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1 June 2014 root Hairs
Claire Grierson, Erik Nielsen, Tijs Ketelaarc, john Schiefelbein
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Claire Grierson,1 Erik Nielsen,2 Tijs Ketelaarc,3 man Schiefelbein4,* 1aSchool of biological Sciences, college of Bristol, Bristol, UK BS8 1UG. 2bDepartment that Molecular, Cellular, and Developmental Biology, college of Michigan, Ann Arbor, MI 3cLaboratory of cell Biology, Wageningen University, 6708 PB Wageningen, The Netherlands. 4dDepartment that Molecular, Cellular, and also Developmental Biology, university of Michigan, Ann Arbor, MI *Address post to schiefel
umich.edu
The Arabidopsis Book, 2014(12): (2014). Https://doi.org/10.1199/tab.0172
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Abstract

Roots hairs room cylindrical expansions of source epidermal cells the are necessary for acquisition of nutrients, microbe interactions, and plant anchorage. The molecule mechanisms associated in the specification, differentiation, and also physiology of root hairs in Arabidopsis are reviewed here. Root hair specification in Arabidopsis is identified by position-dependent signaling and also molecular feedback loops leading to differential buildup of a WD-bHLH-Myb transcriptional complex. The initiation of source hairs is dependency on the RHD6 bHLH gene family and also auxin to define the site of outgrowth. Root hair elongation counts on polarized cell expansion at the farming tip, which requires multiple integrated processes consisting of cell secretion, endomembrane trafficking, cytoskeletal organization, and cell wall modifications. The study of source hair biological in Arabidopsis has noted a design cell kind for insights into many aspects of plant development and cabinet biology.


INTRODUCTION

Root hairs are long tubular-shaped outgrowths from root epidermal cells. In Arabidopsis, root hairs are around 10 µm in diameter and also can prosper to it is in 1 mm or an ext in size (Figure 1). Since they vastly boost the root surface ar area and also effectively boost the root diameter, source hairs are generally thought to help plants in nutrient acquisition, anchorage, and also microbe interaction (Hofer, 1991).

Root hairs in Arabidopsis have actually attracted a an excellent deal of attention from tree biologists because they provide numerous benefits for an easy studies of development, cabinet biology, and also physiology (Schiefelbein and also Somerville, 1990). The visibility of root hairs in ~ the surface of the root and also away native the plant body way that castle are quickly visualized and easily accessible to a variety of speculative manipulations. Further, the lack of a cuticle layer enables physical and chemical probes to be used with ease. Root hairs thrive rapidly, at a rate of an ext than 1 µm/min, which facilitates studies of cell expansion. Perhaps many importantly, source hairs space not vital for tree viability, which patent the recovery and evaluation of all varieties of mutants that transform root hair advance and function. Also, root hairs become visible ~ above seedling roots soon after particle germination, which allows genetic screens and physiological tests to it is in performed rapidly with big numbers of individuals grown on identified media in Petri dishes (Figure 2). Finally, the advance of source hairs (and their resident epidermal cells) wake up in a predictable and also progressive path in cells organized in files emanating native the root guideline (Figure 3). This provides the opportunity for detailed evaluation of the cellular transforms that occur throughout the entire procedure of root hair formation.


Figure 1.


Scanning electron micrograph that a source hair cell. The hair created by this cell is roughly 1/3 that its final length.


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Figure 2.


Development of Arabidopsis seedlings growing on agarose-solidified nutrient medium in vertically-oriented Petri plates. The roots thrive along the surface ar of the medium, and also root hairs are visualized easily using a low-magnification microscope.


This chapter gives a an introduction of the development, structure, and function of source hairs in Arabidopsis. Particular emphasis is placed on recent findings using molecular genes to discover root hair development. Recent reviews emphasizing varied elements of Arabidopsis root hairs have been released (Ishida et al., 2008; Schiefelbein et al., 2009; Tominaga-Wada et al., 2011 ; Benitez et al., 2011 ; Ryu et al., 2013).


ROOT HAIR cabinet SPECIFICATION


Pattern the Epidermal cell in the Root

In Arabidopsis, the epidermal cells that produce root hairs (root hair cells) are interspersed through cells that lack root hair (non-hair cells). Thus, the first step in source hair breakthrough is the specification of a newly-formed epidermal cabinet to differentiate as a source hair cell quite than a non-hair cell. This process has been studied intensively during the past several years since it serves as a straightforward model for expertise the regulation that cell-type patterning in plants.

The Arabidopsis root epidermis is created from a set of 16 initial (stem) cells that are formed during embryogenesis (Dolan et al., 1993; Scheres et al., 1994; Baum and Rost, 1996; see additionally the chapter on root development in this book). This initials are termed epidermal/lateral root lid initials due to the fact that they likewise give rise to the cell of the lateral root lid (Dolan et al., 1993; Scheres et al., 1994). The instant epidermal daughter cells created from these initials undergo secondary transverse departments in the meristematic region of the root, and also these departments (typically 5 or 6 rounds every daughter cell) offer to generate additional cells in ~ the same document (Baum and Rost, 1996; Berger et al., 1998b). Furthermore, anticlinal longitudinal divisions occasionally take place and an outcome in rise in the variety of epidermal cell files; this activity causes the observed number of epidermal cell documents in the seedling source to vary from 18 come 22 (Galway et al., 1994; Baum and Rost, 1996; Berger et al., 1998b). The epidermal cells space symplastically linked during much of their advance (Duckett et al., 1994).


Figure 3.


Photograph of a root reminder showing the progressive advance of source hair cells.


The root epidermis the Arabidopsis, like various other members of the family members Brassicaceae, possesses a distinct position-dependent sample of root hair cells and also non-hair cells (Cormack, 1935; 1949; Bunning, 1951 ; Dolan et al., 1994; Galway et al., 1994). Source hair cell are existing outside the intercellular room between 2 underlying cortical cell (i.e., located outside an anticlinal cortical cabinet wall, referred to as the “H” position), vice versa, non-hair cell are current over a single cortical cell (i.e., located exterior a periclinal cortical cell wall, dubbed the “N” position) (Figure 4). Due to the fact that the Arabidopsis main root repetitively possesses eight papers of cortical cells, there are eight root-hair cell files and also approximately 10 come 14 non-hair cell records (Dolan et al., 1994; Galway et al., 1994). The basic correlation in between cell position and also cell type differentiation indicates that cell-cell communication events are critical for the facility of cell identification in the source epidermis.

An exemption to this pattern exists near the root-hypocotyl junction, in a region containing 3–7 tiers that cells called the collet (Parsons, 2009). Here, every epidermal cell forms a root-hair-like extension throughout early seedling growth (Scheres et al., 1994; Lin and also Schiefelbein, 2007; Sliwinska et al., 2012). Regular with this outstanding pattern, gene that clues the non-hair fate space not active in this region (Lin and also Schiefelbein, 2007). Interestingly, this an ar differs native the rest of the root by own a 2nd (incomplete) class of cortical cell (Lin and also Schiefelbein, 2007), early out to change from the cellular anatomy of the hypocotyl (two cortical layers) to the root (one cortical layer). Furthermore, the source hairs in the collet arise synchronously, quite than the progressive formation of source hairs in ~ cell documents at the source apex (Sliwinska et al., 2012).


Figure 4.


Transverse section of an Arabidopsis root, mirroring the position-dependent pattern of hair cells and also non-hair cells. The hair-bearing cells are located external the space separating 2 cortical cells (the H cell position), whereas the non-hair cells are located outside a solitary cortical cell (the N cell position). 3 hairs are visible in this section; the various other cells in the H position possess hair that space located exterior the field of view.


Nature the the cabinet Patterning Information

The information that directs the position-dependent epidermal cell pattern is noted at very early stage in epidermis development, since immature epidermal cell destined to become root-hair cells (trichoblasts) can be distinguished from immature non-hair cell (atrichoblasts) before hair outgrowth. Specifically, the differentiating root-hair cells display a greater rate the cell division (Berger et al., 1998b), a diminished cell size (Dolan et al., 1994; Masucci et al., 1996), greater cytoplasmic thickness (Dolan et al., 1994; Galway et al., 1994), a reduced rate of vacuolation (Galway et al., 1994), distinctive cell surface ornamentation (Dolan et al., 1994), and also distinct cell wall epitopes (Freshour et al., 1996).

A more-precise knowledge of the timing of the patterning information has been noted by the use of two reporter gene fusions, a GLABRA2 (GL2: At1g79840) gene build (Masucci et al., 1996; Lin and Schiefelbein, 2001) and also an enhancer-trap GFP construct (line J2301; Berger et al., 1998c). Every of these reporters space expressed in the N-cell position (epidermal cell located external a periclinal cortical cell wall) within the meristematic an ar of the source (Figure 5). Mindful examination making use of these sensitive reporters expose position-dependent gene expression within, or just one cell beyond, the epidermal/lateral root lid initials, which indicates that patterning details may be provided (and cabinet fates begin to be defined) within this initial cell and/or their prompt daughters (Masucci et al., 1996; Berger et al., 1998a).

The presence of differential gene expression in the early on meristem resulted in the opportunity that the epidermal cell pattern might be initiated throughout embryogenesis, when the an easy root structure and also meristem initials are developed (Scheres et al., 1994). Indeed, the evaluation of the J2301 enhancer-trap GFP (Berger et al., 1998a) and the GL2::GFP (Lin and Schiefelbein, 2001) reporters present that the epidermal cell specification pattern becomes created during embryonic root breakthrough in Arabidopsis (Figure 6). The GL2::GFP exhibits the earliest expression, start at the early heart stage, i m sorry is prior to the formation of the root meristem. Because that each of this reporters, expression is recognize in a position-dependent epidermal pattern the mirrors the post-embryonic pattern (Berger et al., 1998a; Lin and Schiefelbein, 2001). Thus, it shows up that positional information is provided during embryonic root advance and plot to create the ideal pattern of gene activities that eventually leads to ideal post-embryonic cell form differentiation.


Figure 5.


Expression the the GL2::GUS reporter combination during root development.

(A) surface ar view showing preferential expression in the meristematic region. Bar = 50 µm.

(B) Transverse section reflecting preferential expression in the N-cell place of the epidermis. Bar = 20 µm.


To identify whether positional details is also listed to epidermal cells post-embryonically, 2 sorts of experiments have actually been conducted. In one, a detailed analysis of strange epidermal cell clones was performed (Berger et al., 1998a). The clones examined to be ones obtained from rare post-embryonic longitudinal divisions of epidermal cells, which causes the 2 resulting daughter cell to occupy various positions loved one to the basic cortical cells. The cell within this clones expressed mite genes and exhibited cellular features that are ideal for their new position (Figure 7). In a second set that experiments, details differentiating epidermal cells were based on laser ablation together that neighboring epidermal cells to be able to attack the available an are (Berger et al., 1998a). Regardless of the original state of the ablated cell or invading cabinet (trichoblast or atrichoblast), the ultimate attributes of the invading cabinet were practically always determined by its brand-new location rather than that old. Therefore, in each of these sets of experiments, cell had efficiently undergone a post-embryonic readjust in their position and, in response, displayed a readjust in your developmental fate. This suggests that positional information is noted post-embryonically, not only embryonically, come ensure appropriate cell specification in the Arabidopsis source epidermis.


Figure 6.


Embryonic expression the the GL2::GFP reporter fusion in the torpedo stage embryo. This typical longitudinal view reflects GFP buildup in protodermal cells in the future hypocotyl and root.


Laser ablation of details cells has also provided insight into the directionality of the positional signal that specify the epidermal cell types (Berger et al., 1998a). In one set of experiments, tree harboring the J2301 enhancer-trap GFP reporter were based on ablations in which immature epidermal cells to be isolated from their next-door neighbors within the same file or in nearby files. In nearly every case, the secluded cells, which had lost call with their epidermal neighbors, maintained the very same reporter gene expression and also differentiated follow to their original position (Berger et al., 1998a). In a second set of cabinet ablations, details cortical cell of the J2301 line were ablated such the the overlying immature epidermal cell(s) were isolated. Nevertheless of the initial state the the isolated epidermal cabinet (trichoblast or atrichoblast), the ablation of the basic cortical cell(s) had actually no result on your future GFP expression or morphogenesis (Berger et al., 1998a). This results indicate that continuous signaling between living cortical and/ or epidermal cell is not required to maintain the suitable cell fate decision. However, the is tho unclear whether signaling in between cortical and also epidermal cells may be required to establish cell fates.


Molecular genes of source Hair cabinet Specification

Several mutants have been established in Arabidopsis the possess a disrupted sample of source epidermal cell species ( in Supplemental Table 1 (tab.0172-table-1.docx) ; figure 8). Mutations in the WEREWOLF (WER: At5g14750), TRANSPARENT TESTA GLABRA (TTG: At5g24520), GLABRA3 (GL3: At5g41315)/ENHANCER the GLABRA3 (EGL3: At1g63650), and also GL2 genes reason root hairs to type on basically every source epidermal cell, which indicates that the normal duty of WER, TTG, GL3/EGL3, and also GL2 is to promote non-hair cabinet differentiation and/or repress root hair cabinet differentiation (Galway et al., 1994; DiCristina et al., 1996; Masucci et al., 1996; Lee and Schiefelbein, 1999; Bernhardt et al., 2003). These mutations differ in their certain effects ~ above non-hair cabinet differentiation; because that example, the wer and ttg mutations change all aspects of non-hair differentiation (including the cell division rate, cytoplasmic density, and vacuolation rate) whereas the gl2 mutations only impact the final cell morphology and also do not impact the earlier cellular phenotypes (Galway et al., 1994; Masucci et al., 1996; Berger et al., 1998b; Lee and also Schiefelbein, 1999). Thus, WER and TTG may be previously (and an ext broadly) acting contents required because that position-dependent non-hair cell differentiation.


Figure 7.


Expression that the GL2::GUS reporter fusion in an epidermal cell clone derived from a rare longitudinal division. Note that just one set of cell in the clone expresses the GL2 marker. Bar = 10 µm.


Figure 8.


Root hair manufacturing in wild-type and also cell specification mutants.

(A) Wild-type.

(B) an instance of an ectopic hair mutant (wer).

(C) an instance of a decreased hair mutant (cpc).Bar = 500 µm for all images.


The WER gene encodes a MYB transcription variable of the R2-R3 class (Lee and Schiefelbein, 1999). The is preferentially expressed in occurring epidermal cells in the N position, which space the cells whose fate is mis-specified in the wer mutant. In enhancement to the MYB DNA-binding domains, the WER protein possesses a phosphatidic mountain (PA)-interacting region involved in atom localization (Yao et al., 2013). Uneven TTG and GL2, the WER gene walk not affect trichome development, particle coat mucilage, or anthocyanin production. A closely related gene, MYB23 (At5g40330), encodes a protein through a comparable biochemical function as WER, and it exhibits N-cell-specific expression that is dependent on WER, GL3/EGL3, and TTG (Kang et al., 2009).

The TTG gene encodes a little protein v WD40 repeats (Walker et al., 1999). Although the protein sequence does not carry out a clear mechanistic understanding of that role, TTG is able come physically interact with the simple helix-loop-helix (bHLH) transcriptional activators GL3 and also EGL3, which act in a partly functionally redundancy manner (Bernhardt et al 2003). GL3 and also EGL3 likewise physically interact with WER (Bernhardt et al., 2003; song et al., 2011), which suggests that a tripartite transcription factor complicated is responsible because that directing the non-hair cabinet fate.

The GL2 gene encodes a homeodomain transcription aspect protein (Rerie et al., 1994; DiCristina et al., 1996), and it is preferentially express in the separating non-hair epidermal cells within the meristematic and also elongation regions of the source (Masucci et al., 1996; figure 5). As defined above, GL2 expression initiates during the beforehand heart phase of embryogenesis, wherein it quickly assumes the N-cell-specific expression pattern (Lin and also Schiefelbein, 2001). The embryonic and post-embryonic GL2 gene expression is influenced by the WER, GL3/EGL3, and TTG genes; wer mutations nearly abolish GL2 promoter activity, while ttg and gl3 egl3 mutations alleviate GL2 promoter task (Hung et al., 1998; Lee and also Schiefelbein, 1999; Lin and also Schiefelbein, 2001 ; Bernhardt et al., 2003). Because proper cell position-dependent GL2 expression is current in the ttg and also gl3 egl3 mutants, yet not the wer mutant, it indicates that WER is most vital for conveying positional information for GL2 expression. Taken together, the present view is the WER, TTG, and the GL3/ EGL3 protein act in a transcriptional facility at an early stage in embryonic advance to positively manage the expression that GL2 (and perhaps various other as yet unidentified genes) in a cell position-dependent manner to specify the non-hair cell type.

Another Arabidopsis gene, CAPRICE (CPC: At2g46410), affects source epidermis cabinet specification in a different manner. Fairly than resulting in ectopic source hair cells, the cpc mutant produce a reduced variety of root hair cells (Wada et al., 1997). This means that CPC is a positive regulator that the root hair cell fate. Interestingly, the gl2 mutation is epistatic come cpc, which suggests that CPC plot in the WER/TTG/GL3/EGL3/GL2 pathway as a negative regulator that GL2. A possible explanation because that CPC\"s negative action is provided by the nature of its gene product; CPC encodes a small protein with a solitary R3 Myb repeat v the bHLH- and also DNA-binding domains however without a typical transcriptional activation domain (Wada et al., 1997). Regular with this structure, CPC shows up to inhibit the role of the WER-GL3/EGL3-TTG complex by interfering v WER binding come GL3/EGL3 in a competitive manner (Lee and Schiefelbein, 2002; Tominaga et al., 2007; tune et al., 2011 ; Kang et al., 2013). Interestingly, CPC is capable of moving from cell-to-cell in the emerging root, permitting it to act together a signaling molecule to repress non-hair cell fate specification in neighboring H cell (Kurata et al., 2005; Kang et al., 2013). That preferential buildup in the H cells is proposed to be due to trapping of CPC protein by EGL3 (Kang et al., 2013). Since CPC transcription is positively regulated by the WER-GL3/EGL3-TTG complex, its negative effect ~ above the action of this facility represents an intercellular negative feedback loop (Ryu et al., 2005). In addition to CPC, several related R3 Myb proteins have actually been displayed to plot in a partially redundant manner, including ones encoded by TRIPTYCHON (TRY: At5g53200) and also ENHANCER OF try AND CPC1 (At1g01380) (Schellman et al., 2002; Kirik et al., 2004; Simon et al., 2007; Serna, 2008; Wang etal., 2010).

Another transcriptional feedback loop affect expression of the GL3 and EGL3 genes. Although protein fusions of GL3 accumulate in the N-cell position, the GL3 and EGL3 genes room preferentially transcribed in the H-cell place due to an unfavorable regulation by the WER-GL3/EGL3-TTG complex (Bernhardt et al., 2005). This means that, like the CPC family members proteins, the GL3 and also EGL3 bHLH proteins may also move with plasmodesmata; although in this case, from the H to the N cells. Based on mathematical modeling, these interlocking intercellular feedback loops have been argued to provide stability and also robustness come the facility of the cell-type sample (Savage et al. 2008).

The SCRAMBLED gene (SCM: At1g11130) different from the preceding genes since its mutant phenotype walk not eliminate one the the epidermal cell species but merely alters the circulation of root hair and non-hair cells (Kwak et al, 2005). SCM encodes a leucine-rich-repeat receptor-like kinase (LRR-RLK) that appears to enable epidermal cell to awareness their family member cell position (Kwak et al, 2005), and as a result, they attain distinct gene expression trends and take on appropriate fates. Interestingly, the SCM gene is itself under the transcriptional feedback regulation the the WER-GL3/EGL3-TTG complex, because N-position cells exhibit a complex-dependent reduction in SCM accumulation, family member to H-position cell (Kwak and also Schiefelbein, 2008). This an unfavorable regulatory loop may serve come amplify SCM signaling in the H-position cells.

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These molecular genetic findings led to a basic model because that the regulate of source epidermal cell fate in Arabidopsis (Lee and also Schiefelbein, 2002; number 9). In this model, the root hair cabinet fate is suggest to stand for the default fate because that a source epidermal cell. The pattern of hair and non-hair cell varieties relies top top the relative task of two completing sets of warrior factors, the R2R3 WER and also MYB23 proteins vs. The one-repeat Mybs CPC, TRY, and ETC1. These space able to type an active or inactive complex, respectively, v the TTG and also GL3/EGL3 proteins. In immature epidermal cell in the N position, the is proposed the a reasonably high level that WER is present and also this leads predominately to formation of the energetic complex, expression that GL2 (and probably other genes), and also non-hair cell differentiation. Top top the other hand, immature epidermal cells situated in the H place are proposed to accumulate a fairly high level of CPC, which leads to inactive complexes, repression the GL2, and also hair cabinet differentiation. The SCM receptor is proposed come mediate the effects of a position cue and also initiate differential buildup of the WER and CPC regulators. The downstream feedback loops, consisting of the an adverse effect the CPC and also positive regulation through MYB23, space thought come stabilize and amplify the early stage asymmetry in gene expression patterns.