N IMPERIALI¹, F DENNERT², F MASCHER³, M FESSELET³, T TURLINGS⁴, R CAMPOS HERRERA⁴, M MAURHOFER², C KEEL<sup>1

1</sup>Department of Fundamental Microbiology, University of Lausanne, Switzerland, ²Institute of Integrative Biology, ETH Zurich, Switzerland, ³Institute for Plant Production Sciences, Agroscope Changins, Switzerland, ⁴Laboratory for Fundamental and Applied Research in Chemical Ecology, University of Neuchâtel, Switzerland

In Switzerland there is a lack of knowledge about the impact of soil on the activity of bacteria expressing plant-beneficial traits. In the context of the National Research Programme 68 “Sustainable use of soil as a resource” , we evaluate the possibility of using plant-beneficial bacteria as bio-indicators of soil health. Of particular interest are root-colonizing Pseudomonas protegens and Pseudomonas chlororaphis, both displaying fungicidal and insecticidal activities and both useful to control fungal pathogens and pest insects. We constructed GFP-tagged reporter strains of these pseudomonads, which in addition express mCherry-based fusions reporting the activity of promoters relevant for the production of antifungal compounds (DAPG, HCN, pyrrolnitrin, phenazines) and the insecticidal toxin Fit. In ongoing studies, we test the activity of these bio-reporters in soil samples extracted from representative cereal-oriented fields and from long-term field trials subjected to different agricultural practices (crop rotation, tillage, fertilization). For monitoring, we developed a soil extraction protocol and a flow cytometry approach. Excitingly, our data reveal consistent differences in antifungal gene expression in the different Swiss soils. Another important aspect of our NRP 68 contribution is to develop novel environment-friendly strategies to control crop diseases and pests. To this end, we test combined applications of pseudomonads with other beneficials. In field experiments, seeds of spring wheat were inoculated with P. protegens and P. chlororaphis, alone and combined with entomopathogenic nematodes. The persistence of the inoculants was monitored and the performance and fitness of wheat plants assessed. One experimental field was invaded by Oscinella frit, an insect notorious for causing damage to cereal crops. We observed that plants treated with the two bacterial species were less damaged. In another field trial, seeds of maize were treated with the two Pseudomonas species, alone or in combination with entomopathogenic nematodes and mycorrhizal fungi and a positive effect of the bacteria and nematodes on the grain yield weight was observed.