The bHLH transcription factor SPATULA regulates root growth by controlling the size of the root meristem

An analysis of embryo and root development showed that loss of SPT function causes an increase in quiescent center size in both the embryonic and postembryonic stem cell niches. In addition, root meristem size is larger due to increased division, which leads to a longer primary root. spt mutants exhibit other pleiotropic developmental phenotypes, including more flowers, shorter internodes and an extended flowering period. Genetic and molecular analysis suggests that SPT regulates cell proliferation in parallel to gibberellic acid as well as affecting auxin accumulation or transport.Our data suggest that SPT functions in growth control throughout sporophytic growth of Arabidopsis, but is not necessary for cell fate decisions except during carpel development. SPT functions independently of gibberellic acid during root development, but may play a role in regulating auxin transport or accumulation. Our data suggests that SPT plays a role in control of root growth, similar to its roles in above ground tissues.The primary root of Arabidopsis thaliana has a simple and consistent organization of cell types [1]. Roots are divided into three distinct tissue zones along the proximal-distal axis. The most distal area is the zone of cell division or meristematic zone. A zone of cell elongation occurs just proximal to the division zone and the zone of cell differentiation or zone of maturation is the most proximal [2]. Within the root apical meristem (RAM), stem cells surround a group of four mitotically less active cells called the Quiescent Center (QC; [1]). The QC, together with its surrounding four types of stem cells (columella stem cells, epidermal/lateral root cap stem cells, cortex/endodermal stem cells and vascular stem cells), forms the stem cell niche [3]. The RAM is established during embryogenesis. In Arabidopsis, the zygote divides asymmetrically to form an apical and a basal daughter cell. Three rounds of stereotyped cell divisions in the apical daughter cell give