Shedding the Last Layer: Mechanisms of Root Cap Cell Release

Abstract

The root cap, a small tissue at the tip of the root, protects the root from environmental stress and functions in gravity perception. To perform its functions, the position and size of the root cap remains stable throughout root growth. This occurs due to constant root cap cell turnover, in which the last layer of the root cap is released, and new root cap cells are produced. Cells in the last root cap layer are known as border cells or border-like cells, and have important functions in root protection against bacterial and fungal pathogens. Despite the importance of root cap cell release to root health and plant growth, the mechanisms regulating this phenomenon are not well understood. Recent work identified several factors including transcription factors, auxin, and small peptides with roles in the production and release of root cap cells. Here, we review the involvement of the known players in root cap cell release, compare the release of border-like cells and border cells, and discuss the importance of root cap cell release to root health and survival.

AtPiezo Plays an Important Role in Root Cap Mechanotransduction

“…The growth and development of the root cap are dynamic processes [ 23 ]. The cell wall of outermost columella cells will degrade, causing the cell layer to fall off naturally [ 24 ]. Our result showed AtPiezo has a higher expression in the root cap, especially in the cells processing shedding ( Figure 2 C).…”

“…The root cap, as the outermost part of the root tip, plays a key role in its response to soil mechanical forces [ 22 ]. The important functions of the root cap are to protect the root tip from the impact of the environment, to determine the direction of root growth, and to reduce the friction between the soil by shedding surrounding root cap cells and secreting mucus [ 23 , 24 ]. Previous studies have shown that the soil status causes significant changes in root cap morphology [ 21 , 25 ].…”

Integrative Roles of Phytohormones on Cell Proliferation, Elongation and Differentiation in the Arabidopsis thaliana Primary Root

“…Columella differentiated cells contain amyloplasts (plastids with starch) that participate in root gravitropism and are excellent markers of columella cell differentiation ( Dolan et al, 1993 ; Kumpf and Nowack, 2015 ; Figures 1 , 6A ). Moreover, COL has a controlled cell file number homeostasis with a high cell turnover that maintains a constant cell file number showing the tight coupling of cell proliferation, differentiation and detachment ( Kumpf and Nowack, 2015 ; Kumar and Iyer-Pascuzzi, 2020 ). The regulation of cell proliferation and cell differentiation of the COL is maintained by a well-documented signaling gene regulatory network centered in the activity of WUSCHEL-RELATED HOMEOBOX 5 ( WOX5 ), a homeodomain TF expressed in the QC cells.…”

“…Interestingly, in this regulatory network WOX5 excludes the NAC [NO APICAL MERISTEM (NAM), ATAF, and CUP SHAPED COTYLEDON (CUC)] TF SOMBRERO (SMB), from the CSCs to promote differentiation ( Bennett et al, 2014 .). Moreover, it has been reported that other members of the NAC family of TFs like FEZ inhibits differentiation while BEARSKIN1 (BRN1) and BRN2 , participate in COL cell detachment ( Willemsen et al, 2008 ; Bennett et al, 2010 ; Kumar and Iyer-Pascuzzi, 2020 ). This latter process requires cellulases ( Del Campillo et al, 2004 ; Figure 6A ) and when cell division is repressed by either hydroxyurea or aphidicolin treatments, the cell detachment process stops and the number of differentiated columella cell files remains constant ( Dubreuil et al, 2018 ).…”

“…Auxin homeostasis in the columella is affected by both long-range auxin transport and from local root synthesis ( Dombrecht et al, 2007 ). The polar transport of auxin in the root generates a graded auxin distribution in the MZ which creates a maximum of auxin concentration and response in the QC cells followed by the CSCs and the outermost columella layer, separated by cell files with lower auxin responses ( Petersson et al, 2009 ; De Rybel et al, 2012 ; Dubreuil et al, 2018 ; Kumar and Iyer-Pascuzzi, 2020 ; Figure 6B ). The expression of tryptophan monooxygenase ( iaaM ), an Agrobacterium tumefaciens auxin biosynthesis gene, in the outermost COL cell layers, shows an increased number of attached COL cells ( Dubreuil et al, 2018 ), indicating a role of auxin in the cell-cell separation process.…”