José M. Barea

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

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...

Interactions between arbuscular mycorrhizal fungi and other microbial inoculants (Azospirillum, Pseudomonas, Trichoderma) and their effects on microbial population and enzyme activities in the rhizosphere of maize plants

Arbuscular mycorrhizal (AM) fungi as well as microbial-free inoculants used as phytostimulators (Azospirillum )o r as biological control agents of fungi (Pseudomonasand Trichoderma) have shown beneficial effects on plant growth and health. The study of plants inoculated with biological control agents and AMF requires special attention because of the possibility that these fungal antagonists could also interfere with AM fungi. Our study was performed to test the effects of these inoculants upon mycorrhizal colonization in maize plants inoculated with Glomus mosseae, Glomus deserticola and natural AMF from the test soil. Populations of culturable bacteria and fungi in the rhizosphere soil were also examined since inoculation with AM fungi and other soil microorganisms can affect both quantitatively and qualitatively the microbial communities in the plant rhizosphere. Enzyme activities (esterase, phosphatase, trehalase and chitinase) were used as an index to detect changes in the microbial functioning in soil, as affected by mycorrhizal and other inoculation treatments. None of the microbial inoculants used, even those biocontrol agents of fungi, showed negative effects on AM establishment. Mycorrhizal colonization induced qualitative changes in the bacterial population depending on the inoculant combination involved. Esterase activity was particularly increased by G. mosseae (256%), phosphatase activity by natural AMF (166%), chitinase by G. mosseae (197%), G. deserticola (152%) and natural AMF (151%), and trehalase by G. deserticola (444%). As a result of mycorrhizal colonization and microbial inoculation, modifications of the microbial community structure and ecology were found. An understanding of these effects as part of ecosystem processes is essential for obtaining the maximum benefit for plant growth and health in the context of soil‐plant system sustainability.

β-1,3-Glucanase activities in tomato roots inoculated with arbuscular mycorrhizal fungi and/or Phytophthora parasitica and their possible involvement in bioprotection

b-1,3-Glucanases in tomato roots were studied after arbuscular mycorrhizal (AM) symbiosis establishment and:or pathogenic infection by Phytophthora parasitica by polyacrylamide gel electrophoresis (PAGE). Two species of AM fungi, Glomus mosseae and Glomus intraradices were tested, and Phytophthora inoculation was performed on both non-mycorrhizal and mycorrhizal tomato pre-colonized for 4 weeks with either of the AM fungal species. The protective effect of both AM fungi on tomato plants against Phytophthora was assessed. In control roots two acidic b-1,3-glucanase isoforms were constitutively expressed, and their activity was higher in mycorrhizal roots. Two additional acidic isoforms were detected in extracts from G. mosseae-colonized tomato roots, but not in G. intraradices-colonized roots. Roots infected by P. parasitica displayed stronger activities but the pathogen did not induce the isoforms related to G. mosseae colonization. Only one basic glucanase isoform was detected whether the plants were non-inoculated or colonized by any of the fungi when inoculated singly. However, when plants were pre-inoculated with G. mosseae and post-infected with P. parasitica two additional basic isoforms were clearly revealed. Results are discussed in relation to the possible role of the additional acidic and basic b-1,3-glucanase isoforms in the establishment and development of the AM symbiosis, as well as their putative implication in plant bioprotection.

Communities of P-Solubilizing Bacteria, Fungi and Arbuscular Mycorrhizal Fungi in grass pasture and secondary forest of Paraty, RJ - Brazil*

Communities of P-solubilizing bacteria, fungi and arbuscular mycorrhizal fungi, were evaluated in two different ecosystems. Samplings taken from two areas of Atlantic forest, in Paraty-RJ, Brazil, one with a secondary forest and the other with a grass pasture were studied. Four growth media: GL (glucose and yeast extract), GES (glucose, soil extract, KNO3, CaCl2, MgSO4, NaCl, FeEDTA and micronutrients solution), GAGES (glucose, soil extract, arabinose, glycerol, CaCl2, MgSO4 and NaCl) and GELP (glucose, soil extract, yeast extract, peptone, CaCl2, MgSO4 and NaCl) were evaluated for the isolation of P-solubilizing microorganisms. The identification of P-solubilizing bacteria was based on 16 S rDNA sequence analysis, while the identification of P-solubilizing fungi and arbuscular mycorrhizal fungi was based on morphology. The greatest number of P-solubilizing bacteria was isolated using GL and GELP growth media. The greatest number of P-solubilizing fungi was isolated using GAGES and GES. The bacteria were identified as Enterobacteriaceae and Bacillus sp., while the P-solubilizing fungi were identified as Aspergillus sp. Glomus macrocarpum and Glomus etunicatum were the dominant mycorrhizal fungi in the secondary forest and grass pasture area, respectively.

Interactions of Arbuscular Mycorrhiza and Nitrogen-Fixing Symbiosis in Sustainable Agriculture

The great agricultural and environmental importance of legumes, plus the ability of their rhizosphere system is able to harbour symbionts and other associated microbes of great relevance to plant productivity, make legumes target crops in sustainable agriculture. Current developments in the ecology, physiology, biochemistry, molecular biology, and biotechnology of microbe-plant relationships have given new insights into understanding the formation and functioning of the tripartite arbuscular-mycorrhizal and nitrogen-fixing symbioses of legumes and their interactions with PGPR. Although the technology for the production of rhizobial and free-living PGPR is commercially available, the production of AM fungi inocula and the development of inoculation techniques have limited the manipulation of AM fungi. However, current biotechnology practices now allow the production of efficient AM-fungal inoculants. Therefore, an appropriate management of selected AM fungi, rhizobia, and PGPR is now an available technique for exploiting the benefits of these microorganisms in agriculture, horticulture, and in revegetation of degraded ecosystems.

Impact of a genetically modified Rhizobium strain with improved nodulation competitiveness on the early stages of arbuscular mycorrhiza formation

Arbuscular mycorrhizal (AM) fungi and the AM symbiosis are key components in agroecosystems, thus, they have been used as biosensors to evaluate the impact of a Rhizobium meliloti strain, that was genetically modified to improve its nodulation competitiveness, when applied as inoculant. It was found that such a rhizobial strain did not interfere with any of a series of processes related to mycorrhiza formation by the representative AM fungus Glomus mosseae. The parameters tested include spore germination, mycelial growth from the mycorrhizal propagules and AM entry point formation on the developing root system of the common host plant Medicago sativa L. Indeed, the genetically modified Rhizobium increased the number of AM colonization units and the nutrient acquisition ability in the mycorrhizal plant, with respect to the wild type rhizobial strain.

The interactions between plant life form and fungal traits of arbuscular mycorrhizal fungi determine the symbiotic community.

Arbuscular mycorrhizal (AM) fungi have traditionally been considered generalist symbionts. However, an increasing number of studies are pointing out the selectivity potential of plant hosts. Plant life form, determined by plant life history traits, seems to drive the AM fungal community composition. The AM fungi also exhibit a wide diversity of functional traits known to be responsible for their distribution in natural ecosystems. However, little is known about the role of plant and fungal traits driving the resultant symbiotic assemblages. With the aim of testing the feedback relationship between plant and fungal traits on the resulting AM fungal community, we inoculated three different plant life forms, i.e. annual herbs, perennial herbs and perennial semi-woody plants, with AM fungal communities sampled in different seasons. We hypothesized that the annual climate variation will induce changes in the mean traits of the AM fungal communities present in the soil throughout the year. Furthermore, the association of plants with different life forms with AM fungi with contrasting life history traits will show certain preferences according to reciprocal traits of the plants and fungi. We found changes in the AM fungal community throughout the year, which were differentially disrupted by disturbance and altered by plant growth form and plant biomass. Both plant and fungal traits clearly contributed to the resultant AM fungal communities. The revealed process can have implications for the functioning of ecosystems since changes in dominant plant life forms or climatic variables could influence the traits of AM fungal communities in soil and hence ecosystem processes.

Identification of a cDNA from the arbuscular mycorrhizal fungus Glomus intraradices that is expressed during mycorrhizal symbiosis and up-regulated by N fertilization.

A cDNA library was constructed with RNA from Glomus intraradices-colonized lettuce roots and used for differential screening. This allowed the identification of a cDNA (Gi-1) that was expressed only in mycorrhizal roots and was of fungal origin. The function of the gene product is unknown, because Gi-1 contained a complete open reading frame that was predicted to encode a protein of 157 amino acids which only showed little homology with glutamine synthetase from Helicobacter pylori. The time-course analysis of gene expression during the fungal life cycle showed that Gi-1 was expressed only during the mycorrhizal symbiosis and was not detected in dormant or germinating spores of G. intraradices. P fertilization did not significantly change the pattern of Gi-1 expression compared with that in the unfertilized treatment, whereas N fertilization (alone or in combination with P) considerably enhanced the Gi-1 transcript accumulation. This increase in gene expression correlated with plant N status and growth under such conditions. The possible role of the Gi-1 gene product in intermediary N metabolism of arbuscular mycorrhizal symbiosis is further discussed.

The influence of environmental degradation processes on the arbuscular mycorrhizal fungal community associated with yew (Taxus baccata L.), an endangered tree species from Mediterranean ecosystems of Southeast Spain

AimsTo assess whether the yew roots, which are able to provide a very constant environment due to their long life-span, can maintain the original arbuscular mycorrhizal (AM) fungal community during yew population decline.MethodsThe diversity of AM fungi (AMF) colonizing the roots of yew was analyzed by selecting the small subunit ribosomal RNA genes to construct a database of the overall community of AMF in the experimental area. A terminal restriction fragment length polymorphism (TRFLP) approach was used to identify the AMF communities present in yew roots. Physiological and environmental variables related to topology and soil and plant characteristics were determined as markers of habitat degradation.ResultsThe AMF communities within yew roots were found to be dependent on soil, plant and topological variables indicative of habitat degradation surrounding the yew. The phylogenetic diversity of AMF associated to the yews was lower in habitats more exposed to degradation than in those better conserved.ConclusionsThe target yews can be grouped into two degradation levels. AMF communities were also affected by the degradation processes affecting their hosts. This finding rules out the role of these trees as refugia for their original AMF community, a fact that should be considered in plant reintroduction programs using AMF as bioenhancers.

Spring to autumn changes in the arbuscular mycorrhizal fungal community composition in the different propagule types associated to a Mediterranean shrubland

Background and aimsArbuscular mycorrhizal fungi (AMF) appear differentially represented among propagule forms [intraradical mycelium (IRM) in colonized roots, spores and extraradical mycelium (ERM)]. However, spring to autumn changes in the AMF communities harboured in the different propagule forms has not been studied, being this the aim of the present study.MethodsA terminal restriction fragment length polymorphism approach was used to monitor, in spring and autumn, the AMF community composition present in the three propagule types associated to five shrub species in a semi-arid Mediterranean environment.ResultsThe AMF community composition in roots was significantly different between spring and autumn; however, no significant differences were detected in soil propagules (spores and ERM). Different trends were identified according to the preferential biomass allocation patterns of AMF phylotypes, suggesting different life strategies: those allocating mainly into IRM (belonging to the Glomeraceae), ERM (Diversisporaceae and Gigasporaceae) or spores (Pacisporaceae and Paraglomeraceae).ConclusionsDifferences of AMF taxa in the biomass allocation patterns among propagules are maintained throughout the year. Progress in the knowledge of functional features of AMF communities and their responses to seasonal variations are important for the AMF application in Mediterranean ecosystems.