Cortex proliferation: simple phenotype, complex regulatory mechanisms

Abstract

In plants, the cortex is a relatively undifferentiated cell type. Proliferation of cortex tissues initially appeared to be a simple process of repetitive cell division. However, our recent studies showed that in the Arabidopsis root cortex proliferation involves complex regulatory mechanisms. First, it requires the combined activity of the transcriptional regulators SHORT-ROOT (SHR) and SCARECROW (SCR), but SCR also plays a role in restricting the number of cell divisions. The two opposing activities appear to be mediated by different domains of SCR through physical interaction with distinct partners, and whether SCR behaves as an activator or repressor depends on the relative level of the two protein complexes. We confirmed previous findings that GA plays a major role in cortex proliferation, but also found distinct roles for GA signaling components in this process. We showed that ABA and ethylene also play a role in cortex proliferation, but in an unexpected manner. Finally, we identified an epigenetic component of the regulation, and our data suggested that this is likely the common basis on which various pathways converge. There is evidence that similar mechanisms to those found in Arabidopsis are employed in other plant species.

Figure 1

SCR activates first, but represses subsequent, asymmetric cell divisions in the Arabidopsis root. (A) Schematic depicting the spatial distribution of SHR/SCR and SCR/LHP1 complexes along the longitudinal axis of the Arabidopsis root. The ratio between SCR/SHR is low in CEI, favoring formation of the SHR/SCR activator complex; the ratio would increase rapidly due to SHR/SCR dependent positive feedback regulation of SCR transcription and, above a certain threshold the SCR/LHP1 complex will dominate, thus turning SCR into a repressor. CEI, cortex/endodermis initial cell; Cor, cortex; End, endodermis. (B) in situ hybridization showing the expression pattern of MGP, a common target of SCR and LHP1.

Figure 2

Confocal microscope image of the root of a 6 day-old eto1 seedling, showing middle cortex (MC) formation. End, endodermis; Cor, cortex. Scale bar: 20 µm.