Olga S. Correa

Soil microbial community responses to the fungal endophyte Neotyphodium in Italian ryegrass

Cool-season grasses commonly harbor fungal endophytes in their aerial tissues. However the effects of these symbionts on soil microbial communities have rarely been investigated. Our objective was to explore microbial community responses in soils conditioned by plants of the annual grass Lolium multiflorum with contrasting levels of infection with the endophyte Neotyphodium occultans. At the end of the host growing season, we estimated the functional capacity of soil microbial communities (via catabolic response profiles), the contribution of fungi and bacteria to soil activity (via selective inhibition with antibiotics), and the structure of both microbial communities by molecular analyses. Soil conditioning by highly infected plants affected soil catabolic profiles and tended to increase soil fungal activity. We detected a shift in bacterial community structure while no changes were observed for fungi. Soil responses became evident even without changes in host plant biomass or soil organic carbon or total nitrogen content, suggesting that the endophyte modified host rhizodepositions during the conditioning phase. Our results have implications for the understanding of the reciprocal interactions between above and belowground communities, suggesting that plant-soil feedbacks can be mediated by this symbiosis.

Tomato genotype and Azospirillum inoculation modulate the changes in bacterial communities associated with roots and leaves

Aims:  To evaluate the effect of plant variety and Azospirillum brasilense inoculation on the microbial communities colonizing roots and leaves of tomato (Lycopersicon esculentum Mill.) plants.

Cellular envelopes and tolerance to acid pH in Mesorhizobium loti.

Abstract. Changes in the cell envelopes in response to acidity were studied in two strains of Mesorhizobium loti differing in their tolerance to pH. When the less acid-tolerant strain LL22 was grown at pH 5.5, membrane phosphatidylglycerol decreased and phosphatidylcholine increased, compared with cells grown at pH 7.0. On the other hand, when the more acid-tolerant strain LL56 was grown at pH 5.5, phosphatidylglycerol, phosphatidylethanolamine, and lysophospholipid decreased 25%, 39%, and 51% respectively, while phosphatidyl-N-methylethanolamine and cardiolipin increased 26% and 65% respectively compared with cells grown at pH 7.0. The longest-chain fatty acids (19:0 cy and 20:0) increased in both strains at pH 5.5, while in LL56 these fatty acids increased still further at pH 4.0. Variations in other wall and membrane properties such as cell hydrophobicity, lypopolysaccharides, and protein composition of the outer membrane in relation to acid pH are also discussed.

Pyrosequencing reveals changes in soil bacterial communities after conversion of Yungas forests to agriculture.

The Southern Andean Yungas in Northwest Argentina constitute one of the main biodiversity hotspots in the world. Considerable changes in land use have taken place in this ecoregion, predominantly related to forest conversion to croplands, inducing losses in above-ground biodiversity and with potential impact on soil microbial communities. In this study, we used high-throughput pyrosequencing of the 16S ribosomal RNA gene to assess whether land-use change and time under agriculture affect the composition and diversity of soil bacterial communities. We selected two areas dedicated to sugarcane and soybean production, comprising both short- and long-term agricultural sites, and used the adjacent native forest soils as a reference. Land-use change altered the composition of bacterial communities, with differences between productive areas despite the similarities between both forests. At the phylum level, only Verrucomicrobia and Firmicutes changed in abundance after deforestation for sugarcane and soybean cropping, respectively. In cultivated soils, Verrucomicrobia decreased sharply (~80%), while Firmicutes were more abundant. Despite the fact that local diversity was increased in sugarcane systems and was not altered by soybean cropping, phylogenetic beta diversity declined along both chronosequences, evidencing a homogenization of soil bacterial communities over time. In spite of the detected alteration in composition and diversity, we found a core microbiome resistant to the disturbances caused by the conversion of forests to cultivated lands and few or none exclusive OTUs for each land-use type. The overall changes in the relative abundance of copiotrophic and oligotrophic taxa may have an impact in soil ecosystem functionality. However, communities with many taxa in common may also share many functional attributes, allowing to maintain at least some soil ecosystem services after forest conversion to croplands.

The adaptive acid response in Mesorhizobium sp.

Mesorhizobium huakuii strain LL56 and Mesorhizobium sp. strain LL22, which nodulate Lotus glaber, developed an adaptive acid response during exponential growth upon exposure to sublethal acid conditions. The adaptive acid response was found to be dependent on the sublethal pH and the strain intrinsic acid tolerance: the lowest adaptation pH was 4.0 for strain LL56 and 5.7 for strain LL22, and the lowest pH values tolerated after adaptation were 3.0 and 4.0, respectively. Both complex and minimal medium allowed the development of the adaptive acid response, although in complex medium this response was more effective. Three low molecular weight polypeptides (LMWPs) showed increased expression in strain LL56 during the adaptation to pH 4.0. However, the adaptive acid tolerance was only partially dependent on de novo protein synthesis, and constitutive systems may play a significant role on the acid tolerance of Mesorhizobium huakuii strain LL56.

Genotypic characterization of Azotobacteria isolated from Argentinean soils and plant-growth-promoting traits of selected strains with prospects for biofertilizer production.

The genetic diversity among 31 putative Azotobacter isolates obtained from agricultural and non-agricultural soils was assessed using rep-PCR genomic fingerprinting and identified to species level by ARDRA and partial 16S rRNA gene sequence analysis. High diversity was found among the isolates, identified as A. chroococcum, A. salinestris, and A. armeniacus. Selected isolates were characterized on the basis of phytohormone biosynthesis, nitrogenase activity, siderophore production, and phosphate solubilization. Indole-3 acetic-acid (IAA), gibberellin (GA3) and zeatin (Z) biosynthesis, nitrogenase activity, and siderophore production were found in all evaluated strains, with variation among them, but no phosphate solubilization was detected. Phytohormones excreted to the culture medium ranged in the following concentrations: 2.2–18.2 μg IAA mL−1, 0.3–0.7 μg GA3 mL−1, and 0.5–1.2 μg Z mL−1. Seed inoculations with further selected Azotobacter strains and treatments with their cell-free cultures increased the number of seminal roots and root hairs in wheat seedlings. This latter effect was mimicked by treatments with IAA-pure solutions, but it was not related to bacterial root colonization. Our survey constitutes a first approach to the knowledge of Azotobacter species inhabiting Argentinean soils in three contrasting geographical regions. Moreover, this phenotypic characterization constitutes an important contribution to the selection of Azotobacter strains for biofertilizer formulations.

Potential of Bacilli for Biocontrol and Its Exploitation in Sustainable Agriculture

Plant diseases are caused mainly by fungi, bacteria, viruses, and nematodes, and their control is necessary to feed an increasing population. Control of plant diseases often rely on chemical pesticides, which have contributed to improvements in crop productivity and quality over the past years. However, the intensive use of agrochemical pesticides results in soil and groundwater pollution. Consequently, there are worldwide efforts to develop other alternatives to chemical pesticides for controlling plant diseases. Among them, the use of microorganisms and their products, referred as biological control, are regarded as promissory alternatives to reduce the use of chemical products. Different Bacillus species excrete peptides and lipopeptides to the culture medium, such as fungicine, iturin, bacillomicine and others, that have antifungal antibacterial and surfactant activity. In addition, these species produce spores that are resistant to heat and desiccation, which allows the preparation of more stable and durable formulations. A variety of biological control products based on Bacillus species are available for agronomical use; but in order to translate these developments into a broader and more effective use, a greater understanding of the complex interactions among plants, microorganisms, and the environment is required. This chapter describes some mechanisms of biocontrol exhibited by species of Bacillus, the current status of research and application of biological control using Bacillus species, constraints to microbial biocontrol implementation, and briefly outlines the future directions that might lead to the development of more diverse and effective biological controls for plant diseases.

Protocol for the Quality Control of Azospirillum spp. Inoculants

Azospirillum has been one of the most studied genera of plant growth promoting rhizobacteria (PGPR) worldwide over the past 50 years. The use of these microorganisms in agriculture practices has been adopted due to their ability to associate in rhizospheric, epiphytic, or endophytic ways with roots and promote whole plant growth or crop productivity. The biological treatment of seeds (inoculation) in more than a hundred species of economic or ecological interest has become a common practice in many countries. In Argentina, the Az39 strain of Azospirillum brasilense, belonging to the Culture Collection of the Instituto de Microbiologia y Zoologia Agricola (IMYZA) of INTA Castelar, was selected in the 1980s after an intensive program to isolate and identify microorganisms for agriculture, according to their agronomic behavior. Since then, its ability to cover the premise for which it was selected has determined that Az39 is largely adopted by inoculant companies in Argentina with the aim of producing biological products for the treatment of several crops. In this chapter, those methods developed and standardized by the network Red de Control de Calidad de Inoculantes (REDCAI) of the Asociacion Argentina de Microbiologia (AAM) have been adapted as a guide for the quantification of Azospirillum spp. and the detection of contaminating microorganisms in biological products, as two of the most basic and important quality control parameters of inoculants.