Download The Control of Patterning and Morphogenesis during Root

The Plant Cell, Vol. 9, 1089-1098, July 1997 O 1997 American Society of Plant Physiologists
Building a Root: The Control of Patterning and Morphogenesis
during Root Development
John W. Schiefelbein,’ James D. Masucci, and Haiyang Wang
Department of Biology, University of Michigan, Ann Arbor, Michigan, 48109-1048
INTRODUCTION
Roots are plant organs adapted to acquire water and nutrients from the environment. The rapid exploratory growth
habit of roots makes them well suited for experimental studies of organogenesis in plants. In this regard, one particularly advantageous feature of root development is that
tissues and cell types arise from an apical meristem that is
relatively simple in structure. Moreover, root development is
not complicated by the formation of lateral appendages at the
apex (Esau, 1965). The resulting regular arrangementof tissue
and cell types simplifies the analysis of pattern formation and
enables us to predict accurately the developmental history
and fate of each of the tissues and cells from their location.
Furthermore, cellular studies of root development are aided
by the transparent nature of roots, and genetic analyses profit
from the fact that roots of numerous seedlings growing on
media of defined composition can be observed.
In this review, we describe studies that are beginning to
uncover the fundamental mechanisms controlling root development, with particular emphasis on the analysis of tissue and cell-type patterning. These studies point toward the
general view that patterning in the root is largely guided by
positional cues that are established during embryogenesis
and maintained during postembryonic root development.
Moreover, recent genetic and physical manipulations of developing root cells have led to new insights into the plasticity
of root development.
CELLULAR ORGANIZATION AND STRUCTURE
OF ROOTS
Roots develop in a largely continuous fashion from controlled cell proliferation and morphogenesis at the root apex.
In most roots, the cell lineages are easily identified as orga-
’To whom correspondence should be addressed. E-mail schiefelO
umich.edu; fax 313-647-0884.
nized columns (or files) of cells along the length of the root
that result from repeated transverse divisions in the meristematic region. Thus, the cell files resemble a “cellular
assembly line,” with each cell in a file more developmentally
advanced than the one beneath it (Figure 1B).
A basic feature of roots is their radial pattern, which is
made up of concentric rings (or layers) of tissues. The three
fundamental types of tissue are the protoderm (e.g., the epidermis), the ground tissue (e.g., the cortex), and the vascular
tissue. The Arabidopsis root possesses these tissues in a radial array that is striking in its simplicity and regularity (Figures 1A and 1C). In the Arabidopsis primary root, there are
single cell-thick rings of epidermis, cortex, endodermis, and
pericycle tissues surrounding the central stele, with a constant number of eight cells per ring for the cortex and endodermis layers (Dolan et al., 1993).
In many plant species, including Arabidopsis, the files of
root cells can be traced back to specific sets of cells in the
meristematic region, termed initials (von Guttenberg et al.,
1955; Dolan et al., 1993; Scheres et al., 1994). Based on anatomical and labeling studies, each set of initials is thought to
generate a specific tissue(s) by regulated cell divisions (Figures 1 6 and 1D; Dolan et al., 1993; Baum and Rost, 1996).
Together, the entire cluster of initials and the special cells at
the center of the cluster (termed the quiescent center) make
up the “promeristem,” which is believed to constitute the minimal group of root meristem cells (Clowes, 1961).
To trace back further the lineage of the cells that make up
the Arabidopsis root meristem, Scheres et al. (1994) used
anatomical and clonal analyses to develop a “fate map” that
defines the embryonic origin of the root meristem. This work
showed that the promeristem is composed of cells derived
from each of the two cells (apical and basal) that are generated by the first zygotic division of the embryo. The basal cell
gives rise to the quiescent center and columella root cap regions, whereas cells derived from the apical cell give rise to
the remainder of the promeristem. This study also showed
that the cells generating the three basic tissue types are separated by distinct clonal boundaries during embryogenesis
and that the architecture of the root promeristem is formed
by the heart stage of embryogenesis (Scheres et al., 1994;
see Laux and Jürgens, 1997, in this issue).
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central cells
cortical/endodermal initials
Jjjjg^
endodermis
root cap/epidermal initials
columella initials
columella root cap
••
lateral root cap
Figure 1. Cellular Organization of the Arabidopsis Root.
Patterning in Roots
PAlTERNING OF TISSUE TYPES WlTHlN THE ROOT
A fundamental question in root development centers on the
nature of the radial patterning mechanism that defines the
type of tissue that forms within each concentric ring of cells.
The fact that each tissue alises from a specific set of initials
led to the formulation of the “histogen” concept, which posits that each set of meristem initials is programmed to generate cells that will form only one tissue type (Hanstein, 1870).
An alternative view is that positional information is of major
importance in defining the characteristics of the initials and
the fate of their daughters, a model supported by studies of
shoot development (Poethig, 1987, 1997, in this issue).
Recently, an elegant set of laser ablation studies designed
to address this issue was conducted on cells in the Arabidopsis root promeristem (van den Berg et al., 1995). After
ablation, the developmental fates of root cells were analyzed
by tissue-specific marker gene expression. In one experiment, the ablation of quiescent center cells led them to collapse, and the available space became occupied by cells
originating from the vascular initials. These displaced cells
took on characteristics typical of quiescent center cells, as
assessed by their subsequent expression of a columelld
quiescent center marker gene. Similarly, the ablation of cortical cell initials enabled pericycle cells to expand into the
cortical cell spaces, where they initiated the expression of a
cortex rnarker gene. The position-dependent “switch” in the
fate of cells that cross radial tissue boundaries implies that
positional cues rather than lineage-based determinants direct the tissue-type characteristics of the root meristem initials and their descendants.
Additional evidence that the radial pattern of root tissue
types is not strictly controlled by lineage-dependent factors
has been provided by the analysis of the fass and tonneau
(ton) mutants of Arabidopsis (Torres-Ruiz and Jürgens,
1994; Scheres et al., 1995; Traas et al., 1995). The normally
regular pattern of cell division within the root meristem is
disrupted in these mutants, and unorganized cell divisions
lead to an excessive number of cells and cell layers. If lineage information controlled tissue-type patterning, then the
fass and ton roots would be expected to exhibit a disorganized collection of tissue types in inappropriate cell layers.
However, despite the disruption in cell division patterns, fass
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and ton roots possess an organized radial pattern of tissue
types (Torres-Ruiz and Jürgens, 1994; Scheres et al., 1995;
Traas et al., 1995).
Laser ablation studies were also able to provide insight
into the directionality of the positional signals that define the
root tissue types (Van den Berg et al., 1995). When cells located above (proximal to) a cortical initial were ablated such
that contacts with more mature cortical cells were disrupted,
the cortical initial failed to behave properly @e.,it did not undergo characteristic divisions). These observations imply that
cortical initials require information from differentiatingcortical
cells located above them to guide their fate (van den Berg et
al., 1995). Extrapolating further, it is possible that the entire
radial pattern of root tissues is controlled by a mode of “topdown” transfer of information from more mature tissues that
continuously guides the development of the immature cells
and maintains the previously established pattern.
What is the molecular basis of the positional cues and
how are they conveyed to guide radial tissue patterning?
One promising approach that is likely to shed light on this
question is the identificationand analysis of mutants exhibiting abnormal root tissue patterning; such mutants may define genes with a role in the production or propagation of
positional signals. Severa1 radial patterning mutants have
been identified in Arabidopsis by screening for seedlings with
retarded root growth (Benfey et al., 1993; Scheres et al.,
1995). These include the recessive mutants shortroot (shr),
which lacks the endoderrnis tissue; scarecrow (scr), which
lacks one of the two ground tissue layers; gollum (glm),which
alters the organization of vascular tissue and pericycle; and
wooden leg (wol),which alters the vascular tissue.
The comparative analysis of the shr and scr mutants has
been particularly revealing because it suggests that different
processes related to tissue patterning are affected in each
of these mutants. The shr and scr mutants are similar in that
each has a single ground tissue layer, whereas wild-type
roots possess two layers (cortex and endodermis) that are
derived from a common set of initials via an asymmetric division (Figure 1D). Because this single tissue layer in the shr
mutant has cortex characteristics and because endodermal
cells are not generated in the multicellular fass shr double
mutant, the SHR gene may be required to specify endodermis identity (Scheres et al., 1995). By contrast, the single
ground tissue layer in the scr mutant has characteristics of
Figure 1. (continued).
(A) Transverse section of an Arabidopsis root from the late meristematic region. A single layer of lateral root cap cells surrounds the epidermis.
At this stage of development, the two differentiating epidermal cell types can be distinguished cytologically; the eight trichoblasts possess
densely staining cytoplasm compared with the neighboring atrichoblasts. Bar = 10 pn.
(6)Median longitudinal section of an Arabidopsis root apex. The cell files converge to the promeristem region (outlined in red).
(C) Colorized drawing of the transverse section shown in (A).
(D)Colorized drawing of the promeristem region outlined in (6)The
. red lines indicate the planes of division that occur in the initials; white lines
indicate the secondary divisions that occur in the corticaVendoderma1 and the lateral root cap/epidermal initials.
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both cortex and endodermis, and both tissue types are generated in the scr fass double mutant. These data imply that
SCR activity is required for the generation of two distinct
ground tissue layers (i.e., it affects the asymmetric division
of the cortical/endodermal initial) but that it is not involved in
specifying tissue type (Scheres et al., 1995).
Molecular cloning and sequencing of the SCR gene suggest that it may encode a transcription factor and, as predicted from the genetic analyses, that it is expressed in the
developing ground tissue of the embryo and seedling
(Di Laurenzio et al., 1996). As more genes controlling radial
patterning are analyzed at the molecular level, it will be interesting to investigate the interplay between pathways that
control particular cell divisions and those that control the
specification of tissue type.
An interesting finding from the study of the radial patterning mutants is that the defect in each mutant is manifested
in the tissues of the hypocotyl and the embryonic root as
well as the seedling root (Scheres et al., 1995). This implies
that the radial pattern is established during embryogenesis
and that the mutations cause cell division and/or tissue
specification defects that alter radial patterning throughout
the entire embryonic axis. Other evidence for overlap in the
control of hypocotyl and root development comes from the
embryonic pattern mutant monopteros, which displays defects in both of these regions (Berleth and Jürgens, 1993;
see also Laux and Jürgens, 1997, in this issue).
The findings discussed thus far point to a general explanation of root tissue patterning as being guided by tissuespecific signaling of developing cells by more mature cells,
with the initial architecture established by embryonic patterning genes. The patterning of some of the root tissues,
however, cannot be explained by this sort of scheme. For
example, the quiescent center and the columella root cap tissue do not make up a part of the radial pattern; rather, they
represent portions of the apical-basal pattern of seedling
tissues.
To define the mechanisms responsible for programming
these dista1 root tissues and for organizing the root meristem as a whole, mutants have been identified that lack a
functional root meristem (Scheres et al., 1996). This group of
mutants, which includes hobbit and bombadil, possesses a
seedling- or adult-lethal phenotype and exhibits altered development of the hypophyseal cell and/or its derivatives,
emphasizing the importance of this set of embryonic cells in
generating a functional root meristem (Scheres et al., 1996).
Further analysis of these loci is expected to provide new insights into the embryonic signaling events that are responsible for establishing and organizing the root meristem.
PAlTERNlNG OF CELL TYPES WlTHlN ROOT TISSUE
A central goal in developmental biology is to understand
how diverse cell types are generated. More specifically, in
tissues that contain cells with more than one developmental
fate, how do individual cells acquire their distinct identities?
This question is addressed in severa1 of the reviews in this
issue (see, e.g., Fukuda, 1997; Larkin et al., 1997; Nelson
and Dengler, 1997; Poethig, 1997), but the formation of the
root epidermis during root development provides an exceptional model for studying this problem in plants. In most
species, there are only two cell types in the root epidermis:
cells that produce long hairlike extensions (i.e., root hair
cells, which are derived from trichoblasts) and cells that do
not &e., hairless cells, which are derived from atrichoblasts;
Cormack, 1949, 1962; Cutter, 1978). The arrangement of
these two cell types within the epidermis varies in different
plant species. In some plants, there is no apparent pattern of
root hair and hairless cells, and each newly formed epidermal
cell appears to be capable of differentiating into either cell
type (Leavitt, 1904). In other species, including many monocots, epidermal cell fate is linked to an asymmetric cell division, with the smaller daughter cell differentiating into a root
hair cell and the larger daughter cell generating one or more
mature hairless cells (Sinnott and Bloch, 1939; Avers, 1963;
Cutter and Feldman, 1970). In a third group of plants, which
includes Arabidopsis and other members of the Brassicaceae, a distinct position-dependent pattern of epidermal
cell types is generated (Cormack, 1935; Bunning, 1951;
Dolan et al., 1994; Galway et al., 1994). In this case, trichoblasts form in the crevice between underlying cortical cells
(outside an anticlinal cortical cell wall), whereas atrichoblasts
develop over a cortical cell (outside a periclinal cortical cell
wall; Figures 2A and 26).
The simple correlation between cell position and cell-type
differentiation in the epidermis of Brassica species implies
that lateral signaling events between cortical and epidermal
cells help to define cell identity. At what point during epidermis development does this signaling take place? Trichoblasts and atrichoblasts exhibit cytological differences
within the meristematic region of the root, which indicates
that cell-type specification begins at an early stage of development (Figure 1 A Cormack, 1962; Cutter, 1978; Dolan et
al., 1994; Galway et al., 1994). However, it appears that developmental plasticity is also a feature of root epidermis cell
differentiation. Indeed, three different experimental approaches show that epidermal cells that have embarked on
one differentiation pathway can switch to the alternate pathway at a relatively late stage. These analyses include surgical experiments in which atrichoblasts can be induced to
form root hairs when they are physically separated from the
underlying cortical cells (Bunning, 1951), anatomical studies
in which trichoblast daughter cells from rare longitudinal divisions develop as hairless cells (F. Berger, G. Hung, L.
Dolan, and J.W. Schiefelbein, unpublished results), and
pharmacological experiments in which hormone treatments
induce root hair production on atrichoblasts (Tanimoto et al.,
1995; Masucci and Schiefelbein, 1996).
Arabidopsis mutations that disrupt the normal pattern of
epidermal cell types have also been identified. Two of these
Patterning in Roots
patterning mutants, transparent testa glabra (ttg) and
glabra2 @/2),possess root hairs on essentially every root
epidermal cell. This implies that the role of the TTG and GL2
genes is either to promote hairless cell differentiation or to
repress root hair cell differentiation (Galway et al., 1994;
DiCristina et al., 1996; Masucci et al., 1996). Although the
TTG gene has not been cloned, ttg mutations can be functionally complemented by expression of the maize R cDNA
under the control of the strong cauliflower mosaic virus 35s
promoter (Lloyd et al., 1992; Galway et al., 1994). These
data indicate that TTG is or activates a homolog of R, which
encodes a protein related to basic helix-loop-helix transcriptional activators (Ludwig et al., 1989; see also Larkin et al.,
1997, in this issue). The GL2 gene encodes a homeodomain
protein (Rerie et al., 1994) and is preferentially expressed in
the differentiating hairless epidermal cells within the meristematic and elongation regions of the root (Figures 2C and
2D; Masucci et al., 1996). Thus, the emerging picture is that
TTG and GL2 may encode transcription factors that are expressed in a cell position-dependent manner to specify the
hairless cell type in the root epidermis. Furthermore, because GL2 affects only a subset of the hairless cell characteristics that are affected by TTG (Galway et al., 1994;
Masucci et al., 1996), TTG may act at an early stage to regulate the expression of GL2 and other as yet unidentified
genes (Figure 3).
The TTG and GL2 genes are known also to affect trichorne (shoot epidermal hair) formation in Arabidopsis,
which provides evidence for overlap in the genetic control of
epidermis development in the shoot and root (Galway et al.,
1994; Marks, 1994; Masucci et al., 1996; see Larkin et al.,
1997, in this issue). This outcome was unexpected because
the cell-type specification mechanisms operating in these two
tissues appear to be different. In the root epidermis, cell-type
specification appears to rely on the positional relationshipbetween epidermal cells and underlying cortical cells, whereas
in the shoot epidermis, cell-type specification appears to be
based on the sensing of trichome density. A further interesting aspect of TTG and GL2 is that they affect epidermal hair
formation in opposite ways in root and shoot organs; they are
required for the formation of hairless cells in the root and for
hair-bearing (trichome) cells in the shoot. Although the situation is unclear at present, it is possible that TTG and GL2 encode general epidermis transcription factors that have been
recruited to play distinct roles in cell-type specification in
shoots and roots during angiosperm evolution.
The analysis of another group of Arabidopsis mutants has
been instrumental in linking root hair formation to the action
of plant hormones, particularly ethylene and auxin. Mutations affecting the CONSTITUTIVE TRIPLE RESPONSEl
(CTR7) locus, which encodes a Raf-like protein kinase proposed to negatively regulate the ethylene signal transduction pathway (Kieber et al., 1993), cause root hairs to form on
epidermal cells that normally are hairless (Dolan et al., 1994).
The hairless root phenotype of the dwad (dwf; auxin-resistant)
and auxin resistant2 (a“
auxin-,
;
ethylene-, and abscisic
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acid-resistant) mutants implicate auxin as another possible
regulator of root hair formation (Mizra et al., 1984; Wilson et
al., 1990). In addition, the hairless phenotype of the root hair
defective6 (rhd6) mutant can be suppressed by the inclusion
of 1-arnino-cyclopropane-1-carboxylic acid (ACC; an ethylene precursor) or indole-3-acetic acid (IAA; an auxin) in the
growth media, further implicating ethylene and auxin action in
root hair initiation (Figure 3; Masucci and Schiefelbein, 1994).
Finally, pharmacological studies with wild-type Arabidopsis
seedlings show that root hair formation is enhanced by
treatment with ACC (Tanimoto et al., 1995) and reduced by
treatment with aminoethoxyvinylglycine (AVG), which is an
ethylene biosynthesis inhibitor (Masucci and Schiefelbein,
1994; Tanimoto et al., 1995), or with Ag+, which is an inhibitor
of ethylene perception (Tanimoto et al., 1995).
Given the involvement of hormones in epidermal cell differentiation, one attractive possibility is that ethylene and/or
auxin may act as a diffusible signal responsible for epidermal
cell-type patterning. According to this notion, TTG and GL2
may represent downstream transcription factor genes that
are regulated by the hormones in a cell position-dependent
manner. To test this possibility, the relationship between the
TTGIGL2 pathway (negative regulators of root hair cell differentiation) and the ethylene/auxin-related pathway (positive
regulators of root hair cell differentiation) has been analyzed
(Masucci and Schiefelbein, 1996). By contrast with the above
model, the results from epistasis tests and GL2 promoterreporter gene analyses indicate that the ethylene/auxin pathway does not regulate the TTGIGL2 pathway. In addition,
studies of the developmental timing of the hormone effects
indicate that ethylene and auxin pathways can promote root
hair outgrowth after epidermal cell-type characteristics have
developed (Masucci and Schiefelbein, 1996).
Taken together, the results suggest that the TTGIGL2 pathway acts upstream of or independently from the ethylene/
auxin pathway to define the pattern of cell types in the root
epidermis (Figure 3). lnteresting aspects of this model are
the implications that all newly formed epidermal cells in the
Arabidopsis root may initially have the capacity to differentiate into root hair cells and that the action of the TTGIGL2
pathway is required to induce the hairless cell fate (i.e., a
root hair cell may represent the “default” fate). An important
future goal is to identify additional components of the epidermal cell patterning mechanism, particularly regulators of
the TTG and GL2 genes.
CONTROL OF MORPHOGENESIS IN ROOTS
The molecular mechanisms responsible for regulating the
structure and shape of plant organs and their component
cells are largely unknown. Because roots exhibit a regular
and simple structure and their development is experimentally accessible, they have been widely used in studies of organ and cell morphogenesis. The defined complement of
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A
C
CJ
f.
Epidermis
Cortex
Figure 2. Cell-Type Determination in the Epidermis of the Arabidopsis Root.
Patterning in Roots
cell types and cell shapes and the ease with which genetic
screens and molecular cloning experiments can be conducted make the Arabidopsis root a particularly useful system for these studies.
One of the interesting issues related to morphogenesis in
plants is the relative contribution of cell division to the size
and shape of organs (see also Jacobs, 1997; Poethig, 1997,
in this issue). lnsight into this issue has been obtained from
the analysis of roots from transgenic Arabidopsis that overexpress a mitotic cyclin gene, which normally acts as a key
regulator of cell division (Doerner et al., 1996). These transgenic roots exhibit an increased cell division rate, which
leads to increases in cell number and organ growth &e.,
longer roots) but causes no significant changes in meristem
organization or root morphology (Doerner et al., 1996). Similarly, the basic morphology of Arabidopsis roots is not altered by the application of redox agents that stimulate or
inhibit cell proliferation (Sánchez-Fernández et al., 1997),
nor is it altered in transgenic plants expressing modified versions of a cyclin-dependent kinase (Hemerly et al., 1995).
These observations imply that the form of a plant organ may
be largely insensitive to changes in cell number, which supports the concept of a higher order regulation of organ
shape, rather than control at the leve1 of the cell (Kaplan and
Hagemann, 1991).
Although cell proliferation may not represent a primary
regulator of organ morphology, some cell division activity is
obviously required for organ formation. This point has been
clearly emphasized by the study of the rootmeristemless
(rml7 and rm12) mutations of Arabidopsis (Cheng et al.,
1995). Embryogenic root development is normal in these mutants, but cell divisions in the postembryonic root are lacking
or greatly reduced, which leads to exceedlingly small roots
(Cheng et al., 1995). These analyses suggest that the RML
genes are required to activate cell division in the apical cells
of roots that have reached a critical size. An interesting future
goal is to determine whether the RML proteins act only as a
"trigger" to instigate root meristem activity or whether they
are required continuously to maintain (and perhaps to regulate) cell proliferation in the root meristem.
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TTG
1
GL2
RHD6
ethylene/auxin-regulated
root hair outgrowth
Figure 3. Tentative Model for the Control of Cell-Type Differentiation in Arabidopsis Root Epidermis.
The TTG and GL2 genes are proposed to act on developing epidermal cells in a specific position (those located over cortical cells) to
inhibit the action of the RHD6 gene product and the ethylenelauxin
pathway, thereby preventing these cells from adopting the root hair
fate and generating the wild-type pattern of root hair and hairless
cell types. Evidencefor this model is summarized in the text. Arrows
indicate positive action; T-bars indicate negative regulation; question marks indicate unclear relationships.
The ongoing analysis of severa1 additional root morphology mutants is contributing to our understanding of the genetic and molecular mechanisms that regulate organ
morphogenesis in Arabidopsis (Baskin et al., 1992, 1995;
Benfey et al., 1993; Holding et al., 1994; Hauser et al., 1995).
In many of these mutants, the abnormal root morphology is
Figure 2. (continued).
(A) Schematic drawing of an Arabidopsis root in surface view in the root hair developmentalzone. Files of epidermal cells consist entirely of root
hair cells or hairlesscells.
(B) Schematic drawing of a transverse section from the mature portion of an Arabidopsis seedling root. Root hair cells are located in the clefts
between adjacent cortical cells. Only three root hairs are shown in this figure because the length of epidermal cells generally prevents all eight
root hair projectionsfrom being visible in a single transverse section.
(C)Spatial expression patternof the GL2 promoter-p-glucuronidase reporter gene fusion construct in the root apex of a 4-day-old seedling. The
GL2 promoter directs expression preferentially in a cell file-specific manner in the epidermis. Bar = 20 pm.
(D) Expression pattern of the GL2 promoter-P-glucuronidase reporter gene fusion construct in a transverse root section from the late meristematic region of a 4-day-old seedling. The GL2 promoter directs expression preferentially in epidermal cells located over the cortical cells (atrichoblasts). Bar = 1O pm.
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dueto excessive cell expansion that is largely restricted to a
particular tissue. For example, the major morphological abnormality in the cobra and root epidermal blebbing mutants is
excessive swelling of the epidermis, the sabre mutant displays abnormal expansion primarily in the cortex, and the
lion’s tail mutation principally causes abnormal cell enlargement in the stele (Baskin et al., 1992; Benfey et al., 1993;
Hauser et al., 1995). These results indicate that organ morphogenesis is not likely to be governed by a single overriding
pathway but probably relies on the independent control of the
orientation and extent of cell expansion in different tissues.
Studies of Arabidopsis root morphology mutants have also
called into question the long-held view that the orientation of
microtubules plays a primary role in influencing the orientation of cell expansion (Baskin et al., 1994; Hauser et al., 1995).
The cloning of some of the root morphogenesisgenes is beginning to shed light on the molecules involved in coordinated
organ growth. The SABRE gene product encodes a nove1 protein that appears to be required to counter the effects of ethylene during radial cell expansion (Aeschbacher et al., 1995).
The RHD3 gene, which is required for appropriatecell enlargement throughout the Arabidopsis plant, encodes a protein
with putative GTP binding motifs, and RHD3-related proteins have been identified in yeast, rice, and the parasite
Entamoeba histolytica (Wang et al., 1997). Together, these
studies show that despite the complexity of organ and cell
morphogenesis, the root is a suitable subject for untangling the interrelated molecular mechanisms that control
these processes.
A related feature of root development emphasized by
these studies is the developmental plasticity exhibited by
cells or tissues. There are m w numerous examples in which
root cells that begin to differentiate in one manner switch to
a different program upon a change in their position or in response to externa1 factors. This plasticity may be important
to ensure proper cell and tissue patterning in the event of
rare abnormal cell divisions or organ damage.
An important future goal in studies of root patterning is to
identify the molecular nature of the positional cues. Achieving this objective will probably require more sophisticated
genetic screens and the use of tissue- or cell-specific markers to identify pattern mutants rather than structural (e.g.,
cell division) or morphological (e.g., cell expansion) mutants.
For example, it would be particularly informative to identify
and analyze mutants that possess roots with a normal number of cell layers but inappropriate tissue types occupying
one or more of those layers. With regard to the molecular
basis for the positional cues, there are many possibilities.
Positional information may be communicated by a cell wall
component(s) (Berger et al., 1994; Freshour et al., 1996), by
diffusible signals that act symplastically (Duckett et al., 1994),
or by some other, as yet unknown, signaling mechanism.
CONCLUDING REMARKS
and for providing manuscripts before publication.
A general theme that has emerged from studies of root patterning is that positional cues play a critical role in controlling the identity of the component tissues and cells. This
conclusion may be somewhat unexpected, given the strict
correlation between cell lineage and cell/tissue types in the
roots of some species, such as Arabidopsis. Nonetheless,
the examples of genetic and physical manipulations of root
development outlined in this review show that the history of
a cell or tissue is less important than is its position in determining its developmental fate. Thus, it may be that the strict
correlation between cell lineage and cell-type patterns found
in some roots (e.g., Arabidopsis) is simply the result of a regular array of cell divisions overlaid on a defined set of positional cues. Furthermore, the fact that the roots of some
species lack such a lineage-correlated pattern of cell and
tissue types may reflect a difference in their meristem structure (e.g., different cell division patterns) rather than a difference in the basic patterning mechanism, because the same
tissues are arranged in basically the same way in all roots.
Thus, a common set of positional cues that transcends taxonomic boundaries may be used to direct the patterns of tissue and cell types during root development.
ACKNOWLEDGMENTS
We thank our colleagues in the root development community, including Philip Benfey, Fred Berger, Liam Dolan, Chen-Yi Hung, Jocelyn
Malamy, Mike May, and Ben Scheres, for their helpful discussions
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Patterning in Roots
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