One of the greatest challenges of the next century is to develop new eco-friendly weapons to fight against crop diseases by limiting the input of chemicals. During the past decades, biological control through the use of natural antagonistic microorganisms has emerged as a promising alternative. In that context, plant beneficial rhizobacteria are good candidates, especially members of the Bacillus genus. Indeed, some Bacillus amyloliquefaciens isolates may devote up to 8% of their genetic equipment to the synthesis of antibiotics including cyclic lipopeptides (LPs). Among these LPs, surfactins play a key role in plant-Bacillus interactions since, on one hand, they contribute to the rhizosphere fitness of the producing strain by favoring motility and biofilm formation and on the other hand, they act as elicitor of the host plant immunity. Nevertheless, little information is available on the in situ production of such lipopeptides, allowing to better understand their spatio-temporal accumulation in the root microenvironment.
In this work, we took advantage of the versatility and sensitivity of MALDI mass spectrometry imaging to analyze the metabolites that are released by bacterial cells following their inoculation on roots of tomato plantlets. We developed specific bioassays for time-course monitoring by MALDI MSI. First analyses revealed an efficient secretion of surfactin by Bacillus cells after 7 days when colonization as biofilm-structured populations is well established. Looking at earlier time-points, we observed that the lipopeptide starts to accumulate in the medium surrounding roots as early as 24-48h post-inoculation. However, such a time-period is not sufficient for significant biomass development, suggesting that the surfactin biosynthesis machinery may be up-regulated in Bacillus in the earliest hours following recognition of root cells. Preliminary data on gene expression support this hypothesis and the role of surfactin as keyword in the molecular dialogue between plants and their root-associated Bacillus communities will be discussed.