The production of 3-indoleacetic acid (IAA) by plant growth-promoting bacteria (PGPR) stimulates root development and plant growth. In addition, morphological changes such as an increased root ramification and root hair production improves nutrient absorption and biomass accumulation. The objective of this work was to evaluate the effect of IAA-producing strains on rice in an advanced stage of its vegetative cycle. Rice was inoculated with Gluconacetobacter diazotrophicus PAL 5 and its lao- mutant, deficient in auxin production, Azospirillum baldaniorum Sp 245, and Escherichia coli DH10b. Both the mutant and wild-type G. diazotrophicus stimulated root elongation, area, volume, and diameter. However, the lao- mutant strain was the only one capable of increasing the number of roots. In turn, inoculation with A. baldaniorum had no significant effect on plant development. The inoculation with E. coli led to changes in root volume, area, and diameter, and a response that may be related to the stress caused by its presence. We conclude that the inoculation with G. diazotrophicus stimulates the root system’s growth independently of their IAA production ability, suggesting that a metabolite other than IAA is responsible for this effect at advanced stages of the rice’s vegetative cycle.

Nitrous oxide is a potent greenhouse gas, with N fertilizers being one of its major sources. Plant growth-promoting bacteria have been used to mitigate N₂O emissions by improving N use efficiency in plants. In addition, some of these microorganisms are capable of reducing N₂O to N₂, a process that could be further explored as a complementary mitigation strategy. This study aimed to test whether Azospirillum brasilense strains Ab-V5 and Ab-V6, and strain Wa3, as well as Nitrospirillum viridazoti strain BR 11145, already used in commercial inoculants for N-fertilized crops and known to carry the nosZ gene encoding nitrous oxide reductase, could act as biological sinks for this gas. In the pot experiment, soils fertilized with N and inoculated with N. viridazoti exhibited consistently lower N₂O emissions, but not when A. brasilense strains were used. The mitigation effect was observed both in bare soil (72% emission reduction) and in the presence of millet plants (60% emission reduction), confirming the ability of N. viridazoti strain BR 11145 to consume N₂O. In a field experiment conducted with maize, inoculation with N. viridazoti again reduced N₂O fluxes during the first two weeks after fertilization compared with the urea-only treatment. However, no significant differences were detected comparing emission factors, whose calculation requires consideration of the entire monitoring period, thereby adding more variability. While N₂O mitigation was observed, no significant effects of inoculation or N fertilization were found on maize growth or yield. Nonetheless, the consistent reduction in N₂O emissions achieved with N. viridazoti strain BR 11145 suggests that inoculants with this bacterium represent a promising biological sink for N₂O, offering a novel nature-based solution to enhance the sustainability of tropical crop management.

Soil acidity, specifically the presence of Al ^{+ 3} at toxic levels for most crop plants, is one of the main factors limiting agricultural production. However, the relationship between organic acid exudation and its direct influence and magnitude on aluminium tolerance in rice plants remains poorly elucidated in the literature. In order to verify the complexing effect of organic acids on Al, trials were carried out with rainfed rice seedlings of the Comum Branco and Caiapó varieties, under controlled growth conditions, in addition to evaluating the exudation of organic acids and possible changes in their internal root contents as a function of Al. It can be concluded that citric acid was effective in mitigating the toxic effects of Al³⁺ on the root system of the evaluated rice cultivars. In the absence of aluminium, malic acid promoted an increase in root length. The cultivar Comum Branco exhibited a greater tolerance to Al³⁺ compared to Caiapó, which may be associated with its higher exudation and accumulation of organic acids in the roots. These results suggest that the action of these mechanisms contributes to the greater aluminium tolerance observed in this cultivar.