Effects of “helper” microorganisms on rhizobial and actinorhizal

SOLANS M; VOBIS G; WALL LG

Pucón, Chila

Congreso; ISMOM 2008; 2008

The aim of the present study was to examine the effect of saprophytic actinomycetes as “helper” microorganisms on two different nitrogen-fixing-symbioses: rhizobial and actinorhizal. Experimental assays were carried out under laboratory conditions, employing the strains Streptomyces MM40, Actinoplanes ME3 and Micromonospora MM18, which were previously isolated from the rhizosphere of Ochetophila (Discaria) trinervis and characterized as rhizoactinomycetic microorganisms. Two symbiotic systems, Medicago sativa-Sinorhizobium meliloti and Ochetophila trinervis-Frankia were used. The corresponding plants were inoculated with the symbiotic strains Sinorhizobium meliloti 2011 and Frankia BCU110501, and coinoculated with the above mentioned rhizoactinomycetes strains. Double inoculations were performed either simultaneously or asynchronously, and the plants were fertilized with different N concentrations. In both nitrogen-fixing-systems, the nodulation and, consequently the plant growth were significantly stimulated by simultaneously or asynchronously  coinoculations with rhizoactinomycetes strains, compared to those plants, which were inoculated only with the symbiotic strains. In asynchronous coinoculations, this effect was better when the plants were first inoculated with Sinorhizobium or Frankia, and later coinoculated with rhizoactinomycetes strains. However, the nitrogen-fixation rate, estimated as acetylene reduction activity (ARA/mg-1) was not significantly modified by rhizoactinomycetes strains. The analysis of the nodulation kinetics with simultaneous or asynchronous coinoculations of rhizoactinomycetes with S. meliloti or Frankia suggests, that the effect of saprophytic strains operates in early stage of infection and nodule development, counterresting the autoregulation of nodulation by the plant. The inoculation of rhizoactinomycetes alone did not show any effect on plant growth. In spite of the different infection types -Alfalfa is infected via root hairs and O. trinervis is infected via intercellular invasion-, apparently the rhizoactinomycetes “helper” effect on nodulation may operate by a common step in both systems. As it was expected, a high N-concentration inhibited the nodulation by S. meliloti in single inoculations. In contrary, in all coinoculations with rhizoactinomycetes strains, the development of nodules could be observed, even under high N-fertilization. These results on the rhizoactinomycetes-symbiosis-interaction may open new lines on a topic of potential agronomy application for legumes plants, and an ecological role and their possible use on amenity planting and improvement of the fertility of degenerated forest soil for actinorhizal plants in Patagonia.