Plant growth promoting traits in native actinomycetes from Lotus tenuis rhizosphere

SOLANS M.; VOBIS G.; WALL L.G.

La Falda

Workshop; II LATIN AMERICAN PGPR WORKSHOP; 2014

Earlier results indicated the existence of PGPR strains belonging to the genus Azospirillum associated to the rhizosphere of L. tenuis in the Flooding Pampa. However, there is less information in relation to saprophytic non-Azospirillum bacteria such as actinomycetes. Within the soil microbiota, saprophytic actinomycetes are an important part of the microbial community as they are involved in diverse physiological processes. Actinomycetes have a high capacity to synthesize an array of biodegradative enzymes, antibiotics, phytohormones, siderophores and antifungal metabolites. Previous studies on actinorhizal and rhizobial symbioses showed that co-inoculation of plants with the corresponding microsymbionts (Frankia and rhizobia, respectively) together with native actinomycetes from Patagonia, resulted in plant growth promotion and an enhanced nodulation. In order to study the corresponding conditions in the Flooding Pampa, it is here proposed to isolate saprophytic actinomycete strains from the rhizosphere of Lotus tenuis, and to characterize them as potential Plant Growth Promoting Rhizoactinomycetes. Saprophytic actinomycetes from L. tenuis rhizosphere were isolated applying different methods: a) soil-dilution-plate, using 1 g of rhizospheric soil; b) stamping technique with 1 g of small pieces of roots and nodules, making imprints on agar, and c) chemotactic method, with 2 g of soil. The last one method was used for organisms with zoospores A preliminar taxonomic assignment was mainly based on their morphological traits. Actinomycete isolates were screened for IAA production, degradation of cellulose and hemicellulose, phosphorus solubilization, ability to fix nitrogen, siderophores production, ACC-deaminase and antifungal activitiy. A total of 32 actinomycete isolates was obtained from the rhizosphere of L. tenuis growing in the Flooding Pampa. The majority belongs to the genus Streptomyces (91%), and the rest were assigned to the genera Actinoplanes (6%) and Actinomadura (3%). All isolates were positive for IAA production; 12 isolates were positive for cellulose and hemicellulose degradation, and 9 isolates were positive for phosphate solubilization. Sixteen isolates presented the ability to fix N2. A set of 8 isolates with the higher N2-fixing activity were selected for further physiological tests, such as: ACC-deaminase activity, siderophores production and antagonistic activity. All of them gave positive results, including one Actinoplanes isolate and seven Streptomyces isolate. These results show the presence of actinomycetes with PGP traits in forage soils such as L. tenuis fields. It could be confirmed that these rhizoactinomycetes represent novel soil microorganisms with potential agronomic applications (Plant Growth Promoting Rhizoactinomycetes).