Volatiles emitted by Bacillus sp. BCT9 act as growth modulating agents on Lactuca sativa seedlings.

Chemical products are applied during horticulture to increase food production, but the environmental problems resulting from these applications have led to a search for more sustainable products. Volatile organic compounds (VOCs) demonstrating plant growth promoter (PGP) activity released by bacterial species have emerged as alternatives, but their effects on Lactuca sativa growth are unknown. In this study, VOCs released by Bacillus sp. BCT9 cultures grown in different media (Methyl Red & Voges Proskauer, Murashige & Skoog and nutrient media) at concentrations of 0.1, 0.2, 0.5 and 0.7 (measured as the absorbance, λ=600nm) were tested to evaluate their activity as growth inducers of L. sativa after 10days of exposure. Lower concentrations of BCT9 increased root length, and higher concentrations induced shoot length and lateral root length. The dry weight and number of lateral roots increased similarly, independent of concentration, for VOCs produced in all culture media. BCT9 cultures grown in Methyl Red & Voges Proskauer medium as bioactive compounds with or without lanolin. These VOCs increased shoot length, root length and dry weight at low concentrations, independent of the presence of lanolin. Lateral root length increased with the application of 2-nonanone (50ppm) and 2-undecanone (0.05ppm). Based on these results, the use of bioactive volatiles as growth inducers of horticultural species represents an alternative or complementary strategy.

Growth promotion of Lactuca sativa in response to volatile organic compounds emitted from diverse bacterial species.

Agrochemicals are currently used in horticulture to increase crop production. Nevertheless, their indiscriminate use is a relevant issue for environmental and legal aspects. Alternative tools for reducing fertilizers and synthetic phytohormones are being investigated, such as the use of volatile organic compounds (VOCs) as growth inducers. Some soil bacteria, such as Pseudomonas and Bacillus, stimulate Arabidopsis and tobacco growth by releasing VOCs, but their effects on vegetables have not been investigated. Lactuca sativa was used as model vegetable to investigate bacterial VOCs as growth inducers. We selected 10 bacteria strains, belonging to Bacillus, Staphylococcus and Serratia genera that are able to produce 3-hydroxy-2-butanone (acetoin), a compound with proven growth promoting activity. Two-day old-seedlings of L. sativa were exposed to VOCs emitted by the selected bacteria grown in different media cultures for 7 days. The results showed that the VOCs released from the bacteria elicited an increase in the number of lateral roots, dry weight, root growth and shoot length, depending on the media used. Three Bacillus strains, BCT53, BCT9 and BCT4, were selected according to its their growth inducing capacity. The BCT9 strain elicited the greatest increases in dry weight and primary root length when L. sativa seedlings were subjected to a 10-day experiment. Finally, because acetoin only stimulated root growth, we suggest that other volatiles could be responsible for the growth promotion of L. sativa. In conclusion, our results strongly suggest that bacteria volatiles can be used as growth-inducers as alternative or complementary strategies for application in horticulture species.

Chemical composition and antibacterial activity of Laureliopsis philippiana (Looser) essential oil / Composición química y actividad antibacteriana del aceite esencial de Laureliopsis philippiana (Looser)

Laureliopsis philippiana (Looser) is native evergreen specie from Chile and Argentina used in traditional medicine. In this study, chemical composition as well as its in vitro antibacterial activity against Escherichia coli, Enterobacter aerogenes, Staphylococcus epidermidis and Staphylococcus aureus of essential oil from leaves of this species was determinated. Chemical analysis by GC-MS resulted in the identification of 19 compounds representing 98.8 percent onoterpenes; linalool (32.3 percent) and eucalyptol (37.4 percent) were the main constituents. To evaluate the antibacterial activity disc diffusion method and broth dilution method were used. The essential oil exhibited inhibitory activity against Gram (-) and Gram (+) bacteria, whereas similar activity to essential oil was showed for linalool against E. aerogenes and S. epidermidis whereas linalool alone, achieves an inhibitory effect against E. aerogenes and S. epidermidis comparable to the essential oil.

Laureliopsis philippiana (Looser) es una especie siempre verde nativa de Chile y Argentina usado en medicina tradicional. En este estudio se determinó tanto la composición química del aceite esencial obtenido a partir de hojas de esta especie, así como su actividad antibacterial in vitro contra Escherichia coli, Enterobacter aerogenes, Staphylococcus epidermidis and Staphylococcus aureus. El análisis químico por GC-MS permitió la identificación de 19 compuestos, representando el 98,8 por ciento de la composición del aceite. Monoterpenos oxigenados, eucaliptol y linalol fueron los mayores constituyentes del aceite con un 37,4 por ciento y 32,3 por ciento respectivamente. Para evaluar la actividad antibacteriana se utilizaron los métodos de difusión en agar y dilución en caldo. El aceite esencial muestra actividad inhibitoria contra las bacterias Gram (-) y Gram (+) evaluadas, mientras que linalol por si solo logra un efecto inhibitorio comparable con el aceite esencial contra E. aerogenes y S. epidermidis mientras que el linalol por si solo, logra un efecto inhibitorio contra E. aerogenes y S. epidermidis comparable al del aceite esencial.

Sinorhizobium fredii HH103 does not strictly require KPS and/or EPS to nodulate Glycyrrhiza uralensis, an indeterminate nodule-forming legume.

The Sinorhizobium fredii HH103 rkp-1 region, which is involved in capsular polysaccharide (KPS) biosynthesis, is constituted by the rkpU, rkpAGHIJ, and kpsF3 genes. Two mutants in this region affecting the rkpA (SVQ536) and rkpI (SVQ538) genes were constructed. Polyacrylamide gel electrophoresis and (1)H-NMR analyses did not detect KPS in these mutants. RT-PCR experiments indicated that, most probably, the rkpAGHI genes are cotranscribed. Glycine max cultivars (cvs.) Williams and Peking inoculated with mutants SVQ536 and SVQ538 showed reduced nodulation and symptoms of nitrogen starvation. Many pseudonodules were also formed on the American cv. Williams but not on the Asiatic cv. Peking, suggesting that in the determinate nodule-forming S. fredii-soybean symbiosis, bacterial KPS might be involved in determining cultivar-strain specificity. S. fredii HH103 mutants unable to produce KPS or exopolysaccharide (EPS) also showed reduced symbiotic capacity with Glycyrrhiza uralensis, an indeterminate nodule-forming legume. A HH103 exoA-rkpH double mutant unable to produce KPS and EPS was still able to form some nitrogen-fixing nodules on G. uralensis. Thus, here we describe for the first time a Sinorhizobium mutant strain, which produces neither KPS nor EPS is able to induce the formation of functional nodules in an indeterminate nodule-forming legume.

The rkpU gene of Sinorhizobium fredii HH103 is required for bacterial K-antigen polysaccharide production and for efficient nodulation with soybean but not with cowpea.

In this work, the role of the rkpU and rkpJ genes in the production of the K-antigen polysaccharides (KPS) and in the symbiotic capacity of Sinorhizobium fredii HH103, a broad host-range rhizobial strain able to nodulate soybean and many other legumes, was studied. The rkpJ- and rkpU-encoded products are orthologous to Escherichia coli proteins involved in capsule export. S. fredii HH103 mutant derivatives were contructed in both genes. To our knowledge, this is the first time that the role of rkpU in KPS production has been studied in rhizobia. Both rkpJ and rkpU mutants were unable to produce KPS. The rkpU derivative also showed alterations in its lipopolysaccharide (LPS). Neither KPS production nor rkpJ and rkpU expression was affected by the presence of the flavonoid genistein. Soybean (Glycine max) plants inoculated with the S. fredii HH103 rkpU and rkpJ mutants showed reduced nodulation and clear symptoms of nitrogen starvation. However, neither the rkpJ nor the rkpU mutants were significantly impaired in their symbiotic interaction with cowpea (Vigna unguiculata). Thus, we demonstrate for the first time to our knowledge the involvement of the rkpU gene in rhizobial KPS production and also show that the symbiotic relevance of the S. fredii HH103 KPS depends on the specific bacterium-legume interaction.

Sinorhizobium fredii HH103 mutants affected in capsular polysaccharide (KPS) are impaired for nodulation with soybean and Cajanus cajan.

The Sinorhizobium fredii HH103 rkp-1 region, which is involved in capsular polysaccharides (KPS) production, was isolated and sequenced. The organization of the S. fredii genes identified, rkpUAGHIJ and kpsF3, was identical to that described for S. meliloti 1021 but different from that of S. meliloti AK631. The long rkpA gene (7.5 kb) of S. fredii HH103 and S. meliloti 1021 appears as a fusion of six clustered AK631 genes, rkpABCDEF. S. fredii HH103-Rif(r) mutants affected in rkpH or rkpG were constructed. An exoA mutant unable to produce exopolysaccharide (EPS) and a double mutant exoA rkpH also were obtained. Glycine max (soybean) and Cajanus cajan (pigeon pea) plants inoculated with the rkpH, rkpG, and rkpH exoA derivatives of S. fredii HH103 showed reduced nodulation and severe symptoms of nitrogen starvation. The symbiotic capacity of the exoA mutant was not significantly altered. All these results indicate that KPS, but not EPS, is of crucial importance for the symbiotic capacity of S. fredii HH103-Rif(r). S. meliloti strains that produce only EPS or KPS are still effective with alfalfa. In S. fredii HH103, however, EPS and KPS are not equivalent, because mutants in rkp genes are symbiotically impaired regardless of whether or not EPS is produced.

NolR regulates diverse symbiotic signals of Sinorhizobium fredii HH103.

We have investigated in Sinorhizobium fredii HH103-1 (=HH103 Str(r)) the influence of the nolR gene on the production of three different bacterial symbiotic signals: Nod factors, signal responsive (SR) proteins, and exopolysaccharide (EPS). The presence of multiple copies of nolR (in plasmid pMUS675) repressed the transcription of all the flavonoid-inducible genes analyzed: nodA, nodD1, nolO, nolX, noeL, rhcJ, hesB, and y4pF. Inactivation of nolR (mutant SVQ517) or its overexpression (presence of pMUS675) altered the amount of Nod factors detected. Mutant SVQ517 produced Nod factors carrying N-methyl residues at the nonreducing N-acetyl-glucosamine, which never have been detected in S. fredii HH103. Plasmid pMUS675 increased the amounts of EPS produced by HH103-1 and SVQ517. The flavonoid genistein repressed EPS production of HH103-1 and SVQ517 but the presence of pMUS675 reduced this repression. The presence of plasmid pMUS675 clearly decreased the secretion of SR proteins. Inactivation, or overexpression, of nolR decreased the capacity of HH103 to nodulate Glycine max. However, HH103-1 and SVQ517 carrying plasmid pMUS675 showed enhanced nodulation capacity with Vigna unguiculata. The nolR gene was positively identified in all S. fredii strains investigated, S. xinjiangense CCBAU110, and S. saheli USDA4102. Apparently, S. teranga USDA4101 does not contain this gene.