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

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 1·106 cfu·seed−1 of Bradyrhizobium japonicum E109 and co-inoculating leaves at the V3 stage with 1·108 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.Inst. de Microbiología y Zoología Agrícola IMyZAFil: Puente, Mariana Laura. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Microbiología y Zoología Agrícola. Laboratorio de Bacterias Promotoras del Crecimiento Vegetal; ArgentinaFil: Gualpa, Jose Luis. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Físico-Químicas y Naturales. Departamento de Ciencias Naturales. Laboratorio de Fisiología Vegetal y de la Interacción planta-microorganismo; ArgentinaFil: López, Gastón A. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Físico-Químicas y Naturales. Departamento de Ciencias Naturales. Laboratorio de Fisiología Vegetal y de la Interacción planta-microorganismo; ArgentinaFil: Molina, Romina M. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Físico-Químicas y Naturales. Departamento de Ciencias Naturales. Laboratorio de Fisiología Vegetal y de la Interacción planta-microorganismo; ArgentinaFil: Carletti, Susana. Universidad Nacional de Luján. Departamento de Ciencias Básicas; ArgentinaFil: Cassán, Fabricio D. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Físico-Químicas y Naturales. Departamento de Ciencias Naturales. Laboratorio de Fisiología Vegetal y de la Interacción planta-microorganismo; Argentin

Improvement of soybean grain nutritional quality under foliar inoculation with Azospirillum brasilense strain Az39

Legumes depend on biological nitrogen fixation through symbiosis with rhizobia to meet their nitrogen requirements. Certain plant growth promoting rizobacteria such as Azospirillum may cooperate in the establishment and maintenance of effective legume-rhizobia symbiosis. The aim of this work was to assess if foliar inoculation of soybean with Azospirillum brasilense Az39 has advantages over inoculation with this bacterium at sowing in combination with Bradyrhizobium japonicum E109 inoculation. To test this, glasshouse and field experiments were carried out. Higher shoot biomass was observed at V6 and R2 stages under foliar inoculation of A. brasilense Az39 as compared to coinoculation at sowing. Additionally, increased root dry biomass and higher nodule number and nodule fresh weight per plant were found at V6. Leghemoglobin levels in nodules were significantly greater in foliar-inoculated plants at both stages (V6 and R2) as compared with seed-inoculated plants. In line with these positive effects of foliar inoculation with A. brasilense Az39 on soybean growth and nodulation, grains harvested from foliar-inoculated plants had higher nitrogen and protein contents than those harvested from plants coinoculated at sowing. This was corroborated for two soybean varieties cultivated at two different locations. Our findings may open new insights into soybean agricultural technology.Instituto de Microbiología y Zoología AgrícolaFil: Puente, Mariana Laura. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Microbiología y Zoología Agrícola; ArgentinaFil: Zawoznik, Myriam. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento Química Biológica; ArgentinaFil: Lopez De Sabando, Marcelo. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce. Agencia De Extensión Rural Tandil; ArgentinaFil: Perez, Gonzalo. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Pergamino. Agencia De Extensión Rural Bolivar; ArgentinaFil: Gualpa, José Luis. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Físico-Químicas y Naturales. Departamento de Ciencias Naturales. Laboratorio de Fisiología Vegetal y de la Interacción planta-microorganismo; ArgentinaFil: Carletti, Susana M. Universidad Nacional de Luján. Departamento de Ciencias Básicas; ArgentinaFil: Cassán, Fabricio D. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Físico-Químicas y Naturales. Departamento de Ciencias Naturales. Laboratorio de Fisiología Vegetal y de la Interacción planta-microorganismo; Argentin