Mariana Solans

Effect of actinobacteria on Lotus tenuis - Mesorhizobium loti symbiosis: preliminary study

The effect of the actinobacteria on the legume Lotus tenuis, a model forage plant of the Flooding Pampa region in Argentina was to promote plant growth and root nodulation. L. tenuis seedlings were inoculated either with the symbiotic N2-fixing strain Mesorhizobium loti, or with saprophytic strain of Streptomyces MM40, Actinoplanes ME3, or Micromonospora MM18, or co-inoculated with a combination of one, two or three of them, together with the rhizobium. The plants were grown in pouches and fertilized with three nitrogen levels (low, medium and high). L. tenuis plants co-inoculated with the symbiotic M. loti and actinobacteria showed an increase in plant biomass and nodulation. Plants fertilized with high N levels (inhibitory for nodulation by M. loti) were nodulated if the M. loti strain was added simultaneously with actinobacteria. These preliminary results show that actinobacteria have potential agronomic application which should be further explored.

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.

Synergy of Actinomycete Co-inoculation

Historically the symbioses between leguminous plants and rhizobia have attracted the attention of researchers due to the incidence of legumes for sustaining nutritional requirements to humans and animals. There have been large efforts to increase the ability to symbiotic N2 fixing and productivity of legumes. New research is focusing on increasing the legume–rhizobia symbiosis with increased biological nitrogen fixation (BNF), growth, and productivity. The inoculation of legumes with rhizosphere bacteria has often been found to increase symbiotic properties, plant biomass, and yields under greenhouse or field conditions. The potential to enhance plant growth, nodulation, nitrogen fixation, productivity of legumes by plant growth-promoting rhizobacteria (PGPR), and Rhizobium co-inoculation does exist, although most of studies have been conducted with Bacillus spp., Pseudomonas spp., or other genera and few with actinomycetes. The latter, a group of actinobacteria widely distributed in terrestrial ecosystems contribute to soil nutrient cycling and live in association with plants and are considered as one of the most important communities in the rhizosphere. They have a great ability to synthesize a series of bioactive metabolites and potential within the agroecosystem, where they play important roles in disease suppression and plant growth promotion in cultivated plants. In this sense, the purpose of this chapter is to show the synergistic effect of actinomycete co-inoculation on N2-fixing symbioses and their potential use in agriculture.