Michael F. Allen

Patterns and regulation of mycorrhizal plant and fungal diversity

The diversity of mycorrhizal fungi does not follow patterns of plant diversity, and the type of mycorrhiza may regulate plant species diversity. For instance, coniferous forests of northern latitudes may have more than 1000 species of ectomycorrhizal (EM) fungi where only a few ectomycorrhizal plant species dominate, but there are fewer than 25 species of arbuscular mycorrhizal (AM) fungi in tropical deciduous forest in Mexico with 1000 plant species. AM and EM fungi are distributed according to biome, with AM fungi predominant in arid and semiarid biomes, and EM fungi predominant in mesic biomes. In addition, AM fungi tend to be more abundant in soils of low organic matter, perhaps explaining their predominance in moist tropical forest, and EM fungi generally occur in soils with higher surface organic matter.EM fungi are relatively selective of host plant species, while AM tend to be generalists. Similar morphotypes of AM fungi collected from different sites confer different physiological benefits to the same plant species. While the EM fungi have taxonomic diversity, the AM fungi must have physiological diversity for individual species to be so widespread, as supported by existing studies. The environmental adaptations of mycorrhizal fungi are often thought to be determined by their host plant, but we suggest that the physiology and genetics of the fungi themselves, along with their responses to the plant and the environment, regulates their diversity. We observed that one AM plant species,Artemisia tridentata, was associated with different fungal species across its range, indicating that the fungi can respond to the environment directly and must not do so indirectly via the host. Different species of fungi were also active during different times of the growing season on the same host, again suggesting a direct response to the environment.These patterns suggest that even within a single “functional group” of microorganisms, mycorrhizal fungi, considerable diversity exists. A number of researchers have expressed the concept of functional redundancy within functional groups of microorganisms, implying that the loss of a few species would not be detectable in ecosystem functioning. However, there may be high functional diversity of AM fungi within and across habitats, and high species diversity as well for EM fungi. If one species of mycorrhizal fungus becomes extinct in a habitat, field experimental data on AM fungi suggest there may be significant shifts in how plants acquire resources and grown in that habitat.

The Spread of Va Mycorrhizal Fungal Hyphae in the Soil: Inoculum Types and External Hyphal Architecture

Different forms of vesicular-arbuscular mycorrhizal fungal inoculum are found in soil patches with differing histories. These inoculum types include spores, infected root fragments, and extramatrical hyphae. Utilizing root observation chambers, detailed observations were made of the growth dynamics of mycorrhizal hyphae from inoculum sources to roots and the subsequent radiation out from individual colonized root segments. There were several types of hyphal architecture classified by whether the external hyphae were entering (infecting) a plant root or developing from the root surface and growing out into the soil matrix. Morphologically, individual hyphal filaments within all architectural types looked quite similar and clear architectural differences could only be identified when distinct sets of interconnected hyphal filaments were studied. The primary hyphal architectural types observed in the soil were classified as runner hyphae, hyphal bridges, absorptive hyphal networks, germ tubes, and infection networks produced by spores and root fragments. The specialized hyphal architectures of these fungal endophytes appear to be linked to the unique function of each hyphal type. Runner hyphae and infection networks from either spores or root fragments were observed to be capable of infecting new root segments. Absorptive hyphal networks were never observed to act as units of infection and were classified as structures primarily involved in the acquisition of soil resources. This high degree of specialization may help explain why vesicular-arbuscular mycorrhizal fungi are so efficient in their beneficial role of nutrient and water transport to the host plant.

COUPLING FINE ROOT DYNAMICS WITH ECOSYSTEM CARBON CYCLING IN BLACK SPRUCE FORESTS OF INTERIOR ALASKA

Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs, and mosses were measured relative to soil CO2 fluxes. Fine root production, measured over a two-year period using minirhizotrons, varied from 0.004 ± 0.001 mm·cm–2·d–1 over winter, to 0.051 ± 0.015 mm·cm–2·d–1 during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. M...

Competition for phosphorus: differential uptake from dual-isotope-labeled soil interspaces between shrub and grass

Two species of Agropyron grass differed strikingly in their capacity to compete for phosphate in soil interspaces shared with a common competitor, the sagebrush Artemisia tridentata. Of the total phosphorus-32 and -33 absorbed by Artemisia, 86 percent was from the interspace shared with Agropyron spicatum and only 14 percent from that shared with Agropyron desertorum. Actively absorbing mycorrhizal roots of Agropyron and Artemisia were present in both interspaces, where competition for the labeled phosphate occurred. The results have important implications about the way in which plants compete for resources below ground in both natural plant communities and agricultural intercropping systems.

Rise in carbon dioxide changes soil structure

Carbon in soil affects the formation and stabilization of aggregates (groups of primary particles that adhere to each other more strongly than to surrounding soil particles). Soil aggregation is important for preventing soil loss through wind and water erosion, and the size distribution and abundance of water-stable aggregates influences a range of physical, chemical, biological and agricultural properties of soil. The effects on soil biota and nutrient cycling of increases in soil carbon availability, brought about by increased CO2, are well studied, but the consequences for soil aggregation and structure have not been examined. Here we show for three ecosystems that the water stability and size distribution of aggregates is affected by long-term CO2fumigation, and we propose a mechanism for this that involves the production by fungi of the glycoprotein glomalin.

Arabidopsis CHL27, located in both envelope and thylakoid membranes, is required for the synthesis of protochlorophyllide

CHL27, the Arabidopsis homologue to Chlamydomonas Crd1, a plastid-localized putative diiron protein, is required for the synthesis of protochlorophyllide and therefore is a candidate subunit of the aerobic cyclase in chlorophyll biosynthesis. δ-Aminolevulinic acid-fed antisense Arabidopsis plants with reduced amounts of Crd1/CHL27 accumulate Mg-protoporphyrin IX monomethyl ester, the substrate of the cyclase reaction. Mutant plants have chlorotic leaves with reduced abundance of all chlorophyll proteins. Fractionation of Arabidopsis chloroplast membranes shows that Crd1/CHL27 is equally distributed on a membrane-weight basis in the thylakoid and inner-envelope membranes.

Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands

Abstract Root, arbuscular-mycorrhizal (AM), soil faunal (protozoa and microarthropods), and microbial responses to field exposure to CO2 for six growing seasons were measured in spring 1997 in two adjacent grassland communities. The grasslands showed contrasting root responses to CO2 enrichment: whereas root length was not affected in the sandstone grassland, it was greater in the serpentine grassland, as was specific root length. AM fungal hyphal lengths were greater in the sandstone, but were unaffected in the serpentine community. This lent support to the hypothesis that there may be a tradeoff in resource allocation to more fine roots or greater mycorrhizal extraradical hyphal length. AM root infection was greater in both communities at elevated CO2, as was the proportion of roots containing arbuscules. Our data on total hyphal lengths, culturable and active fungi, bacteria, and protozoa supported the hypothesis that the fungal food chain was more strongly stimulated than the bacterial chain. This study is one of the first to test these hypotheses in natural multi-species communities in the field.

Abrupt rise in atmospheric CO2 overestimates community response in a model plant-soil system.

Attempts to understand the ecological effect of increasing atmospheric CO2 concentration, [CO2], usually involve exposing todays ecosystems to expected future [CO2] levels. However, a major assumption of these approaches has not been tested—that exposing ecosystems to a single-step increase in [CO2] will yield similar responses to those of a gradual increase over several decades. We tested this assumption on a mycorrhizal fungal community over a period of six years. [CO2] was either increased abruptly, as is typical of most [CO2] experiments, or more gradually over 21 generations. The two approaches resulted in different structural and functional community responses to increased [CO2]. Some fungi were sensitive to the carbon pulse of the abrupt [CO2] treatment. This resulted in an immediate decline in fungal species richness and a significant change in mycorrhizal functioning. The magnitude of changes in fungal diversity and functioning in response to gradually increasing [CO2] was smaller, and not significantly different to those with ambient [CO2]. Our results suggest that studies may overestimate some community responses to increasing [CO2] because biota may be sensitive to ecosystem changes that occur as a result of abrupt increases.

DISPERSAL AGENTS OF VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI IN A DISTURBED ARID ECOSYSTEM

An evaluation of wind, small mammals and grasshoppers as dispersal agents of vesicular-arbuscular mycorrhizal (VAM) fungi (Endogonaceae) was conducted in a shrub-steppe community disturbed by strip-mining in southwestern Wyoming. Wind-dispersed VAM spores were collected on the study site with spore traps arranged in transects and in snow drifts across the site. Small mammals and grasshoppers were captured using traps and nets, respectively, and examined for the presence of VAM spores. To determine the source of the spores blown onto the study site, deposition patterns of spore mimics released from two potential source areas were assessed. Wind accounted for the movement of large numbers of spores onto the site from distances up to 2 km. Small mammals also appeared to move spores but at a lower magnitude. The importance of these vectors to the establishment of the VAM symbiosis remains to be elucidated.

The ecology of arbuscular mycorrhizas: a look back into the 20th century and a peek into the 21st

Arbuscular mycorrhizal symbioses have been an integral part of terrestrial ecosystems since the invasion of land by plants. Studies on the descriptions, phylogenetic relationships, and world-wide distributions date back to the late 1800s. By the 1950s, the basic systematic position of the fungi was known. By the early 1980s the underlying functions of AM were documented. During the last decade, most research has focused on detailing the varying forms that AM functions could take as the species compositions environments and environment/species interactions varied. Research into the next century should begin to develop a better understanding of population genetics of the fungi and how those influence phylogeny and ecological functioning, a better evaluation of the subtle changes in ‘mycorrhizal functioning’ given the large divergence in environmental and biological interactions, and the ability to link mycorrhizal dynamics into the changing global conditions.