Agaves are major biotic resources in arid and semi-arid ecosystems. Despite their ecological, economical and cultural relevance, many aspects of the microbial communities associated with agaves are still unknown. Here, we investigated the bacterial communities associated with two Agave species by 16S rRNA- Denaturing gradient gel electrophoresis fingerprinting and sequencing. We also evaluated the effects of biotic and abiotic factors in the structure of the bacterial communities. In parallel, we isolated and characterized diazotrophic bacteria associated with agaves, as Agave soils are characterized by their low nitrogen content. Our results demonstrate that in Agave, the structure of prokaryotic assemblages was mostly influenced by the community group, where the soil, episphere, and endosphere were clearly distinct. Proteobacteria (γ and α), Actinobacteria, and Acidobacteria were the dominant phyla. Bacterial communities in the episphere of agaves were mainly influenced by the host species, whereas in the endosphere were affected by the season. Fifteen bacterial taxa were common and abundant in the endosphere of both Agave species during the dry season. Notably, some of the confirmed diazotrophic strains belonged to this group, suggesting a possible beneficial role in planta.

Based on partial 16S sequences, we previously described a novel group of nonsymbiotic, acetylene reduction activity-positive actinomycetes which were isolated from surface-sterilized roots of Casuarina equisetifolia growing in Mexico. An amplified rRNA restriction analysis confirmed that these actinomycetes are distinct from Frankia, a finding substantiated by a 16S rRNA gene phylogenetic analysis of two of the Mexican isolates. Further support for these actinomycetes being separate from Frankia comes from the very low DNA-DNA homology that was found. Nevertheless, the Mexican isolates may be diazotrophs based not only on their ability to grow in N-free medium and reduce acetylene to ethylene but also on the results from 15 N isotope dilution analysis and the finding that a nifH gene was PCR amplified. A comparison of the nifH sequences from the various isolates showed that they are closely related to nifH from Frankia; the similarity was 84 to 98% depending on the host specificity group. An analysis of complete 16S rRNA gene sequences demonstrated that the two strains analyzed in detail are most closely related to actinobacteria in the Thermomonosporaceae and the Micromonosporaceae.In the course of isolating Frankia from surface-sterilized root nodules of Casuarina equisetifolia growing in Mexico, additional filamentous bacteria were obtained (13). The filaments of these bacteria are smaller in diameter than Frankia hyphae, the filaments do not develop typical nitrogen-fixing diazovesicles ex planta under aerobic conditions, and the strains are unable to reinfect Casuarina or other tested actinorhizal plants upon reinoculation (20). In addition, of 24 fatty acids examined, only 7 were shared by Frankia and the Mexican isolates (17). Based on an analysis of a partial sequence of the 16S rRNA gene, we proposed that these bacteria lie outside of the Frankia clade (20).In this report, we examined the possibility that these nonFrankia Mexican actinomycetes have nitrogen-fixing genes based on their previously discovered abilities to reduce acetylene to ethylene, an indicator of nitrogenase activity, and to grow in N-free medium (13, 36). Nitrogenase, which catalyzes the reduction of dinitrogen to ammonia, is encoded by the genes of the nifHDK operon and is universally found in diazotrophs. Preliminary results with Southern blot analysis suggested that nifH-and nifD-like sequences were present in some of the Mexican isolates, but the bands weakly hybridized to a Klebsiella pneumoniae nifHD probe (unpublished results).Weak hybridization to DNA isolated from two Mexican strains (L4 and 7702B) was also observed when a Frankia CcI3 nifH probe was used (19).Because nif genes are conserved among a broad spectrum of bacteria, the use of universal primers has enabled the amplification and analysis of nifH sequences from very different microorganisms and environmental samples (21,38,40). However, when we used the universal degenerate primers Zf and Zr (39), the expected nifH gene products were not amplified. Instead, a PCR fra...

Colletotrichum gloeosporioides is the causal agent of anthracnose in mango. Burkholderia cepacia XXVI, isolated from mango rhizosphere and identified by 16S rDNA sequencing as a member of B. cepacia complex, was more effective than 6 other mango rhizosphere bacteria in inhibiting the model mango pathogen, C. gloeosporioides ATCC MYA 456. Biocontrol of this pathogen was demonstrated on Petri-dishes containing PDA by > 90 % reduction of surface colonization. The nature of the biocontrol metabolite(s) was characterized via a variety of tests. The inhibition was almost exclusively due to production of agar-diffusible, not volatile, metabolite(s). The diffusible metabolite(s) underwent thermal degradation at 70 and 121 °C (1 atm). Tests for indole acetic acid production and lytic enzyme activities (cellulase, glucanase and chitinase) by B. cepacia XXVI were negative, indicating that these metabolites were not involved in the biocontrol effect. Based on halo formation and growth inhibition of the pathogen on the diagnostic medium, CAS-agar, as well as colorimetric tests we surmised that strain XXVI produced a hydroxamate siderophore involved in the biocontrol effect observed. The minimal inhibitory concentration test showed that 0.64 μg ml(-1) of siderophore (Deferoxamine mesylate salt-equivalent) was sufficient to achieve 91.1 % inhibition of the pathogen growth on Petri-dishes containing PDA. The biocontrol capacity against C. gloeosporioides ATCC MYA 456 correlated directly with the siderophore production by B. cepacia XXVI: the highest concentration of siderophore production in PDB on day 7, 1.7 μg ml(-1) (Deferoxamine mesylate salt-equivalent), promoted a pathogen growth inhibition of 94.9 %. The growth of 5 additional strains of C. gloeosporioides (isolated from mango "Ataulfo" orchards located in the municipality of Chahuites, State of Oaxaca in Mexico) was also inhibited when confronted with B. cepacia XXVI. Results indicate that B. cepacia XXVI or its siderophore have the potential to be used as a biological control agent against C. gloeosporioides; thus diminishing environmental problems caused by the current practices to control this disease.

Este trabajo es el producto de un ejercicio multidisciplinario y examina el significado de la microbiología en la agricultura, haciendo énfasis en el beneficio de los biofertilizantes. Una evaluación de los biofertilizantes puede tratar un gran número de tópicos, cuya relevancia depende del interés del lector o tema concreto; sin embargo, el contenido principal está centrado en los tipos, modos de acción, factores que afectan su eficiencia, producción e impacto en la agricultura. La tecnología relativamente simple de la biofertilización se practica desde hace siglos y en la mayoría de los casos se reportan respuestas positivas sobre el rendimiento; sin embargo, resulta preocupante que en México no ha sido transferida y gran parte de los productores la desconoce. Por lo tanto, se requiere una mayor vinculación entre la industria y los científicos con el fin de colaborar en mejorar los sistemas de producción y calidad de los biofertilizantes. Además, es indispensable contar con asesoría, orientación y capacitación sobre su uso a los productores agrícolas.

Grain amaranth is an emerging crop that produces seeds having high quality protein with balanced amino-acid content. However, production is restricted by agronomic limitations that result in yields that are lower than those normally produced by cereals. In this work, the use of five different rhizobacteria were explored as a strategy to promote growth and yields in Amaranthus hypochondriacus cv. Nutrisol and A. cruentus cv. Candil, two commercially important grain amaranth cultivars. The plants were grown in a rich substrate, high in organic matter, nitrogen (N), and phosphorus (P) and under greenhouse conditions. Burkholderia ambifaria Mex-5 and B. caribensis XV proved to be the most efficient strains and significantly promoted growth in both grain amaranth species tested. Increased grain yield and harvest index occurred in combination with chemical fertilization when tested in A. cruentus. Growth-promotion and improved yields correlated with increased N content in all tissues examined. Positive effects on growth also occurred in A. cruentus plants grown in a poor soil, even after N and P fertilization. No correlation between non-structural carbohydrate levels in roots of inoculated plants and growth promotion was observed. Conversely, gene expression assays performed at 3-, 5- and 7-weeks after seed inoculation in plants inoculated with B. caribensis XV identified a tissue-specific induction of several genes involved in photosynthesis, sugar- and N- metabolism and transport. It is concluded that strains of Burkholderia effectively promote growth and increase seed yields in grain amaranth. Growth promotion was particularly noticeable in plants grown in an infertile soil but also occurred in a well fertilized rich substrate. The positive effects observed may be attributed to a bio-fertilization effect that led to increased N levels in roots and shoots. The latter effect correlated with the differential induction of several genes involved in carbon and N metabolism and transport.