Lessons from the root apex - Intercellular Communication in Plants - page 214

Figure 8.5 Model of root epidermal patterning in the growing root. Positional cues from underlying
cells generate a bias in WER expression either because of inhibition of WER in H cells or because of
activation of WER in N cells. This bias is enhanced by increased CPC levels, which carry an inhibitory
signal from N cells to H cells. Dark grey tint corresponds to cell files that will produce non-hair cells
(N cells). Light grey tint corresponds to cell files that will produce hair cells (H cells). Adapted from
Larkin et al. (2003).
in the heart stage embryo, but it is not completed until the embryo reaches the
mature stage. The model is based on the complex regulatory interactions between
WER, CPC and GL2 that occur during the formation of epidermal pattern in the
embryo. GL2 is first expressed in the heart stage embryo in a subset of cells in
the protoderm and WER positively regulates its expression. Then, by the torpedo
stage, GL2 expression has spread to all cells in the future epidermis. WER and
CPC expression is then detectable and WER promotes GL2 expression throughout
the epidermis. CPC is in turn required for the preferential accumulation of GL2
transcript in future N cells, perhaps by negatively regulating GL2 transcription in H
cells position. In the mature embryo, GL2 negatively regulates WER transcription
whereas WER positively regulates CPC expression from the torpedo to mature
stages. These events result in GL2 being expressed at high levels in the future
N cells and being absent from the future H cells in the mature embryo. The pattern
of GL2 expression is then maintained in the root of the seedling and accounts for the
pattern of H cells and N cells in the root epidermis, where GL2 negatively regulates
hair formation in cells located in the N position (Costa & Dolan, 2003).