Strains, Growth Conditions, and Genotyping

Arabidopsis thaliana (L.) HEYNH. ecotypes Col-0, C24 and Wassilewskija were grown in the greenhouse in potting soil or under long-day conditions (16 h light/8 h dark) at 22°C and 60% relative humidity in a phytochamber. Plants for the generation of protoplasts were cultivated under a short-day regime (8 h light/16 h dark). For analyses of seedlings or roots, surface-sterilized seeds were cultivated on MS plates (Murashige and Skoog, 1962) under long-day conditions at 22°C. The T-DNA insertion lines Atsuc6.1 (SALK_132450; Alonso et al., 2003), Atsuc6.2 (SALK_108259; Alonso et al., 2003), Atsuc6.3 (SM_3.18900; Tissier et al., 1999), Atsuc6.4 (SM_3.41113; Tissier et al., 1999), Atsuc7.1 (GABI_054G04; Kleinboelting et al., 2012), Atsuc7.2 (SAIL_221_C05; Sessions et al., 2002), and Atsuc7.3 (GABI_374G11; Kleinboelting et al., 2012) were obtained from the Nottingham Stock Centre¹. The Attmt1/tmt2 double mutant line (Wormit et al., 2006) was kindly provided by Ekkehard Neuhaus (Division of Plant Physiology, University of Kaiserslautern). The companion cell marker lines pMH5a and pEPS1 have been described by Imlau et al. (1999) and Stadler et al. (2005b), respectively. The primers used for genotyping are listed in Supplementary Table 2. Segregation analyses of the Atsuc6.3 and Atsuc7.3 alleles were performed by PCR-based genotyping with the respective primer pairs. The positions of the T-DNA insertions were reviewed by sequencing of PCR products obtained with the primer pair for the mutant allele (Supplementary Table 2). Transformations of Arabidopsis thaliana were performed via floral dip with Agrobacterium tumefaciens SMITH AND TOWNSEND strain GV3101 (Holsters et al., 1980; Clough and Bent, 1998). Escherichia coli (MIGULA) CASTELLANI AND CHALMERS strain DH5α (Hanahan, 1983) was used for all cloning steps. Heterologous expression analyses were performed in Saccharomyces cerevisiae MEYEN ex E.C. HANSEN strains CSY4000 (Rottmann et al., 2016) or SEY2102 (Emr et al., 1983). SEY2102 containing the sucrose transporter Srt1 (Wahl et al., 2010) was used as a positive control in uptake experiments.