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Endodermal
fate
Endodermal
fate
Endodermal
fate
Endodermal
fate
Cell division
Cell division
Cell division
Figure 8.7 Radial patterning of the ground tissue in the root meristem. (A) SHR mRNA localization
(left), SHR protein (middle) and SCR mRNA (right). (B) Intercellular signalling mechanism in radial
pattern formation of wild type (left) and pSCR::SHR transgenic plants (right). In wild type, SHR
protein moves from the stele to a single adjacent layer. In the adjacent layer it activates SCR
transcription, which is essential for the asymmetric cell divisions that form the cortex and endodermis.
In the mature region SHR presence in the adjacent layer confers endodermal cell fate. How SHR
protein movement is restricted to a single cell layer is as yet unknown. In pSCR::SHR transgenic plants
(right) the SCR promoter is repeatedly activated in adjacent layers by the production of SHR protein
from the pSCR::SHR transgene, resulting in supernumerary layers due to cortex-endodermis initial
daughter cell (Ceid) divisions and the acquisition of endodermal fate in these new layers. Abbreviations
for the cell types are given in Figure 8.4A. Adapted from Nakajima et al. (2001).
2000). SCR expression is down-regulated in the shr background, indicating that
SHR is upstream of SCR .Onthe other hand, ectopic SHR expression under the
35S promoter results in supernumerary cell divisions and altered cell specification
including ectopic SCR expression, indicating that SHR is both necessary and suffi-
cient to regulate cell division and cell specification in the root meristem.
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