The plant meristem is the ultimate source of all tissues in the plant body, and these generative activities are supported by
a pool of stem cells residing within the meristem. Thus, it is not surprising that strong activation of these meristematic activities leads to ectopic callus induction. The homeodomain-containing transcription factor WUSCHEL (WUS) is expressed in the stem cell organizing center of shoot meristems and is required to maintain stem cells in a relatively undifferentiated state (Laux et al., 1996; Mayer et al., 1998). WUS is also strongly expressed in several callus lines (Iwase et al., 2011a), and Arabidopsis plants overexpressing WUS generate callus as well as somatic embryos (Zuo et al., 2002) (Figures 3I and 4D).