BGIV

High-altitude hypersaline Andean wetlands are considered stressful environments by the prevalence of extreme abiotic conditions affecting both plant host and fungal root endophytes. Puccinellia frigida (Phil.) Johnston, a dominant plant species inhabiting the margins of these wetlands over 4000 m.a.s.l. in Northwest Argentina, is frequently colonized by fungal root endophytes. Here we examined the nature and dynamics of fungal root colonization on this plant species in three different wetlands (six sampling sites) and two growing seasons. Morphologically diverse septate fungi and chytrids were observed in root samples of P. frigida in all sampling sites, whereas arbuscular mycorrhizal fungi were found only in two sites. The level of colonization of chytrids and septate fungi differed significantly both between sites and seasons. Soil organic matter and electrical conductivity were the only habitat parameters significantly related to the observed differences. The widespread occurrence of non-destructive fungal associations suggest an important role for plant survival in extreme environments.

The growth dynamics of extraradical mycelium and spore formation of 14 "Rhizophagus" isolates from different sites in Argentina were evaluated under monoxenic conditions. A modified Gompertz model was used to characterize the development of mycelium and spores for each isolate under the same conditions. The lag time, maximal growth rate and total quantity of both extraradical hyphae and spores were determined. Wide variability among isolates was detected, and all growth parameters were significantly altered by fungal isolate. Discriminant analysis differentiated isolates primarily based on the extent of extraradical hyphae produced, yet such differences did not conclusively correspond to phylogenetic relationships among closely related isolates based on partial SSU sequences. Given that the "Rhizophagus" isolates were grown under controlled conditions for many generations, the expression of phenotypic variability could be attributed to genetic differences that are not completely resolved by phylogenetic analysis employing the small ribosomal gene.

In this thesis, 30 bacterial strains belonged to Firmicutes and Proteobacteria phylum were isolated, associated to AMF spores germination and intraradical mycelia re-growth. Some of their metabolic characteristics related to plant growth promotion (PGPR), and their effects as mycorrhiza helper bacteria (MHB), were analyzed. Pre symbiotic and symbiotic parameters, In Vitro conditions, of these bacterial strains were evaluated, and Azospirillum, Rhizobium, Bacillus and Paenibacillus strains were selected in order to verify their function as HMB and PGPR In Vivo conditions. The results of the interactions between HMB and AMF, and their effects on plant hosts, depended on the characteristics of the microbial strains, and the conditions in which the co-inoculation was made. The results confirm the importance of correctly choosing the HMB-HMA combinations, according to the permanent effects of promotion that the first ones demonstrated about the survival and development of the HMA.