Download Evolution and genetics of root hair stripes in the root epidermis

Journal of Experimental Botany, Vol. 52, Roots Special Issue,
pp. 413±417, March 2001
Evolution and genetics of root hair stripes
in the root epidermis
Liam Dolan1 and Silvia Costa
Department of Cell Biology, John Innes Centre, Norwich NR4 7UH, UK
Received 10 June 2000; Accepted 9 September 2000
Abstract
Root hair pattern develops in a number of different
ways in angiosperm. Cells in the epidermis of some
species undergo asymmetric cell divisions to form
a smaller daughter cell from which a hair grows, and
a larger cell that forms a non-hair epidermal cell.
In other species any cell in the epidermis can form
a root hair. Hair cells are arranged in files along the
Arabidopsis root, located in the gaps between underlying cortical cell files. Epidermal cells overlying a
single cortical cell file develop as non-hair epidermal
cells. Genetic analysis has identified a transcription
factor cascade required for the formation of this
pattern. WEREWOLF (WER) and GLABRA2 (GL2) are
required for the formation of non-hair epidermal cells
while CAPRICE (CPC) is required for hair cell development. Recent analyses of the pattern of epidermal
cells among the angiosperms indicate that this
striped pattern of cell organization evolved from
non-striped ancestors independently in a number of
diverse evolutionary lineages. The genetic basis for
the evolution of epidermal pattern in angiosperms
may now be examined.
Key words: Root epidermis, root hair stripes, evolution,
genetics, epidermal pattern.
Cellular patterning diversity in the root
epidermis
The root epidermis of most angiosperms is composed
of hair cells and non-hair cells that develop in de®ned
patterns. Hairs are tip-growing extensions of epidermal
cells that play a variety of functions including anchorage,
water absorption, nutrient uptake etc. Some species have
lost the ability to make root hairs while in other species
1
every cell in the epidermis forms a hair. Exceptional
root hairs are found in the Commelinaceae (which
includes Tradescantia) where they may originate in the
cortex (Pinkerton, 1936). Hairs may fail to develop in
older roots of many species with the development of
symbiotic mycorrhizae, or their formation may be inhibited by environmental factors. For example, the roots of
Elodea canadensis form hairs when in physical contact
with a soil substrate, but roots are hairless when the plant
is free ¯oating (Cormack, 1937). Nevertheless, hairs are
generally epidermal in origin and their patterning re¯ects
their mode of development.
The patterns of cellular organization in the root
epidermis have been described (Leavitt, 1904; Cormack,
1947; Clowes, 2000). The main types are summarized
here.
Alternate patterns resulting from asymmetric
cell divisions
Asymmetric cell division in an epidermal cell gives rise
to a large cell (atrichoblast) that develops into a hairless
epidermal cell and a shorter `specialized' cell that forms
a root hair (trichoblast). This pattern of development is
widespread among monocot taxa but restricted to a small
group of dicots, the paleoherbs (such as water lillies),
which recent DNA-based phylogenies have shown to
be closely related to the monocots (Chase et al., 1993).
Among the monocots there are at least two distinct
modes of development associated with asymmetric cell
division. In the ®rst case, the daughter cell nearest the
meristem forms the root hair (Vd in the Clowes, 2000,
notation). Root hairs of the Alismataceae, Hydrocharitaceae, Araceae, Commelinaceae, Typhaceae, Zingiberaceae, Haemodoraceae, and Pontederiaceae develop in
this way. In the second mode, the daughter cell furthest
from the meristem (Vp) forms a hair cell. The latter
pattern is found among the Restionaceae, Juncaceae,
To whom correspondence should be addressed. Fax: q44 1603 456844. E-mail: [email protected]
ß Society for Experimental Biology 2001
414
Dolan and Costa
Cyperaceae, and Poaceae. These families constitute a
major derived clade within the monocots (Chase et al.,
1995). It is therefore possible that the Vp asymmetric
mode of epidermal development arose once in a common
ancestor to this group. Examination of epidermal pattern
in key groups can be used to test this hypothesis.
Random pattern
Root hairs can develop in epidermal cells in any position, relative to the underlying cortical cells, and morphologically distinguishable trichoblasts do not form. This
pattern of hair cell development is prevalent among the
dicots and is found in many monocot taxa. The proportion of cells that develop root hairs depends on environmental factors (Cormack, 1947). Hairs may develop on
every epidermal cell, no cells or on a subset of cells
(Cormack, 1935; Clowes, 2000).
Striped pattern
Plants with the striped pattern develop hairs in cell
®les interspersed with ®les of non-hair cells (Fig. 1).
Cell ®les (T in Fig. 2) overlying anticlinal cortical cell
walls (ACCWs) form root hairs and cells overlying
periclinal cortical cell walls (PCCW) (A in Fig. 2) form
non-hair epidermal cells. The cells over the ACCWs
are shorter and less vacuolated than cells overlying the
PCCW because of their slightly shorter cell cycle time.
This difference in cell size between the two cell types
is visible in the meristem and maintained through
the mature region of the root (Fig. 3). This pattern
was ®rst described for members of the Brassicaceae
(Cormack, 1935; BuÈnning, 1951). It has recently been
described in other families including the Capparaceae,
Resedaceae, Caryophylaceae, Portulacaceae, Aizoaceae,
Salicaceae, Euphorbiaceae, Boraginaceae, Hydrophyllaceae, and Acanthaceae (Clowes, 2000). Interestingly,
Onagraceae and Urticaceae contain species with striped
and non-striped epidermal patterns (Clowes, 2000).
Evolution of the striped pattern
Analysis of the pattern of epidermal cells among diverse
groups of angiosperms indicates that the striped pattern
of hair cell organization evolved independently in a
number of lineages (Clowes, 2000) (Fig. 4). The striped
pattern evolved at least once within the Capparales
(the order that includes the Brassicaceae) after the
emergence of the Tropaeolaceae. Within the Capparales,
the Brassicaceae, Capparaceae, and Resedaceae exhibit
the striped pattern. The Capparaceae and Resedaceae
are sister groups of the Brassicaceae and the three
taxa form a monophyletic group (Rodman et al., 1998).
The striped pattern has also been observed in the
Limnanthaceae, but the epidermal cell patterns of
groups more derived than the Limnanthaceae such as
Fig. 1. Cellular organization of the Arabidopsis root. Scanning electron
micrograph showing the organization of epidermal cell types in the
epidermis. Orange cells are atrichoblasts and non-hair cells. Blue cells
are trichoblasts and root hairs.
Fig. 2. Schematic representation of a transverse section through a root
in the meristematic zone, showing the position of trichoblasts (T) and
atrichoblasts (A) relative to underlying cortical cells. Yellow indicates
the position of lateral root cap cells and blue cells are cortical cells.
Fig. 3. Meristematic cellular organization of wild-type and mutant
roots. (A) Wild-type root with ®les of shorter trichoblasts in the ACCW
position (arrowhead) and atrichoblasts in the PCCW position (arrow).
(B) Cellular organization of the epidermis of the root of a plant
homozygous for the cpc mutation showing that cells are more
atrichoblast-like in morphology. (C) Cellular organization of the
epidermis of the root of a plant homozygous for the wer mutation
showing that cells are more trichoblast-like in morphology. The
arrowhead indicates the location of the cell ®le located in the ACCW
position and the arrow indicates the location of cells in the PCCW
position. Cells were stained in propidium iodide which ¯uorescently
stains the intercellular spaces, and imaged with a confocal microscope.
Images are presented in reverse contrast to enhance resolution.
Root hair pattern in angiosperms
415
Fig. 4. The distribution of species with striped (S) and non-striped (NS) epidermis in the Capparales. The epidermal pattern has not been determined
in the Tovariaceae. The phylogeny is based on Rodman et al. (Rodman et al., 1998). Branch lengths are not indicative of distance.
the Gyrostemonaceae, Tovariaceae, Pentadiplandraceae,
Koeberliniaceae, Bataceae, and Salvadoraceae, have not
yet been described. Root hairs can form in any position
in the Tropaeolaceae, and this pattern of development
is therefore considered ancestral. The simplest explanation is that the striped pattern evolved once among the
Capparales in a taxon ancestral to Limnanthaceae but
more derived than the Tropaeolaceae. Nevertheless, the
possibility cannot yet be ruled out that the striped pattern
arose more than once in the Capparales. Characterization
of root hair development in other taxa within the
Capparales is required to distinguish between single and
multiple origin models.
This phylogenetic analysis suggests that the derived,
striped pattern evolved from an ancestral non-striped
state among the Capparales and independently in a number of other dicot families. Alternatively, it is possible
that the striped pattern is ancestral and was progressively
lost in many clades. The prevalence of the random pattern
throughout the whole of the ¯owering plants (monocots
and dicots) would suggest that the random patterning
is the ancestral condition and it is more parsimonious to
suggest that the striped pattern has arisen independently
in many plant groups. The development of more reliable
phylogenies for these groups, and further characterization
of the organization of root epidermal cells in key groups
identi®ed by these phylogenies, will be instructive in
distinguishing between these alternatives.
If the striped pattern evolved a number of times,
independently, it will be instructive to determine if the
same regulatory genes were involved in morphological
change in each case. The characterization of genes
required for the development of pattern in the Arabidopsis
epidermis is providing useful tools to begin such an
analysis.
Cellular organization of the Arabidopsis root
epidermisÐa model system
The Arabidopsis root epidermis consists of 16±24 cell ®les
and is derived from a ring of 16 initials that also gives
rise to lateral root cap cells (Figs 1, 2, 3A; Dolan et al.,
1993). Variation in the number of cell ®les in the
epidermis occurs as a result of rare longitudinal anticlinal
divisions that take place in trichoblasts (Berger et al.,
1998a, b). At the end of the meristematic zone (when cell
division ceases), lateral root cap cells die and the epidermis emerges at the root surface. The epidermal cells
undergo rapid elongation and initiate hairs when elongation (in the direction of the long axis of the root) ceases.
Root hairs develop from trichoblasts that are arranged
in ®les overlying the ACCWs (Fig. 2). At maturity hair
cells are shorter than the adjacent non-hair epidermal
cells and this difference in length can be traced back to
the meristem where the two cell types can be easily
distinguished.
Laser microsurgical experiments indicate that positional information directs cell fate in the epidermis
(Berger et al., 1998a). It is likely that this information
is in place by the torpedo stage of embryogenesis and
maintained in the developing meristem during postembryonic growth of the root. A clonal analysis of
epidermal development shows that the positional information may be located in the cell wall, indicating that
protoplast±cell wall interactions are necessary for the
establishment of cell pattern in the root (Berger et al.,
1998a). The molecular basis of this information remains
to be de®ned.
A cascade of transcription factors regulated
the development of epidermal pattern
Genetic analysis of epidermal development in Arabidopsis
has identi®ed genes required for the development of
the characteristic striped pattern of hair cell development.
To date, a cascade of transcriptional regulators has been
identi®ed that speci®es the identities of cells in the
epidermis. CPC (CAPRICE) and WER (WEREWOLF)
are the earliest acting genes in this pathway and both
are required for cell speci®c transcription of GLABRA2
(GL2), which encodes a homeodomain protein expressed
in atrichoblasts required for the development of non-hair
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Dolan and Costa
cells (Di Cristina et al., 1996; Masucci et al., 1996; Wada
et al., 1997; Lee and Schiefelbein, 1999).
Plants homozygous for loss of function mutations
in WER have a hairy phenotype, i.e. all epidermal cells
develop root hairs, suggesting that WER is a positive
regulator of non-hair cell development. Epidermal
cells in the meristem of plants homozygous for wer are
indistinguishable morphologicallyÐthere are no clearly
differentiated trichoblasts and atrichoblasts (Fig. 3C),
indicating that WER activity is required for the repression
of hair cell identity early, in the meristem, before root
hairs have formed. The WER protein is a member of
the MYB family of transcriptional regulators, suggesting
that WER is required for the transcription of genes
involved in non-hair cell development and is expressed
in non-hair cells.
CPC, on the other hand, mutates to a hairlessu
decreased hair cell density phenotype. The differences
between atrichoblasts and trichoblasts are reduced in
plants homozygous for cpc mutation (Fig. 3B). This
suggests that CPC is either a positive regulator of hair
cell development or a negative regulator of non-hair cell
development, i.e. it promotes the development of root
hair cells. CPC is also a member of the MYB family
of transcriptional regulators but it lacks the transcriptional activator domain, which suggests that it may act as
a transcriptional repressor, repressing genes that promote
non-hair cell identity.
A model has been proposed in which the ratio of
the levels of WER and CPC can specify epidermal cell
identity (Lee and Schiefelbein, 1999). Cells with high
WER:CPC levels develop as non-hair cells and those
with lower ratios develop as root hair cells (Lee and
Scheifelbein, 1999). A possible target for the WERuCPCmediated regulation is the GL2 gene that encodes a
homeodomain, transcriptional regulator. GL2 mutates
to a recessive, hairy phenotype, suggesting that GL2 is
a transcriptional regulator required for the development of the non-hair cell (Di Cristina et al., 1996;
Masucci et al., 1996).
Possible roles for CPC and WER in the
evolution of hair cell pattern
The distribution of hair cell patterns among the angiosperms indicates that the striped pattern characteristic
of the BrassicaceaeuCapparaceaeuResedaceae is derived
from an ancestral state in which hairs could develop
in any epidermal position relative to the underlying
cortex. It is possible that changes in gene expression
of key regulatory genes accompanied the evolution of
the striped trait. At least two hypotheses (there are
others) are proposed here to explain the evolution of
pattern in the epidermis of the angiosperm root.
(1) CPC and WER are expressed in every epidermal cell in
the ancestral root. Root hairs develop in each location
(over PCCW and ACCW) in the ancestral root and WER
and CPC are not involved in the speci®cation of cellular
identity in the ancestral species. It is proposed that the
expression of these genes could have become restricted to
particular cell types (i.e. in `stripes') at the same time as
acquiring the ability to promote non-hair cell fate in cells
over PCCW.
(2) CPC and WER are already exclusively expressed
in the epidermal cells located over the PCCW in the
ancestral type. These genes then acquired the ability to
transcriptionally activate genes that repress hair cell fate
in cells in this location. The striped pattern of gene
expression therefore already existed in the ancestral type
and the cell fate mechanism co-opted the pre-existing
pattern.
Perspectives
The recent deciphering of the molecular basis of the
patterning of cell types in the root epidermis provides
a mechanistic understanding of the development of
pattern at the cellular level in plants. This information
can now be used to examine the roles of key regulatory
genes in the evolution of epidermal patterns in angiosperms. To meet this challenge, more detailed information is needed about the cellular patterns in the root
epidermis in species from a number of key taxa. For
example, if the epidermal cell patterns of some key
families within the Capparales were known, it could
more on®dently be stated how many times the striped
pattern evolved in this group of plants. Having identi®ed
important regulatory genes in Arabidopsis it is now
important to identify orthologues in other species with
different patterns of epidermal development. This will
be instructive in understanding the role of these regulatory genes in the evolution and development of cell
pattern in these other species. Similarly more detailed
knowledge of the patterns of epidermal cells in the
commelinoids (grasses, rushes, sedges etc.) will be instructive in terms of how many times the Vp pattern of cell
division occurred. Understanding the molecular mechanisms underpinning the development of epidermis
with asymmetric divisions is still some way off, but the
analysis of root epidermal development in model monocot genetic systems will be instructive in this respect. This
combined evolutionary and developmental analysis will
offer insights into the molecular mechanism underpinning
morphological change during evolution.
Acknowledgements
We are grateful to Jackie Nugent for comments on the manuscript and Ned Friedman for helpful comments and guidance.
Root hair pattern in angiosperms
We owe much to two very patient referees and Keith Skene for
help in putting a comprehensible manuscript together. We are
grateful to the BBSRC and the Gatsby Foundation for funding research in our laboratory. We are grateful to the Nottingham
and Ohio Arabidopsis stock centres for seed stocks.
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