Growth promotion of maize and metabolomic profiling of Azospirillum brasilense cultures at varying incubation times

Azospirillum brasilense Ab-V5, a well-known plant growth-promoting bacteria (PGPB), is widely used in commercial inoculant formulations for crops of economic and nutritional relevance. The mechanisms underlying its plant-growth-promoting activity are largely associated with secondary metabolic products such as exopolysaccharides (EPS), polyhydroxyalkanoates (PHA), phytohormones, siderophores, enzymes, and other bioactive metabolites. In this study, cultures of A. brasilense Ab-V5 at 12, 36 and 72 h (representing the exponential, stationary and decay growth phases) were sampled for untargeted metabolomic analysis using gas chromatography-mass spectrometry (GC-MS), and to evaluate inoculation responses in maize (Zea mays). After 15 days of growth in a greenhouse, the maize plants responded differently to inoculation depending on the growth stage of the bacterial inoculum, with the most pronounced increases in shoot dry weight and root system development observed in plants inoculated with 72-hour cultures. Metabolomic profiling revealed distinct metabolic patterns across growth phases, with a different expression of metabolites associated with physiological maturation and potential plant interaction capacity. Cultures from the 72-hour decaying stage displayed significant qualitative and quantitative shifts in metabolic compounds that were putatively assigned to carbohydrates, carboxylic acids, and aromatic and phenolic compounds. The role of putatively assigned metabolites on inoculation efficiency and the plant response is discussed.