Untargeted Metabolomics to Explore the Bacteria Exo-Metabolome Related to Plant Biostimulants

The control and development of plant growth promoters is a key factor for the agro-nomy industry in its economic performance. Different genera of bacteria are widely used as natural biostimulants with the aim of enhancing nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of their nutrients content. However, the complete exo-metabolome of the bacteria responsible for the biostimulant effect is still unknown and needs to be investigated. Three bacteria with different biostimulant effects were studied by untargeted metabolomics in order to describe the metabolites responsible for this effect. The pentose phosphate pathway, tryptophan metabolism, zeatin biosynthesis, vitamin B6 metabolism and amino acid metabolism were the highlighted pathways related to bacteria biostimulant activity. These results are related to the plant hormones biosynthesis pathway for auxins and zeatins biosynthesis. Fourteen metabolites were identified as biomarkers of the biostimulant activity. The results suggest a greater relevance of auxins than cytokinin pathways due the importance of the precursors identified. The results show a clear trend of using indole-3-pyruvate and 3-Indoleglycolaldehyde pathways to produce auxins by bacteria. The results demonstrate for the first time that 4-Pyridoxic acid, the fructosamines N-(1-Deoxy-1-fructosyl)phenylalanine and N-(1-Deoxy-1-fructosyl)isoleucine and the tripeptides diprotin A and B are metabolites related to biostimulant capabilities. This study shows how untargeted metabolomic approaches can be useful tools to investigate the bacteria exo-metabolomes related to biostimulant effects.