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Variations in arbuscular mycorrhizae (AM) effects on plant growth (MGR) are commonly assumed to result from cost : benefit balances, with C as the cost and, most frequently, P as the benefit. The trade-balance model (TBM) adopts these assumptions and hypothesizes that mycorrhizal benefit depends on C : N : P stoichiometry. Although widely accepted, the TBM has not been experimentally tested. We isolated the parameters included in the TBM and tested these assumptions using it as framework. Oryza sativa plants were supplied with different N : P ratios at low light level, establishing different C : P and C : N exchange rates, and C, N or P limitation. MGR and effects on nutrient uptake, %M, ERM, photosynthesis and shoot starch were measured. C distribution to AM fungi played no role in MGR, and N was essential for all AM effects, including on P nutrition. C distribution to AM and MGR varied with the limiting nutrient (N or P), and evidence of extensive interplay between N and P was observed. The TBM was not confirmed. The results agreed with the exchange of surplus resources and source–sink regulation of resource distribution among plants and AMF. Rather than depending on exchange rates, resource exchange may simply obey both symbiont needs, not requiring further regulation.

Arbuscular mycorrhiza (AM) is a widespread symbiosis in nature whose establishment and function is influenced by environmental factors. Mineral nutrients are known to influence AM. In this work, we investigated the importance of iron homeostasis in the Rhizophagus irregularis‐Solanum lycopersicuminteraction by using the iron inefficient mutant chloronerva (chln) of tomato. AM development was impaired in the chln mutant, but the symbiosis partially reverted its growth defect. RNA sequencing revealed a lower number of differentially expressed genes in the “chln vs wild‐type” comparison in mycorrhizal than in non‐inoculated roots. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathways enrichment analyses revealed differential metabolic responses to AM colonization between roots of wild‐type and chln mutant plants. Expression of a subset of genes that have been previously shown to be required for AM establishment and function was lower in the mycorrhizal roots of chln mutants compared to the wild‐type. The lower expression levels of the phosphate starvation‐induced genes in non‐mycorrhizal chln roots compared to wild‐type and the higher phosphorus concentration of the chln plants suggests that reduced AM colonization of chln may involve phosphate signalling pathways. Over‐expression of iron‐deficiency response genes in chln roots was mitigated by AM colonization, which suggests that root AM colonization contributes to increase Fe availability and distribution in the chln mutants. All these data together indicate that there exists a connection between AM and Fe homeostasis.

Phosphorus (P) and iron (Fe) are two essential nutrients for plant growth and development. Both elements are abundant in most soils, but poorly available to plants. Recent evidence shows that P availability can affect plant Fe homeostasis and vice versa, but the regulatory mechanisms involved in the P‐Fe signalling crosstalk are still poorly characterized. To cope with P or Fe deficiency, plants have evolved a suit of adaptive strategies oriented towards increasing acquisition and use efficiency of these elements. A widespread strategy engaged by plants to overcome nutrient deficiencies is the formation of a mutualistic symbiotic interaction, referred as arbuscular mycorrhiza, with some soil‐borne fungi belonging to the subphylum Glomeromycotina within the phylum Mucoromycota. Arbuscular mycorrhizal fungi are obligate biotrophs that colonize the root cortex and develop an external mycelium that overgrows the rhizosphere. This hyphal network represents, therefore, an adaptation strategy to increase the supply of mineral nutrients to the plant. The contribution of the AM symbiosis to plant P uptake has been extensively studied. However, very little is known about its contribution to Fe nutrition and on the interactive effects of P and Fe in the symbiosis. Here, using the Solanum lycopersicum‐ Rhizophagus irregularis mycorrhizal system, we analysed the effect of different Pi and Fe regimes on plant performance and physiology as well as on P‐ and Fe‐deficiency related signalling by analysing the expression of key genes regulating Pi and Fe starvation signalling. Both P and Fe fertilization impacted plant performance, but the effects we modulated by the mycorrhizal status of the plant. The role of AM in the regulation of the P‐Fe interplay will be presented.

Transporters of the NRAMP family are ubiquitous metal-transition transporters, playing a key role in metal homeostasis, especially in Mn and Fe homeostasis. In this work, we report the characterization of the NRAMP family members (RiSMF1, RiSMF2, RiSMF3.1 and RiSMF3.2) of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis. Phylogenetic analysis of the NRAMP sequences of different AM fungi showed that they are classified in two groups, which probably diverged early in their evolution. Functional analyses in yeast revealed that RiSMF3.2 encodes a protein mediating Mn and Fe transport from the environment. Gene-expression analyses by RT-qPCR showed that the RiSMF genes are differentially expressed in the extraradical (ERM) and intraradical (IRM) mycelium and differentially regulated by Mn and Fe availability. Mn starvation decreased RiSMF1 transcript levels in the ERM but increased RiSMF3.1 expression in the IRM. In the ERM, RiSMF1 expression was up-regulated by Fe deficiency, suggesting a role for its encoded protein in Fe-deficiency alleviation. Expression of RiSMF3.2 in the ERM was up-regulated at the early stages of Fe toxicity but down-regulated at later stages. These data suggest a role for RiSMF3.2 not only in Fe transport but also as a sensor of high external-Fe concentrations. Both Mn-and Fe-deficient conditions affected ERM development. While Mn deficiency increased hyphal length, Fe deficiency reduced sporulation.

Arbuscular mycorrhizal (AM) fungi, symbionts of most land plants, increase plant fitness in metal contaminated soils. To further understand the mechanisms of metal tolerance in the AM symbiosis, the expression patterns of the maize Heavy Metal ATPase (HMA) family members and the ionomes of non-mycorrhizal and mycorrhizal plants grown under different Cu supplies were examined. Expression of ZmHMA5a and ZmHMA5b, whose encoded proteins were predicted to be localized at the plasma membrane, was up-regulated by Cu in non-mycorrhizal roots and to a lower extent in mycorrhizal roots. Gene expression of the tonoplast ZmHMA3a and ZmHMA4 isoforms was up-regulated by Cu-toxicity in shoots and roots of mycorrhizal plants. AM mitigates the changes induced by Cu toxicity on the maize ionome, specially at the highest Cu soil concentration. Altogether these data suggest that in Cu-contaminated soils, AM increases expression of the HMA genes putatively encoding proteins involved in Cu detoxification and balances mineral nutrient uptake improving the nutritional status of the maize plants.

An in vivo whole-plant bi-dimensional experimental system has been devised and tested with different host plants, in order to obtain extraradical mycelium (ERM) produced by different arbuscular mycorrhizal fungi (AMF). In this system, a host plant germling is inoculated with AMF to establish mycorrhizal symbiosis, and, after colonization, newly formed extraradical hyphae and spores are removed. Then the mycorrhizal root system is wrapped in a nylon net and placed between membranes in a Petri dish, allowing ERM to grow on the membrane surface. Such extraradical hyphae may be used for in situ morphometric analyses or collected for molecular or biochemical assays: in the latter case, the plant with its root sandwich may be reassembled to renew mycelium production. In this experimental system, which was tested with diverse host plant species and lines, values of explored membrane surface areas and densities of ERM showed wide ranges of variation, and its length ranged from 9.7 2.0 to 48.8 9.9 m per plant, depending on host and AMF identity. Across the different plant-AMF combinations tested, the whole-plant system produced 2.0 0.6 to 5.3 0.3 mg of ERM fresh biomass per plant per harvest. This experimental system can be used for a wide range of AMF and host plants species, either establishing arbuscular mycorrhizas or other mycorrhizal interactions. ERM produced and collected in the whole-plant system is suitable for morphological, physiological, and molecular analyses, facilitating studies on the different aspects of mycorrhizal symbiotic interactions.

The obligate symbiotic nature of arbuscular mycorrhizal (AM) fungi makes extremely difficult their genetic manipulation or transformation. For this reason, a heterologous system has been traditionally used for functional analysis of AM fungal genes, being the budding yeast Saccharomyces cerevisiae an organism suitable for this purpose. Here we present the yeast methods required for the functional analysis of AM fungal genes, including protocols for yeast transformation, heterologous gene expression, functional complementation assays, preparation of yeast extracts, and subcellular localization of the encoded protein.

Cadmium is a heavy metal (HM) that inhibits plant growth and leads to death, causing great losses in yields, especially in Cd hyperaccumulator crops such as Glycine max (L.) Merr. (soybean), a worldwide economically important legume. Furthermore, Cd incorporation into the food chain is a health hazard. Oxidative stress (OS) is a plant response to abiotic and biotic stresses with an intracellular burst of reactive oxygen species (ROS) that causes damage to lipids, proteins, and DNA. The arbuscular mycorrhizal fungal (AMF) association is a plant strategy to cope with HM and to alleviate OS. Our aim was to evaluate the mitigation effects of mycorrhization with AMF Rhizophagus intraradices on soybean growth, nutrients, Cd accumulation, lipid peroxidation, and the activity of different antioxidant agents under Cd (0.7–1.2 mg kg bioavailable Cd) induced OS. Our results suggest that glutathione may act as a signal molecule in a defense response to Cd-induced OS, and mycorrhization may avoid Cd-induced growth inhibition and reduce Cd accumulation in roots. It is discussed that R. intraradices mycorrhization would act as a signal, promoting the generation of a soybean cross tolerance response to Cd pollution, therefore evidencing the potential of this AMF association for bioremediation and encouragement of crop development, particularly because it is an interaction between a worldwide cultivated Cd hyperaccumulator plant and an AMF–HM–accumulator commonly present in soils.

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Arbuscular mycorrhizal fungi increase fitness of their host plants under Cu deficient and toxic conditions. In this study, we have characterized two Cu transporters of the CTR family (RiCTR1 and RiCTR2) and a CTR-like protein (RiCTR3A) of Rhizophagus irregularis. Functional analyses in yeast revealed that RiCTR1 encodes a plasma membrane Cu transporter, RiCTR2 a vacuolar Cu transporter and RiCTR3A a plasma membrane protein involved in Cu tolerance. RiCTR1 was more highly expressed in the extraradical mycelia (ERM) and RiCTR2 in the intraradical mycelia (IRM). In the ERM, RiCTR1 expression was up-regulated by Cu deficiency and down-regulated by Cu toxicity. RiCTR2 expression increased only in the ERM grown under severe Cu-deficient conditions. These data suggest that RiCTR1 is involved in Cu uptake by the ERM and RiCTR2 in mobilization of vacuolar Cu stores. Cu deficiency decreased mycorrhizal colonization and arbuscule frequency, but increased RiCTR1 and RiCTR2 expression in the IRM, which suggest that the IRM has a high Cu demand. The two alternatively spliced products of RiCTR3, RiCTR3A and RiCTR3B, were more highly expressed in the ERM. Up-regulation of RiCTR3A by Cu toxicity and the yeast complementation assays suggest that RiCTR3A might function as a Cu receptor involved in Cu tolerance.

Phosphorus (P) is a poorly available macronutrient essential for plant growth and development and consequently for successful crop yield and ecosystem productivity. To cope with P limitations plants have evolved strategies for enhancing P uptake and/or improving P efficiency use. The universal 450-million-yr-old arbuscular mycorrhizal (AM) (fungus-root) symbioses are one of the most successful and widespread strategies to maximize access of plants to available P. AM fungi biotrophically colonize the root cortex of most plant species and develop an extraradical mycelium which overgrows the nutrient depletion zone of the soil surrounding plant roots. This hyphal network is specialized in the acquisition of low mobility nutrients from soil, particularly P. During the last years, molecular biology techniques coupled to novel physiological approaches have provided fascinating contributions to our understanding of the mechanisms of symbiotic P transport. Mycorrhiza-specific plant phosphate transporters, which are required not only for symbiotic P transfer but also for maintenance of the symbiosis, have been identified. The present review provides an overview of the contribution of AM fungi to plant P acquisition and an update of recent findings on the physiological, molecular and regulatory mechanisms of P transport in the AM symbiosis.

Arbuscular mycorrhizal (AM) fungi can improve iron (Fe) acquisition of their host plants. Here, we report a characterization of two components of the high-affinity reductive Fe uptake system of Rhizophagus irregularis, the ferric reductase (RiFRE1) and the high affinity Fe permeases (RiFTR1-2). In the extraradical mycelia (ERM), Fe deficiency induced activation of a plasma membrane-localized ferric reductase, an enzyme that reduces Fe(III) sources to the more soluble Fe(II). Yeast mutant complementation assays showed that RiFRE1 encodes a functional ferric reductase and RiFTR1 an iron permease. In the heterologous system, RiFTR1 was expressed in the plasma membrane while RiFTR2 was expressed in the endomembranes. In the ERM, the highest expression levels of RiFTR1 were found in mycelia grown in media with 0.045 mM Fe, while RiFTR2 was upregulated under Fe-deficient conditions. RiFTR2 expression also increased in the intraradical mycelia (IRM) of maize plants grown without Fe. These data indicate that the Fe permease RiFTR1 plays a key role in Fe acquisition and that RiFTR2 is involved in Fe homeostasis under Fe-limiting conditions. RiFTR1 was highly expressed in the (IRM), which suggests that the maintenance of Fe homeostasis in the IRM might be essential for a successful symbiosis.

The Bradford-reactive soil protein (BRSP) fraction includes glomalin, a glycoprotein produced by arbuscular mycorrhizal (AM) fungi able to bind some metals, such as copper (Cu), which could promote the bioremediation of Cu-polluted soils. This study aimed to analyze the Cu-binding capacity of BRSP in Oenothera picensis that was inoculated or not inoculated with AM fungi. O. picensis plants were established in a Cu contaminated sterilized soil and treated with the following: i) uninoculated (-M); ii) inoculated with native AM fungal propagules (+M); or iii) inoculated with a Claroideoglomus claroideum (CC) strain isolated from non-contaminated soil. In each case, five Cu levels were applied to the soil (basal level 497.3 mg Cu kg): 0 (T1); 75 (T2); 150 (T3); 225 (T4); and 300 mg Cu kg (T5). A high BRSP accumulation in AM inoculated treatments, especially with CC, was observed. A higher Cu-bound-to-BRSP content was found with increasing Cu concentrations, representing up to 20-22% of the total Cu in the soil. Moreover, a higher root Cu concentration in +M was observed. These results suggest a high Cu binding capacity by BRSP, which is a relevant aspect to consider in the design of bioremediation programs together with the selection of endemic metallophytes and AM fungal strains, which are able to produce glomalin at high quantities.

Glutaredoxins (GRXs) are small ubiquitous oxidoreductases involved in the regulation of the redox state in living cells. In an attempt to identify the full complement of GRXs in the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis, three additional GRX homologs, besides the formerly characterized GintGRX1 (renamed here as RiGRX1), were identified. The three new GRXs (RiGRX4, RiGRX5 and RiGRX6) contain the CXXS domain of monothiol GRXs, but whereas RiGRX4 and RiGRX5 belong to class II GRXs, RiGRX6 belongs to class I together with RiGRX1. By using a yeast expression system, we observed that the newly identified homologs partially reverted sensitivity of the GRX deletion yeast strains to external oxidants. Furthermore, our results indicated that RiGRX4 and RiGRX5 play a role in iron homeostasis in yeast. Gene expression analyses revealed that RiGRX1 and RiGRX6 were more highly expressed in the intraradical (IRM) than in the extraradical mycelium (ERM). Exposure of the ERM to hydrogen peroxide induced up-regulation of RiGRX1, RiGRX4 and RiGRX5 gene expression. RiGRX4 expression was also up-regulated in the ERM when the fungus was grown in media supplemented with a high iron concentration. These data indicate the two monothiol class II GRXs, RiGRX4 and RiGRX5, might be involved in oxidative stress protection and in the regulation of fungal iron homeostasis. Increased expression of RiGRX1 and RiGRX6 in the IRM suggests that these GRXs should play a key role in oxidative stress protection of R. irregularis during its in planta phase.

Arbuscular mycorrhizal symbioses that involve most plants and Glomeromycota fungi are integral and functional parts of plant roots. In these associations, the fungi not only colonize the root cortex but also maintain an extensive network of hyphae that extend out of the root into the surrounding environment. These external hyphae contribute to plant uptake of low mobility nutrients, such as P, Zn, and Cu. Besides improving plant mineral nutrition, arbuscular mycorrhizal fungi (AMF) can alleviate heavy metal (HM) toxicity to their host plants. HMs, such as Cu, Zn, Fe, and Mn, play essential roles in many biological processes but are toxic when present in excess. This makes their transport and homeostatic control of particular importance to all living organisms. AMF play an important role in modulating plant HM acquisition in a wide range of soil metal concentrations and have been considered to be a key element in the improvement of micronutrient concentrations in crops and in the phytoremediation of polluted soils. In the present review, we provide an overview of the contribution of AMF to plant HM acquisition and performance under deficient and toxic HM conditions, and summarize current knowledge of metal homeostasis mechanisms in arbuscular mycorrhizas.

Arbuscular mycorrhiza can increase plant tolerance to heavy metals. The effects of arbuscular mycorrhiza on plant metal tolerance vary depending on the fungal and plant species involved. Here, we report the effect of the arbuscular mycorrhizal fungus Rhizophagus irregularis on the physiological and biochemical responses to Cu of two maize genotypes differing in Cu tolerance, the Cu-sensitive cv. Orense and the Cu-tolerant cv. Oropesa. Development of the symbiosis confers an increased Cu tolerance to cv. Orense. Root and shoot Cu concentrations were lower in mycorrhizal than in non-mycorrhizal plants of both cultivars. Shoot lipid peroxidation increased with soil Cu content only in non-mycorrhizal plants of the Cu-sensitive cultivar. Root lipid peroxidation increased with soil Cu content, except in mycorrhizal plants grown at 250 mg Cu kgsoil. In shoots of mycorrhizal plants of both cultivars, superoxide dismutase, ascorbate peroxidase, catalase and glutathione reductase activities were not affected by soil Cu content. In Cu-supplemented soils, total phytochelatin content increased in shoots of mycorrhizal cv. Orense but decreased in cv. Oropesa. Overall, these data suggest that the increased Cu tolerance of mycorrhizal plants of cv. Orense could be due to an increased induction of shoot phytochelatin biosynthesis by the symbiosis in this cultivar.

Arbuscular mycorrhizal fungi (AMF), belonging to the Glomeromycota, are soil microorganisms that establish mutualistic symbioses with the majority of higher plants. The efficient uptake of low mobility mineral nutrients by the fungal symbiont and their further transfer to the plant is a major feature of this symbiosis. Besides improving plant mineral nutrition, AMF can alleviate heavy metal toxicity to their host plants and are able to tolerate high metal concentrations in the soil. Nevertheless, we are far from understanding the key molecular determinants of metal homeostasis in these organisms. To get some insights into these mechanisms, a genome-wide analysis of Cu, Fe and Zn transporters was undertaken, making use of the recently published whole genome of the AMF Rhizophagus irregularis. This in silico analysis allowed identification of 30 open reading frames in the R. irregularis genome, which potentially encode metal transporters. Phylogenetic comparisons with the genomes of a set of reference fungi showed an expansion of some metal transporter families. Analysis of the published transcriptomic profiles of R. irregularis revealed that a set of genes were up-regulated in mycorrhizal roots compared to germinated spores and extraradical mycelium, which suggests that metals are important for plant colonization.

Arbuscular mycorrhizal fungi (AMF), which form an ancient and widespread mutualistic symbiosis with plants, are a crucial but still enigmatic component of the plant micro biome. Nutrient exchange has probably been at the heart of the success of this plant-fungus interaction since the earliest days of plants on land. To characterize genes from the fungal partner involved in nutrient exchange, and presumably important for the functioning of the AM symbiosis, genome-wide transcriptomic data obtained from the AMF Rhizophagus irregularis were exploited. A gene sequence, showing amino acid sequence and transmembrane domains profile similar to members of the PTR2 family of fungal oligopeptide transporters, was identified and called RiPTR2. The functional properties of RiPTR2 were investigated by means of heterologous expression in Saccharomyces cerevisiae mutants defective in either one or both of its di/tripeptide transporter genes PTR2 and DAL5. These assays showed that RiPTR2 can transport dipeptides such as Ala-Leu, Ala-Tyr or Tyr-Ala. From the gene expression analyses it seems that RiPTR2 responds to different environmental clues when the fungus grows inside the root and in the extraradical phase.

The stability and productivity of agro-ecosystems is largely dependent on maintaining the quality of soil resources through a sustainable soil management, which aims to conserve its productive capacity, minimizing energy and resource use by optimizing the rate of turnover and recycling of nutrients (Altieri 2004). These are fundamental issues not only for a sustainable production of healthy crops, or to ensure ecosystem selfsustainability, but also to prevent erosion and to minimize negative environmental impacts (Buscot 2005).

Mycorrhizas are worldwide symbiotic associations established between certain soil fungi and most vascular plants and are fundamental in optimizing plant fitness and soil quality. Mycorrhizal symbioses improve the resilience of plant communities against environment stresses, including nutrient deficiency, drought and soil disturbance. Since these stresses are paramount in the degradation of semi-arid ecosystems in the SE Spain, a series of basic, strategic and applied studies have been made to ascertain how the activity and diversity of mycorrhizal fungi affect plant community composition, structure and dynamics in this region. These investigations are reviewed here in terms of: (i) analysing the diversity of mycorrhizal fungi; (ii) assessing the ecological and functional interactions among plant communities and their associated mycorrhizal fungal populations; and (iii) using mycorrhizal inoculation technology for the restoration of degraded semi-arid areas in Southeast Spain. Disturbance of the target semi-arid ecosystems decreases the density and diversity of mycorrhizal fungust populations. Nevertheless, the mycorrhizal propagules do not disappear completely suggesting a certain degree of stress adaptation, and these remaining, resilient ecotypes are being used as plant inoculants. Numerous field experiments, using plant species from the natural succession inoculated with a community of indigenous mycorrhizal fungi, have been carried out in revegetation projects in the semi-arid Iberian Southeast. This management strategy improved both plant development and soil quality, and is a successful biotechnological tool to aid the restoration of self-sustaining ecosystems. However, despite a 20-year history of this work, we lack a comprehensive view of the mycorrhizal potential to improve the composition, diversity, structure and functionality of drought-adapted plant communities in the Region.

In the symbiotic association of plants and arbuscular mycorrhizal (AM) fungi, the fungus delivers mineral nutrients, such as phosphate and nitrogen, to the plant while receiving carbon. Previously, we identified an transporter in the AM fungus Glomus intraradices (GintAMT1) involved in uptake from the soil when preset at low concentrations. Here, we report the isolation and characterization of a new G. intraradices transporter gene (GintAMT2). Yeast mutant complementation assays showed that GintAMT2 encodes a functional transporter. The use of an anti-GintAMT2 polyclonal antibody revealed a plasma membrane location of GintAMT2. GintAMT1 and GintAMT2 were differentially expressed during the fungal life cycle and in response to N. In contrast to GintAMT1, GintAMT2 transcript levels were higher in the intraradical than in the extraradical fungal structures. However, transcripts of both genes were detected in arbuscule-colonized cortical cells. GintAMT1 expression was induced under low N conditions. Constitutive expression of GintAMT2 in N-limiting conditions and transitory induction after N re-supply suggests a role for GintAMT2 to retrieve leaked out during fungal metabolism

[ES] Las micorrizas arbusculares, simbiosis hongo-planta beneficiosa para ambos organismos, han jugado un papel clave en el origen y evolución de las plantas sobre la Tierra, así como en el desarrollo de la estructura y diversidad de los ecosistemas terrestres. La mayoría de las plantas dependen de estar micorrizadas para sobrevivir, particularmente en ambientes frágiles y sometidos a estreses, como son los característicos de alta montaña. Así ocurre probablemente en Sierra Nevada, un refugio excepcional para la flora y uno de los enclaves con mayores niveles de biodiversidad de Europa. Sierra Nevada alberga 66 endemismos exclusivos, algunos de ellos en peligro de extinción. Para investigar el posible papel de las micorrizas en la conservación de flora amenazada de Sierra Nevada se han llevado a cabo estudios con los siguientes objetivos: (i) determinar el estado micorricíco de especies de plantas amenazadas de Sierra Nevada, (ii) analizar la diversidad de los hongos micorrícicos asociados a dichas especies, (iii) establecer un banco de germoplasma de hongos micorrícicos de Sierra Nevada, y (iv) verificar los efectos de la micorrización dirigida con dichos hongos autóctonos sobre la producción de plantas de calidad. Los resultados obtenidos muestran que la mayoría (76%) de las especies seleccionadas son micorrícicas. La densidad de esporas de hongos micorrícicos en la rizosfera de las plantas seleccionadas es relativamente baja. Sin embargo, la diversidad de hongos es bastante elevada. Se han detectado unos 50 morfotipos distintos, pertenecientes a los géneros Glomus, Acaulospora, Entrophospora, Gigaspora, Scutellospora, Pacispora, Diversispora y Ambispora. Algunos morfotipos no corresponden a ninguna especie descrita hasta la fecha. Los géneros mejor representados en Sierra Nevada son Glomus y Acaulospora. En las altas cumbres y suelos ácidos predominan las especies del género Acaulospora, mientras que a medida que se desciende en altitud y en suelos alcalinos y co, [EN] Arbuscular mycorrhizal symbioses have played a key role in plant evolution on Earth as well as on the development and maintenance of the structure and diversity of terrestrial ecosystems. Most of the plants depend on mycorrhizas to thrive, particularly in fragile and stressed environments, as those in certain areas of the high Mediterranean mountains of Sierra Nevada (Granada, Spain). Sierra Nevada constitutes an exceptional refuge for the flora and one of the enclaves with higher biodiversity levels of the European continent. It presents about 2100 plant species and 66 exclusive endemisms, some of them in serious extinction danger. With the objective of ascertaining the impact of mycorrhizal associations at facilitating the conservation of species from the threatened flora of Sierra Nevada a research programme was initiated aimed at (i) determining the mycorrhizal status of selected species of the endangered flora of Sierra Nevada, (ii) analysing the diversity of the mycorrhizal fungi associated to the selected species, (iii) to establish a mycorrhizal fungi germplasm bank and (iv) to study the effect of a tailored mycorrhizal inoculation with autochthonous mycorrhizal fungi on the development of the target plants. It was found that most of the selected species (76%) form arbuscular mycorrhizas in natural conditions. The spore density of mycorrhizal fungi in the rhizosphere of the selected plants was relatively low. However, the diversity of mycorrhizal fungi was quite high. About 50 different morphotypes were detected, belonging to the genera Glomus, Acaulospora, Entrophospora, Gigaspora, Scutellospora, Pacispora, Diversispora and Ambispora. Some morphotypes do not correspond to any of the species described up to now. The most frequent genera in Sierra Nevada are Glomus and Acaulospora. Acaulospora species are the most frequent in the high mountains and in acidic soils, whereas Glomus species predominate at lower altitudes and in neutral and basic soils. All

Lipoxygenase (linoleate: oxygen oxidoreductase, EC 1.13.11.12) and lipid fatty acid composition of cotyledons and oil bodies of two near-isogenic high and low oleic acid sunflower hybrids (Helianthus annuus L.) were investigated during seedling growth. Triacylglycerol fatty acid content decreased strongly in cotyledons of both hybrids between days 1 and 7 of seedling growth. Oil bodies contained mostly neutral lipids and small amounts of phospholipids and proteins. Between days 1 and 7 of germination the degradation rate of oil body neutral lipids was lower for the high oleic acid hybrid compared to the low oleic acid hybrid. From days 0 to 9 of germination, neutral lipids and phospholipids of cotyledons and oil bodies from the high oleic acid hybrid contained more oleic acid and less linoleic acid than those of the low oleic acid hybrid. The presence of an oil body lipoxygenase was shown enzymatically and by immunodetection. In both hybrids, lipoxygenase activity of oil bodies and whole cotyledons was lower in 7 than in 1 day-old seedlings. Moreover, the high oleic acid hybrid showed a lower oil body lipoxygenase activity than the low oleic acid hybrid, both at 1 and 7 days of germination. Immunoblot analysis indicated that differences in enzyme activity between the two hybrids were not due to changes in the amount of antigenic protein. Oleic acid was found to be a competitive inhibitor of oil body lipoxygenase. Based on these results it is concluded that oleic acid could act as a natural inhibitor of sunflower oil body lipoxygenase.