Evaluation of indigenous bacterial strains for biocontrol of the frogeye leaf spot of soya bean caused by Cercospora sojina.

AIMS: Assessment of biological control of Cercospora sojina, causal agent of frogeye leaf spot (FLS) of soya bean, using three indigenous bacterial strains, BNM297 (Pseudomonas fluorescens), BNM340 and BNM122 (Bacillus amyloliquefaciens). METHODS AND RESULTS: From cultures of each bacterial strain, cell suspensions and cell-free supernatants were obtained and assayed to determine their antifungal activity against C. sojina. Both mycelial growth and spore germination in vitro were more strongly inhibited by bacterial cell suspensions than by cell-free supernatants. The Bacillus strains BNM122 and BNM340 inhibited the fungal growth to a similar degree (I ≈ 52-53%), while cells from P. fluorescens BNM297 caused a lesser reduction (I ≈ 32-34%) in the fungus colony diameter. The foliar application of the two Bacillus strains on soya bean seedlings, under greenhouse conditions, significantly reduced the disease severity with respect to control soya bean seedlings and those sprayed with BNM297. This last bacterial strain was not effective in controlling FLS in vivo. CONCLUSIONS: Our data demonstrate that the application of antagonistic bacteria may be a promising and environmentally friendly alternative to control the FLS of soya bean. SIGNIFICANCE AND IMPACT OF THE STUDY: To our knowledge, this is the first report of biological control of C. sojina by using native Bacillus strains.

Antifungal activity of selected indigenous pseudomonas and bacillus from the soybean rhizosphere.

The purpose of this study was to isolate and select indigenous soil Pseudomonas and Bacillus bacteria capable of developing multiple mechanisms of action related to the biocontrol of phytopathogenic fungi affecting soybean crops. The screening procedure consisted of antagonism tests against a panel of phytopathogenic fungi, taxonomic identification, detection by PCR of several genes related to antifungal activity, in vitro detection of the antifungal products, and root colonization assays. Two isolates, identified and designated as Pseudomonas fluorescens BNM296 and Bacillus amyloliquefaciens BNM340, were selected for further studies. These isolates protected plants against the damping-off caused by Pythium ultimum and were able to increase the seedling emergence rate after inoculation of soybean seeds with each bacterium. Also, the shoot nitrogen content was higher in plants when seeds were inoculated with BNM296. The polyphasic approach of this work allowed us to select two indigenous bacterial strains that promoted the early development of soybean plants.

Assessment of the role of chemotaxis and biofilm formation as requirements for colonization of roots and seeds of soybean plants by Bacillus amyloliquefaciens BNM339.

This article correlates colonization with parameters, such as chemotaxis, biofilm formation, and bacterial growth, that are believed to be connected. We show here, by using two varieties of soybean plants that seeds axenically produced exudates, induced a chemotactic response in Bacillus amyloliquefaciens, whereas root exudates did not, even when the exudates, also collected under axenic conditions, were concentrated up to 200-fold. Root exudates did not support bacterial cell division, whereas seed exudates contain compounds that support active cell division and high cell biomass at stationary phase. Seed exudates of the two soybean varieties also induced biofilm formation. B. amyloliquefaciens colonized both seeds and roots, and plant variety significantly affected bacterial root colonization, whereas it did not affect seed colonization. Colonization of roots in B. amyloliquefaciens occurred despite the lack of chemotaxis and growth stimulation by root exudates. The data presented in this article suggest that soybean seed colonization, but not root colonization, by B. amyloliquefaciens is influenced by chemotaxis, growth, and biofilm formation and that this may be caused by qualitative changes of the composition of root exudates.

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. METHODS AND RESULTS: Seeds of cherry and fresh-market tomato were inoculated with A. brasilense BNM65. Sixty days after planting, plants were harvested and the microbial communities of the rhizoplane and phyllosphere were analysed by community-level physiological profiles (CLPP) using BIOLOG EcoPlates and denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes. Differences on the rhizoplane and phyllosphere bacterial communities between the two tomato types were detected by principal component analysis of the CLPP; DGGE fingerprints also showed differences at the phyllosphere level. Fresh-market tomato had a more complex phyllosphere bacterial community than cherry tomato, as determined by DGGE profiles. Physiological and genetic changes on phyllosphere and rhizoplane bacterial communities by Azospirillum seed inoculation were evident only on cherry tomato. CONCLUSIONS: Tomato genotype affects the response of native bacterial communities associated with the roots and leaves to A. brasilense seed inoculation. SIGNIFICANCE AND IMPACT OF THE STUDY: The successful implementation of Azospirillum inoculation requires not only the consideration of the interactions between A. brasilense strains and plant genotypes, but also the plant-associated microflora.

Cellular envelopes and tolerance to acid pH in Mesorhizobium loti.

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.

Effect of Azospirillum-mediated plant growth promotion on the development of bacterial diseases on fresh-market and cherry tomato.

AIMS: Plant growth-promoting (PGP) activity of two Azospirillum strains and their effects on foliar and vascular bacterial diseases were evaluated on fresh market and cherry tomato. METHODS AND RESULTS: Tomato seeds were inoculated with A. brasilense Sp7 or Azospirillum sp. BNM-65. Four-week-old plants were challenge-inoculated with Clavibacter michiganensis subsp. michiganensis (bacterial canker) or with Xanthomonas campestris pv. vesicatoria (bacterial spot). Azospirillum-induced PGP was greater on cherry than on fresh-market tomato. Cherry tomato was more resistant to bacterial canker but more susceptible to bacterial spot than the fresh-market tomato. Canker severity was not affected by Azospirillum seed treatments. However, leaf- and plant-death were delayed on Azospirillum-treated plants compared with nontreated controls. Azospirillum increased the bacterial spot severity on cherry but not on fresh-market tomato. CONCLUSIONS: PGP was observed on both tomato genotypes, although growth effects were larger on cherry tomato. Also, Azospirillum treatments may alter tomato susceptibility to bacterial diseases. SIGNIFICANCE AND IMPACT OF THE STUDY: The interaction between PGP rhizobacteria like Azospirillum spp., not known to induce systemic resistance, with plant pathogens distantly located is frequently overlooked. This work demonstrates the importance of this kind of evaluation.

Genetic and functional characterization of a Bacillus sp. strain excreting surfactin and antifungal metabolites partially identified as iturin-like compounds.

AIMS: A bacterial strain producing antifungal compounds active against the plant pathogenic fungi Fusarium, Rhizoctonia and Sclerotinia has been characterized and shown to control Rhizoctonia root rot of soya bean. METHODS AND RESULTS: The metabolites excreted by Bacillus BNM 122 remained active after autoclaving, were resistant over a wide pH range and to hydrolytic enzymes. By (1)H-NMR and thin-layer chromatography analyses surfactin and iturin-like compounds were partially identified. Moreover, soya bean seeds bacterization with BNM 122 in a compost-based formulation was as effective controlling Rhizoctonia solani as pentachloronitrobenzene. According to its 16S rDNA sequence BNM 122 was closely related to Bacillus amyloliquefaciens and Bacillus subtilis. PCR analysis of the 16S-23S rRNA intergenic spacer region and repetitive sequence-based PCR (rep-PCR) genomic fingerprinting revealed a close genetic relationship to B. amyloliquefaciens. However, by physiological characterization using API tests, this strain resembled more B. subtilis. CONCLUSIONS: This is the first report describing the co-production of surfactin and iturin-like compounds by a putative strain of B. amyloliquefaciens. The synergistic effect of both lipopetides is a remarkable trait for a candidate biocontrol agent. SIGNIFICANCE AND IMPACT OF THE STUDY: This kind of research has relevance in order to minimize the use of synthetic fungicides and surfactants, contributing to the preservation of the environment.

Inoculación artificial de hongos micorríticos de los géneros Boletus y Scleroderma de especies de plantas del género Eucalyptus / Artificial symbiosis of mycorrhizal fungi of genera Boletus and Scleroderma between some species of plants of Eucalyptus genera

Dada la creciente importancia que van adquiriendo especies de plantas del género Eucalyptus en la explotación forestal argentina y al considerar los benefícios de la asociación micorrítica, se realizó este trabajo que consistió en desarrollar, en cultivos puros, la simbiosis entre especies de plantas forestales del género Eucaliptus con hongos micorríticos de los géneros Boletus sp2 con Eucalyptus viminalis y entre Boletus sp. y Scleroderma sp. (f) con Eucalyptus rostrata. Los análisis estadísticos confirmaron el efecto beneficioso de las asociaciones desarrolladas para el parámetro peso seco de parte aérea. Estos resultados deberían ser confirmados con ensayos de vivero, con el fin de tener evidencias sobre la respuesta de estas asociaciones en el medio ecológico natural