Boundary formation during organ development: the transition zone establishment in the Arabidopsis thaliana root apical meristem

Coherent organ growth is ensured by signaling pathways that position boundaries of cell proliferation-to-differentiation transition. The mature root is composed of three developmental zones: the meristematic zone, including a stem cell niche, where stem cell daughters transit- amplify; the transition zone (TZ), where cells stop dividing and begin elongating; the elongation/differentiation zone. Undetermined post-embryonic root growth starts after germination at the root apical meristem (RAM). At the end of embryogenesis, the root is composed only of a stem cell niche (SCN) and meristematic cells. Therefore, the TZ is established post-embryonically and, during RAM growth, the TZ shifts along the root until the RAM reaches its final size. In this mechanisms, auxin promotes meristematic cell proliferation, prevailing over cell differentiation, which is controlled by cytokinins. This thesis aims to unveil the molecular mechanisms that lead to the TZ establishment and, hence, the processes that drive the developmental zonation of the root during its morphogenesis. To define the time-point and the site at which the TZ is detectable, a morphological analysis was performed. Moreover, in the incipient TZ, the expression of the cytokinin-responsive factor ARR12 was found. To clarify ARR12 role in the establishment of the TZ, ChIP-Seq assay was performed and many genes involved in regulating cell proliferation/cell differentiation balance resulted as ARR12-direct targets. In particular, it was discovered that ARR12 directly represses and confines two morphogenetic factors, auxin and PLETHORAs, that, on the other hand, are positive regulators of cell division. Thus, the data presented in this thesis suggest that cytokinins, via ARR12, contribute to establish the TZ and modulate the formation of root zonation at early stages of root meristem development after germination.