Andres Wiemken

Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity

The functioning and stability of terrestrial ecosystems are determined by plant biodiversity and species composition. However, the ecological mechanisms by which plant biodiversity and species composition are regulated and maintained are not well understood. These mechanisms need to be identified to ensure successful management for conservation and restoration of diverse natural ecosystems. Here we show, by using two independent, but complementary, ecological experiments, that below-ground diversity of arbuscular mycorrhizal fungi (AMF) is a major factor contributing to the maintenance of plant biodiversity and to ecosystem functioning. At low AMF diversity, the plant species composition and overall structure of microcosms that simulate European calcareous grassland fluctuate greatly when the AMF taxa that are present are changed. Plant biodiversity, nutrient capture and productivity in macrocosms that simulate North American old-fields increase significantly with increasing AMF-species richness. These results emphasize the need to protect AMF and to consider these fungi in future management practices in order to maintain diverse ecosystems. Our results also show that microbial interactions can drive ecosystem functions such as plant biodiversity, productivity and variability.

DIFFERENT ARBUSCULAR MYCORRHIZAL FUNGAL SPECIES ARE POTENTIAL DETERMINANTS OF PLANT COMMUNITY STRUCTURE

Almost all natural plant communities contain arbuscular mycorrhizal fungi (AMF). We hypothesized that the species composition of AMF communities could have the potential to determine plant community structure if the growth response to different AMF species or to communities of AMF species varies among plant species. To test the existence of such a differential response we conducted a pot experiment where each of three plant species, Hieracium pilosella, Bromus erectus, and Festuca ovina were inoculated with each of four AMF species, or with a mixture of these four AMF species, or were uninoculated. The AMF species originated from a calcareous grassland in which the three plant species also coexisted. We obtained three pieces of evidence suggesting that AMF have the potential to de- termine plant community structure. First, plant species differed in their dependency on AMF, thus varying in degree of benefit received. Second, specific AMF species and a mixture of these AMF species had significantly different effects on several plant growth variables, and these effects were not the same on each plant species. Third, the amount of variation in the growth response of a plant species to four AMF species and to the mixture of AMF species differed among the plant species. Hieracium differed greatly in its growth response to several AMF species while Bromus did not exhibit much variation in its response to different AMF species. The varying mycorrhizal dependency of different plant species has previously been proposed as a mechanism determining plant community structure. However, we found that the mycorrhizal dependency of a plant species can vary greatly because of differential growth responses to specific AMF species compared to the growth of the uninoculated plants. Consequently mycorrhizal dependency, as a measure indicating how much a plant depends on AMF for its growth, is not necessarily a fixed value and therefore cannot be used as a definitive term. In addition, those plant species with highly variable responses to single AMF species or to combinations of AMF species (AMF communities) will be strongly affected by the specific species of AMF that occupy their roots, in contrast to plant species that do not respond differently to different AMF species. We conclude that, through their differential effects on plant growth, AMF species that co-occur as natural AMF communities have the potential to determine plant community structure, and that future studies on plant population and community structure need to consider the strength of their role as a determinant.

Impact of Land Use Intensity on the Species Diversity of Arbuscular Mycorrhizal Fungi in Agroecosystems of Central Europe

ABSTRACT The impact of land use intensity on the diversity of arbuscular mycorrhizal fungi (AMF) was investigated at eight sites in the “three-country corner” of France, Germany, and Switzerland. Three sites were low-input, species-rich grasslands. Two sites represented low- to moderate-input farming with a 7-year crop rotation, and three sites represented high-input continuous maize monocropping. Representative soil samples were taken, and the AMF spores present were morphologically identified and counted. The same soil samples also served as inocula for “AMF trap cultures” with Plantago lanceolata, Trifolium pratense, and Lolium perenne. These trap cultures were established in pots in a greenhouse, and AMF root colonization and spore formation were monitored over 8 months. For the field samples, the numbers of AMF spores and species were highest in the grasslands, lower in the low- and moderate-input arable lands, and lowest in the lands with intensive continuous maize monocropping. Some AMF species occurred at all sites (“generalists”); most of them were prevalent in the intensively managed arable lands. Many other species, particularly those forming sporocarps, appeared to be specialists for grasslands. Only a few species were specialized on the arable lands with crop rotation, and only one species was restricted to the high-input maize sites. In the trap culture experiment, the rate of root colonization by AMF was highest with inocula from the permanent grasslands and lowest with those from the high-input monocropping sites. In contrast, AMF spore formation was slowest with the former inocula and fastest with the latter inocula. In conclusion, the increased land use intensity was correlated with a decrease in AMF species richness and with a preferential selection of species that colonized roots slowly but formed spores rapidly.

Trehalose in yeast, stress protectant rather than reserve carbohydrate

Trehalose and glycogen are generally regarded as the two main reserve carbohydrates in yeast. However, several lines of evidence suggest that trehalose does not primarily function as a reserve but as a highly efficient protecting agent to maintain strutural integrity of the cytoplasm under environmental stress conditions.

Rapid changes of heat and desiccation tolerance correlated with changes of trehalose content in Saccharomyces cerevisiae cells subjected to temperature shifts

The trehalose content of exponentially growing Saccharomyces cerevisiae cells rapidly increased in response to a temperature shift from 27 to 40°C and decreased again when the temperature was shifted back from 40 to 27°C. These changes were closely correlated with increases and decreases in the thermotolerance and desiccation tolerance of the cells. Our results support the hypothesis that trehalose functions as a protectant against heat and desiccation.

Communities of arbuscular mycorrhizal fungi in arable soils are not necessarily low in diversity

Communities of arbuscular mycorrhizal fungi (AMF) in five agricultural field sites of different management intensities were studied. Variable regions of the ribosomal RNA genes were used to detect and identify AMF directly within colonized roots. Roots from a continuous maize monoculture showed low AMF diversity, in agreement with previous reports on molecular diversity of AMF in agricultural soils. In contrast, a substantially higher diversity of AMF was found throughout the long term ‘DOK’ field experiment, where organic and conventional agricultural practices have been compared side by side since 1978. In this experiment, a 7‐year crop rotation is performed under lower levels of inorganic fertilizer input and chemical pest control. These results are in good agreement with analyses of the spore community previously conducted in these field sites. In a third site, an organically managed leek field with soil of very high phosphate content reflecting the highly intensive conventional field history and intensive tillage, we detected a low‐diversity community comparable to the maize monoculture. In addition to fungi from Glomus group A, which have previously been reported to dominate arable soils, we regularly found members of the genera Scutellospora, Paraglomus and Acaulospora. The genus Acaulospora was shown to occur more frequently early in the growing season, suggesting that the life history strategy of AMF may influence the active community at a given time. These data show that the diversity of AMF is not always low in arable soils. Furthermore, low‐input agriculture involving crop rotation may provide better conditions to preserve AMF diversity, by preventing the selection for the few AMF taxa tolerating high nutrient levels.

Arbuscular mycorrhizae in a long-term field trial comparing low-input (organic, biological) and high-input (conventional) farming systems in a crop rotation

Abstract Arbuscular mycorrhizal (AM) root colonization was studied in a long-term field trial in which four farming systems currently in use in Switzerland were continuously applied to a randomized set of plots at a single field site from 1978 till 1993. There were two low-input farming systems (organic and bio-dynamic) and two high-input (conventional) farming systems (according to Swiss guidelines of integrated plant production with and without farmyard manure). The systems had an identical 7-year crop rotation and tillage scheme and differed essentially only in the amount and type of fertilizer supplied and in plant protection management. The percentage of root colonization by AM fungi was determined in field samples 2–3 times over the growing season in crops in the rotation, namely in winter wheat (Triticum aestivum L. cv. Sardona), vetch-rye and grass-clover. We found the percentage of root length colonized by AM fungi to be 30–60% higher (P≤0.05) in the plants grown in soils from the low-input farming systems than in those grown in conventionally farmed soils. Approximately 50% of the variation of AM root colonization was explained by chemical properties of the soils (pH, soluble P and K, exchangeable Mg), the effect of soluble soil P being most pronounced. The potential of the field soils from the differently managed plots to cause symbiosis with AM fungi was tested in a glasshouse experiment, using wheat as a host plant. Soils from the low-input farming systems had a greatly enhanced capacity to initiate AM symbiosis. The relative differences in this capacity remained similar when propagules of the AM fungus Glomus mosseae were experimentally added to the soils, although overall root colonization by AM fungi was 2.8 times higher.

Trehalose and trehalase in plants: recent developments

Abstract Trehalose is a non-reducing disaccharide consisting of two α-glycosidically linked glucose units. It accumulates in many microorganisms and invertebrate animals when they are exposed to various forms of stress, and it may serve as a protectant of enzymes and membranes, particularly under conditions of heat and desiccation stress. Most vascular plants lack the capacity to produce trehalose, except for a small number of desiccation tolerant plants, such as some ferns and the angiosperm Myrothamnus flabellifolia . In contrast, a highly specific trehalase activity has been described in many plants. The enzyme does not cleave other common α-glucosides, and it is highly sensitive to the inhibitor validamycin A. Trehalases have been found in various tissues; particularly high activities occur in pollen and legume root nodules. The possible functions of plant trehalase are discussed, focussing on its significance in the interaction of plants with trehalose-accumulating microorganisms.

Differential Expression of Eight Chitinase Genes in Medicago truncatula Roots During Mycorrhiza Formation, Nodulation, and Pathogen Infection

Expression of eight different chitinase genes, representing members of five chitinase classes, was studied in Medicago truncatula roots during formation of arbuscular mycorrhiza with Glomus intraradices, nodulation with Rhizobium meliloti, and pathogen attack by Phytophthora megasperma f. sp. medicaginis, Fusarium solani f. sp. phaseoli (compatible interactions with root rot symptoms), Ascochyta pisi (compatible, symptomless), and F. solani f. sp. pisi (incompatible, nonhost interaction). In the compatible plant-pathogen interactions, expression of class I, II, and IV chitinase genes was enhanced. The same genes were induced during nodulation. Transcripts of class I and II chitinase genes accumulated transiently during early stages of the interaction, and transcripts of the class IV chitinase gene accumulated in mature nodules. The pattern of chitinase gene expression in mycorrhizal roots was markedly different: Expression of class I, II, and IV chitinase genes was not enhanced, whereas expression of three class III chitinase genes, with almost no basal expression, was strongly induced. Two of these three (Mtchitinase III-2 and Mtchitinase III-3) were not induced at all in interactions with pathogens and rhizobia. Thus, the expression of two mycorrhiza-specific class III chitinase genes can be considered a hallmark for the establishment of arbuscular mycorrhiza in Medicago truncatula.

Rhizobial Nodulation Factors Stimulate Mycorrhizal Colonization of Nodulating and Nonnodulating Soybeans

Legumes form tripartite symbiotic associations with noduleinducing rhizobia and vesicular-arbuscular mycorrhizal fungi. Co-inoculation of soybean (Glycine max [L.] Merr.) roots with Bradyrhizobium japonicum 61-A-101 considerably enhanced colonization by the mycorrhizal fungus Glomus mosseae. A similar stimulatory effect on mycorrhizal colonization was also observed in nonnodulating soybean mutants when inoculated with Bradyrhizobium japonicum and in wild-type soybean plants when inoculated with ineffective rhizobial strains, indicating that a functional rhizobial symbiosis is not necessary for enhanced mycorrhiza formation. Inoculation with the mutant Rhizobium sp. NGR[delta]nodABC, unable to produce nodulation (Nod) factors, did not show any effect on mycorrhiza. Highly purified Nod factors also increased the degree of mycorrhizal colonization. Nod factors from Rhizobium sp. NGR234 differed in their potential to promote fungal colonization. The acetylated factor NodNGR-V (MeFuc, Ac), added at concentrations as low as 10–9 M, was active, whereas the sulfated factor, NodNGR-V (MeFuc, S), was inactive. Several soybean flavonoids known to accumulate in response to the acetylated Nod factor showed a similar promoting effect on mycorrhiza. These results suggest that plant flavonoids mediate the Nod factor-induced stimulation of mycorrhizal colonization in soybean roots.