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AD22-043

Root Hairs

Authors: Grierson, Claire, Nielsen, Erik, Ketelaarc, Tijs, and ​Schiefelbein, John

Source: The Arabidopsis Book, 2014(12)

Published By: The American Society of Plant Biologists

URL: https://doi.org/10.1199/tab.0172

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The Arabidopsis Book

© 2014 American Society of Plant Biologists

First published on June 25, 2014: e0172. doi: 10.1199/tab.0172

This is an updated version of a chapter originally published on April 4, 2002, e0060. doi:10.1199/tab.0060

Root Hairs

Claire Griersona, Erik Nielsenb, Tijs Ketelaarc, and John Schiefelbein

d,1

a

School of Biological Sciences, University of Bristol, Bristol, UK BS8 1UG.

b

Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, USA 48109.

c

Laboratory of Cell Biology, Wageningen University, 6708 PB Wageningen, The Netherlands.

d

Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, USA 48109.

1

Address correspondence to schiefel@umich.edu

Roots hairs are cylindrical extensions of root epidermal cells that are important for acquisition of nutrients, microbe interac- ​tions, and plant anchorage. The molecular mechanisms involved in the specification, differentiation, and physiology of root ​hairs in Arabidopsis are reviewed here. Root hair specification in Arabidopsis is determined by position-dependent signaling ​and molecular feedback loops causing differential accumulation of a WD-bHLH-Myb transcriptional complex. The initiation of ​root hairs is dependent on the RHD6 bHLH gene family and auxin to define the site of outgrowth. Root hair elongation relies ​on polarized cell expansion at the growing tip, which involves multiple integrated processes including cell secretion, endo- ​membrane trafficking, cytoskeletal organization, and cell wall modifications. The study of root hair biology in Arabidopsis has ​provided a model cell type for insights into many aspects of plant development and cell biology.

INTRODUCTION

Root hairs are long tubular-shaped outgrowths from root epider- ​mal cells. In Arabidopsis, root hairs are approximately 10 μm in ​diameter and can grow to be 1 mm or more in length (Figure 1). ​Because they vastly increase the root surface area and effec- ​tively increase the root diameter, root hairs are generally thought ​to aid plants in nutrient acquisition, anchorage, and microbe inter- ​actions (Hofer, 1991).

Root hairs in Arabidopsis have attracted a great deal of at- ​tention from plant biologists because they provide numerous ​advantages for basic studies of development, cell biology, and ​physiology (Schiefelbein and Somerville, 1990). The presence ​of root hairs at the surface of the root and away from the plant ​body means that they are easily visualized and accessible to a ​variety of experimental manipulations. Further, the lack of a cu- ​ticle layer allows physical and chemical probes to be applied with ​ease. Root hairs grow rapidly, at a rate of more than 1 μm/min, ​which facilitates studies of cell expansion. Perhaps most impor- ​tantly, root hairs are not essential for plant viability, which permits ​the recovery and analysis of all types of mutants that alter root ​hair development and function. Also, root hairs become visible ​on seedling roots shortly after seed germination, which enables ​genetic screens and physiological tests to be performed rapidly ​with large numbers of individuals grown on defined media in Petri ​dishes (Figure 2). Finally, the development of root hairs (and their

Figure 1. Scanning electron micrograph of a root hair cell. The hair pro- ​duced by this cell is approximately 1/3 of its final length.

resident epidermal cells) occurs in a predictable and progressive ​manner in cells organized in files emanating from the root tip (Fig- ​ure 3). This provides the opportunity for detailed analysis of the ​cellular changes that occur during the entire process of root hair ​formation.

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Report

Stimulation of Hair Growth by Small Molecules that ​Activate Autophagy

Graphical Abstract

Authors

Min Chai, Meisheng Jiang,

Laurent Vergnes, ..., Gay M. Crooks, ​Karen Reue, Jing Huang

Corresponden ce

jingh uang.uc la@gmail. com

In Brief

Hair regeneration requires reactivating ​dormant hair follicle stem cells. Chai et al. ​discover that pharmacological induction ​of autophagy is sufficient to activate ​quiescent telogen hair follicles, initiating ​new anagen hair growth. Hair loss ​resulting from shortened anagen, ​lengthened telogen, and/or impeded ​anagen induction may thereby be

rescued by activating autophagy.

Highlights

d

mTOR and AMPK modulation by rapamycin, metformin, and ​a-KG induces anagen hair growth

Autophagy induction is necessary and sufficient for anagen

entry and hair growth

Autophagy is increased during anagen phase of the natural

hair follicle cycle

Aged mice fed the autophagy-inducing metabolite a-KB are

protected from hair loss

d

d

d

27

Chaietal.,2019,CellReports ,3413–3421 ​June 18, 2019 ª 2019 The Authors. ​https://doi.org/10.1016/j.celrep.2019.05.070

Current Biology Vol 23 No 2 ​R52

and take over the discipline. If we

want to make sure that the biology

of the future preserves our hard-won ​biological perspectives, knowledge ​and insights, we need to be able to

do the analyses and deal with all

these data ourselves.

For young scientists embarking

on a PhD, make sure your PhD topic

is something you love, and that

your question is one whose answer ​you care deeply about. Don’t settle

for less than this, or you’ll lack the ​drive needed to work to your own

full potential. And if there’s some

body of knowledge or theory that is ​important to your question, whatever ​the discipline, just roll up your

sleeves and learn it. For post-docs, ​one word: ‘publish!’ And don’t spend ​months trying to get a paper perfect ​in every detail before submitting: your ​reviewers will find flaws no matter ​what. Spend your time getting the ​experiments and analysis right, not ​perfecting the writing.

Your website says you play music ​and paint: does science influence ​your creative work? While in college

I seriously considered a career in the ​arts, and many of my closest friends ​and band-mates went on to become ​professional musicians. I’ve played in ​rock and salsa bands and an African ​drumming ensemble, and I still play

a lot of guitar and write and record ​songs. And recently, we’ve been ​studying the biology and evolution of ​music, and I’ve really been enjoying ​the opportunity to combine music

with scientific research.

Regarding the influence of

science, I draw the figures for a lot

of my publications (ink drawings or ​watercolors, reworked with Adobe ​Illustrator). I’ve also written some ​biological songs, including “I Don’t ​Believe in Evolution”, which pokes

fun at creationists and has been live- ​broadcast on Italian radio (and even ​served as a ring tone on some of my ​students’ phones!).

But frankly, I’m happy to have

science as my ‘day job’ and music ​and painting as hobbies: I think the ​pressure to make money with art ​would take the fun out of it.

So you’re glad you became a ​scientist? Absolutely. I feel incredibly ​fortunate to be a scientist. Sure, ​scientists’ salaries are not usually ​commensurate to their education

and ability. But how many people are ​lucky enough to be paid to follow

their interests and satisfy their own ​curiosity every day?

What are the most exciting topics ​you are researching right now?

At the moment I’m very excited

about our new research program

in empirical aesthetics, trying to ​understand the biological roots of the ​visual arts, and in particular of the ​human love for symmetry and order. ​Humans around the planet surround ​themselves with decorative patterns, ​with no obvious function, such as ​weaving, quilting, decorated pottery, ​clothes, tattoos and architectural ​ornament. Oddly, art historians have ​largely focused on representational

art by great geniuses, and neglected ​this much more widespread, popular ​and presumably ancient form of art ​(often relegated to ‘craft’). We’ve

been bringing ordinary people into

the lab and studying the kinds of ​patterns they make using computer ​interfaces (as well as what they like, ​and what kind of rules they can ​perceive). It looks like there is a deep ​biological drive in our species — what ​the art historian Ernst Gombrich

called our ‘sense of order’ — that ​hasn’t received enough attention.

I’m also very excited about our

work in bioacoustics, trying to ​understand how animals produce

their sounds. This is a truly inter- ​disciplinary bridging area, spanning

an amazing breadth of disciplines

from physics to physiology to

behavior, cognition, and evolution. It ​also relies on comparative anatomy, ​so you get to dig out old anatomical ​papers documenting weird and ​wonderful adaptations for sound ​production that were forgotten long ​ago, and then try to understand

them from the viewpoint of modern ​acoustics and nonlinear dynamics. ​We’re studying vocal production in ​alligators, deer, primates, ravens, ​parrots, and lots of other species,

but it is amazing how the same ​physical phenomena and principles ​(mostly originally discovered in

human speech) seem to underlie

all this diversity. It’s a comparative ​biologist’s dream come true.

Quick guide

Aquaporins

A.S. Verkman

What are aquaporins? Aquaporins ​(often called aquaporin water

channels) are a family of small,

integral membrane proteins that are ​expressed broadly throughout the ​animal and plant kingdoms. They

have a similar basic structure, with ​aquaporin monomers consisting of

six transmembrane helical segments ​and two short helical segments that ​surround cytoplasmic and extracellular ​vestibules connected by a narrow ​aqueous pore (Figure 1A). They contain ​several conserved motifs, including ​NPA sequences in their short helical ​segments. Aquaporin monomers ​assemble as tetramers in membranes, ​with each monomer functioning ​independently. Some aquaporins,

such as mammalian AQP4, can further ​aggregate in cell membranes to form ​supramolecular crystalline assemblies ​called orthogonal arrays of particles.

What do aquaporins do at the

molecular level? The primary function

of most aquaporins is to transport

water across cell membranes in

response to osmotic gradients created

by active solute transport. Because the

water transport capacity of aquaporin

monomers is low, membranes often

contain a high density of aquaporins,

up to 10,000 per square micron,

to increase water permeability

substantially above that in the absence

of aquaporins. Molecular dynamics

simulations suggest that steric

factors and electrostatic interactions

in the aqueous pore are responsible

for the selectivity of aquaporins for

water. A subset of aquaporins, called

aquaglyceroporins also transport

glycerol. The pore diameter of the

aquaglyceroporins is slightly greater

than that of the water-selective

aquaporins, and the pore is lined

by relatively hydrophobic residues

compared with the pore of a water-

selective aquaporin. In addition to

water and glycerol, there is evidence,

some of which is controversial, that

some aquaporins pass gases (CO2,

NH3, NO, O2), various small solutes

such as H2O2 and arsenite, and

Department of Cognitive Biology, Faculty ​of Life Sciences, University of Vienna, 14 ​Althanstrasse, A-1090 Vienna, Austria. ​E-mail: tecumseh.fitch@univie.ac.at

2 of 25

The Arabidopsis Book

Figure 2. Development of Arabidopsis seedlings growing on agarose-so- ​lidified nutrient medium in vertically-oriented Petri plates. The roots grow ​along the surface of the medium, and root hairs are visualized easily using ​a low-magnification microscope.

This chapter provides a summary of the development, struc- ​ture, and function of root hairs in Arabidopsis. Particular empha- ​sis is placed on recent findings using molecular genetics to ex- ​plore root hair development. Recent reviews emphasizing varied ​aspects of Arabidopsis root hairs have been published (Ishida et ​al., 2008; Schiefelbein et al., 2009; Tominaga-Wada et al., 2011;

Benitez et al., 2011; Ryu et al., 2013).

ROOT HAIR CELL SPECIFICATION

Pattern of Epidermal Cells in the Root

In Arabidopsis, the epidermal cells that produce root hairs (root ​hair cells) are interspersed with cells that lack root hairs (non-hair ​cells). Thus, the first step in root hair development is the specifi- ​cation of a newly-formed epidermal cell to differentiate as a root ​hair cell rather than a non-hair cell. This process has been studied ​intensively during the past several years because it serves as a ​simple model for understanding the regulation of cell-type pattern- ​ing in plants.

The Arabidopsis root epidermis is generated 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 also ​the chapter on root development in this book). These initials are ​termed epidermal/lateral root cap initials because they also give ​rise to the cells of the lateral root cap (Dolan et al., 1993; Scheres ​et al., 1994). The immediate epidermal daughter cells produced ​from these initials undergo secondary transverse divisions in the ​meristematic region of the root, and these divisions (typically 5 or 6 ​rounds per daughter cell) serve to generate additional cells within ​the same file (Baum and Rost, 1996; Berger et al., 1998b). Further- ​more, anticlinal longitudinal divisions occasionally occur and result ​in an increase in the number of epidermal cell files; this activity ​causes the observed number of epidermal cell files in the seedling ​root to vary from 18 to 22 (Galway et al., 1994; Baum and Rost,

Figure 4. Transverse section of an Arabidopsis root, showing the position- ​dependent pattern of hair cells and non-hair cells. The hair-bearing cells ​are located outside the space separating two cortical cells (the H cell posi- ​tion), whereas the non-hair cells are located outside a single cortical cell ​(the N cell position). Three hairs are visible in this section; the other cells ​in the H position possess hairs that are located outside the field of view.

Figure 3. Photograph of a root tip showing the progressive development ​of root hair cells.

Nature of the Cell Patterning Information

1996; Berger et al., 1998b). The epidermal cells are symplastically ​connected during much of their development (Duckett et al., 1994). ​The root epidermis of Arabidopsis, like other members of the ​family Brassicaceae, possesses a distinct position-dependent ​pattern of root hair cells and non-hair cells (Cormack, 1935; 1949; ​Bunning, 1951; Dolan et al., 1994; Galway et al., 1994). Root ​hair cells are present outside the intercellular space between two ​underlying cortical cells (i.e., located outside an anticlinal corti- ​cal cell wall, called the “H” position), whereas non-hair cells are ​present over a single cortical cell (i.e., located outside a periclinal ​cortical cell wall, called the “N” position) (Figure 4). Because the ​Arabidopsis primary root consistently possesses eight files of cor- ​tical cells, there are eight root-hair cell files and approximately 10 ​to 14 non-hair cell files (Dolan et al., 1994; Galway et al., 1994). ​The simple correlation between cell position and cell type differ- ​entiation implies that cell-cell communication events are critical

for the establishment of cell identity in the root epidermis.

An exception to this pattern exists near the root-hypocotyl junc- ​tion, in a region containing 3-7 tiers of cells called the collet (Par- ​sons, 2009). Here, every epidermal cell forms a root-hair-like exten- ​sion during early seedling growth (Scheres et al., 1994; Lin and ​Schiefelbein, 2007; Sliwinska et al., 2012). Consistent with this ex- ​ceptional pattern, genes that specify the non-hair fate are not active ​in this region (Lin and Schiefelbein, 2007). Interestingly, this region ​differs from the rest of the root by possessing a second (incomplete) ​layer of cortical cells (Lin and Schiefelbein, 2007), due to transition ​from the cellular anatomy of the hypocotyl (two cortical layers) to ​the root (one cortical layer). Furthermore, the root hairs in the collet ​arise synchronously, rather than the progressive formation of root

hairs within cell files at the root apex (Sliwinska et al., 2012).

The information that directs the position-dependent epidermal cell ​pattern is provided at an early stage in epidermis development, ​because immature epidermal cells destined to become root-hair ​cells (trichoblasts) can be distinguished from immature non-hair ​cells (atrichoblasts) prior to hair outgrowth. Specifically, the dif- ​ferentiating root-hair cells display a greater rate of cell division ​(Berger et al., 1998b), a reduced cell length (Dolan et al., 1994; ​Masucci et al., 1996), greater cytoplasmic density (Dolan et al., ​1994; Galway et al., 1994), a lower rate of vacuolation (Galway et ​al., 1994), unique cell surface ornamentation (Dolan et al., 1994), ​and distinct cell wall epitopes (Freshour et al., 1996).

A more-precise understanding of the timing of the patterning ​information has been provided by the use of two reporter gene fu- ​sions, a GLABRA2 (GL2: At1g79840) gene construct (Masucci et ​al., 1996; Lin and Schiefelbein, 2001) and an enhancer-trap GFP ​construct (line J2301; Berger et al., 1998c). Each of these report- ​ers are expressed in the N-cell position (epidermal cells located ​outside a periclinal cortical cell wall) within the meristematic re- ​gion of the root (Figure 5). Careful examination using these sensi- ​tive reporters reveals position-dependent gene expression within, ​or just one cell beyond, the epidermal/lateral root cap initials, ​which implies that patterning information may be provided (and ​cell fates begin to be defined) within these initial cells and/or their ​immediate daughters (Masucci et al., 1996; Berger et al., 1998a). ​The presence of differential gene expression in the early meri- ​stem led to the possibility that the epidermal cell pattern may be ​initiated during embryogenesis, when the basic root structure and ​meristem initials are formed (Scheres et al., 1994). Indeed, the ​analysis of the J2301 enhancer-trap GFP (Berger et al., 1998a) ​and the GL2::GFP (Lin and Schiefelbein, 2001) reporters show ​that the epidermal cell specification pattern becomes established

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Figure 5. Expression of the GL2::GUS reporter fusion during root devel- ​opment.

(A) Surface view showing preferential expression in the meristematic re-

gion. Bar = 50 μm.

(B) Transverse section showing preferential expression in the N-cell posi-

tion of the epidermis. Bar = 20 μm.

during embryonic root development in Arabidopsis (Figure 6). The ​GL2::GFP exhibits the earliest expression, beginning at the early ​heart stage, which is prior to the formation of the root meristem. ​For each of these reporters, expression is detected in a position- ​dependent epidermal pattern that mirrors the post-embryonic pat- ​tern (Berger et al., 1998a; Lin and Schiefelbein, 2001). Thus, it ​appears that positional information is provided during embryonic ​root development and acts to establish the proper pattern of gene ​activities that ultimately leads to appropriate post-embryonic cell ​type differentiation.

To determine whether positional information is also provided

to epidermal cells post-embryonically, two sorts of experiments ​have been conducted. In one, a detailed analysis of peculiar ​epidermal cell clones was performed (Berger et al., 1998a). The ​clones examined were ones derived from rare post-embryonic ​longitudinal divisions of epidermal cells, which causes the two re- ​sulting daughter cells to occupy different positions relative to the ​underlying cortical cells. The cells within these clones expressed ​marker genes and exhibited cellular characteristics that are ap- ​propriate for their new position (Figure 7). In a second set of ex- ​periments, specific differentiating epidermal cells were subjected ​to laser ablation such that neighboring epidermal cells were able