Two T-DNA insertion mutant lines for AtSBT1.1 were obtained from the Arabidopsis Biological Resource Center (ABRC, Columbus, OH). Seeds were surface-sterilized, rinsed with sterile water, and stratified at 4°C for at least 2 days in 0.1% agar. Seeds were germinated and grown vertically on agar plates containing Gamborg's B5 medium (Gamborg et al., 1968) . Root segments (5 mm) were cut and transferred to callus induction medium (CIM), which consisted of B5 medium with 5 g l )1 MES, 2.2 lM 2,4-dichlorophenoxyacetic acid, 0.2 lM kinetin and 0.8% agarose. Explants were incubated on CIM for 4 days under constant light conditions, and then transferred to shoot induction medium (SIM). SIM is prepared similarly to CIM except that it contains the hormones isopentenyladenine (5.0 lM) and 3-indoleacetic acid (0.9 lM). For AtSBT1.1-YFP and AtPSK4-YFP localization experiments, transgenic seedlings were grown on B5 plates for 7 days, and roots were used for confocal microscope examination. Screening for homozygous plants was carried out by PCR using left border (LB) T-DNA primers and the gene-specific primer pair ScrSBT1.1 (Table S1 ). The transcript level of AtSBT1.1 was evaluated by RT-PCR using primer pair sqRT-SBT1.1 listed in Table S1 .