LAUSR

Abstract Six Bacillus subtilis strains collected from broad-leaved trees rhizosphere and exhibiting an ability of surfactin production were investigated as potent plant biofertilizers with biocontrol activity. Lipopeptide (LP) biosurfactants were extracted by a modified QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) procedure proposed as a quick and effective method of isolating LP biosurfactants. Matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry (MALDI-TOF-TOF) was applied to detect the lipopeptides. Predominant peaks at m/z 1016, 1030, 1044, 1058 and 1072 were putatively assigned as surfactin Na adducts possess a Glu-Leu-Leu-Val-Asp-Leu–Leu peptide and the β-OH fatty acid with a different length (C13-C16) whereas m/z 1016 was C13 surfactin Na adduct with Val in position 7 on the peptide moiety. Four other ions m/z 1051, 1065, 1079 and 1093 were recognized as iturin A Na adducts. Five fengycin homologues with Na adducts (m/z 1471, 1485, 1499, 1513 and 1527) were also determined. The quantitative analyses of the compounds were performed with the use of liquid chromatography-mass spectrometry (LC–MS/MS). Strains IM 7, IM 16 and IM 17 cultivated in Luria-Bertani medium for 48 h produced surfactin alone at the concentrations below 10 mg l−1. In contrast, strains KP7, IM 13 and IM 14 produced surfactin much more efficiently. Surfactin concentrations in 72-h-old cultures reached 100.3, 55.3 and 124.4 mg l−1, respectively and were accompanied by the production of iturin in concentrations 5.7, 142.5 and 2.3 mg l−1, respectively. Additionally, in Cooper’s medium, strain KP7 increased twofold the production of surfactin and iturin. Only surfactin overproducers (KP7, IM 13 and IM 14) exhibited swarming motility and formed biofilms, limited the growth of Botrytis cinerea and Aspergillus flavus strains, produced cellulases and inulinases and enhanced cucumber seed germination in vitro. All examined surfactin overproducers produced siderophores and, additionally, KP7 and IM 14 solubilized phosphates. Among all examined surfactin overproducers only IM 13 during the growth in the presence of F. oxysporum mycelium (in dual liquid culture) increased about twofold surfactin and iturin secretion and almost completely suppressed fungal biomass growth. The obtained results revealed that the studied surfactin overproducers were equipped with various plant growth-promoting (PGP) traits allowing them to promote plant development in a diverse manner.