Plasmid construction ppAtPSK4 and AtSBT1.1 were amplified from root RNA of 1-weekold Arabidopsis seedlings by RT-PCR (primer pairs pSKMPSK4 and pSKMSBT1.1, respectively, Table S1 ), and cloned into the AscI and SpeI sites of pSKM36 in-frame with a 4 x epitope myc tag (EQKLISEEDLRN). cDNA clones with error-free copies were named ppSKAtPSK4 and pSKAtSBT1.1, respectively. A mutated form of AtSBT1.1 (S552A) was generated using a QuickChange site-directed mutagenesis kit (Stratagene, http://www.stratagene.com/) with primer pairs SDM1.1 and pSKAtSBT1.1 as template. YFP C-terminal fusions were created by inserting cDNAs from above at the AscI and SpeI sites of pSKY36. The clones were named 35S:AtSBT1.1-YFP and 35S:ppAtPSK4-YFP. Promoter-GUS constructs for AtSBT1.1 and AtPSK4 were generated by amplifying 984 and 934 nucleotides using the primers pCAMSBT1.1 and pCAMPSK4, respectively. The promoters were ligated into the BamHI and PstI sites of pCAM-BIA3300.