Cellular localization of a plant protein PSAM 1 in arbuscular mycorrhizas of Pisum sativum

Psam 1 is a single-copy gene which is activated during early plant-fungal interaction in wild-type pea inoculated with Glomus mosseae and which codes for PSAM 1, a putative protein of 108 amino acids. A synthetic peptide was designed in an antigenic region of this protein to produce a polyclonal antibody against PSAM 1 and to investigate its cellular localization. Western blot analysis revealed that a polypeptide of about 14.5 kDa accumulated more in mycorrhizal than non-mycorrhizal pea roots. The PSAM 1 antigen was immunolocated in planta in arbuscule-containing cells of mycorrhizal roots and especially in the cytoplasm surrounding young arbuscules in cortical cells, which suggests that its accumulation is somehow related to the symbiotic state of these cells. Received: 27 June 1998 / Accepted: 27 July 1998     

Rock-eating mycorrhizas: their role in plant nutrition and biogeochemical cycles

Rates of water uptake by individual trees in a native Australian forest were measured on the Liverpool Plains, New South Wales, Australia, using sapflow sensors. These rates were up-scaled to stand transpiration rate (expressed per unit ground area) using sapwood area as the scalar, and these estimates were compared with modelled stand transpiration. A modified Jarvis-Stewart modelling approach (Jarvis 1976), previously used to calculate canopy conductance, was used to calculate stand transpiration rate. Three environmental variables, namely solar radiation, vapour pressure deficit and soil moisture content, plus leaf area index, were used to calculate stand transpiration, using measured rates of tree water use to parameterise the model. Functional forms for the model were derived by use of a weighted non-linear least squares fitting procedure. The model was able to give comparable estimates of stand transpiration to those derived from a second set of sapflow measurements. It is suggested that short-term, intensive field campaigns where sapflow, weather and soil water content variables are measured could be used to estimate annual patterns of stand transpiration using daily variation in these three environmental variables. Such a methodology will find application in the forestry, mining and water resource management industries where long-term intensive data sets are frequently unavailable.     

Plant and fungal identity determines pathogen protection of plant roots by arbuscular mycorrhizas

1.  A major benefit of the mycorrhizal symbiosis is that it can protect plants from below-ground enemies, such as pathogens. Previous studies have indicated that plant identity (particularly plants that differ in root system architecture) or fungal identity (fungi from different families within the Glomeromycota) can determine the degree of protection from infection by pathogens. Here, we test the combined effects of plant and fungal identity to assess if there is a strong interaction between these two factors.
2.  We paired one of two plants ( Setaria glauca , a plant with a finely branched root system and Allium cepa , which has a simple root system) with one of six different fungal species from two families within the Glomeromycota. We assessed the degree to which plant identity, fungal identity and their interaction determined infection by Fusarium oxysporum , a common plant pathogen.
3.  Our results show that the interaction between plant and fungal identity can be an important determinant of root infection by the pathogen. Infection by Fusarium was less severe in Allium (simple root system) or when Setaria (complex root system) was associated with a fungus from the family Glomeraceae. We also detected significant plant growth responses to the treatments; the fine-rooted Setaria benefited more from associating with a member of the family Glomeraceae, while Allium benefited more from associating with a member of the family Gigasporaceae.
4.   Synthesis . This study supports previous claims that plants with complex root systems are more susceptible to infection by pathogens, and that the arbuscular mycorrhizal symbiosis can reduce infection in such plants – provided that the plant is colonized by a mycorrhizal fungus that can offer protection, such as the isolates of Glomus used here.     

Functional diversity of arbuscular mycorrhizal fungal isolates in relation to extraradical mycelial networks

We investigated the functional significance of extraradical mycorrhizal networks produced by geographically different isolates of the arbuscular mycorrhizal fungal (AMF) species Glomus mosseae and Glomus intraradices. A two-dimensional experimental system was used to visualize and quantify intact extraradical mycelium (ERM) spreading from Medicago sativa roots. Growth, phosphorus (P) and nitrogen (N) nutrition were assessed in M. sativa plants grown in microcosms. The AMF isolates were characterized by differences in extent and interconnectedness of ERM. Phenotypic fungal variables, such as total hyphal length, hyphal density, hyphal length per mm of total or colonized root length, were positively correlated with M. sativa growth response variables, such as total shoot biomass and plant P content. The utilization of an experimental system in which size, growth rate, viability and interconnectedness of ERM extending from mycorrhizal roots are easily quantified under realistic conditions allows the simultaneous evaluation of different isolates and provides data with a predictive value for selection of efficient AMF.     

Organization and expression of genes involved in the biosynthesis of 987P fimbriae

Summary A chromosomal DNA segment encoding the biosynthesis of 987P fimbriae was isolated by cosmid-cloning and subsequent subcloning into pBR322. The 12 kb DNA segment expressed five polypeptides with apparent molecular weights of 81,000, 39,000, 28,500, 20,500, and 16,500, respectively. The location of the corresponding genes was determined by insertional mutagenesis using Tn5. The 20.5 K polypeptide was identified as the 987P fimbrial subunit by its reaction with specific anti-987P antibodies. The 81, 39, and 28.5 K polypeptides appeared to be accessory proteins involved in 987P production.     

Mechanisms of nutrient transport across interfaces in arbuscular mycorrhizas

Bidirectional nutrient transfer between the plant and the fungus is a key feature of arbuscular mycorrhizal symbiosis. The major nutrients exchanged between the symbiotic partners are reduced carbon, assimilated through the plant photosynthesis and phosphate, taken up by the fungal hyphae exploring soil microhabitats. This nutrient exchange takes place across the symbiotic interfaces which are bordered by the plant and fungal plasma membranes. This review provides an overview of the current knowledge of the mechanisms underlying nutrient transport processes in the symbiosis, with special emphasis on recent developments in the molecular biology of the plant and fungal primary (H⁺-ATPases) and secondary transporters.     

Molecular approaches to study plasma membrane H+-ATPases in arbuscular mycorrhizas

The activity of H⁺-ATPases of plant and fungi generates an electrochemical gradient of H⁺ across the cell plasma membrane that drives a number of secondary transport systems, including those responsible for the translocation of cations, anions, amino acids and sugars. During the last years, several studies have been aimed at elucidating the role of plasma membrane H⁺-ATPases in the nutrient exchange processes taking place between the plant and the fungus in arbuscular mycorrhizal (AM) symbiosis. This paper reviews present knowledge about plasma membrane H⁺-ATPases and experimental evidence supporting the involvement of H⁺-ATPases of both organisms in the bidirectional transport of nutrients between partners. Molecular strategies that will provide further information on the function and regulation of plasma membrane H⁺-ATPases in AM symbiosis are presented and discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Preliminary assessment of plant community structure and arbuscular mycorrhizas in rangeland habitats of Cholistan desert, Pakistan

Plant species dominance and arbuscular mycorrhizas (AMs) were evaluated in four different habitats of Cholistan desert, Pakistan. The sites were selected on the basis of variations in topography, floristic composition and biotic interference. The phytosociological data revealed obvious differences in the floristic composition, plant species diversity, AM colonization and arbuscular mycorrhizal fungi (AMF) spore densities in the rhizospheres. The vegetation pattern and composition was governed by topography and edaphic characteristics of a particular site. Grasses were dominant and main components of vegetation at all the study sites and were invariably mycorrhizal. Some plant species were mycorrhizal at one site but non-mycorrhizal at the other. The AMF survey reported here offers an important starting point from which to analyse AMF community structure in different phytosociological habitats and land uses of Cholistan desert.