The benefits of foliar inoculation with Azospirillum brasilense in soybean are explained by an auxin signaling model

Abstract

Azospirillum sp. is one of the most studied genera of plant growth-promoting rhizobacteria (PGPR). The ability of Azospirillum sp. to promote plant growth has been associated with its ability to produce several phytohormones, such as auxins, gibberellins and cytokinins, but mainly indole-3-acetic acid (IAA). It has been propoosed that the production of IAA explains the positive effects of co-inoculation with Azospirillum sp. on the rhizobia-legume symbiosis. In this study, we constructed an IAA-deficient mutant of A. brasilense Az39 (ipdC − ) by using a restriction-free cloning method. We inoculated soybean seeds with 1•10 6 cfu•seed −1 of Bradyrhizobium japonicum E109 and co-inoculating leaves at the V3 stage with 1•10 8 cfu.plant −1 of A. brasilense Az39 wt or ipdC − or inoculated leaves with 20 μg.plant −1 synthetic IAA. The results confirmed soybean growth promotion as there was increased total plant and root length, aerial and root dry weight, number of nodules on the primary root, and an increase in the symbiosis established with B. japonicum E109. Nodule weight also increased after foliar co-inoculation with the IAA-producer A. brasilense Az39. The exogenous application of IAA decreased aerial and root length, as well as the number of nodules on primary roots in comparison with the Az39 wt strain. These results allow us to propose a biological model of response to foliar co-inoculation of soybean with IAA-producing rhizobacteria. This model clearly shows that both the presence of microorganism as part of the colonization process and the production of IAA in situ are co-responsible, via plant signaling molecules, for the positive effects on plant growth and symbiosis establishment.

Effects of plant growth-promoting rhizobacteria on co-inoculation with Bradyrhizobium in soybean crop: a meta-analysis of studies from 1987 to 2018

“…in maize, interesting results were found by Zeffa et al (2018), where the inoculated treatment out-yielded the control by 651 kg ha −1 . In general, it is believed that the production of phytohormones by PGPR is one of the main mechanisms of action on the development of the host plant, whose effects are more prominent on the root system (Olanrewaju, Glick & Babalola, 2017;Puente et al, 2018). Interestingly, the symbiotic relationship between rhizobia and legumes is also mediated by bacterial phytohormones (Stacey et al, 1995;Imada et al, 2017).…”

“…In this context, auxins produced by PGPR are believed to increase the number of root hairs, leading to the formation of rhizobia-soybean interaction sites (Schmidt, Messmer & Wilbois, 2015). Puente et al (2018) examined the effect of IAA on the co-inoculation response of soybean with Bradyrhizobium and A. brasilense and demonstrated that the increase in root system growth, which improves the soybean-Bradyrhizobium interaction, is a result of the action of phytohormones. Moreover, the authors co-inoculated soybean with A. brasilense Az39 (ipdC+) and with its respective mutant deficient in IAA biosynthesis (ipdC-).…”

Azospirillum: benefits that go far beyond biological nitrogen fixation

“…Remarkably, inoculation with Azospirillum , a typical rhizospheric bacterium, via foliar spray can also increase plant growth (Fukami et al 2016 ), attributable to the synthesis of IAA by the bacterium, i.e. a plant-signaling process mediated by the bacterium, far stronger than when synthetic IAA was applied (Puente et al 2017 ). Intriguingly, in maize, foliar application of Azospirillum also elicited genes related to tolerance of abiotic stresses ( APX1 , APX2 , SOD4 ), as well as defense genes (PR -genes), which has also been attributed to phytohormones signaling (Fukami et al 2017 ).…”

Application methods of Azospirillum brasilense in first- and second-crop corn

“…Among the metabolic responses resulting from the action of A. brasilense, it is possible to highlight the greater development of the roots, which allows for a greater recovery of N applied (Martins et al, 2017). This increase in roots is due to higher hormonal activity, mainly auxins (Puente et al, 2017) and the symbiotic fixation of N 2 (Brusamarello-Santos et al, 2017). It should be noted that the colonization and metabolic production capacity of the bacteria are dependent on the genotypes (Brusamarello-Santos et al, 2017), edaphoclimatic conditions (Martins et al, 2017) and soil N availability (Pereira-Defilippi et al, 2017).…”