R2a medium

The bacterial strains used in this study were Sphingomonas sediminicola (DSM-18106) [43 (link)], kanamycin-resistant DH5-α Escherichia coli carrying GusA under the constitutive promoter pNeo in the pOPS0385 plasmid (Addgene ID #133235) [44 (link)], and Rhizobium leguminosarum bv. viciae Rlv3841 [45 (link)]. S. sediminicola was grown in R2A medium (VWR, Fontenay-sous-Bois, France) for 72 h at 30 °C (constant shaking at 150 rpm), R. leguminosarum at 28 °C (150 rpm) in tryptone-yeast extract (TY) medium [46 (link)], and the E. coli strain was grown at 37 °C for 24 h (200 rpm) in LB medium (Luria–Bertani, Sigma Aldrich, St. Louis, MO, USA). Bacterial concentration (CFU mL⁻¹) was determined by the spiral method using an EasySpiral^® automatic plater (Intersciences, Mourjou, France) and counted using a Scan^®500 (Interscience). At DO_{600 nm} = 0.75, the S. sediminicola culture corresponded to 2 10⁶ CFU mL⁻¹, while 1 10⁸ CFU mL⁻¹ was obtained with R. leguminosarum culture.
Mutation of S. sediminicola conferring a resistance to rifampicin was generated spontaneously by plating the bacteria on R2A medium containing rifampicin (25 mg L⁻¹) [46 (link)]. Using bacterial conjugation [47 (link)], the plasmid pOPS0385 carrying GusA was transferred into S. sediminicola^Rif, giving S. sediminicola^Rif [pOPS0385]. This strain was grown in R2A containing 20 mg L⁻¹ kanamycin and 25 mg L⁻¹ rifampicin.

Sphingomonas sediminicola (DSM-18106) was grown in R2A medium (VWR, Fontenay-sous-Bois, France) for 72 h at 30 °C under constant shaking at 150 rpm. Pea (Pisum sativum, cv. Douce Provence, Jardiland, Paris, France) seeds were surface-sterilized with a 3.5% (v/v) bleach solution, cold-stratified for 48 h, and germinated in the dark on a 1% (w/v) agar medium at 21 °C. Five days after germination, etiolated seedlings were transferred to hydroponic system.