Autonomous activator12/27/2022 ![]() Second, the QC is a source of non-cell-autonomous signals, which prevent progression of stem cell differentiation, henceĮxcluding periclinal division in the stem cell ( Fig. More mature ground tissue cells may thus reinforce periclinal cell division, and SCR may provide such a “top-down” signalīecause SCR expression persists in the maturing endodermis ( Fig. First, when ground tissue stemĬells that normally give rise to cortex and endodermis are isolated from more mature daughters by laser ablation, their daughtersĭo not perform their characteristic periclinal division ( van den Berg et al. Additional extrinsic factors influencing the asymmetric cell division may exist. One candidate extrinsic factor is the stele-derived SHR protein, whereas no genetic evidence exists to date for cortexĭeterminants. ![]() 1b, 1), or rotation to a periclinal division plane per se, which could be sufficient for postdivision asymmetry mediated by extrinsicįactors ( Fig. Mutant analysis has notĭetermined whether SCR is involved in asymmetric fate separation ( Fig. ![]() The mixed fate of ground tissue cells in scr suggests that it does not encode a fate determinant but that it controls asymmetric cell division. 1996), and SHR is required for up-regulation of SCR expression in the ground tissue ( Helariutta et al. SCR is expressed in the endodermis, the ground tissue stem cells, and the QC ( Scheres et al. Scr mutants form a single ground tissue layer that expresses both cortical and endodermal markers. Color-filled objects represent active promoters (green), transcribed genes (blue, green, orange, yellow), and Heat-shock activation of CRE recombinase mediates excision of SCR, resulting in the 35S-driven GAL4VP16 and transactivation of GFP ER expression, thereby marking the scr-4 mutant cells. ( e) Clonal deletion of SCR using pCB1-SCR complemented scr-4 and pG7HSCRE. The 35S constitutive promoter then drives GAL4VP16 expression, resulting in transactivation of SCR expression and GFP ER, thereby marking the cells with recombination events. Heat-shock activation of CRE recombinase mediates excision of the sequences between the direct repeat lox recombination ( d) Clonal SCR gene activation in scr-4 using vectors pCB1 and pG7HSCRE-USCR. ![]() ( 2) QC signaling prevents differentiation and periclinal division in the stem cell. ( 1) Mature cells act as a patterning template and reinforce periclinal cell division. ( c) Two possible signaling pathways allowing periclinal cell division in the ground tissue stem cell daughters. ( 3) During periclinal division, SCR separates endodermis and cortex fates. ( 2) SCR plays no part in division asymmetry, only external factors determine endodermal and cortex fate. ( 1) SCR and SHR are required to maintain endodermis fate C determines cortex fate. SCR is requiredįor the execution of the periclinal division ( 1,2,3). ( b) Possibilities for SCR function in establishing asymmetry during periclinal ground tissue cell division. (gtsc) Ground tissue stem cell (scd) stemĬell daughter. In the proximal-distal plane, quiescent center (QC) and columella (Col). Radially organized are the stele (St), endodermis (En), cortex (Cor), epidermis (Epi), and lateral root cap ( a) Schematic representation of cell types present in the Arabidopsis root meristem. Site-specific SCR gene activation and deletion to investigate alternative possibilities for SCR function in asymmetric division. The loss of endodermal fate in shr mutants makes SHR a candidate cell fate determinant, but SHR protein is transiently detected in both daughter cells afterĪsymmetric cell division and thus is not an intrinsically segregated determinant. SHR is transcribed in stele cells, but SHR:GFP fusions and immunohistochemistry revealed that the protein resides in the steleĪnd in the adjacent endodermis, ground tissue stem cells, and QC ( Helariutta et al. shr mutants form a single ground tissue layer without endodermis characteristics ( Benfey et al. This asymmetric division requires the putative transcription factors SCARECROW ( SCR) and SHORTROOT ( SHR). In the Arabidopsis root meristem, ground tissue stem cells flank the quiescent center (QC), which functions as a stem cell organizer, and endodermisĪnd cortex are formed through a periclinal division of stem cell daughters ( Fig. Asymmetric cell divisions generate daughter cells with different fates that can be specified by intrinsic fate determinantsĭifferentially separated over daughter cells, by extrinsic cues, or by a combination of both ( Horvitz and Herskowitz 1992 Scheres and Benfey 1999).
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