Biology of Actinomycetes in the Rhizosphere of Nitrogen-Fixing Plants

SOLANS, M., VOBIS, G.

ACTINOBACTERIA. Application in Bioremediation and Production of Industrial Enzymes

CRC Press

Lugar: Boca Ratón; Año: 2012;

Actinomycetes produce a diverse range of secondary metabolites including numerous antibiotics, antitumor and immunosuppresive agents and enzymes, that tradicionally play an important role in pharmaceutical industry. More recently, other aspects of application complete the investigations with actinobacteria, which are the most widely distributed microorganisms as saprophytic soil inhabitants. The aim of this chapter is to present new data of the knowledge of those actinomycetes, that live in the rhizosphere interacting with nitrogen-fixing plants. A total number of 122 saprophytic strains of actinomycetes with multiple enzymatic activities were isolated from the rhizosphere of the nitrogen-fixing actinorhizal plant, Ochetophila trinervis, named here as “rhizoactinomycetes”. Three strains out of them, Streptomyces sp. (BCRU-MM40), Actinoplanes sp. (BCRU-ME3) and Micromonospora sp. (BCRU-MM18), were selected for plant growth experiments, using the actinorhizal O. trinervis - Frankia symbiosis, and the rhizobial Medicago sativa - Sinorhizobium meliloti symbiotic system. All three strains stimulated the nodulation in both systems, improving the growth of young plants under laboratory conditions. It could be shown that these strains produce the phytohormones zeatine, indole 3-acetic acid and gibberellic acid. The promoting effect apparently operates at the autoregulation level of the plant nodulation process in both symbiotic systems. The strains are interpreted as “helper” rhizoactinomycetes. Additionally, they demonstrate antifungal effects in cross cultures. The “helper” effect of the saprophytic rhizoactinomycetes opens a wide range of application in agriculture, land reclamation and landscape ecology. For example, the growth rate and production of nitrogen-fixing legume plants may be improved by the presence of those helper rhizoactinomycetes, or in the case of pioneer actinorhizal shrubs and trees, degraded soil systems can be fertilized and stabilized with higher effectivity than with the symbiotic actinomycete Frankia alone.