Like animals, plants are known to host a community of endophytic microorganisms that live inside their tissues, without causing harm. Starting from the observation that some plants seem to tolerate winter cold quite well, we intend to study the contribution that endophytic bacteria in roots make to plant cold tolerance. By looking directly at the DNA of the endophytes without cultivating them, we will first make an inventory of the endophytic root bacteria enriched in thale cress, corn salad and meadow grass grown under cold conditions. We will then bring these bacteria in culture from the roots of cold grown plants, identify and characterize them. Experiments will be performed to assess the effect of different bacteria, or combinations of bacteria, on plant growth in the cold for thale cress, corn salad and meadow grass. Bacteria with a positive effect on plant cold tolerance will be studied further to find out which genetic pathways and functions are involved in cold tolerance. To do this, we will perform experiments that will reveal which genes are activated to higher or lower level in both the plants and the bacteria when growing in the cold. For those genes that seem to be important in cold conditions, we will confirm our observations by studying what happens to cold tolerance when we block those genes with molecular tools.