In vitro co-inoculation of rhizobacteria from the semi-arid aiming at their implementation as bio-inoculants

The use of nitrogen fertilizers is of paramount importance for the supply of this nutrient to plants. However, the application of these fertilizers brings numerous environmental and health problems. An alternative to these chemical products would be the use of rhizobia — plant growth-promoting rhizobacteria naturally present in the rhizosphere and capable of carrying out biological nitrogen fixation. Through the present work, we propose the co-inoculation of Actinobacteria and rhizobia, aiming at the production of a new bio-inoculant that replaces, at least in part, nitrogen fertilization in legumes. It is expected that Actinobacteria, by producing exoenzymes, enable the growth of rhizobia in non-specific culture media for these microorganisms. Ten strains of Actinobacteria with statistically distinct cellulolytic and xylanolytic activity and seven strains of rhizobia without the aforementioned enzymatic activities were used. A co-inoculation of these microorganisms was performed in culture media containing carboxymethylcellulose (CMC) and xylan as sole carbon sources, and then their compatibility indexes (CI) were calculated. Actinobacteria strains A139 and A145 (both with CI = 0.857 in the medium with CMC and CI = 1 in the medium with xylan) showed remarkable facilitation of rhizobia growth, and had only one antagonistic relation each (both with rhizobia L9 in the medium with CMC). This biological interaction, called cross-feeding, occurs when microorganisms stimulate each other’s growth and is promising for prospecting a bio-inoculant, in addition to providing an overview of the ecological relationships that occur between plant growth-promoting rhizobacteria in the semi-arid region.