Up-regulation of AtSBT1.1 gene expression As pAtPSK4 cleavage was induced approximately 8 h following the explanting of root segments, we wished to determine whether expression of the gene encoding AtSBT1.1 was similarly up-regulated. Real-time quantitative RT-PCR was performed on extracts from root segments at various times following explanting. It was found that AtSBT1.1 was up-regulated approximately 3.5-fold starting at approximately 8 h after explanting root segments ( Figure 2a) . Thus, the up-regulation of AtSBT1.1 expression was similar to the kinetics for the acquisition of cleavage activity. The endogenous gene encoding AtPSK4 was also up-regulated after explanting, but not as much as AtSBT1.1 (approximately twofold, Figure 2a ).