Inoculation and growing conditions
We then performed in-vitro root organ culture experiments. To create a three compartment in-vitro system, we modified squared a 4-well compartment system by removing the central barrier, creating a plate with a large central fungal compartment and two smaller root compartments (Olssonet al. 2014). We filled each compartment with Modified Strullu Romand (MSR) media (0.4 % phytogel, pH 5.5, 55 nM sucrose, 3980 µM N, 30 µM P, Fortin et al. 2002). To each focal and partner root compartment, we transplanted a branching, two cm long, section of in-vitro Ri T-DNADaucus carota L. transformed root organ culture. We inoculated the roots with an 1x1 cm2 agar plug containing ~700 fungal spores. We again randomly designated one root as the “focal root”, and inoculated it with R. irregularisA5. The partner root was inoculated with either A5, B12 or Agg (Fig. 1b). In the A5-A5 treatment, the two compartments were randomly assigned as focal or partner. We sealed the plates with parafilm and stored them in the dark at 25 ˚C. We placed any roots crossing into the central compartment back into the root compartment using sterile equipment.