Inoculation of plants with bacteria that produce indole acetic acid (IAA) and 1aminocyclopropane-1-carboxylate deaminase (ACCD) often has a positive effect on alleviation of salt stress in plants. Here, we isolated, characterized and formulated halotolerant bacterial consortia from avocado trees with the aim of developing biofertilizers to improve avocado production on saline soils.Using wheat as a test plant, experiments were conducted to investigate the effects of selected bacterial consortia on growth, biomass and superoxide dismutase (SOD) activity of wheat seedlings exposed to salt stress (0.25 M and 0.45 M NaCl) under greenhouse conditions. Among 309 isolates, 17.4% were characterized as halotolerant IAA-and ACCD-producing bacteria. Based on differences in their IAA production and ACCD activities, four consortia were formulated using members of five genera: Enterobacter,Serratia,Microbacterium, Pseudomonas andAchromobacter. Inoculation with selected halotolerant bacterial consortia significantly (P≥0.05) increased the emergence, growth, biomass and SOD activity of wheat seedlings exposed to salt stress. Avocado trees and their rhizosphere soils harbor halotolerant IAA-and ACCD-producing bacteria with the potential to mitigate the salt stress effects on plants. While wheat was useful for screening, further studies are necessary to validate the effects of selected bacterial consortia on avocado growth and yields under saline conditions.

No abstract

Gaeumannomyces graminis var. tritici (Ggt) is the main soilborne factor that affects wheat production around the world. Recently we reported the occurrence of six suppressive soils in monoculture areas from indigenous “Mapuche” communities, and evidenced that the suppression relied on the biotic component of those soils. Here, we compare the rhizosphere and endosphere microbial community structure (total bacteria, actinomycetes, total fungi, and ascomycetes) of wheat plants grown in suppressive and conducive soils. Our results suggested that Ggt suppression could be mediated mostly by bacterial endophytes, rather than rhizosphere microorganisms, since the community structure was similar in all suppressive soils as compared with conducive. Interestingly, we found that despite the lower incidence of take-all disease in suppressive soils, the Ggt concentration in roots was not significantly reduced in all suppressive soils compared to those growing in conducive soil. Therefore, the disease suppression is not always related to a reduction of the pathogen biomass. Furthermore, we isolated endophytic bacteria from wheat roots growing in suppressive soils. Among them we identified Serratia spp. and Enterobacter spp. able to inhibit Ggt growth in vitro. Since the disease, but not always pathogen amount, was reduced in the suppressive soils, we propose that take all disease suppressiveness is not only related to direct antagonism to the pathogen.

The presence of fungi in pristine Antarctic soils is of particular interest because of the diversity of this microbial group. However, the extreme conditions that coexist in Antarctica produce a strong selective pressure that could lead to the evolution of novel mechanisms for stress tolerance by indigenous microorganisms. For this reason, in recent years, research on cold-adapted microorganisms has increased, driven by their potential value for applications in biotechnology. Cold-adapted fungi, in particular, have become important sources for the discovery of novel bioactive secondary metabolites and enzymes. In this study, we studied the fungal community structure of 12 soil samples from Antarctic sites, including King George Island (including Collins Glacier), Deception Island and Robert Island. Culturable fungi were isolated and described according to their morphological and phenotypical characteristics, and the richness index was compared with soil chemical properties to describe the fungal community and associated environmental parameters. We isolated 54 fungal strains belonging to the following 19 genera: Penicillium, Pseudogymnoascus, Lambertella, Cadophora, Candida, Mortierella, Oxygenales, Geomyces, Vishniacozyma, Talaromyces, Rhizopus, Antarctomyces, Cosmospora, Tetracladium, Leptosphaeria, Lecanicillium, Thelebolus, Bjerkandera and an uncultured Zygomycete. The isolated fungi were comprised of 70% Ascomycota, 10% Zygomycota, 10% Basidiomycota, 5% Deuteromycota and 5% Mucoromycota, highlighting that most strains were associated with similar genera grown in cold environments. Among the culturable strains, 55% were psychrotrophic and 45% were psychrophilic, and most were Ascomycetes occurring in their teleomorph forms. Soils from the Collins Glacier showed less species richness and greater species dominance compared with the rest of the sites, whereas samples 4, 7, and 10 (from Fildes Bay, Coppermine Peninsula and Arctowski Station, respectively) showed greater species richness and less species dominance. Species richness was related to the C/N ratio, whereas species dominance was inversely related to C and N content. Thus, the structure of the fungal community was mainly related to soil chemical parameters more than sample location and altitude.

Cereal production in southern Chile is based on ash-derived volcanic Andisols, which present suboptimal levels of available selenium (Se). Strategies are needed to improve Se content in cereal crops and concomitantly improve the nutritional quality of grain. Here, we investigated the occurrence of Se-tolerant bacteria (STB) in Andisols and evaluated Se tolerance and accumulation in STB. The inoculation of wheat with STB and the contributions of these bacteria to Se content in plants were also evaluated under greenhouse conditions. The results showed that Se amendment of Andisols stimulated some bacterial groups (Paenibacillaceae and Brucellaceae) but inhibited others (Clostridia, Burkholderiales, Chitinophagaceae and Oxalobacteraceae), as revealed by denaturing gradient gel electrophoresis. Furthermore, we found four STB isolates that displayed 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase activity) and that carried the acdS gene as revealed by PCR. The selected STB were characterised as Stenotrophomonas, Bacillus, Enterobacter and Pseudomonas according to partial sequencing of the 16S rRNA gene. After 24 h of culture in nutrient broth, the selected STB showed the ability to grow in high Se concentrations (5 and 10 mM) and to accumulate elemental Se in micro-and nanospherical deposits, transforming 50-80 % of the Se initially added. Greenhouse experiments with wheat showed that Se associated with STB (micro-and nanospheres of elemental Se and other intracellular forms) can be translocated into leaves of wheat plantlets.

Phosphobacteria, secreting organic acids and phosphatases, usually favor plant performance in acidic soils by increasing phosphorus (P) availability and aluminum (Al) complexing. However, it is not well-known how P-deficiency and Al-toxicity affect the phosphobacteria physiology. Since P and Al problems often co-occur in acidic soils, we have therefore proposed the evaluation of the single and combined effects of P-deficiency and Al-toxicity on growth, organic acids secretion, malate dehydrogenase (mdh) gene expression, and phosphatase activity of five Al-tolerant phosphobacteria previously isolated from ryegrass. These phosphobacteria were identified as Klebsiella sp. RC3, Stenotrophomona sp. RC5, Klebsiella sp. RCJ4, Serratia sp. RCJ6, and Enterobacter sp. RJAL6. The strains were cultivated in mineral media modified to obtain (i) high P in absence of Al–toxicity, (ii) high P in presence of Al–toxicity, (iii) low P in absence of Al–toxicity, and (iv) low P in presence of Al–toxicity. High and low P were obtained by adding KH2PO4 at final concentration of 1.4 and 0.05 mM, respectively. To avoid Al precipitation, AlCl3 × 6H2O was previously complexed to citric acid (sole carbon source) in concentrations of 10 mM. The secreted organic acids were identified and quantified by HPLC, relative mdh gene expression was determined by qRT-PCR and phosphatase activity was colorimetrically determined using p-nitrophenyl phosphate as substrate. Our results revealed that although a higher secretion of all organic acids was achieved under P–deficiency, the patterns of organic acids secretion were variable and dependent on treatment and strain. The organic acid secretion is exacerbated when Al was added into media, particularly in the form of malic and citric acid. The mdh gene expression was significantly up–regulated by the strains RC3, RC5, and RCJ6 under P–deficiency and Al–toxicity. In general, Al–tolerant phosphobacteria under P deficiency increased both acid and alkaline phosphatase activity with respect to the control, which was deepened when Al was present. The knowledge of this bacterial behavior in vitro is important to understand and predict the behavior of phosphobacteria in vivo. This knowledge is essential to generate smart and efficient biofertilizers, based in Al–tolerant phosphobacteria which could be expansively used in acidic soils.

Wheat seedling inoculated with rhizobacterial consortia obtained from an undisturbed Chilean arid ecosystem showed improved growth in phosphorus-poor and partly dry soil. Arid ecosystems should be considered in further studies as an alternative source of microbial inoculants for agro-ecosystems subjected to stressful conditions by low nutrients and/or adverse climate events.

Chile is among the largest avocado producers worldwide; however, currently, its production has diminished mainly attributed to abiotic stresses such as drought and soil salinity. Here, we evaluated the contribution of halotolerant bacterial consortia to water shortage and salt stress tolerance of avocado seedlings (Persea americana Mill.) under field conditions. Inoculation trials were conducted in a commercial nursery to study the effects of two endophytic (C1 and C2) and two rhizosphere (C3 and C4) halotolerant bacterial consortia on growth, biomass, superoxide dismutase (SOD) and thiobarbituric acid reactive substances (TBARS) of avocado seedlings under salt (2% NaCl) stress and water shortage (50% less irrigation). Ours results revealed that avocado inoculation with C4 significantly (P≤0.05) increased aerial and root length; aerial and root fresh weight and chlorophyll content of salt-stressed seedlings; and the aerial length and root fresh weight of seedlings under water shortage. Similarly, the C4 significantly (P≤0.05) increased SOD activity in leaves of both the control and seedlings grown under salt stress and water shortage and also decreased TBARS content in leaves of control plants and of seedlings grown under salt stress. Whereas, C3 increased significantly (P≤0.05) aerial and root length and root fresh weight of salt-stressed seedlings; and also increased the trunk diameter and chlorophyll content of seedlings under water shortage. Similarly, C3 significantly (P≤0.05) stimulated SOD activity of leaves as compared to the control seedlings and also reduced the TBARS content of leaves and roots of avocado seedlings under salt stress. In contrast, the endophytic consortia were less efficient than rhizosphere consortia. Thus, C1 only increased the trunk diameter and chlorophyll content of salt-stressed seedlings and C2 increased the chlorophyll content of avocado seedlings under water shortage. Our study showed the favorable effect of bacterial 3 inoculation on avocado nursey in nursery conditions under water shortage and salt stress, and identified consortia that potentially could be used as avocado biofertilizers.