Metabolismo de nitrogênio em plantas de milho inoculadas com dois gêneros de bactérias diazotróficas

Abstract

Maize (Zea mays L) is one of the world’s leading produced cereals. Its inoculation with diazotrophic bacteria described as plant growth-promoters can reduce the demand for nitrogen fertilizers and at the same time the costs associated with this practice. This study investigated how inoculation of the diazotrophic bacteria Azospirillum brasilense, Azospirillum baldaniorum and Herbaspirillum seropedicae in maize plants would influence the modulation of root architecture and the parameters associated with the absorption rate of nutrients, especially nitrogen (N), under different conditions of availability of N. Experiments were developed in a hydroponic system and sterile substrate (sand+vermiculite) in a greenhouse, in order to verify the metabolism and expression of genes involved in the absorption and assimilation of N. For this purpose, nitrogen fractions, the activity of the enzymes nitrate reductase and glutamine synthetase, the gene expression of proton pump isoforms (ZmHA2 and ZmHA4), high-affinity nitrate transporters (NO3-) (ZmNRT2.3, ZmNRT2.5 and ZmNRT3.1A) and N-reduction and assimilation enzymes (ZmNR1, ZmNR2, ZmGS1.1, ZmGS1.5, ZmGS2 and ZmNADH-GOGAT) were evaluated. Parameters such as biomass accumulation, total N, N use efficiency (NUE), leaf area and chlorophyll index were also evaluated. The inoculation with A. baldaniorum (Ab)-Sp245 and H. seropedicae (Hs)-ZAE94 strains promoted the greatest changes in root architecture, with improvement in all parameters evaluated, which was in agreement with the increase in dry mass and accumulation of N, but they were dependent on the date of sampling and experiment. Increases in the accumulation of biomass and total N of up to 150 and 180%, respectively, were found in the treatment inoculated with Ab-Sp245 in relation to the control. Inoculation with both strains also promoted increments greater than 90% for length, volume and root área. The parameters improved due to greater availability of N. The Ab-Sp245 was more effective in the greater absorption of macronutrients, with higher speed of absorption of NO3- and provided a greater NUE under low N. The changes caused by inoculation in the gene expression profile were less consistent, only after 2 h of induction of the NO3- transport system did Hs-ZAE94 increase the expression of two high-affinity NO3- transporters (ZmNRT2.3 and ZmNRT2.5) and two isoforms of glutamine synthetase (ZmGS1.1 and ZmGS2) When compared to the control, but those found in the content of soluble metabolites and enzymatic activity indicated an improvement in the N assimilation process in the inoculated plants. The shift in root architecture and the maintenance of N uptake per unit of root area through inoculation with Ab-Sp245 was probably the main effect.