Lessons from the root apex - Intercellular Communication in Plants - page 216

A B

C

Figure 8.6 Schematics of wild-type and mutant Arabidopsis root structures. (A) Wild-type
longitudinal and transverse schematic; (B) the QC keeps surrounding cells in an undifferentiated state;
(C) radial patterns of three Arabidopsis mutants, scr , shr and wol. Adapted from Nakajima et al. (2001).

that both loci are essential for the periclinal asymmetric cell division that gives
rise to the two distinct cell files (Fig. 8.6C). In scr mutants, the remaining layer
has differentiated attributes of both endodermis and cortex, indicating that SCR is
required for cell division but not differentiation of the ground tissue (Scheres et al. ,
1995; Di Laurenzio et al. , 1996). In contrast to scr , the single layer of ground tissue
in shr mutants is missing endodermal differentiation markers, indicating that SHR
is essential for both cell division and cell specification (Benfey et al. , 1993).
Both SCR and SHR genes have been identified at the molecular level (Di Lauren-
zio et al. , 1996; Helariutta et al. , 2000). They encode members of the GRAS family
of putative transcription factors, indicating that they regulate the asymmetric cell
division (and subsequent endodermal differentiation in the case of SHR) at a tran-
scriptional level (Pysh et al. , 1999). The SCR gene is expressed in the initial daughter
cell before its asymmetric division and remains expressed in the endodermal cell
layer after the division (Fig. 8.7A) (Di Laurenzio et al. , 1996; Wysocka-Diller et al. ,