As shoot regeneration in culture is dependent on cytokinin and auxin hormones, we determined whether induction of pAtPSK4-myc cleavage required hormone treatment during CIM pre-incubation. To test this, root segments were explanted to hormone-free B5 medium as well as to CIM, and it was found that pAtPSK4-myc was cleaved on basal B5 medium at 1 and 4 days after explantation ( Figure 1b ). We concluded that the processing activity was probably induced by the wounding or handling of root tissue during explanting. We then determined how rapidly processing was induced. Cleavage of pAtPSK4-myc was first detected approximately 8 h after explanting ( Figure 1c ). Thus induction is fairly rapid, but not immediate as one might expect for a post-translational activation mechanism.