| dc.description.abstract | Plant development requires a tightly controlled balance between undifferentiated dividing cells and cells, which are subjected to undergo differentiation. In the shoot apical meristem, this process is governed by a complex signaling network involving several classes of transcription factors, which are often expressed in highly distinct patterns (Shani et al., 2006). Key-players of differentiation control are the KNOX genes (Knotted-1 like genes; e.g. KNAT1, KNAT2, STM), which comprise a small gene family with eight members in Arabidopsis thaliana. KNOX genes can be divided into two subclasses, class I and II KNOX genes, based on phylogenetic analyses (Scofield and Murray, 2006). A well-characterized member of the class I KNOX genes is SHOOT MERISTEMLESS (STM), which is expressed in the centre of the shoot apical meristem (SAM) but not in the newly formed leaf primordia and in the incipient leaf (Long et al., 1996). Loss-offunction mutations in STM lead to premature differentiation of meristematic cells and eventually to cessation of the SAM (Long et al., 1996); but its simultaneous over-expression together with the homeodomain transcription factor WUSCHEL induces meristem formation at ectopic places (Lenhard et al., 2002). Taken together these findings indicate that STM is a critical regulator of differentiation, whose expression is required to keep cells in an undifferentiated state. The other characterized members of the class I KNOX genes fulfill partly redundant functions to STM and are generally suggested to be involved in preventing differentiation of the tissue where they are expressed (Scofield and Murray, 2006) . In contrast to the class I KNOX genes, the members of class II KNOX genes are only scarcely described and functional data is mostly lacking. | |
