Mechanisms of stomatal development: an
Anne Vatén1,2and Dominique C Bergmann1,3*
Plant development has a significant postembryonic phase that is guided heavily by interactions between the plant
and the outside environment. This interplay is particularly evident in the development, pattern and function of
stomata, epidermal pores on the aerial surfaces of land plants. Stomata have been found in fossils dating from
more than 400 million years ago. Strikingly, the morphology of the individual stomatal complex is largely
unchanged, but the sizes, numbers and arrangements of stomata and their surrounding cells have diversified
tremendously. In many plants, stomata arise from specialized and transient stem-cell like compartments on the leaf.
Studies in the flowering plant Arabidopsis thaliana have established a basic molecular framework for the acquisition
of cell fate and generation of cell polarity in these compartments, as well as describing some of the key signals and
receptors required to produce stomata in organized patterns and in environmentally optimized numbers. Here we
present parallel analyses of stomatal developmental pathways at morphological and molecular levels and describe
the innovations made by particular clades of plants.
Keywords: Stomata, Plant evolution, bHLH transcription factors, Arabidopsis, Maize, Physcomitrella, Rice, Ligand
receptor signaling, Cell polarity, Asymmetric cell division
Introduction to stomata and stomatal pattern
Plants conquered land more than 400 million years
ago. In the fossil record, the appearance of these pion-
eer species is contemporaneous with the appearance
of structures on their surfaces called stomata. Each
stoma (plural, stomata) consists of paired epidermal
guard cells, a pore between them and an airspace in
the photosynthetic mesophyll tissue subtending it. The
function of stomata is to regulate gas exchange
between the plant and its surroundings. On short
timescales (minutes to hours), the opening and closing
of the stomatal pore by turgor-driven changes in guard
cell shape is a key regulatory step in maintaining water
and carbon dioxide balance. Work from many labora-
tories has defined the intracellular signal transduction
cascades that mediate changes in pore size in response
to hormone and environmental signals .
The current view is that stomata arose only once during
evolution . In early land plants, stomatal density was
low . During intervening millennia, the stomatal density
(SD, number of stomata/unit leaf area) increased, probably
in response to reduced aerial CO2concentration . The
stomatal complex has been fine-tuned by several innova-
tions including recruitment of neighboring subsidiary cells
to facilitate stomatal opening/closing, relocation of stoma-
tal complexes under protective epidermal cells and incorp-
oration of multiple asymmetric cell divisions in precursors
to create a variety of stomatal distributions. Despite the
variation, the basic core structure has remained un-
changed: two guard cells flank the stomal pore. In nearly
all species, two stomata are separated at least by one non-
stomatal cell, an arrangement thought to be essential for
efficient opening and closing. Stomata are located on aerial
organs including leaves, stems, flowers, fruits and seeds
and they develop gradually during organ growth such that
young organs have fewer total stomata than mature
organs, though SD often decreases as the neighboring epi-
dermal cells expand during maturation. The frequency
and positioning of stomata are organ and species-specific
characters, but are also affected by environmental factors.
* Correspondence: email@example.com
1Department of Biology, Stanford University, Stanford, CA 94305-5020, USA
3Howard Hughes Medical Institute, Stanford, USA
Full list of author information is available at the end of the article
© 2012 Vatén and Bergmann; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the
Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
Vatén and Bergmann EvoDevo 2012, 3:11
Paleobotanical analyses utilizing the fossils of the early
land plants and their currently living descendants have
been combined with phylogenetic analyses to address
the origins of stomata (Figures 1 and 2A). Liverworts,
mosses and hornworts comprise the bryophytes, a basal
land plant group. Liverworts do not have stomata; gas
exchange is facilitated by epidermal air pores, structures
whose development and morphology differ from stomata.
Stomata are found in mosses and hornworts, making it
likely that liverworts diverged from other bryophytes be-
fore the origin of stomata. Intriguingly, in extant bryo-
phytes, both guard cell morphology and regulation of pore
aperture can closely resemble higher plant stomata.
In the most recently derived plant group (angiosperms,
or flowering plants) there is a dedicated epidermal lineage
that produces stomata. In dicot plants, such as the research
model Arabidopsis , these lineages are initiated from
various sites on the leaf (Figure 3). In each lineage, a com-
mitted protodermal cell called the meristemoid mother cell
(MMC) divides asymmetrically to give rise to a larger sto-
matal lineage ground cell (SLGC) and smaller meristemoid.
The meristemoid undergoes one to three asymmetric divi-
sions (amplifying divisions) before it differentiates into a
guard mother cell (GMC). Later, SLGCs can also divide
asymmetrically and produce more meristemoids (spacing
divisions). The GMC divides symmetrically to create two
guard cells, and in some species the GMCs recruit neigh-
boring subsidiary cells. These subsidiary cells can provide
mechanical assistance and a source of ions required for
guard cell movement. Amplifying and spacing divisions and
subsidiary recruitment all require cell to cell communica-
tion and together they contribute to pattern. The frequency
with which cells participate in these division types can be
modified to yield the extraordinary diversity of stomatal
patterns seen in nature  (Figure 2B, 2D and 2F).
Monocots exhibit a strong base to tip gradient of leaf dif-
ferentiation with stomata-producing cell lineages forming
at the base of the leaf. Asymmetric cell divisions produce
GMCs without prior transit through a self-renewing meris-
temoid stage (Figure 4). Protodermal cells in files flanking
the GMC polarize towards the GMC and divide asymmet-
rically giving rise to subsidiary cells. After this, the GMC
divides to produce guard cells that exhibit a novel flattened
or dumbbell-shaped morphology (Figure 2G). In monocots
like Tradescantia, overall stomatal pattern can be refined
when GMCs change fate and differentiate into epidermal
cells . This fate change is dependent on distance from
neighboring stomata, suggesting an inhibitory communica-
Given the current patterns and developmental processes
associated with stomata of flowering plants, what were
their origins? In the simplified ontogeny seen in some
mosses, stomatal development involves a single asymmet-
ric cell division giving rise directly to a GMC (Figure 4).
This GMC may not divide completely, as seen in Funaria
hygrometrica where two guard cell nuclei are separated by
an incomplete cell wall , or may break the spacing rule
First specialized structure for gas exchange?
(Group IA and III bHLHs, EPFs)
(Divergence of group IA bHLHs)
450 420390380 360150
Figure 1 Divergence of major land plant lineages and appearance of stomatal characteristics. Phylogenetic tree of extant land plants
indicating positions of major innovations in the evolution of stomata, following Ruszala et al. . Those in brackets indicate predicted appearance
of stomatal development regulatory genes. Numbers on the x-axis refer to multiples of millions of years.
Vatén and Bergmann EvoDevo 2012, 3:11
Page 2 of 9
Figure 2 Representative stomatal complexes and patterns from different species. (A) Scanning electron micrograph (SEM) of Silurian fossil
stoma displaying common morphology. Scale bar, 20 μm . (B) SEM of moss Bryum capillare sporangium with stomata visible on the lower half.
Scale bar, 600 μm . (C) SEM of moss Bryum capillare sporangium stoma sunken below epidermal cells. Scale bar, 50 μm . (D) SEM of fern
Thelypteris ovata var. lindheimeri (sporophyte) leaf with stomata separated by pavement cells. Scale bar, 10 μm; s, stomata . (E) Left panel, field
emission SEM of Pinus koraiensis (gymnosperm) stomata arranged in rows on needle surface; granular material is surface wax. Scale bar, 10 μm.
Upper right, dewaxed stomata. Scale bar, 10 μm. Lower right, dewaxed guard cells (arrows) within an epistomatal chamber. Scale bar, 2 μm .
(F) SEM of dicot Arabidopsis thaliana stomatal pattern in the sepal. (G) SEM of monocot Poa annua stoma, with subsidiary cells (sc) flanking the
narrow guard cells. Scale bar, 10 μm .
SCRM1/2 SCRM1/2 SCRM1/2
Figure 3 Stomatal development in Arabidopsis. Diagram of major stages in stomatal development with place of action of the subset of
regulatory genes discussed in this review noted. Positive regulators are written in green, negative regulators in red and polarity regulators in blue.
Not all genes known to regulate stomata are presented. The image of the young leaf in the lower right corner is to represent the dispersed nature
of stomatal lineage initiation. Color code: yellow, meristemoid; orange, guard mother cell; red, guard cell; grey, meristemoid mother cell (MMC).
Vatén and Bergmann EvoDevo 2012, 3:11
Page 3 of 9
as in Polytrichastrum formosum where stomata sometimes
form next to each other. Already in mosses, diverse sto-
matal morphologies are seen . Several (both early- and
late-divergent) moss species also lack stomata . The
function of stomata in these plants may also be unusual,
for example the basal genus Sphagnum displays pseudos-
tomata which might function in spore desiccation rather
than typical CO2acquisition .
The appearance of amplifying divisions in ferns provided
novel mechanisms to control cell number as well stomatal
density and to produce specialized subsidiary cells [15,16].
Here an epidermal cell may go through one or two asym-
metric cell divisions before it differentiates into a GMC.
Subsidiary cells in gymnosperms (for example, pines) can
arise from meristemoid divisions or division of protoder-
mal cells next to stomata, or both. In Pinus strobus and in
Pinus banksiana, meristemoids divide once symmetrically
to generate a GMC and a subsidiary cell . The subsid-
iary cell, as well as neighboring epidermal cells, expands
in a polar fashion over the GMC. As a result the GMC,
and later the guard cell pair, is overlaid by a group of epi-
dermal subsidiary cells of mixed origin and in addition, is
closely connected to hypodermal subsidiary cells. Also in
gymnosperms, we begin to see stomata incorporating
some of the biochemical innovations of this group
(Figure 2E). For example, subsidiary cells display thick,
waterproof cuticles, and the guard cells become reinforced
with lignin, a cell wall polymer that is not present in
Despite the benefits of stomata-mediated gas-exchange,
some plant lineages have lost stomata. This is sometimes
facultative; for example among heterophyllic species, two
alternate leaf forms are made, depending on whether the
leaf is submerged in water or airborne. In these species,
leaf submergence leads to elimination of stomata .
Some parasitic plants, whose sources of fixed carbon are
their hosts, may also lose or inactivate their stomata .
Other plant groups, like the small, predominantly aquatic
isoetes, have members that have lost stomata completely.
The astomatous isoetes gain CO2from the sediment via
their extensive root system . Isoetes perform a variant
of photosynthesis common among cacti (crassulacean acid
metabolism (CAM)), in which separation of particular bio-
chemical reactions allow (stomatous) plants to only open
stomata during the night to decrease water loss. Use of a
root-derived carbon source enabled astomatous isoetes to
fix carbon continuously without a threat of stomata-
related water loss. In general, astomatous species are small
and only exist in a narrow growth environment. It has
been suggested that functional stomata allow plants to
develop to larger sizes and to adapt to a wider range of
growth conditions .
Pathways for stomatal development in Arabidopsis
The regulation of stomatal development is best under-
stood at a molecular level in Arabidopsis. Here, individual
cell fate transitions in the stomatal lineage are promoted
by three closely related basic helix-loop-helix (bHLH)
transcription factors, SPEECHLESS (SPCH), MUTE and
FAMA [22-24] (Figure 3). These bHLHs are expressed in
the stomatal lineage, each in a specific developmental win-
dow, and each of them is absolutely required for stomata
formation. SPCH is expressed in subset of young epider-
mal cells, often in two adjacent cells  and SPCH
expression is dynamic. After an asymmetric cell division,
SPCH disappears from the SLGC, but remains in the mer-
istemoid, which continues asymmetric cell divisions .
Loss of SPCH leads to a complete loss of the stomatal
lineage whereas overexpression of SPCH leads to ectopic
asymmetric cell divisions [23,24,26]; thus it is required for
entry into the stomatal lineage. MUTE is expressed in late
meristemoids and is required for exit from the amplifying
Figure 4 Comparison of the molecular and morphological features of stomatal development in Arabidopsis and representatives of the
grasses and mosses for which molecular data exist. Presentation of a simplified stomatal lineage displaying only cell identities (in the same
color codes as Figure 3), with the addition of blue to mark the subsidiary cells in monocots. Genetic regulators of the processes are included at
their points of action, with black text indicating that there is direct functional evidence supporting the placement and grey text representing
inferences from cross-species complementation tests. The curved arrow in the dicot lineage represents the continued asymmetric amplifying
divisions made by meristemoids.
Vatén and Bergmann EvoDevo 2012, 3:11
Page 4 of 9
division stage, and it promotes the meristemoid to GMC
transition [23,24,26]. FAMA is expressed in the GMC and
in immature guard cells. Overexpression of FAMA leads
to ectopic formation of unpaired guard cells indicating
that FAMA promotes stomatal cell fate while restricting
(symmetric) divisions .
Proteins encoded by the paralogous bHLHs, INDUCER
OF CBF EXPRESSION1/SCREAM (ICE1/SCRM) and
SCRM2, form heterodimers with SPCH, MUTE and
FAMA and promote all three stomatal fate transitions
. A semidominant scrm-D mutant converts the epider-
mis into stomata, a phenotype identical to MUTE overex-
pression, whereas double mutants of ICE1/SCRM and
SCRM2 resemble spch . Interestingly, ICE1/SCRM has
been shown to be involved in cold stress response .
Since stomatal development is regulated by both environ-
mental  and developmental factors , it is possible
that ICE1/SCRM is a cross-regulatory node where several
signaling pathways are integrated to direct stomatal
More signal integration occurs via mitogen-activated pro-
tein kinases (MAPKs) which regulate stomatal development
and stress responses through a three-step phosphorylation
cascade. MAPK kinase kinase YODA, MAPK kinases
(MKK4/5/7/9) and MAPKs (MPK3/6) are essential for nor-
mal stomatal spacing [30-32]. SPCH is a direct target of
MAPK-mediated phosphorylation and this serves to nega-
tively regulate SPCH activity . The MAPK pathway also
regulates the later stages of stomatal development, but the
targets have not been identified. More complexity arises
from the recent finding that signaling intermediates from
the steroid hormone brassinosteroid (BR) pathway phos-
phorylate both YODA  and SPCH . Interestingly,
YODA and SPCH actually produces opposite stomatal phe-
notypes. Combined with other evidence that SPCH is dif-
conditions , we are seeing just hints of the complex
interactions and precise tuning to which the early parts of
the stomatal pathway may be subjected.
Upstream of the intracellular signalling cascades, genetic
studies have revealed that stomatal spacing is regulated by
secreted peptides of the EPIDERMAL PATTERNING
FACTOR-LIKE (EPFL) family [37-41], by three leucine-
rich repeat receptor kinases (LRR-RLKs), ERECTA (ER),
ERECTA-LIKE 1 (ERL1) and ERL2  and one LRR-re-
ceptor-like protein, TOO MANY MOUTHS (TMM)
[43,44]. Members of the ER-family (ERf) are broadly
expressed and their absence leads to severe stomatal over-
proliferation and mispatterning, as well as pleiotropic
growth phenotypes, indicating that they regulate multiple
developmental processes [42,45]. ER acts predominantly
as a negative regulator of entry divisions whereas ERL1
and ERL2 control later stages . TMM is expressed in
the early stomatal lineage and, thus far, only roles in sto-
matal development have been described .
EPF1 and EPF2 peptides are stomatal lineage-expressed
and regulate the number and orientation of asymmetric
divisions [37-41]. Loss of either EPF1 or EPF2 results in
more stomata, but mutant and overexpression phenotypes
indicate that EPF2 prevents entry into the stomatal lineage
whereas EPF1 acts later. Their paralogue, STOMAGEN/
EPFL9, by contrast, is expressed in the underlying cell layer
(mesophyll) and travels to the epidermis to promote stoma-
tal differentiation [46,47]. EPF1, EPF2 and STOMAGEN
require the receptor TMM for full activity [37-41,46,47].
Surprisingly, the function of three other EPFLs, EPFL6/
CHALLAH (CHAL), EPFL4 and EPFL5, is inhibited by the
presence of TMM . Although CHAL was originally
identified by its stomatal phenotype in a tmm background
, CHAL/EPFL4/EPFL5 are expressed in internal tissues
and their loss leads to a compromised growth phenotype
resembling loss of ER . Thus, they likely represent
ligands for ER’s non-stomatal roles. The identification of
EPF family members with distinct developmental roles has
led to interesting models of how signaling specificity is
achieved by using the non-kinase receptor TMM to
modulate ligand interactions with the ERf kinases in spe-
cific tissues . Recently, elegant biosensor approaches
demonstrated ER and ERL1 primarily bind EPF2 and
EPF1, respectively, in vitro  and that in planta, TMM
can heterodimerize with ER and ERL1. Clarifying the
physical interactions and in vivo activities of the four
receptors with the 11 members of the EPF family looks to
be an exciting future area of research in Arabidopsis.
Homologues of ERf,TMM and EPFLs are found in diverse
species, including monocots and mosses, indicating that
the potential for conserved signaling systems exist. To
date, however, no experimental information is available
outside of Arabidopsis.
Stomatal lineages in Arabidopsis are established by asym-
metric cell divisions, and these unusual and unequal divi-
sions involve several other novel, plant-specific, proteins:
BREAKING OF ASYMMETRY IN THE STOMATAL
LINEAGE (BASL)  and POLAR LOCALIZATION
DURING ASYMMETRIC DIVISION AND REDISTRI-
BUTION (POLAR) . BASL displays dynamic spatio-
temporal localization in the stomatal lineage. Before
asymmetric cell division, BASL is detected in both the nu-
cleus and at the cell periphery distal to the cell division
plane. After the division, daughter cells inherit BASL in a
manner that defines their fate: nuclear localization (differ-
entiation to guard cells), peripheral localization (differenti-
ation to pavement cell), or both (continued asymmetric
cell divisions) . BASL mutants display misoriented
asymmetric cell division and overexpression of BASL leads
to ectopic outgrowths in the positions where BASL is per-
ipherally concentrated . Hence, it seems possible that
Vatén and Bergmann EvoDevo 2012, 3:11
Page 5 of 9
BASL controls or mediates cell polarity during asymmetric
cell division in the stomatal lineage. POLAR shares some
features of the BASL localization pattern; it is peripheral
and distal to the cell division site before asymmetric cell
division and shows unequal behaviors in the daughters,
disappearing from the larger daughter and being upregu-
lated in the smaller, meristematic, daughter . Although
no phenotypes have been ascribed to loss of POLAR, its
localization is dependent on BASL suggesting that they act
in the same pathway .
Additional rules for monocot stomata
One of the major differences between dicot and monocot
(specifically, grass) stomatal pathways is that, in the latter,
subsidiary cells are recruited from cell files flanking the sto-
matal lineage. This process requires the generation of a
highly polarized cell division that is specifically oriented
toward the GMC. After formation of a GMC (itself formed
by asymmetric division within the stomatal lineage), neigh-
boring subsidiary mother cells (SMCs) divide asymmetric-
ally to produce small subsidiary cells next to the GMC
(Figures 2G and 4). SMC polarization involves localization
of F-actin patches along the cell wall flanking the GMC,
and nuclear migration towards the actin patches . In
maize, actin patches co-localize with an LRR-RLK protein,
PANGLOSS 1 (PAN1) . Despite shared roles in stoma-
tal development, PAN1 is not in the same LRR-kinase
family as ERf and there are no polarly localized LRR-
kinases implicated yet in Arabidopsis stomatal develop-
ment. Loss of PAN1 leads to mislocalization of actin and
the nucleus. This disrupts asymmetric cell divisions and
results in abnormal subsidiary cells [52,53]. Recently, the
actin regulators Rho of plants 2 (ROP2) and ROP9 were
shown to localize polarly in SMCs and to promote SMC
polarization . PAN1, ROP2 and ROP9 interact and
localization of ROP2 and ROP9 is dependent on PAN1,
but PAN1 localization is independent of ROPs. It is at-
tractive to speculate that these proteins work in a com-
mon pathway to receive polarity cues and translate them
into the cellular reorganization necessary for SMC
Evolution of stomatal regulators
Arabidopsis stomatal bHLH genes are in stomatal-producing
As described above, in Arabidopsis, five bHLH genes are
major determinants of the identities and behaviors of differ-
ent stomatal lineage precursors. SPCH, MUTE and FAMA
are fairly restricted in their expression pattern to subsets of
the stomatal lineage whereas ICE1/SCRM and SCRM2 are
expressed throughout the lineage and in additional non-
stomatal lineage cells. When considering the diversity of
stomatal pattern in nature, it is interesting to think about
how the expression, regulation and function (and existence)
of this class of regulators may change. Moreover, one might
ask whether the heterodimeric partnership between SPCH,
MUTE and FAMA with ICE1/SCRM and SCRM2 could be
ancient or whether this is a new innovation.
The bHLH family is characterized by a conserved
DNA binding region, but there are easily recognizable
sub-families within. SPCH, MUTE and FAMA belong to
the group IA bHLHs  whose genic intron/exon
structure and protein C-termini are distinctive enough
to serve as high confidence group characters through-
out the flowering plants and out to Selaginella (a model
lycophyte) and Physcomitrella (a model moss) . Dis-
tinction among individual 1A members is only clear
within the flowering plants. ICE1/SCRM and SCRM2
are members of group III and representatives of this
group are found in many clades back to the mosses
(http://www.phytozome.net/). In the incomplete tran-
scriptome and genome sequences from plant lineages
predating the emergence of stomata, neither group IA nor
group III bHLH genes are obvious  (C MacAlister, per-
sonal communication). Based on sequences currently
available, in all cases where group 1A members can be dis-
tinguished, there is also a group III bHLH, suggesting that
their partnership can be ancient. A group 1A homologue
from Physcomitrella can partially complement Arabidopsis
mute and fama, but not spch mutants . These cross
species complementation results are interesting in light of
the shortened pathway for development of stomata in
Physcomitrella; in this moss, no early asymmetric divisions
are evident and instead a single GMC is specified and
undergoes incomplete cytokinesis to form two connected
guard cells (Figure 4). This pathway would require
MUTE-and FAMA-like fate promoting activities, but not
the division-promoting activity of SPCH .
In grasses, the positions of stomata are determined and
fixed at early stages of leaf development and amplifying
divisions are not present. In fact, only the differentiation
(GMC to guard cells) step is similar between grasses and
Arabidopsis (Figure 4). Nonetheless, SPCH, MUTE and
FAMA genes can be identified in the genomes of maize,
rice and Brachypodium. There has also been a duplication
of SPCH in these plants . Perhaps due to the different
stomatal ontogenies, however, the rice homologues
OsSPCH1/2 are expressed very early during plant devel-
opment, possibly before the production of stomatal
lineage . OsSPCH2 mutants in rice, do, however, have
reduced stomatal numbers and resemble weak mutant
alleles of AtSPCH . Overexpression of OsMUTE and
OsFAMA recapitulates overexpression phenotypes of the
Arabidopsis genes, indicating that their GMC and guard
cell identity-promoting functions are conserved. Of the
three genes, the only one acting at a stage common to sto-
matal development in both plant groups (the GMC to
stomatal guard cell transition), FAMA, is most highly
Vatén and Bergmann EvoDevo 2012, 3:11
Page 6 of 9
conserved in terms of expression pattern and loss of func-
tion phenotypes in both rice and Arabidopsis .
New appearance of polarity regulators
Homologues of the cell fate regulators and many of the
signaling components discussed above appear in many
plant species [10,58,59]. In contrast, the two proteins
shown to exhibit polarized localization in stomatal lineage
cells of Arabidopsis, POLAR and BASL, do not. BASL
does not resemble any other Arabidopsis proteins and
only in the congeneric A. lyrata is there a significantly
similar sequence. In Arabidopsis, POLAR is moderately
similar to another gene (POLAR-LIKE1) and homologues
of POLAR and POLAR-LIKE1 can be found in closely
related (dicot) species such as poplar (POPTR B9IL54).
Already in rice, however, the sequence similarity becomes
restricted to a very small domain of the proteins. It is
interesting to consider whether this inability to find such
homologues is because the function of BASL and POLAR
is required only in the dicots, or because similar functions
are carried out by different genes and the apparent
uniqueness of these proteins represents either fast substi-
tution rates or that their roles can be served by other pro-
teins. For example, scaffold proteins that bind others
together into complexes play important roles in polarity
generation in yeast and animals, yet these scaffolds are
often not well conserved at the sequence level and consist
primarily of multiple interaction surfaces.
Evolution of regulated stomatal pore opening
A developmental approach concerns itself with the correct
specification and pattern of stomata. From a physiological
point of view, however, the behavior of these final pro-
ducts is key. Modulation of the stomatal pore aperture
depends on coordinated morphologies of the guard cell
pair and, particularly in the case of the grasses, on the co-
ordination of guard cells and the specialized subsidiary
cells that are obligate parts of the stomatal complex .
For stomatal pore aperture to be optimized for daily and
seasonal fluctuations in light, temperature, humidity and
CO2availability, the guard cells must be able to sense such
environmental factors. Guard cells in angiosperms appear
to sense many of these factors autonomously, and key
kinases (OST1), phosphatases (PP2C) and receptors for
the “drought stress” hormone, ABA (PYR1) have been
identified in Arabidopsis . Recent studies of CO2and
ABA responsiveness in non-vascular plants have come to
different conclusions about when the sensing of these
different environmental cues arose. Monitoring stomatal
pore closure in response to ABA,  concluded that re-
sponsiveness to this hormone was a new feature and was
absent in fern and lycophyte species. Other studies, how-
ever, provide evidence that ABA sensing may have arisen
quite early. By cross-species complementation, Ruszala 
and Chater  showed that the OST1 homologues from
Selaginella moellendorffii and Physcomitrella patens could
partially restore the ability of Arabidopsis ost1 stomata to
respond to ABA. Moreover, knockout of PpOST1-1 sig-
nificantly attenuated ABA response in P. patens stomata
. The differing conclusions from these studies could
be due to the different “representative” species chosen, a
general caution in evolutionary studies of this system that
is also echoed in the behavior of maize and rice bHLHs
Stomatal development in Arabidopsis has been used as a
model genetic system for the analysis of cell fate, cell po-
larity and cell to cell communication. The nature of the
gene products identified in such analysis, coupled with the
long tradition of evaluating the numbers and patterns of
stomata in diverse plants for taxonomic purposes makes
this system a useful natural laboratory to look at the paral-
lel evolution of genes and developmental trajectories. As
the number of completed plant genomes increases and
tools for experimental manipulation of non-model species
develop, we believe there will be an excellent opportunity
to test the roles of candidate cell fate- and cell signaling
factor-encoding genes in creating developmental diversity.
The authors declare that they have no competing interests.
Funding for work on stomata in the author’s laboratory is provided by grants
from the US National Science Foundation (IOS-0845521) and the National
Institutes of Health (R01GM086632). DCB is a Gordon and Betty Moore
Foundation investigator of the Howard Hughes Medical Institute.
1Department of Biology, Stanford University, Stanford, CA 94305-5020, USA.
2Institute of Biotechnology/Department of Bio and Environmental Sciences,
University of Helsinki, Helsinki FIN-00014, Finland.3Howard Hughes Medical
Institute, Stanford, USA.
AV and DCB and designed the study, wrote the manuscript and prepared
the figures. All authors read and approved the final manuscript.
Received: 10 April 2012 Accepted: 12 June 2012
Published: 12 June 2012
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Cite this article as: Vatén and Bergmann: Mechanisms of stomatal
development: an evolutionary view. EvoDevo 2012 3:11.
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