Lessons from the root apex - Intercellular Communication in Plants - page 215

In addition to WER , GL2 and CPC , yet other genetic loci have been implicated in
the root hair patterning. The TTG gene encodes a small WD40 repeat protein (Walker
et al. , 1999). It has been proposed to mediate protein-protein interactions (Galway
et al. , 1994) and to be involved in signal transduction to downstream transcription
factors (Walker et al. , 1999). Expression of the maize R gene (a transcription factor
involved in anthocyanin biosynthetic pathway) under the CaMV 35S promoter com-
plemented the ttg1 mutant phenotype, suggesting that TTG1 might regulate an R -like
gene in Arabidopsis to adopt a hairless cell fate (Lloyd et al. , 1992; Galway et al. ,
1994). Indeed, yeast two-hybrid assays indicate that GLABRA3 (GL3), an R ho-
mologous bHLH protein expressed in shoot epidermal cells in Arabidopsis , interacts
with TTG (Payne et al. , 2000). Thus, TTG may act as a general regulator of epidermal
cell patterning. However, its precise role in root epidermis remains to be elucidated.
In addition, the TRY ( TRIPTHYCON ) gene encodes a CPC-homologous MYB-
related transcription factor that lacks a recognizable activation domain. TRY has
been shown to function as a negative regulator of trichome development in the shoot
butitisalso expressed in roots, suggesting that it has a role in root hair patterning.
Indeed, over-expression of TRY results in formation of extra root hairs. Furthermore,
various expression studies with TRY have shown that TRY levels are controlled by
TRY and CPC, and that TRY is likely to be expressed in N cells (Schellmann et al. ,
2002).
Although TTG and TRY involvement in root epidermal pattern formation has
not been rigorously examined, several studies indicate that they have a role in this
process. The emerging picture is that TTG and an as yet unknown bHLH-related
transcription factor, together with WER, begin to act an early stage in embryonic
development to positively regulate the expression of GL2 (and perhaps other as yet
unidentified genes) in a cell position-dependent manner to specify the N cell type
(Hung et al. , 1998; Lin & Schiefelbein, 2001). Furthermore, it is likely that TRY,
in addition to CPC, is also involved in the lateral suppression of GL2 expression
in root hair files (Schellmann et al. , 2002). The control of epidermal patterning in
aerial tissue and its overlap with the system described in the root is explored further
in Chapter 9, this volume.

8.4.4 Patterning of ground tissue

As discussed above, pattern formation in the Arabidopsis root is the result of highly
asymmetric cell divisions and subsequent cell specification events. One of the best
characterized examples is the radial pattern formation of ground tissue that originates
from a set of stem cells ('initials') that undergo asymmetric cell divisions to give rise
to the youngest cells of the endodermis and cortex cell lineages (Figs. 8.7A and 8.7B).
Subsequently, cells in both lineages undergo differential expansion resulting in cell
types with distinct morphologies and differentiated features. Two loci, SHORT-
ROOT ( SHR ) and SCARECROW ( SCR ), controlling ground tissue patterning during
root development have been identified in Arabidopsis (Benfey et al. , 1993; Scheres
et al. , 1995). Mutations in these loci result in a single ground tissue layer, indicating