Silvio Gianinazzi

Agroecology: the key role of arbuscular mycorrhizas in ecosystem services

The beneficial effects of arbuscular mycorrhizal (AM) fungi on plant performance and soil health are essential for the sustainable management of agricultural ecosystems. Nevertheless, since the ‘first green revolution’, less attention has been given to beneficial soil microorganisms in general and to AM fungi in particular. Human society benefits from a multitude of resources and processes from natural and managed ecosystems, to which AM make a crucial contribution. These resources and processes, which are called ecosystem services, include products like food and processes like nutrient transfer. Many people have been under the illusion that these ecosystem services are free, invulnerable and infinitely available; taken for granted as public benefits, they lack a formal market and are traditionally absent from society’s balance sheet. In 1997, a team of researchers from the USA, Argentina and the Netherlands put an average price tag of US

Cell Defense Responses Associated with Localized and Systemic Resistance to Phytophthora parasitica Induced in Tomato by an Arbuscular Mycorrhizal Fungus

33 trillion a year on these fundamental ecosystem services. The present review highlights the key role that the AM symbiosis can play as an ecosystem service provider to guarantee plant productivity and quality in emerging systems of sustainable agriculture. The appropriate management of ecosystem services rendered by AM will impact on natural resource conservation and utilisation with an obvious net gain for human society.

In planta histochemical staining of fungal alkaline phosphatase activity for analysis of efficient arbuscular mycorrhizal infections

The arbuscular mycorrhizal fungus Glomus mosseae is able to confer bioprotection against Phytophthora parasitica in tomato roots. Localized and induced systemic resistance (ISR) have been demonstrated to be involved in pathogen control in mycorrhizal and nonmycorrhizal roots with a split root experimental system. Decreased pathogen development in mycorrhizal and nonmycorrhizal parts of mycorrhizal root systems is associated with accumulation of phenolics and plant cell defense responses. G. mosseae-containing cortical cells in the mycorrhizal tissues are immune to the pathogen and exhibit a localized resistance response with the formation of cell wall appositions reinforced by callose adjacent to intercellular hyphae. The systemically induced resistance in nonmycorrhizal root parts is characterized by elicitation of host wall thickenings containing non-esterified pectins and PR-1a protein in reaction to intercellular pathogen hyphae, and by the formation of callose-rich encasement material around P. paras...

Proteome analysis and identification of symbiosis‐related proteins from Medicago truncatula Gaertn. by two‐dimensional electrophoresis and mass spectrometry

A histochemical procedure was developed to visualize and estimate the proportion of arbuscular mycorrhizal infections showing fungal alkaline phosphatase activity, as compared to the total amount of fungal tissue (trypan blue staining) and of living mycelium, indicated by succinate dehydrogenase activity. In roots of Allium porrum and Platanus acerifolia, only a small proportion of living intraradical mycelium showed alkaline phosphatase activity during early infection but this increased greatly just before the mycorrhizal growth response of the host plant. Infection revealed by all three stains reached a maximum at 6 weeks after inoculation, after which the level of trypan blue stained infection remained constant but the proportion showing succinate dehydrogenase and alkaline phosphatase activity declined as the infection aged. Alkaline phosphatase activity was absent from virtually all abortive entry point hyphae formed on roots of a resistant myc−, nod− mutant of Pisum sativum, although succinate dehydrogenase activity was detected. Observations suggest that the alkaline phosphatase activity is induced by colonization of host roots and that this fungal enzyme could provide a useful marker for analyzing the symbiotic efficiency of arbuscular mycorrhizal infections.

Cellular localization and cytochemical probing of resistance reactions to arbuscular mycorrhizal fungi in a locus a myc- mutant of Pisum sativum L.

Time‐course analysis of root protein profiles was studied by two‐dimensional gel electrophoresis and silver staining in the model plant Medicago truncatula, inoculated either with the arbuscular mycorrhizal fungus Glomus mosseae or with the nitrogen fixing bacterium Sinorhizobium meliloti. Protein modifications in relation to the development of both symbioses included down‐ and upregulations, as well as newly induced polypeptides. Matrix assisted laser desorption/ionization‐time of flight‐mass spectrometry after trypsin digestion clearly identified one polypeptide induced in nodulated roots as a M. truncatula leghemoglobin. Internal sequencing with a quadrupole time‐of‐flight mass spectrometer and database searches confirmed the induction of proteins previously described in root symbioses, and revealed the implication of other proteins. In nodulated roots, one polypeptide was identified as an elongation factor Tu from S. meliloti, while another one could not be assigned a function. In mycorrhizal roots, analyzed proteins also included a protein of unknown function, as well as a glutathione‐S‐transferase, a fucosidase, a myosin‐like protein, a serine hydroxymethyltransferase and a cytochrome‐c‐oxidase. These results emphasize the usefulness of proteome analysis in identifying molecular events occurring in plant root symbioses.

Colonisation patterns of root tissues byPhytophthora nicotianae var.parasitica related to reduced disease in mycorrhizal tomato

Pisum sativum L. myc− mutants which fail to form arbuscular mycorrhiza have recently been identified amongst nod− mutants (Duc et al., 1989, Plant Sci. 60, 215–222). The reason for this resistance to symbiotic fungi has been investigated in the case of a ‘locus a’ mutant (P2) inoculated with Glomus mosseae (Nicol. and Gerd.) Gerd, and Trappe. The fungal symbiont formed viable appressoria in contact with the root surface but its development was stopped at the root epidermis. Abundant material was deposited on the inner face of root cell walls adjacent to the appressoria in the P2 mutant, but not in the wild-genotype parent cultivar (Frisson) forming a symbiotic mycorrhizal infection. Fluorescence, histochemical, cytochemical and immunocytological approaches were used to characterize the paramural deposits in epidermal and hypodermal cells of the mutant. Strong fluorescence under blue light indicated the accumulation of phenolic compounds although polymers like lignin or suberin were not localized. Proteins and glycoproteins were homogeneously distributed within the paramural deposits. In the latter, the periodic acid-thiocarbohydrazide-silver proteinate (PATAg) reaction for 1,4-polysaccharide detection showed a heterogeneous composition with electron-dense points surrounded by non-reactive material, but cytological tests for cellulose and pectin gave weak responses as compared to epidermal and hypodermal walls of the wild genotype. β-1,3-Glucans indicative of callose were detected by in-situ immunolocalization in the paramural deposits below appressoria on mutant roots, but not in walls of the wild genotype. Thus, appressorium formation by G. mosseae on roots of the ‘locus a’ P. sativum mutant elicits wall modifications usually associated with activation of defence responses to pathogens. It is proposed that this locus must be involved in a key event in symbiotic infection processes in P. sativum, and the possible role of complex regulatory interactions between symbiosis and defence genes in endomycorrhiza development is discussed.

Fungal Elicitation of Signal Transduction-Related Plant Genes Precedes Mycorrhiza Establishment and Requires the dmi3 Gene in Medicago truncatula

Tomato plants pre-colonised by the arbuscular mycorrhizal fungusGlomus mosseae showed decreased root damage by the pathogenPhytophthora nicotianae var.parasitica. In analyses of the cellular bases of their bioprotective effect, a prerequisite for cytological investigations of tissue interactions betweenG. mosseae andP. nicotianae v.parasitica was to discriminate between the hyphae of the two fungi within root tissues. We report the use of antibodies as useful tools, in the absence of an appropriate stain for distinguishing hyphae ofP. nicotianae v.parasitica from those ofG. mosseae inside roots, and present observations on the colonisation patterns by the pathogenic fungus alone or during interactions in mycorrhizal roots. Infection intensity of the pathogen, estimated using an immunoenzyme labelling technique on whole root fragments, was lower in mycorrhizal roots. Immunogold labelling ofP. nicotianae v.parasitica on cross-sections of infected tomato roots showed that inter or intracellular hyphae developed mainly in the cortex, and their presence induced necrosis of host cells, the wall and contents of which showed a strong autofluorescence in reaction to the pathogen. In dual fungal infections of tomato root systems, hyphae of the symbiont and the pathogen were in most cases in different root regions, but they could also be observed in the same root tissues. The number ofP. nicotianae v.parasitica hyphae growing in the root cortex was greatly reduced in mycorrhizal root systems, and in mycorrhizal tissues infected by the pathogen, arbuscule-containing cells surrounded by intercellularP. nicotianae v.parasitica hyphae did not necrose and only a weak autofluorescence was associated with the host cells. Results are discussed in relation to possible processes involved in the phenomenon of bioprotection in arbuscular mycorrhizal plants.

Defense Genes Are Differentially Induced by a Mycorrhizal Fungus and Rhizobium sp. in Wild-Type and Symbiosis-Defective Pea Genotypes

Suppressive subtractive hybridization and expressed sequence tag sequencing identified 29 plant genes which are upregulated during the appressorium stage of mycorrhiza establishment between Medicago truncatula J5 (Myc+) and Glomus mosseae. Eleven genes coding plant proteins with predicted functions in signal transduction, transcription, and translation were investigated in more detail for their relation to early events of symbiotic interactions. Expression profiling showed that the genes are activated not only from the appressorium stage up to the fully established symbiosis in the Myc+ genotype of M. truncatula, but also when the symbionts are not in direct cell contact, suggesting that diffusible fungal molecules (Myc factors) play a, role in the induction of a signal-transduction pathway. Transcript accumulation in roots of a mycorrhiza-defective Myc- dmi3 mutant of M. truncatula is not modified by appressorium formation or diffusible fungal molecules, indicating that the signal transduction pathway is required for a successful G. mosseae-M. truncatula interaction leading to symbiosis development. The symbiotic nodulating bacterium Sinorhizobium meliloti does not activate the 11 genes, which supposes early discrimination by plant roots between the microbial symbionts.

Mycorrhizas : functional processes and ecological impact

Mycorrhiza-resistant and non-nodulating pea mutants provide a model system for identifying common genes regulated during the early events in mycorrhiza and nodule establishment. Inoculation of pea roots with Glomus mosseae or Rhizobium leguminosarum can induce overex-pression of seven defense-related genes (pI 206, pI 49, pI 176, PR 10, basic A1-chitinase, transcinnamic acid 4-hydroxylase, chalcone isomerase), depending on the plant genotype and the time point of interaction between the plant and the microsymbiont. Expression of the pI 206 gene is closely correlated with appressorium formation by the mycorrhizal fungus on both mutant and wild-type pea roots. The gene is also induced by the pathogen Aphanomyces euteiches. Transcript accumulation was higher in mutant than in wild-type genotypes for five and six of the studied genes during early stages of root interactions with G. mosseae and R. leguminosarum, respectively, and this is discussed in relation to the symbiotic-defective phenotype of Myc-1Nod¯ p...

Hydrolytic enzyme activity of Paenibacillus sp. strain B2 and effects of the antagonistic bacterium on cell integrity of two soil-borne pathogenic fungi.

1 Soil microbial resources and agricultural policies David Atkinson 2 Interspecies altruism in plant-microbe symbioses: use of group selection models to resolve the evolutionary paradoxes Nikolai A. Provorov and Nikolai I. Vorobyov 3 Dissection of genetic cell programmes driving early arbuscular mycorrhiza interactions Vivienne Gianinazzi-Pearson, Marie Tollot and Pascale Seddas 4 Analysis of mycorrhizal functioning using transcriptomics Matthias Arlt, Dietmar Schwarz and Philipp Franken 5 Protein profiling analyses in arbuscular mycorrhizal symbiosis Ghislaine Recorbet and Eliane Dumas-Gaudot 6 Coordinated nutrient exchange in arbuscular mycorrhiza Nuria Ferrol and Jacob Perez-Tienda 7 Deciphering the arbuscular mycorrhizal pathway of P uptake in non-responsive plant species Emily J. Grace, F. Andrew Smith and Sally E. Smith 8 Mechanisms underlying heavy metal tolerance in arbuscular mycorrhizas Manuel Gonzalez-Guerrero, Karim Benabdellah, Nuria Ferrol and Concepcion Azcon-Aguilar 9 Priming plant defences against pathogens by arbuscular mycorrhizal fungi Maria J. Pozo, Adriaan Verhage, Javier Garcia-Andrade, Juan M. Garcia and Concepcion Azcon-Aguilar 10 Reconciliation of conflicting phenotypic and rRNA gene phylogenies of fungi in Glomeromycota based on underlying patterns and processes Joseph B. Morton 11 Spatial ecology of ectomycorrhizas: analytical strategies Brian J. Pickles, David Genney, Ian C. Anderson and Ian J. Alexander 12 Spatial heterogeneity in mycorrhizal populations and communities: scales and mechanisms Benjamin E. Wolfe, Jeri L. Parrent, Alexander M. Koch, Benjamin A. Sikes, Monique Gardes and John N. Klironomos 13 Response diversity of ectomycorrhizas in forest succession following disturbance Suzanne W.Simard 14 AMF associated with indigenous and non indigenous plants at urban and desert sites in Arizona Robert J. Bills and Jean C. Stutz 15 Desert truffle cultivation in semiarid Mediterranean areas Asuncion Morte, Mar Zamora, Almudena Gutierrez and Mario Honrubia.