has been suggested that the benefit of a plant associating with fungal symbionts depends not only on the identity of AM fungi⁴ and plant genotypes⁶ but also on hyphal colonization density in roots.⁷ In a recent greenhouse study, we examined components of the conditionality of plant interactions with soil biota.⁸ We were interested in knowing how the performance and tolerance to defoliation of the annual plant Datura stramonium varied along a concentration gradient of commercial AM fungal inoculum containing four Glomus species (Mycorrhizal Applications, Grants Pass, OR USA).We found a curvilinear relationship between AM fungal inoculum concentration and plant performance, as predicted by previous models.⁷ The quadratic decelerating function between inoculum concentration and plant performance indicates an optimum level of AM fungal concentration (1/24^th total pot volume) that maximizes AM fungal benefit (Fig. 1A). This result suggests that, in D. stramonium, positive associations between AM fungi and plant fitness may not be proportional and, that at high colonization densities, mycorrhizae may have detrimental effects, perhaps by competing with plants for nutrients, or by interfering with other essential interactions.^4,5 We also found, from root examination, that hyphal colonization of roots increased linearly with AM fungi inoculum concentration. Moreover, we found that tolerance to herbivory decreased linearly with increasing AM fungal inoculum concentration (r² = −0.40; F_1,27 = 5.89; p = 0.0222; Fig. 1B), suggesting that, in our system, at high densities, mycorrhizae may become parasitic and may compete for resources (e.g., carbon) with the induced host plant response to leaf damage.Open in a separate windowFigure 1Effect of a gradient in AM fungi inoculum concentration on D. stramonium performance. (A) Non-linear relation between seed production and inoculum concentration. In general, plants achieved their maximal performance at an inoculum concentration of 1/24^th total pot volume. (B) Tolerance to defoliation decreased linearly with inoculum concentration. Tolerance was calculated as the difference in standardized seed production between related damaged and undamaged genetically related plants corresponding to six genetic full-sib families.In order to know whether the effects we found in the greenhouse using commercial inoculum could be expected in the field, we addressed whether or not D. stramonium performance and tolerance were similarly influenced by whole soil field communities; including AM fungi, pathogens, root herbivores, etc. Unfortunately, D. stramonium is not native to the area where this research was undertaken, so we collected soil immediately below plants of a native congener Datura wrightii, a perennial herb that grows at the Putah Creek Reserve (UC, Davis). Pots were inoculated at a 1/12^th total pot volume with this live soil and plants were grown concurrently with those in the previous experiment. We compared plant performance and tolerance under the live soil treatment and the last level of the commercial AMF inoculum gradient (both inoculated at a 1/12^th total pot volume). Results indicated no differences in foliar area (F_1,94 = 1.18; p = 0.2782), root mass (F_1,94 = 0.99; p = 0.3222), flowering day (χ² = 0.31; p = 0.5804) and fitness (χ² = 0.03; p = 0.8691). Moreover, root colonization levels were (F_1,94 = 0.75; p = 0.3877) in both 1/12^th volume vs. live soil, as well as in the 0 AMF and sterilized soil (F_1,94 = 2.56; p = 0.1130). Despite these similarities, plant tolerance did differ significantly between AMF and live soil treatments (F_1,94 = 5.49; p = 0.0411), tolerance being greater under the live soil treatment (0.3755 ± 0.0311 tolerance) relative to the 1/12^th AM fungal treatment (−0.5744 ± 0.2714 tolerance). This result suggests that the expression of plant tolerance may also depend on the identity of AMF colonizing roots or the number and identities of soil bacteria. We did not know which microbial species were in the soils we collected.We show that, when inoculated over a gradient of abundance, Glomus AM fungal colonization consistently decreased tolerance to herbivory. The presence of mycorrhizae could therefore decrease the adaptive value of traits increasing tolerance. We also show here that though live soil inoculum had similar effects in magnitude and direction to those of commercial AMF incoculum on growth and fitness, live soil biota collected under a congener of D. stramonium increased tolerance to herbivory at the same levels of root colonization. Overall, the results of this study indicate that the interaction between soil biotic components and the response of D. stramonium to leaf damage is highly conditional; and can depend on amounts of root colonization, as well as perhaps identities of AM fungi and bacteria. In both cases, soil biota affected the impact of damage to leaves aboveground. AM fungi may mediate the efficacy of tolerance as a defense, and this effect may be especially important in light of herbivore adaptation, when tolerance may be favored over resistance as a plant defense strategy.¹⁰     

Microbial ecology of the arbuscular mycorrhiza

Arbuscular mycorrhizal (AM) fungi interact with a wide variety of organisms during all stages of their life. Some of these interactions such as grazing of the external mycelium are detrimental, while others including interactions with plant growth promoting rhizobacteria (PG PR) promote mycorrhizal functioning. Following mycorrhizal colonisation the functions of the root become modified, with consequences for the rhizosphere community which is extended into the mycorrhizosphere due to the presence of the AM external mycelium. However, we still know relatively little of the ecology of AM fungi and, in particular, the mycelium network under natural conditions. This area merits attention in the future with emphasis on the fungal partner in the association rather than the plant which has been the focus in the past.     

Synthesis of mycorrhiza on Pinus oocarpa

Mycorrhiza was synthesized using Pinus oocarpa and Rhizopogon luteolus. After one year's successful growth in the laboratory, the seedlings raised were planted out in the field where the inoculated plants continued to grow normally. The uninoculated plants died after planting out. The results are believed to be significant in that a field establishment of mycorrhiza synthesized in controlled conditions is possible. The experiment also demonstrates the absence of suitable mycorrhizal fungi for pine establishment in Nigerian soils.     

Growth models of cultures with two liquid phases. II. Pure substrate in dispersed phase

Mathematical models which can be used to describe batch frowth in fermentations with two liquid phases are developed for systems in which the growth limiting substrate is the dispersed liquid phase. Three special cases are considered assuming pure substrate in the dispersed phase and a decreasing interfacial area due to substrate consumption. In the first, it is assumed that all growth occurs at the surface of the dispersed phase. In the second and third growth occurs at the interface and in the continuous phase. The second case assumes substrate equilibrium between the two phases while the third assumes substrate consumption in the continuous phase is limited by rate of substrate transport to that phase. Since the amount of growth at the interface and substrate transport to the continuous phase depend on the interfacial area, two limiting cases for the decrease of interfacial area with substrate consumption are also considered in this investigation. The first and third models are compared with available experimental data.     

The occurrence of vesicular-arbuscular mycorrhiza in agro-ecosystems

In a long-lasting field experiment at the Research Station Hanninghof at Dülmen (Westphalia), laid out in 1971, the inoculum potential (IP) of vesicular-arbuscular mycorrhizal (VAM) fungi of soil from winter wheat in a continuous monoculture (CM) and in a four-years cereal crop rotation (CR, winter wheat, winter barley, winter rye, oats) was investigated from 1980 to 1982. The influence of green manure and two levels of introgen fertilization (100 and 200 kg ha⁻¹) on VAM-IP was also assessed. It turned out that VAM is frequent also in intensively cultivated soils, but varies this with the crop rotation system and form and intensity of fertilization. Compared to CR the VAM-IP was significantly reduced in CM. A cumulation of this effect, with a particularly strong decrease of VAM occurred when other factors unfavourable for the symbiosis coincided in CM. Among these factors were the use of non-mycorrhizalRaphanus sativus andBrassica napus for green manure together with a N-fertilization of 200 kg ha⁻¹. On the other hand green manure and the higher N-fertilization resulted in less disturbance of the VAM-IP in the CR. A correlation between VAM-IP in the soil and of wheat yields could neither be observed in the CR nor in the CM.     

Occurrence of arbuscular mycorrhiza on aged Eucalyptus

Eucalyptus despite the fact that Eucalyptus seedlings do form both endomycorrhiza and ectomycorrhiza early during their developement. In the present study, all the structures of arbuscular mycorrhiza were observed within roots of four Eucalyptus species of 15, 17 and more than 50 years old at three different sites in northern Algeria. Arbuscular mycorrhiza frequency was assessed in roots of 15-years old Eucalyptus camaldulensis species, during two periods in 2 consecutive years (July and November of 1996 and 1997). Intensity of root colonization was dependent on the time of sampling and attained 42% in July 1997. Accepted: 17 September 1999