Iver Jakobsen

Mycorrhizal Fungi Can Dominate Phosphate Supply to Plants Irrespective of Growth Responses

Arbuscular mycorrhizal (AM) fungi are vital components of nearly all terrestrial ecosystems, forming mutually beneficial (mutualistic) symbioses with the roots of around 80% of vascular plants and often increasing phosphate (P) uptake and growth. We present novel data showing that AM fungi can

The use of phospholipid and neutral lipid fatty acids to estimate biomass of arbuscular mycorrhizal fungi in soil

Cucumber seedlings associated with the arbuscular mycorrhizal (AM) fungi Glomus WUM10 or G. caledanium (BEG 15) were grown in PVC tubes with a lateral root-free compartment and an identical compartment containing both hyphae and roots. The amounts of specific fatty acids, in the neutral lipid and phospholipid fractions, were measured in both compartments and compared with controls without mycorrhiza. The phospholipid fatty acids (PLFAs) 16:1 omega 5, 18:1 omega 7c, 20:4 and 20:5 were present in higher amounts in soil with mycorrhizal hyphae than in soil without mycorrhizal hyphae. The largest relative difference was found in 20:5, but a good correlation existed between 16:1 omega 5 and 20:5 in soil with hyphae. Amounts of these fatty acids were correlated both with length of mycorrhizal hyphae and with amounts of ATP in soil. Conversion factors to calculate hyphal length and AM fungal biomass carbon using the phospholipid fatty acids could thus be estimated; 38 nmol PLFA 16:1 omega 5 mg(-1) AM fungal biomass C (Glomus WUM10) and 22 nmol PLFA 20:5 mg(-1) biomass C. The fatty acid 16:1 omega 5 from the neutral lipid fraction, containing triglycerides, dominated in soils with mycorrhizal hyphae. The amount of 16:1 omega 5 in the neutral lipid fraction decreased during storage of soils, indicating a decrease in storage lipids, while the proportion of 16:1 omega 5 in the phospholipid fraction was almost unaffected. (Less)

Role of Arbuscular Mycorrhizal Fungi in Uptake of Phosphorus and Nitrogen From Soil

AbstractColonization of plant roots by arbuscular mycorrhizal fungi can greatly increase the plant uptake of phosphorus and nitrogen. The most prominent contribution of arbuscular mycorrhizal fungi to plant growth is due to uptake of nutrients by extraradical mycorrhizal hyphae. Quantification of hyphal nutrient uptake has become possible by the use of soil boxes with separated growing zones for roots and hyphae. Many (but not all) tested fungal isolates increased phosphorus and nitrogen uptake of the plant by absorbing phosphate, ammonium, and nitrate from soil. However, compared with the nutrient demand of the plant for growth, the contribution of arbuscular mycorrhizal fungi to plant phosphorus uptake is usually much larger than the contribution to plant nitrogen uptake. The utilization of soil nutrients may depend more on efficient uptake of phosphate, nitrate, and ammonium from the soil solution even at low supply concentrations than on mobilization processes in the hyphosphere. In contrast to ectomy...

Estimation of the biomass of arbuscular mycorrhizal fungi in a linseed field

Abstract Linseed was grown in field plots included in a long-term P fertilisation experiment (0, 15 or 30 kg P ha −1 yr −1 for 20 yr). Two months before sowing, half of each plot was applied with dazomet to prevent the formation of arbuscular mycorrhiza (AM). The biomass of different groups of micro-organisms was estimated 28, 51 and 72 d after sowing based on amounts of certain fatty acids extracted from the soil. Dazomet application strongly suppressed colonisation of the linseed roots by AM fungi throughout the experiment. In plots with no dazomet application, root colonisation by the AM fungi increased from harvests 1 to 3 as judged both from microscopical estimates and from quantitative analysis of the AM fungal indicative fatty acid 16:1ω5. These methods also revealed that AM formation was reduced in P-fertilised plots. The phospholipid fatty acid (PLFA) 16:1ω5 decreased in dazomet-treated soil, and it was assumed that the PLFA 16:1ω5 remaining in treated soil originated from bacteria. The biomass of the extraradical AM mycelium could then be estimated by multiplying the difference in PLFA 16:1ω5 between dazomet treated and nontreated soils by a conversion factor. This calculation indicated that the biomass of the extraradical mycelium of AM fungi was about 10 times as high as the biomass of intraradical mycelium and that the extraradical mycelium constituted the largest fraction of the soil microbial biomass. Dazomet application also decreased the biomass of saprophytic fungi in the soil as indicated by the amount of PLFA 18:2ω6,9, while analyses of bacteria-specific fatty acids indicated that the bacterial biomass in the soil was not affected by either dazomet or P application.

Growth and extracellular phosphatase activity of arbuscular mycorrhizal hyphae as influenced by soil organic matter

Abstract Two experiments were set up to investigate the influence of soil organic matter on growth of arbuscular mycorrhizal (AM) hyphae and concurrent changes in soil inorganic P, organic P and phosphatase activity. A sandy loam soil was kept for 14 months under two regimes (outdoor where surplus precipitation leached through the soil, or indoor at constant moisture) with or without 9% (w/w) chopped wheat straw plus mineral N. Then the soils were partially sterilized and placed in two-compartment pots where mycorrhizal or non-mycorrhizal cucumber plants were grown in one root compartment (RC), and soils differing in organic matter were placed in six parallel hyphal compartments (HC) separated from the RC with a 37 μm mesh. In the first experiment, using Glomus caledonium, hyphal length densities were measured in the HC after 31 days. Added straw increased hyphal length densities by 34 and 62% for soil kept outdoors and indoors, respectively. In the second experiment, using G. invermaium and only soil kept outdoors, three treatments were included: soil with no added straw with or without a new addition of 0.5% (w/w) of ground clover leaves, and soil with 9% straw plus mineral N. After 41 days hyphal length density was twice as high in soil with added straw compared to the two other treatments. Mycorrhizal colonization resulted in lower activity of acid phosphatase in the HC for two out of three treatments. Alkaline phosphatase activity was only decreased by mycorrhiza in soil without organic matter additions. In soil with added clover alkaline phosphatase activity increased due to the presence of mycorrhizal hyphae. We suggest that mycorrhizas may influence the exudation of acid phosphatase by roots. Hyphae of G. invermaium did apparently not excrete extracellular phosphatases, but their presence may have influenced alkaline phosphatase excreted by other microorganisms, probably through competition for nutrients. Phosphatase activity was not correlated with the concentration of labile organic P in soil extracts.

Nonredundant Regulation of Rice Arbuscular Mycorrhizal Symbiosis by Two Members of the PHOSPHATE TRANSPORTER1 Gene Family

The arbuscule-specific expression of two rice PHOSPHATE TRANSPORTER1 (PHT1) genes, PT11 and PT13, suggests that both of these genes play a role in symbiotic phosphate acquisition. Indeed, they are both important for the development of arbuscular mycorrhizal symbioses in rice; however, PT11 is the key player in fungus-delivered phosphate uptake. Pi acquisition of crops via arbuscular mycorrhizal (AM) symbiosis is becoming increasingly important due to limited high-grade rock Pi reserves and a demand for environmentally sustainable agriculture. Here, we show that 70% of the overall Pi acquired by rice (Oryza sativa) is delivered via the symbiotic route. To better understand this pathway, we combined genetic, molecular, and physiological approaches to determine the specific functions of two symbiosis-specific members of the PHOSPHATE TRANSPORTER1 (PHT1) gene family from rice, ORYsa;PHT1;11 (PT11) and ORYsa;PHT1;13 (PT13). The PT11 lineage of proteins from mono- and dicotyledons is most closely related to homologs from the ancient moss, indicating an early evolutionary origin. By contrast, PT13 arose in the Poaceae, suggesting that grasses acquired a particular strategy for the acquisition of symbiotic Pi. Surprisingly, mutations in either PT11 or PT13 affected the development of the symbiosis, demonstrating that both genes are important for AM symbiosis. For symbiotic Pi uptake, however, only PT11 is necessary and sufficient. Consequently, our results demonstrate that mycorrhizal rice depends on the AM symbiosis to satisfy its Pi demands, which is mediated by a single functional Pi transporter, PT11.

The mycorrhizal fungus (Glomus intraradices) affects microbial activity in the rhizosphere of pea plants (Pisum sativum)

Pea plants were grown in g-irradiated soil in pots with and without addition of the AM fungus Glomus intraradices at sufficient N and limiting P. Depending on the growth phase of the plant presence of AM had negative or positive effect on rhizosphere activity. Before flowering during nutrient acquisition AM decreased rhizosphere respiration and number of protozoa but did not affect bacterial number suggesting top-down regulation of bacterial number by protozoan grazing. In contrast, during flowering and pod formation AM stimulated rhizosphere respiration and the negative effect on protozoa decreased. AM also affected the composition of the rhizosphere bacterial community as revealed from DNA analysis (DGGE). With or without mycorrhiza, rhizosphere respiration was P-limited on very young roots, not nutrient limited at more mature roots and C-limited at withering. This suggests changes in the rhizosphere community during plant growth also supported by changes in the bacteria (DGGE). q 2003 Elsevier Ltd. All rights reserved.

Arbuscular mycorrhiza reduces susceptibility of tomato to Alternaria solani

Mycorrhiza frequently leads to the control of root pathogens, but appears to have the opposite effect on leaf pathogens. In this study, we studied mycorrhizal effects on the development of early blight in tomato (Solanum lycopersicum) caused by the necrotrophic fungus Alternaria solani. Alternaria-induced necrosis and chlorosis of all leaves were studied in mycorrhizal and non-mycorrhizal plants over time course and at different soil P levels. Mycorrhizal tomato plants had significantly less A. solani symptoms than non-mycorrhizal plants, but neither plant growth nor phosphate uptake was enhanced by mycorrhizas. An increased P supply had no effect on disease severity in non-mycorrhizal plants, but led to a higher disease severity in mycorrhizal plants. This was parallel to a P-supply-induced reduction in mycorrhiza formation. The protective effect of mycorrhizas towards development of A. solani has some parallels to induced systemic resistance, mediated by rhizobacteria: both biocontrol agents are root-associated organisms and both are effective against necrotrophic pathogens. The possible mechanisms involved are discussed.

Enzymatic Evidence for the Key Role of Arginine in Nitrogen Translocation by Arbuscular Mycorrhizal Fungi

Key enzymes of the urea cycle and 15N-labeling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen translocation by arbuscular mycorrhizal (AM) fungi. Mycorrhiza was established between transformed carrot (Daucus carota) roots and Glomus intraradices in two-compartment petri dishes and three ammonium levels were supplied to the compartment containing the extraradical mycelium (ERM), but no roots. Time courses of specific enzyme activity were obtained for glutamine synthetase, argininosuccinate synthetase, arginase, and urease in the ERM and AM roots. 15NH4+ was used to follow the dynamics of nitrogen incorporation into and turnover of Arg. Both the absence of external nitrogen and the presence of l-norvaline, an inhibitor of Arg synthesis, prevented the synthesis of Arg in the ERM and resulted in decreased activity of arginase and urease in the AM root. The catabolic activity of the urea cycle in the roots therefore depends on Arg translocation from the ERM. 15N labeling of Arg in the ERM was very fast and analysis of its time course and isotopomer pattern allowed estimation of the translocation rate of Arg along the mycelium as 0.13 μg Arg mg−1 fresh weight h−1. The results highlight the synchronization of the spatially separated reactions involved in the anabolic and catabolic arms of the urea cycle. This synchronization is a prerequisite for Arg to be a key component in nitrogen translocation in the AM mycelium.

The occrrence of vesicular-arbuscular mycorrhiza in barley and wheat grown in some Danish soils with different fertilizer treatments

SummarySoil samples, roots and shoots were collected from barley crops at three locations which had received different combinations of N and P for 10 years, and from long-term fertilizer experiments on barley in a sandy soil and on barley and wheat in a loamy soil.Soils were analysed for available P by an anion exchange resin procedure, roots were examined for intensity of VAM infection, and shoots wee analysed for N and P.Vesicualr-arbuscular mycorrhizal infection was found at all locations. It was most abundant at the three locations with least soil-P and lightest at the two locations high in soil-P. Within loocations an inverse relation was found between soil-P level and intensity of infection. Infection was also intensity. by increasing N-fertilizer. Spore counts from selected samples correlated well with infection intensity. Shoot-P did not differ significantly between treatments in spite of significant differences in soil-P. This points to the significance of vesicular-arbuscular mycorrhiza: a lower soil-P level accompanied by a higher infection intensity seem to counterbalance each other to a certain extent.