Plant Development: YODA the
Stomatal Switch
Dispatch
Julie E. Gray1 and Alistair M. Hetherington2
The appearance of stomatal pores during plant
evolution is believed to have been a crucial step in
land colonisation. A recent screen for genes involved
in stomatal development has identified for the first
time a mutant plant with no stomata; the results implicate
a MAP kinase cascade in stomatal development.
Stomata are small pores on the surfaces of leaves that
control the exchange of gasses between the interior of
the plant and the atmosphere [1]. They make a significant
contribution to global water and carbon cycles, as
an estimated 440 x 1015 g CO2 and 32 x 1015 kg of
water vapour pass through stomatal pores annually [2].
Stomatal pore opening and closure are regulated by
plant hormones and environmental signals. The presence
of these control systems allows the plant to conserve
water during periods of drought and to maximise
CO2 uptake during the day. It has been convincingly
argued that the acquisition of stomata is a key element
in the evolution of advanced terrestrial plants [3], as
they allow the plant to inhabit different environments
while maintaining control over their water content.
Stomatal development and patterning are also influenced
by environmental factors, and the identity of
some of the components involved in controlling these
processes is beginning to emerge [4–6].
In this context, recent work from Bergmann, Lukowitz
and Somerville [7] is of great interest, as these authors
have shown that a newly characterised mitogen activated
protein (MAP) kinase kinase kinase known as
YODA is an important negative regulator of stomatal
development. Of equal significance, however, is their
observation that plants engineered to express a constitutively
active YODA have no stomata and die before
they reach reproductive maturity. Although further work
is required before it is possible to reach definitive conclusions,
this result is potentially very important
because it suggests that the possession of stomata is
vital to plant survival and as such lends support to
Raven’s suggestion [3] that the acquisition of these
structures was a key element in the evolution of
advanced terrestrial plants.
In the Arabidopsis epidermis, stomatal development
starts with the initiation of a meristemoid mother cell.
When this cell divides asymmetrically the smaller
product is known as the meristemoid and this, after
several asymmetric divisions, is eventually converted
into a guard mother cell, which divides symmetrically to
form the pair of guard cells that gate the stomatal pore
[4–6]. At least three rules govern stomatal development
and patterning. First, their development follows the
series of asymmetric divisions described above;
second, a patterning rule ensures that stomata are separated
from each other by at least one non-stomatal
epidermal cell; and third, the number of stomata that
form, relative to the non-stomatal epidermal cells, is
subject to control by environmental factors. The isolation
of Arabidopsis mutants that carry lesions in stomatal
development and patterning has provided
significant insights into the signal transduction pathways
that underlie these processes [4,5]. Arabidopsis
mutant screening also formed the basis of the work
which allowed Bergmann et al. [7] to uncover the role of
a putative MAP kinase kinase kinase encoded by the
YODA gene (YDA) in stomatal development.
Bergmann et al. [7] carried out a visual screen for
aberrant stomatal patterning and out of the 19 mutants
recovered, six were alleles of YDA [8]. This gene had
recently been identified as a site of mutations that affect
early embryo development. After fertilization, the Arabidopsis
zygote undergoes an asymmetric division to
form a small apical cell that goes on to form the embryo
and a larger lower cell that develops into an extra
embryonic structure known as the suspensor. In yda
mutants, the suspensor does not form and the cells of
the lower cell lineage are incorporated into the embryo.
On the basis of these observations Lukowitz et al. [8]
concluded that the putative MAP kinase kinase kinase
encoded by the YDA gene acts as a switch that promotes
extra-embryonic fate. From what we know of
MAP kinase cascades, it seems reasonable to conclude
that a MAP kinase signalling cassette is involved in this
example of the control of post asymmetric division cell
fate. The six yda alleles identified by Bergmann et al. [7]
also cause defects in post-embryonic development;
additionally, they result in plants developing with
extreme overproduction and clustering of stomata in
the epidermis of the cotyledons and hypocotyls. Most
yda seedlings fail to survive on transfer to soil, but
those that do were found to be dwarfed and characterized
by compact leaves filled with clusters of stomata.
When the yda phenotype was investigated in detail,
it was found that, compared to wild type, more yda epidermal
cells had entered the stomatal fate pathway.
This indicates that just as in embryos, YDA plays a role
in cell fate choice in the leaf epidermis. To investigate
the role of YDA further, Bergmann et al. [7] made transgenic
plants expressing a gene (∆∆N-YDA) encoding a
form of the YDA MAP kinase kinase kinase designed to
be constitutively active. Seedlings homozygous for ∆∆NYDA
totally lacked stomata, supporting the idea that
YDA acts as a switch that negatively regulates stomatal
fate. The authors also used genetic means to ‘place’
YDA in the stomatal development pathway relative to
other gene products that affect stomatal development.
The TOO MANY MOUTH (TMM) and STOMATAL
DENSITY AND DISTRIBUTION (SDD1) loci encode a
leucine-rich repeat receptor-like protein and a putative
Current Biology, Vol. 14, R488–R490, June 22, 2004, ©2004 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cub.2004.06.019
1Department of Molecular Biology and Biotechnology,
University of Sheffield, Sheffield S10 2TN, UK. 2Department of
Biology, LEC, Lancaster University, Lancaster LA1 4YQ, UK.
subtilisin-like serine protease, respectively [9,10]. Mutations
in these genes result in plants that exhibit an
increase in stomatal number, and current models to
account for these phenotypes are based on the idea
that the SDD1 protein generates a peptide signal that is
recognized by TMM receptor [5]. As indicated in Figure
1, genetic analysis places YDA downstream of both
these gene products.
Finally, Bergmann et al. [7] used whole-genome transcript
analysis to identify further genes involved in the
control of stomatal patterning and development. They
reasoned that transcripts up-regulated in yda plants
(mostly stomata) and down-regulated in ∆∆N-YDA
plants (no stomata), compared to wild-type, would be
good candidate regulators of stomatal patterning and
development. Indeed, expression of three genes
known to be involved in stomatal development, TMM,
SDD1 and HIC (involved in the control of stomatal
development by atmospheric CO2 concentration [11]),
followed this expression pattern [7]. Analysis of genes
that clustered with these three known stomatal genes
revealed 109 that are strong candidates for involvement
in the control of stomatal patterning and development.
This was confirmed by selecting one of these
genes known as FAMA, which encodes a putative transcription
factor; when plants homozygous for a T-DNA
insertion in FAMA were analysed it was found that they
too lack stomata [7].
The impact of the work from Bergmann et al. [7] is
likely to be profound. They have identified over 100
candidate genes involved in guard cell development
and patterning which are set to be a rich source for
uncovering the intricacies of these developmental pathways
for a considerable time. In addition, they have
provided a tantalizing glimpse into the possible significance
of the acquisition of stomata for the evolution of
advanced terrestrial plants [9]. Of course, because YDA
is implicated in multiple developmental responses, it is
not possible to conclude that ∆∆N-YDA plants fail to
reach reproductive maturity because of their lack of
stomata. But the fact that fama plants are small and
sterile certainly adds weight to this possibility.
The issue should be resolved in the near future as
Bergmann et al. [7] are currently constructing plants in
which ∆∆N-YDA is expressed only in the epidermis
(D. Bergmann, personal communication). Phenotypic
analysis of these plants might result in the production
of key data supporting Raven’s suggestion [3] that the
acquisition of stomata is key to the success of the
advanced terrestrial plants. Even though a tobacco
MAP kinase kinase kinase had previously been implicated
in the control of guard cell development [12], the
identification of YDA MAP kinase kinase kinase and the
fate switch it controls is a major advance. Indeed, it is
an advance that prompts many new questions. We
need to know whether YDA functions in the same (as
in Figure 1) or a separate pathway to TMM and SDD. If
they are in the same pathway how does TMM feed into
the MAP kinase signalling cassette? Might FAMA be a
phosphorylation target for the putative MAP kinase signalling
cassette? And how do components involved in
the environmental control of stomatal development, such
as HIC, interact with the YDA stomatal development
pathway? Finally, we urgently need to identify the other
components, such as the receptor-like kinase that one
assumes interacts with TMM to inactivate YDA and
activate stomatal development, and the MAP kinase
kinase and MAP kinase scaffold proteins that will presumably
be responsible for controlling the specificity
of YDA interactions in the cells of the zygote and stomatal
precursors [13].
References
1. Hetherington, A.M. and Woodward, F.I. (2003). The role of stomata
in sensing and driving environmental change. Nature 424, 901-908.
2. Ciais, P., Denning, A.S., Tans, P.P., Berry, J.A., Randall, D.A. Collatz,
C.J., Sellers, P.J., White, J.W.C., Trolier, M., Meijer, H.A.J., et al.
(1997). A three-dimensional synthesis study of dO18 in atmospheric
CO2. I. Surface fluxes. J. Geophy. Res. — Atmosphere 102, 5857-
5872.
3. Raven, J. (2002). Selection pressures on stomatal evolution. New
Phytol. 153, 371-386.
4. Bergmann D.C. (2004). Integrating signals in stomatal development.
Curr. Opin. Plant Biol. 7, 26-32.
5. Nadeau, J.A. and Sack, F.D. (2003). Stomatal development: cross
talk puts mouths in place. Trends Plant Sci. 8, 294-299.
6. Bird, S.M. and Gray, J.E. (2003). Signals from the cuticle affect epidermal
cell differentiation. New Phytol. 157, 9-23.
7. Bergmann, D.C., Lukowitz, W. and Somerville, C.R. (2004). Stomatal
development and pattern controlled by a MAPKK kinase. Science
304, 1494-1497.
8. Lukowitz W., Roeder, A., Parmenter, D. and Somerville C. (2004). A
MAPKK kinase gene regulates extra-embryonic cell fate in Arabidopsis.
Cell 116, 109-119.
Current Biology
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Figure 1. Hypothetical scheme linking components known to
be involved in the control of stomatal development.
Genetic analysis places the MAPKKK YDA downstream of
TMM, a leucine-rich repeat receptor-like protein, and SDD1, a
subtilisin-like protease. Coloured components represent gene
products known to affect stomatal development. Black components
are predicted to be present on the basis of knowledge
gained from other better characterised signalling pathways.
Although this model for signalling during stomatal development
assumes that all components shown act in the same pathway,
it is quite possible, on the basis of our current knowledge, that
some components act in separate signalling pathways.
P
P
P
YDA
Receptor-like
protein kinase
MAPK
MAPKK
Mature peptide
signal
SDD1
Stomatal development
TMM
–ve
P
Current Biology
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9. Nadeau, J.A. and Sack, F.D. (2000). Control of stomatal distribution
on the Arabidopsis leaf surface. Science 296, 1697-1700.
10. Berger, D. and Altmann, T. (2000). A subtilisin-like serine protease
involved in the regulation of stomatal density and distribution in
Arabidopsis thaliana. Genes Dev. 14, 1119-1131.
11. Gray, J.E., Holroyd, G.H., van der Lee, F., Bahrami, A.R., Sijmons,
P.C., Woodward, F.I., Schuch, W., and Hetherington A.M. (2000).
The HIC signalling pathway links CO2 perception to stomatal development.
Nature 408, 713-716.
12. Nishihama, R., Ishikawa, M., Araki, S., Soyano, T., Asada, T. and
Machida, Y. (2001). The NPK1 mitogen-activated protein kinase
kinase kinase is a regulator of cell-plate formation in plant cytokinesis.
Genes Dev. 15, 352-363.
13. Ptashne M. and Gann, A. (2003). Imposing specificity on kinases.
Science 299, 1025-1027.