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Many studies have established that arbuscular mycorrhizal fungi transfer N to the host plant. However, the role and importance of arbuscular mycorrhiza (AM) in plant N nutrition is still uncertain, as are the C/N interactions within the symbiosis. Published reports provide differing, and often contradictory, results that are difficult to combine in a coherent framework. This review explores questions such as: What makes the difference between a positive and a negative effect of AM on plant N nutrition? Is the mycorrhizal N response (MNR) correlated to the mycorrhizal growth response (MGR), and how or under which conditions? Is the MNR effect on plant growth C mediated? Is plant C investment on fungal growth related to N needs or N benefit? How is the N for C trade between symbionts regulated? The patternless nature of current knowledge is made evident, and possible reasons for this are discussed. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AM) symbioses are mutualistic associations between soil fungi and most vascular plants. Modulation of the hormonal and transcriptional profiles, including changes related to defense signalling, has been reported in many host plants during AM symbioses. These changes have been often related to the improved stress tolerance common in mycorrhizal plants. However, results on the alterations in phytohormones content and their role on the symbiosis are controversial. Here, an integrative analysis of the response of phylogenetically diverse plants (i.e., tomato, soybean, and maize) to two mycorrhizal fungi - Funneliformis mosseae and Rhizophagus irregularis- was performed. The analysis of the defense-related hormones salicylic acid, abscisic acid, and jasmonates, and the expression of marker genes of the pathways they regulate, revealed significant changes in the roots of mycorrhizal plants. These changes depended on both the plant and the AM fungus (AMF) involved. However, general trends can be identified: roots associated with the most effective colonizer R. irregularis showed fewer changes in these defense-related traits, while the colonization by F. mosseae led to significant modifications in all plants tested. The up-regulation of the jasmonate pathway by F. mosseae was found to be highly conserved among the different plant species, suggesting an important role of jasmonates during this AM interaction. Our study evidences a strong influence of the AMF genotype on the modulation of host defense signalling, and offers hints on the role of these changes in the symbiosis. [ABSTRACT FROM AUTHOR]

The kinetics and energetics of NH uptake by the extraradical mycelium of the arbuscular mycorrhizal fungus Rhizophagus irregularis were investigated. NH uptake increased with increasing substrate concentration over the concentration range of 0.002 to 25 mM. Eadie-Hofstee plots showed that ammonium (NH) uptake over this range was biphasic. At concentrations below 100 μM, NH uptake fits a Michaelis-Menten curve, typical of the activity of a saturable high-affinity transport system (HATS). At concentrations above 1 mM, NH influx showed a linear response typical of a nonsaturable low-affinity transport system (LATS). Both transport systems were dependent on external pH. The HATS and, to a lesser extent, the LATS were inhibited by the ionophore carbonylcyanide m-chlorophenylhydrazone (CCCP) and the ATP-synthesis inhibitor 2,4-dinitrophenol. These data indicate that the two NH transport systems of R. irregularis are dependent on metabolic energy and on the electrochemical H gradient. The HATS- and the LATS-mediated NH influxes were also regulated by acetate. This first report of the existence of active high- and low-affinity NH4 transport systems in the extraradical mycelium of an arbuscular mycorrhizal fungus and provides novel information on the mechanisms underlying mycosymbiont uptake of nitrogen from the soil environment. [ABSTRACT FROM AUTHOR]

Discusses mechanisms of nutrient transport across interfaces in arbuscular mycorrhizas (AM). Structure and composition of AM symbiotic interfaces; Supply of carbon compounds to the fungus; Transfer of phosphate from the fungus to the plant.

Presents molecular method to examine plasma membrane H[sup +]-adenosine triphosphatases in arbuscular mycorrhizas (AM). Translocation of cations, anions, amino acids and sugars; Electrochemical gradient of H[sup +] across the cell plasma membrane; Nutrient exchange processes taking place between the plant and the fungus in AM symbiosis.

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