Fungal spore diversity of arbuscular mycorrhizal fungi associated with spring wheat: effects of tillage

Assessment of mycorrhizal association of a threatened medicinal plant Clerodendrum indicum (L.) O. Kuntze (Verbenaceae) in different ecological variations

Mycorrhizae association is reported to enhance the survivability of the host plant under adverse environmental conditions. The present study aims to explore the mycorrhizal association in the roots of different ecotypes of a threatened medicinal plant, Clerodendrum indicum (L.) O. Kuntze (Verbenaceae), collected from W.B., India, which correlates the degree of root colonization to the nutritional status of the native soil. Ten ecotypes of C. indicum having diverse morphological variations were collected. The mycorrhizae were characterized by both morphological and molecular methods. The nutritional status of the native soils was estimated. The study revealed that all the ecotypes have an association with mycorrhizal forms like hyphae, arbuscules, and vesicles. The molecular analysis showed Glomus intraradices and Rhizophagus irregularis as the associated arbuscular mycorrhizal fungi (AMF). A significant variation in arbuscule and vesicle formation was found growing in the varied nutritional statuses concerning soil parameters. The arbuscule was found negatively correlated with pH, conductivity, and potassium and positively correlated with organic carbon, nitrogen, and phosphorus. The vesicle was found positively correlated with pH, organic carbon, and potassium and negatively correlated with conductivity, nitrogen, and phosphorus. The interaction between conductivity: nitrogen, conductivity: phosphorus, organic-carbon: nitrogen, and pH: conductivity was significant in influencing vesicle formation. However, none of the interactions between parameters was found significant in influencing arbuscule formation. Thus, the study concludes that G. intraradices and R. irregularis are the principle mycorrhizae forming the symbiotic association with the threatened medicinal plant, C. indicum. They form vesicles and arbuscules based on their soil nutritive factors. Therefore, a large-scale propagation through a selective AMF association would help in the conservation of this threatened species from extinction.

Density- and moisture-dependent effects of arbuscular mycorrhizal fungus on drought acclimation in wheat

Arbuscular mycorrhizal fungus (AMF) is widely viewed as an ecosystem engineer to help plants adapt to adverse environments. However, a majority of the previous studies regarding AMF's eco-physiological effects are mutually inconsistent. To clarify this fundamental issue, we conducted an experiment focused on wheat (Triticum aestivum L.) plants with or without AMF (Funneliformis mosseae) inoculation. Two water regimes (80% and 40% field water capacity, FWC80 (CK) and FWC40 (drought stress) and four planting densities (6 or 12 plants per pot as low densities, 24 or 48 plants per pot as high densities) were designed. AMF inoculation did not show significant effects on shoot biomass, grain yield, and water use efficiency (WUE) under the low densities, regardless of water regimes. However, under the high densities, AMF inoculation significantly decreased shoot biomass, grain yield and WUE in FWC80, while it significantly increased these parameters in FWC40, showing density and/or moisture-dependent effects of AMF on wheat performance. In FWC40, the relationships between reproductive biomass (y-axis) vs. vegetative biomass (x-axis) (R-V), and between grain biomass (y-axis, sink) vs. leaf biomass (x-axis, source) fell into a typical allometric pattern (α > 1, P < 0.001), and the AMF inoculation significantly increased the values of α. Yet in FWC80, they were in an isometric pattern (α ≈ 1, P < 0.001) and AMF addition had no significant effects on α. Similarly, AMF did not significantly change the isometric relationship between leaf biomass (i.e., metabolic rate) and shoot biomass (body size) in FWC80, while it significantly decreased the α of allometric relationship between both of them in FWC40 (α > 1, P < 0.001). We therefore, sketched a generalized model of R-V and sink-source relationships as affected by AMF, in which AMF inoculation might enhance the capabilities of sink acquisition and utilization under drought stress, while having no significant effect under the well watered conditions. Our findings demonstrate dual density- and moisture-dependent effects of AMF on plant development and provide new insights into current ecological applications of AMF as an ecosystem engineer.

[Mycotrophic capacity and efficiency of microbial consortia of arbuscular mycorrhizal fungi native of soils from Buenos Aires province under contrasting management]

We characterized the infective and sporulation capacities of microbial consortia of arbuscular mycorrhizal fungi (AMF) native of Buenos Aires province (Argentina) and determined if some soil characteristics and mycorrhizal parameters could allow to select potentially beneficial inocula. Soil samples were selected from seven locations in Buenos Aires province all under agricultural (A) and pristine (P) conditions. The AMF were multiplied and mycorrhizal root colonization of trap plants was observed at 10 weeks of growth. Spore number in field was low; however, after multiplication spore density accounted for 80-1175 spores per 100g of soil. The principal component analysis showed that the P and Fe soil contents are the main modulators of infectivity and sporulation capacity. The mycorrhizal potential was determined in three locations, being high in Pristine Lobería and Agricultural Trenque Lauquen and low in Junín. Agricultural Lobería (AL) and Pristine Lobería (PL) inocula were selected and their efficiency was evaluated under controlled conditions. Even though shoot dry matter increases after inoculation was not significant (p>0.05) mycorrhizal response was greater than 40% for tomato and 25% for corn, particularly after inoculation with inocula from the agricultural management. These results could be associated to the incipient development of mycorrhizae in both species. Additional research should be conducted to further develop our findings in order to determine the factors involved in the selection of efficient inocula.

Agricultural practices indirectly influence plant productivity and ecosystem services through effects on soil biota

It is well established that agricultural practices alter the composition and diversity of soil microbial communities. However, the impact of changing soil microbial communities on the functioning of the agroecosystems is still poorly understood. Earlier work showed that soil tillage drastically altered microbial community composition. Here we tested, using an experimental grassland (Lolium, Trifolium, Plantago) as a model system, whether soil microbial communities from conventionally tilled (CT) and non-tilled (NT) soils have different influences on plant productivity and nutrient acquisition. We specifically focus on arbuscular mycorrhizal fungi (AMF), as they are a group of beneficial soil fungi that can promote plant productivity and ecosystem functioning and are also strongly affected by tillage management. Soil microbial communities from CT and NT soils varied greatly in their effects on the grassland communities. Communities from CT soil increased overall biomass production more than soil communities from NT soil. This effect was mainly due to a significant growth promotion of Trifolium by CT microorganisms. In contrast to CT soil inoculum, NT soil inoculum increased plant phosphorus concentration and total plant P content, demonstrating that the soil microbial communities from NT fields enhance P uptake. Differences in AM fungal community composition resulting, for instance, in twofold greater hyphal length in NT soil communities when compared to CT, are the most likely explanation for the different plant responses to CT and NT soil inocula. A range of field studies have shown that plant P uptake increases when farmers change to conservation tillage or direct seeding. Our results indicate that this enhanced P uptake results from enhanced hyphal length and an altered AM fungal community. Our results further demonstrate that agricultural management practices indirectly influence ecosystem services and plant community structure through effects on soil biota.

Various forms of organic and inorganic P fertilizers did not negatively affect soil- and root-inhabiting AM fungi in a maize-soybean rotation system

Arbuscular mycorrhizal (AM) fungi are key components of most agricultural ecosystems. Therefore, understanding the impact of agricultural practices on their community structure is essential to improve nutrient mobilization and reduce plant stress in the field. The effects of five different organic or mineral sources of phosphorus (P) for a maize-soybean rotation system on AM fungal diversity in roots and soil were assessed over a 3-year period. Total DNA was extracted from root and soil samples collected at three different plant growth stages. An 18S rRNA gene fragment was amplified and taxa were detected and identified using denaturing gradient gel electrophoresis followed by sequencing. AM fungal biomass was estimated by fatty acid methyl ester analysis. Soil P fertility parameters were also monitored and analyzed for possible changes related with fertilization or growth stages. Seven AM fungal ribotypes were detected. Fertilization significantly modified soil P flux, but had barely any effect on AM fungi community structure or biomass. There was no difference in the AM fungal community between plant growth stages. Specific ribotypes could not be significantly associated to P treatment. Ribotypes were associated with root or soil samples with variable detection frequencies between seasons. AM fungal biomass remained stable throughout the growing seasons. This study demonstrated that roots and soil host distinct AM fungal communities and that these are very temporally stable. The influence of contrasting forms of P fertilizers was not significant over 3 years of crop rotation.

Composition and structure of arbuscular-mycorrhizal communities in El Palmar National Park, Argentina

The arbuscular-mycorrhizal-fungal (AMF) communities from the El Palmar National Park of Entre Ríos Province, Argentina, were investigated and characterized. The species of AMF present in five distinct vegetation types-gallery forest, grassland, marsh, palm forest, and scrubland-were isolated, identified and quantified over 2 y. Forty-six AMF morphotaxa were found. The composition of the AMF communities differed between the seasons, soil and vegetation types. Seasonal variations were observed in members of the Acaulosporaceae, Archaeosporaceae, Claroideoglomeraceae, Gigasporaceae and Pacisporaceae. Depending on soil type, the AMF-spore communities were dominated by members of one of the two main orders of the Glomeromycota. AMF communities from grassland and palm forest, which occur on sandy soils, comprised primarily members of the Diversisporales, with a high percentage of species of Acaulospora and of Gigasporaceae. Communities from the gallery forest, marsh and scrubland, which occur on loam-clay soils, were composed of members of the Glomerales, with a high percentage of spores from species of Glomus. Thus, both AMF and plant communities would appear to be strongly and similarly influenced by edaphic conditions.

Short-term temporal variation in sporulation dynamics of arbuscular mycorrhizal (AM) fungi and physico-chemical edaphic properties of wheat rhizosphere

In this study, we investigated the pattern of short-term temporal variation in the arbuscular mycorrhizal (AM) fungi and physico-chemical edaphic properties of some wheat growing areas of the Bundelkhand region, Central India. Rhizospheric soil samples were collected every month from December 2007 to May 2008 from four wheat growing sites around Jhansi (Bundelkhand region). AM fungal root colonization, sporulation and physico-chemical edaphic properties during this period were determined and compared to evaluate the dynamics of response of wheat towards the AMF along crop maturation. Maximum AMF root colonization recorded was 54.3% indicating that AMF, particularly in low phosphorus (P) soils, can be important even in case of less responsive crop like wheat. In the two out of four sites studied, the AMF spore density increased with the increase in soil temperature. Absence of this type of pattern in remaining two sites indicated that site-specific environmental and agricultural conditions may affect the degree of wheat response to AMF. It also suggested that AMF communities inhabiting agroecosystems may exhibit considerable temporal sporulation patterns. The maximum AMF colonization was observed during February-March 2008, whereas maximum AMF sporulation was noticed during March-April 2008. Statistically significant negative correlation of AMF spore density with pH, organic carbon (OC) and available P was observed in the one of the sites studied. Overall assessment of the data indicated that season and location significantly affected the interaction of AM fungi with winter wheat necessitating the further need to understand the ecology of AMF populations with reference to specific host species under different micro-climatic conditions of Bundelkhand region.

Native arbuscular mycorrhizal fungi in the Yungas forests, Argentina

The arbuscular mycorrhizal fungal (AMF) communities from the Yungas forests of Argentina were studied. The AMF species present in the rhizosphere of some dominant native plants (one tree: Alnus acuminata; three herbaceous species: Duchesnea indica, Oxalis conorrhiza, Trifolium aff. repens; and one shrub: Sambucus peruviana) from two sites (Quebrada del Portugués and Narváez Range) of the Yungas forests were isolated, identified and quantified during the four seasons of the year. Twenty-two AMF morphotaxa were found. Spore density of some AMF species at each site varied among seasons. The genera that most contributed to the biodiversity index were Acaulospora for Quebrada del Portugués and Glomus for Narváez Range. High diversity values were observed in the Yungas forests, particularly in the spring (rainy season). We concluded AMF differed in species composition and seasonal sporulation dynamics in the Yungas forests.

Order of plant host establishment alters the composition of arbuscular mycorrhizal communities

The causes of local diversity and composition remain a central question in community ecology. Numerous studies have attempted to understand community assembly, both within and across trophic levels. However, little is known about how community assembly aboveground influences soil microbial communities belowground. We hypothesized that plant establishment order can affect the community of arbuscular mycorrhizal fungi (AMF) in roots, with the strength of this effect dependent on both host plant identity and neighboring plant identity. Such priority effects of plants on AMF may act through host-specific filters of the initial species pool that limit the available pool for plants that established second. In a greenhouse experiment with four plant hosts, we found that the strength of the priority effect on AMF communities reflected both host plant characteristics and interactions between host and neighbor plant species, consistent with differential host specificity among plants. These patterns were independent of plant biomass and root colonization. Functional studies of AMF associated with a wide array of host plants will be required to further understand this potential driver of community dynamics.