Growth and biochemical responses of soybean to double and triple microbial associations with Bradyrhizobium, Azospirillum and arbuscular mycorrhizae

Abstract

Bradyrhizobium (B), Azospirillum (A) and arbuscular mycorrhizal fungi (M) are plant beneficial rhizospheric microorganisms often applied as biofertilizers. The objectives of the present work were to study the effects of double (B+A and B+M) and triple (B+A+M) microbial combinations on soybean yield (Glycine max [L.] Merr.), nodulation, mycorrhization and plant metabolites concentrations, as compared to Bradyrhizobium alone, after 28 and 56days of growth under controlled conditions. All microbial combinations increased soybean root biomass, but not shoot biomass. The highest total biomass of soybean (root+shoot) was observed with the double association Azospirillum and Bradyrhizobium (B+A). This treatment reduced the total number of nodules but seems to increase their capacity to fix nitrogen, as shown by their high starch concentration during establishment. The presence of mycorrhizae (B+M) had a transitory adverse effect on depth of nodulation as compared to Bradyrhizobium alone, which could indicate competition between these two symbionts during establishment (day 28). The triple association (B+A+M) reduced shoot growth, as well as the number of small nodules. The higher concentration of the stress-induced amino acid proline in nodules and leaves in response to B+A+M indicates that soybean plants are under stress when in presence of the combination of three symbionts. During establishment, the concentration of coumestrol in soybean roots was lower with microbial combinations than with Bradyrhizobium alone, which could indicate a common regulating signal between soybean and both Azospirillum and mycorrhiza. Our results show that complex interactions and competition between the three microorganisms induced differential growth and nodulation responses, which can be linked to metabolic changes.