A Genetic and Metabolomic Perspective on the Production of Indole-3-Acetic Acid byPantoea agglomeransand Use of Their Metabolites as Biostimulants in Plant Nurseries

The speciesPantoea agglomeransincludes strains that are agronomically relevant for their growth-promoting or biocontrol traits. Molecular analysis demonstrated that the IPDC pathway involved in the conversion of tryptophan (Trp) to indole-3-acetic acid (IAA) is highly conserved amongP. agglomeransstrains at both gene and protein levels. Results also indicated that the promoter region controlling the inducible expression ofipdCgene differs from the model systemEnterobacter cloacae, which is in accordance with the observation thatP. agglomeransaccumulates higher levels of IAA when cells are collected in the exponential phase of growth. To assess the potential applications of these microorganisms for IAA production,P. agglomeransC1, an efficient auxin-producer strain, was cultivated in 5 L fermenter so as to evaluate the effect of the medium formulation, the physiological state of the cells, and the induction timing on the volumetric productivity. Results demonstrated that higher IAA levels were obtained by using a saline medium amended with yeast extract and saccharose and by providing Trp, which acts both as a precursor and an inducer, to a culture in the exponential phase of growth. Untargeted metabolomic analysis revealed a significant effect of the carbon source on the exometabolome profile relative to IAA-related compounds and other plant bioactive signaling molecules. The IAA-enriched metabolites secreted in the culture medium byP. agglomeransC1 were used as plant biostimulants to run a series of trials at a large-scale nursery farm. Tests were carried out within vitroandex vitrosystems following the regular protocols used for large-scale plant tree agamic propagation. Results obtained with 4,540 microcuttings ofPrunus rootstock GF/677 and 1,080 plantlets ofCorylus avellanaL. showed that metabolites from strain C1 improved percentage of rooted-explant, number of adventitious root formation, plant survival, and quality of plant as vigor, with an increase in the leaf area between 17.5 and 42.7% compared to IBA-K (indole-3-butyric acid potassium salt)-treated plants.