Paola Durán

Bacterial alkaline phosphomonoesterase in the rhizospheres of plants grown in Chilean extreme environments

Bacterial alkaline phosphomonoesterases (APases) are relevant for organic phosphorus (Po) recycling in many soils. However, the abundance and diversity of bacterial APase in the rhizospheres of native plants are poorly known, particularly in extreme environments. In this research work, we studied the composition of total and APase-harboring bacterial communities, abundances of selected APase genes (phoD and phoX), and APase activities in rhizosphere soils from native plants grown in extreme environments of northern (Atacama Desert), central (Andes volcano; Quetrupillan and Mamuil Malal) and hot spring (Liquiñe), and southern polar (Patagonia and Antarctic) regions of Chile. Differences in the composition of bacterial communities in the rhizosphere soils were revealed by denaturing gradient gel electrophoresis (DGGE) and quantitative PCR (qPCR) of 16S ribosomal RNA (rRNA), phoD, and phoX genes. In general, the significant lowest bacterial diversities, APase gene abundances, and APase activities were observed in rhizosphere soils from Atacama Desert, whereas the highest values were observed in rhizosphere soils of Patagonia. In addition, APase gene abundances were positively correlated among them and with APase activity of rhizosphere soils, but negatively correlated with phosphorus (P) availability in rhizosphere soils. Although bacterial APases were observed in all studied rhizosphere soils, their relevance to soil Po recycling in soils of extreme environments remains unclear and further studies are needed.

Improving selenium status in plant nutrition and quality

Abstract Selenium (Se) is an essential micronutrient for human health due to its antioxidant capabilities. The Se content around the world is highly variable from 0.005 mg kg -1 in areas from China and Finland to 8,000 mg kg in seleniferous soils from Tuva-Russia. However, about one billion of people in the world wide are Se deficient. During the last decade, studies related with strategies for Se biofortification in food plants for human nutrition have significantly increased because this metalloid is incorporated into human metabolism mainly as a constituent of food plants. Similarly, Se biofortification is important in pastures for increasing the Se content in cattle to enrich meat and to prevent disease associated to Se deficiency as white muscle disease. In China, two endemic diseases have been related to Se deficiency: Keshan and Keshin–Beck diseases.Agronomic biofortification by using inorganic Se sources is a current practice in countries as China, Finland, and USA. In Chile, fertilization by using chemical compounds with Se is an uncommon practice due the edaphoclimatic characteristics of Andisols, which represent around 60% of agricultural soils of southern Chile. Recent studies showed that microorganisms as bacteria and arbuscular mycorrhizal fungi play an important role in the transformations and Se availability, representing an interesting biotechnological alternative to Se biofortification.This review is focalized to describing Se behavior in soil-plant system and the possible strategies to improving Se content, including the use of microorganisms as biotechnological tools for increasing plant nutrition and quality. Specific attention will be devoted to volcanic soils of Southern Chile, where different factors concur to enhance the Se-deficiency problem.

Inoculation with selenobacteria and arbuscular mycorrhizal fungi to enhance selenium content in lettuce plants and improve tolerance against drought stress

This study evaluated the co-inoculation effect of the endophytic selenobacteria Bacillus sp., Klebsiella sp. or Acinetobacter sp. and the arbuscular mycorrhizal (AM) fungus Rhizophagus intraradices on lettuce plants grown under drought conditions. Plants inoculated with bothnmicroorganisms were able to enhance the Se content in their shoots (1 to 6 pg plant-1) and promote macro-and micronutrient uptake. Moreover, the inoculated plants showed significantntolerance to drought stress, as determined by their adaptation to physiological parameters(relative water content and stomatal conductance), increase in photosynthetic pigments (chlorophyll and carotenoids) and improvement inantioxidant enzyme levels (catalase, ascorbate peroxidase and glutathione reductase). The selenobacteria increased the Se content in lettuce plants and enhanced the effect of AM fungus in controlling the antioxidant systems that play a role as elicitors of plant drought responses and improving the nutritional quality and physiological and biochemical processes involved in plant drought tolerance.

Assessment of plant growth promoting activities and abiotic stress tolerance of Azotobacter chroococcum strains for a potential use in sustainable agriculture

This study aimed to select and characterize bacterial strains with multiple plant growth promotion properties able to tolerate drought and salinity stress in order to improve plant growth performance under abiotic stresses. Plant growth promoting activities were evaluated in 106 bacterial strains isolated by soil, compost, lignocellulosic biomasses and food matrices. Two strains A. chroococcum 67B and 76A were selected for their multiple plant growth promotion activities (N2-fixing activity, synthesis of siderophores and indole-3-acetic acid) as well as antimicrobial activity against Sclerotinia minor. Moreover, the selected strains were tested for their tolerance to drought and salinity stress in in vitro experiments evaluating their growth, production of SOD, CAT, proline and ACC deaminase. The two Azotobacter chroococcum strains exhibited high tolerance to salt and drought stresses and could alleviate the negative effects exerted by abiotic stress on tomato plants. Overall results showed that the strains Azotobacter chroococcum 67B and Azotobacter chroococcum 76A could be interesting candidates to be employed as bio-effector in sustainable agricultural systems also under stressful conditions leading to improved economic and environmental benefits.

Screening and Characterization of Potentially Suppressive Soils against Gaeumannomyces graminis under Extensive Wheat Cropping by Chilean Indigenous Communities

Wheat production around the world is severely compromised by the occurrence of “take-all” disease, which is caused by the soil-borne pathogen Gaeumannomyces graminis var. tritici (Ggt). In this context, suppressive soils are those environments in which plants comparatively suffer less soil-borne pathogen diseases than expected, owing to native soil microorganism activities. In southern Chile, where 85% of the national cereal production takes place, several studies have suggested the existence of suppressive soils under extensive wheat cropping. Thus, this study aimed to screen Ggt-suppressive soil occurrence in 16 locations managed by indigenous “Mapuche” communities, using extensive wheat cropping for more than 10 years. Ggt growth inhibition in vitro screenings allowed the identification of nine putative suppressive soils. Six of these soils, including Andisols and Ultisols, were confirmed to be suppressive, since they reduced take-all disease in wheat plants growing under greenhouse conditions. Suppressiveness was lost upon soil sterilization, and recovered by adding 1% of the natural soil, hence confirming that suppressiveness was closely associated to the soil microbiome community composition. Our results demonstrate that long-term extensive wheat cropping, established by small Mapuche communities, can generate suppressive soils that can be used as effective microorganism sources for take-all disease biocontrol. Accordingly, suppressive soil identification and characterization are key steps for the development of environmentally-friendly and efficient biotechnological applications for soil-borne disease control.

Understanding the Strategies to Overcome Phosphorus–Deficiency and Aluminum–Toxicity by Ryegrass Endophytic and Rhizosphere Phosphobacteria

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.

Microbial Community Composition In Take-All Suppressive Soils

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.