A sustainable alternative to improve yield and the nutritive value of forage is the use of plant growth-promoting bacteria (PGPB) that release nutrients, synthesize plant hormones and protect against phytopathogens (among other mechanisms). Azospirillum genus is considered an important PGPB, due to the beneficial effects observed when inoculated in several plants. The aim of this study was to evaluate the diversity of new Azospirillum isolates and select bacteria according to the plant growth promotion ability in three forage species from the Brazilian Pantanal floodplain: Axonopus purpusii, Hymenachne amplexicaulis and Mesosetum chaseae. The identification of bacterial isolates was performed using specific primers for Azospirillum in PCR reactions and partial sequencing of the 16S rRNA and nifH genes. The isolates were evaluated in vitro considering biological nitrogen fixation (BNF) and indole-3-acetic acid (IAA) production. Based on the results of BNF and IAA, selected isolates and two reference strains were tested by inoculation. At 31 days after planting the plant height, shoot dry matter, shoot protein content and root volume were evaluated. All isolates were able to fix nitrogen and produce IAA, with values ranging from 25.86 to 51.26 mg N mL and 107-1038 µmol L, respectively. The inoculation of H. amplexicaulis and A. purpusii increased root volume and shoot dry matter. There were positive effects of Azospirillum inoculation on Mesosetum chaseae regarding plant height, shoot dry matter and root volume. Isolates MAY1, MAY3 and MAY12 were considered promising for subsequent inoculation studies in field conditions.

The purpose of this study was to evaluate the contribution of the inoculation of the strain Azospirillum MAY12 to the development of rice plants of the cultivar BRS Esmeralda. A completely randomized experiment was performed in a greenhouse, with 8 treatments and 6 repetitions, namely, commercial inoculant (CI) containing strains of Azospirillum brasilense ABV5 and ABV6 (T1), MAY12 (T2), nitrogen control with 50 mg of N or full dose (T3), CI + 30% of N (percentage referring to full dose) (T4), CI + 60% N (T5), MAY12 + 30% N (T6), MAY12 + 60% N (T7) and absolute control (T8). There was a significant increase in the fresh and dry mass of the shoot and roots, height, root volume, and the number of tillers and leaves of plants with the MAY12 inoculation (with and without N added) compared to T8. Treatments containing MAY12 + N increased the total dry mass by ~920% compared to T8, matching the T3 yield. The inoculation of MAY12 in isolation was as efficient as the application of the full dose of N, resulting in an increase of ~115% in height and 373% in root volume of the plants compared to T8. The MAY12 strain demonstrated good performance in the development of the cultivar, which was comparable to the CI in the variables tested. Therefore, inoculation with the MAY12 strain is promising for the partial reduction of nitrogen fertilizers in upland rice cultivation.