Unique functions of root cap cells
The root cap consists of cell layers and covers the root tip. This relatively inconspicuous tissue actually executes a number of important functions to direct root growth, such as reducing friction against the soil, sensing gravity, and protecting the root meristem. How are these root cap-specific functions regulated?
The root cap is a unique tissue not only for its physiological functions but also for its development. Root cap consists a few to tens of cell layers depending on plant species. A new root cap cell layer is formed via periclinal division of proximal stem cells, while the oldest cell layer located at the outermost position dehisces. Due to this continuous turning over, the root cap is always composed of viable cells. Importantly, root cap cell detachment is an autonomous process, with the cells detaching at a constant interval while they are still alive.
Using the activation tagging system described in the preceding page, we identified a dominant mutant in which root epidermis is converted to root cap-like cells. The causal gene was found to be identical to SOMBRERO (SMB) which was reported by Ben Scheres's group to encodes a NAC transcription factor. Two closely related NAC transcription factors, BEARSKIN1とBEARSKIN2 (BRN1とBRN2), are encoded in the Arabidopsis genome. However, it was unclear how these three NAC transcription facotrs coordinate root cap differentiation and functions.
After careful expression analyses, we found that while SMB is expressed in all root cap cell layers, BRN1 and BRN2 expression is confined to the 1-2 cell layers located at the outermost positions of the root cap. Furthermore, our comparative transcriptome analysis identified 60 genes acting downstream of the SMB/BRN1/BRN2 regulatory module. These 60 genes included those encoding cell wall-modifying enzymes, lipid synthesizing enzymess and membrane trafficking factors.
As a first step, we focused on a gene encoding a putative polygalacturonase (PG) and named it RCPG (ROOT CAP POLYGALACTURONASE). Expression of RCPG is induced in detaching root cap cells. RCPG proteins were found to localize to the cell walls.
While detaching root cap cell layers of wild-type plants take an open plate-like morphology, those of rcpg mutant roots retain a close bowl-like morphology. On the other hand, overexpression of RCPG caused the root cap cells to dehisce individually rather than as a cell layer. Expression of RCPG was found to be directly activated by BRN1.
Based on these results, a scheme was drawn for the gene expression control for the root cap cell detachment in which BRN transcription factors activate RCPG expression to promote pectin degradataion in the outermost root cap cell layer and thereby promote their detachment. Our study revealed a unique behavior of the outermost root cap layer, in which the cells somehow sense their position in the tissue and degrade their own cell walls to detach from the root tip.
Kamiya, M., Higashio, S., Isomoto, A., Kim, J-M., Seki, M., Miyashima, S. and Nakajima, K., Control of root cap maturation and cell detachment by BEARSKIN transcription factors in Arabidopsis. Development 143, 4063-4072. (2016). PubMed Publisher Press