Exploring the response of Actinobacteria to the presence of phosphorus salts sources: Metabolic and co-metabolic processes.

This study tested the solubilization of phosphorus by five actinobacterial strains in liquid media containing Ca3 PO4 ; AlPO4 or FePO4 as the sole phosphate source, and discusses the possible mechanisms involved in this process. P solubilization by different strains was accompanied by a significant drop in pH from 7.0 to 2.15-5.0 after 14 days. The efficiency of different strains depended on the P-source. Streptomyces spp. MM140 and MM141 were the most efficient in solubilizing Ca3 PO4 , MM136, and MM141 were the most efficient in solubilizing AlPO4 , while all strains were equally efficient in solubilizing FePO4 . Gluconic, oxalic, citric, malic, succinic, formic, and acetic acid were detected in the medium with Ca3 PO4 , while all except acetic acid were detected in the media with FePO4 or AlPO4 . Although we did not use an organic source of phosphorus in the media, all strains produced acid and alkaline phosphatase. It is concluded from this study that actinobacteria produced multiple organic acids followed by a decrease in the pH to solubilize phosphate salts. As well as producing phosphatase, these microorganisms were found to have different ways of making P available, suggesting an ecological advantage as they form part of soil microbiomes important for plants.

Potential biocontrol actinobacteria: Rhizospheric isolates from the Argentine Pampas lowlands legumes.

Control of fungal plant diseases by using naturally occurring non-pathogenic microorganisms represents a promising approach to biocontrol agents. This study reports the isolation, characterization, and fungal antagonistic activity of actinobacteria from forage soils in the Flooding Pampa, Argentina. A total of 32 saprophytic strains of actinobacteria were obtained by different isolation methods from rhizospheric soil of Lotus tenuis growing in the Salado River Basin. Based on physiological traits, eight isolates were selected for their biocontrol-related activities such as production of lytic extracellular enzymes, siderophores, hydrogen cyanide (HCN), and antagonistic activity against Cercospora sojina, Macrophomia phaseolina, Phomopsis sp., Fusarium oxysporum, and Fusarium verticilloides. These actinobacteria strains were characterized morphologically, physiologically, and identified by using molecular techniques. The characterization of biocontrol-related activities in vitro showed positive results for exoprotease, phospholipase, fungal growth inhibition, and siderophore production. However, none of the strains was positive for the production of hydrogen cyanide (HCN). Streptomyces sp. MM140 presented the highest index for biocontrol, and appear to be promising pathogenic fungi biocontrol agents. These results show the potential capacity of actinobacteria isolated from forage soils in the Argentine Pampas lowlands as promising biocontrol agents, and their future agronomic applications.

Discaria trinervis - Frankia symbiosis promotion by saprophytic actinomycetes.

The influence of saprophytic actinomycetes strains on the Discaria trinervis - Frankia actinorhizal symbiosis was investigated. Three strains out of 122 isolated from the rhizosphere and rhizoplane of D. trinervis with multiple enzymatic activities, were selected for plant growth experiments: Streptomyces (BCRU-MM40), Actinoplanes (BCRU-ME3) and Micromonospora (BCRU-MM18). Inoculated seedlings of Discaria trinervis were grown in glass tubes with vermiculite-sand for 12 weeks. They were inoculated either with a single saprophytic strain or a combination of one or two of them together with the symbiotic N(2) fixing strain Frankia BCU110501. The saprophytic strains were applied in two experimental series, i.e. mycelium + supernatant simultaneously or mycelium and supernatant (growth medium free of cells) separately. Micromonospora strain MM18 showed a direct promotion effect on shoot growth, when plants were inoculated with mycelium and supernatant together. Streptomyces strain MM40 and Actinoplanes strain ME3 promoted the actinorhizal symbiosis with Frankia and consequently the development of plant shoots, when supernatant was involved as inoculum. It is supposed, that the strains MM18, MM40 and ME3 produce bioactive metabolites, which are released into the culture medium. The saprophytic strains studied could be considered as "promoting or helper rhizoactinomycetes" of the actinorhizal plant D. trinervis.