Volatile organic compounds profile synthesized and released by endophytes of tomato (Solanum lycopersici L.) and their antagonistic role.

The endophytic microbiome uses mechanisms such as the secretion of diffusible antibiotic molecules, synthesis and release of volatile organic compounds, and/or toxins to protect plants. The aim of this research was to study the volatile organic compounds (VOCs) profile as well as the diffusible secondary metabolites produced and released by endophytic bacteria isolated from tomato plants that in in-vitro assays prevented growth of pathogenic fungi. Bacteria belonging to seven genera (Acinetobacter, Arthrobacter, Bacillus, Microbacterium, Pantoea, Pseudomonas, and Stenotrophomonas) were isolated from different tissues of tomato plants with and without symptoms of Gray leaf spot, a disease provoked by Stemphylium lycopersici. In vitro, antagonistic assays were performed and the effect of volatile and soluble compounds released by endophytic bacteria on the growth of pathogenic fungi was determined. The VOCs synthesized by the endophytes were extracted, identified and quantified. These isolates representatives of seven bacterial genera inhibited the growth of three fungal pathogens of tomato S. lycopersici, Alternaria alternata and Corynespora cassiicola, which was related to the synthesis of soluble compounds as well as VOCs. Endophytes synthesize and release different VOCs, probably due to the different type of interaction that each bacterium establishes with the fungus, presenting a range of fungal growth inhibition.

The endosphere bacteriome of diseased and healthy tomato plants.

Here we analyze the microbial community of healthy and diseased tomato plants to evaluate its impact on plant health. The organisms found in all samples mainly belonged to 4 phyla: Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria. The Proteobacteria were the highest relative abundant within the endophytic communities of different plant organs of diseased tomato. Among endophytic bacteria of tomato, only a few taxa could be cultured. Here we showed that only a few taxa of bacteria inhabiting tomato plants could be cultured and that all plant organs have a highly diverse endophytic bacterial, whose activity might affect plant growth and development as well as health. The roots seem to be an important barrier for microbes and leaves appear to be the organs with the higher diversity which is incidentally related to plant health. Fruits also contain a complex bacterial community that appeared to be unaffected by foliar diseases such as gray leaf spot at least under the conditions studied.

Nodulation and Delayed Nodule Senescence: Strategies of Two Bradyrhizobium Japonicum Isolates with High Capacity to Fix Nitrogen.

The purpose of this work was to study further two Bradyrhizobium japonicum strains with high nitrogen-fixing capacity that were identified within a collection of approximately 200 isolates from the soils of Argentina. Nodulation and nitrogen-fixing capacity and the level of expression of regulatory as well as structural genes of nitrogen fixation and the 1-aminocyclopropane-1-carboxylate (ACC) deaminase gene of the isolates were compared with that of E109-inoculated plants. Both isolates of B. japonicum, 163 and 366, were highly efficient to fix nitrogen compared to commercial strain E109. Isolate 366 developed a higher number and larger biomass of nodules and because of this fixed more nitrogen. Isolate 163 developed the same number and nodule biomass than E109. However, nodules developed by isolate 163 had red interiors for a longer period, had a higher leghemoglobin content, and presented high levels of expression of acdS gene, that codes for an ACC deaminase. In conclusion, naturalized rhizobia of the soils of Argentina hold a diverse population that might be the source of highly active nitrogen-fixing rhizobia, a process that appears to be based on different strategies.

A Survey on Occurrence of Cladosporium fulvum Identifies Race 0 and Race 2 in Tomato-Growing Areas of Argentina.

The presence of Cladosporium fulvum (syn. Passalora fulva), causal agent of tomato leaf mold, was confirmed in the two main greenhouse-production areas for tomato in Argentina. Using both morphological characters and internal transcribed spacer sequencing, we confirmed the presence of physiological races of this pathogen. A diagnostic multiplex polymerase chain reaction (PCR) was also developed, using primers derived from C. fulvum avirulence (Avr) genes. In all, 20 isolates of Cladosporium spp. were obtained as monospore cultures and 12 were identified as C. fulvum. By this method, we showed that, of these 12 isolates, 5 were race 0 (carrying functional Avr2, Avr4, Avr4E, and Avr9 genes) and 7 were race 2 (lacking the Avr2 gene). Race identity was confirmed by testing their virulence on a set of tomato differentials carrying different Cf resistance genes. All Avr genes could be amplified in single or multiplex PCR using DNA isolated from in vitro grown monospore cultures but only three Avr could be amplified when genomic DNA was isolated from C. fulvum-infected necrotic leaf tissue.