SCARECROW is involved in positioning the stem cell niche in the Arabidopsis root meristem

Abstract

Stem cells self-renew and produce daughter cells that differentiate. How stem cells are specified and maintained is a central question in developmental biology. Plant stem cells occupy a small region or niche in larger zones of mitotic activity called meristems. Here we provide molecular evidence that in the Arabidopsis root meristem, the stem cell population depends on a central group of cells, the quiescent center (QC), which positions the stem cell niche. We show that the putative transcription factor SCARECROW (SCR), first identified by its role in radial patterning, is required cell-autonomously for distal specification of the QC, which in turn regulates stem cell fate of immediately surrounding cells.

Figure 1

Defects in QC identity and columella stem cell activity in the scr-1 mutant. (a–c,g–i) Wild-type (wt) roots. (d–f,j–l) scr-1 roots. (a) SCR∷GFP expression in wild-type (wt) roots. The white line includes the stem cells surrounding the QC (arrowheads). c, cortex; e, endodermis. (d) SCR∷GFP expression in scr-1 roots. (b,e) QC25 expression. (c,f) QC46 expression. Lack of QC marker expression in scr-1 roots (d–f) indicates defect in QC identity. (g,j) QC184 expression retained in scr-1. (h,k) Double labeling of QC and differentiated columella cells visualized by QC184 and amyloplast staining, respectively, reveal defective columella stem cells ( arrows) in scr-1. (i,l) Columella-specific enhancer trap Q1630 also reveals defective columella stem cells (arrows) in scr-1.

Figure 2

SCR expression in the QC is required and sufficient to establish QC identity and stem cell activity. (a,b,d) GFP expression in roots of wild-type (wt) plants carrying the driver J2341 (a), J1092 (b), and N9094 (d), each with endogenous GFP reporter. (c) Root tip of wild-type (wt) plant carrying the driver 184∷GAL4VP16 without GFP reporter. (e,f,h) GFP expression in roots of scr-1 plants carrying the driver J2341 (e), J1092 (f), and N9094 (h). (g) Root tip of scr-1 plant carrying the UAS∷SCR,UAS∷ERGFP construct. (i,j,l) GFP expression marking SCR activity in roots of scr-1,UAS∷SCR,QC46 plants carrying the driver J2341 (i), J1092 (j), and N9094 (l). (k) Idem in roots of scr-1,UAS∷SCR,UAS∷ERGFP,QC46 plants carrying the driver 184∷GAL4VP16. (m–p) Double labeling of QC and differentiated columella cells visualized by QC46 and amyloplast staining of roots with genotypes depicted in (i–l). Arrowheads indicate position of QC cells, arrows indicate position of columella stem cells. Restoration of functional QC and columella stem cells (indicated by QC46 staining and absence of amyloplasts at the position of columella stem cells) are observed with SCR expression in QC cells, driven by J2341 (m) and 184∷GAL4VP16 (o).

Figure 3

SCR expression in the QC is required and sufficient to maintain indeterminate growth and meristematic cells. Root length (a,b) and root meristem cell number (c) measured in time of wild-type (wt), scr-1,UAS∷SCR, scr-1,UAS∷SCR,J2341, scr-1, UAS∷SCR,J1092, scr-1,UAS∷SCR,N9094, and scr-1,UAS∷SCR, J2341,N9094 plants. Root growth and stem cell activity was only maintained when SCR expression was restored in QC cells, i.e., genotypes scr-1,UAS∷SCR,J2341 and scr-1,UAS∷SCR,J2341, N9094. SCR expression in any other cell type failed to prevent meristem cell differentiation and cessation of root growth (c). Arrow in scr-1,UAS∷SCR root indicates a root hair, characteristic of fully differentiated epidermal cells. Other signs of cell differentiation are the GFP expression in the vasculature of scr-1,UAS∷ SCR,J1092 roots (vascular GFP expression in the J1092 enhancer trap line is found only in fully differentiated vascular cells) and cell elongation in scr-1,UAS∷SCR,N9094 root (arrowheads). Root tips depicted in the right panel in c were analyzed 24 dpg. wt, wild-type.

Figure 3

SCR expression in the QC is required and sufficient to maintain indeterminate growth and meristematic cells. Root length (a,b) and root meristem cell number (c) measured in time of wild-type (wt), scr-1,UAS∷SCR, scr-1,UAS∷SCR,J2341, scr-1, UAS∷SCR,J1092, scr-1,UAS∷SCR,N9094, and scr-1,UAS∷SCR, J2341,N9094 plants. Root growth and stem cell activity was only maintained when SCR expression was restored in QC cells, i.e., genotypes scr-1,UAS∷SCR,J2341 and scr-1,UAS∷SCR,J2341, N9094. SCR expression in any other cell type failed to prevent meristem cell differentiation and cessation of root growth (c). Arrow in scr-1,UAS∷SCR root indicates a root hair, characteristic of fully differentiated epidermal cells. Other signs of cell differentiation are the GFP expression in the vasculature of scr-1,UAS∷ SCR,J1092 roots (vascular GFP expression in the J1092 enhancer trap line is found only in fully differentiated vascular cells) and cell elongation in scr-1,UAS∷SCR,N9094 root (arrowheads). Root tips depicted in the right panel in c were analyzed 24 dpg. wt, wild-type.

Figure 3

SCR expression in the QC is required and sufficient to maintain indeterminate growth and meristematic cells. Root length (a,b) and root meristem cell number (c) measured in time of wild-type (wt), scr-1,UAS∷SCR, scr-1,UAS∷SCR,J2341, scr-1, UAS∷SCR,J1092, scr-1,UAS∷SCR,N9094, and scr-1,UAS∷SCR, J2341,N9094 plants. Root growth and stem cell activity was only maintained when SCR expression was restored in QC cells, i.e., genotypes scr-1,UAS∷SCR,J2341 and scr-1,UAS∷SCR,J2341, N9094. SCR expression in any other cell type failed to prevent meristem cell differentiation and cessation of root growth (c). Arrow in scr-1,UAS∷SCR root indicates a root hair, characteristic of fully differentiated epidermal cells. Other signs of cell differentiation are the GFP expression in the vasculature of scr-1,UAS∷ SCR,J1092 roots (vascular GFP expression in the J1092 enhancer trap line is found only in fully differentiated vascular cells) and cell elongation in scr-1,UAS∷SCR,N9094 root (arrowheads). Root tips depicted in the right panel in c were analyzed 24 dpg. wt, wild-type.