Fig. 6.

Model for the SCR-based regulatory circuits that control root meristem size. The model shows different tissue-specific activities of SCR on the control of ARR1 expression. Yellow cells represent SCR expression domain (endodermis and quiescent center, QC). From the QC, SCR represses ARR1 in two different ways: (1) SCR directly represses ARR1 in the QC cells, which in turn controls auxin production, thus enabling stem cell division. (2) With the same molecular mechanism SCR, via polar auxin transport (PAT), also exerts a long-distance control on ARR1 at the transition zone (TZ) enabling cytokinin to sustain cell differentiation via the SHY2/PINs (polar auxin transporters) pathway (Dello Ioio et al. 2008; Moubayidin et al. 2013). (3) From the root endodermis SCR positively transduces gibberellin (GA) signaling, presumably by controlling the F-box SNEEZY (SNE) (Cui et al. 2007), thus in turn controlling the DELLA protein RGA degradation (Ariizumi and Steber 2011; Ariizumi et al. 2011). In this way SCR tightly controls the level of ARR1-mediated differentiation, via RGA (Moubayidin et al. 2010 and this work)