conditions (5, 6). Moreover, it suggests that the
prevailing view by which mechanisms associated
with local resource competition best explain patterns in local plant diversity along resource gradients should be revisited (6). Our results do not
imply that local resource factors and niches are
unimportant in promoting local plant species coexistence within individual communities (31);
rather, they suggest that such factors may not
explain variation in plant diversity among communities. Our finding about the importance of
environmental filtering and species pools in
shaping local plant diversity patterns along resource gradients might also help to explain why
studies find inconsistent responses of plant diversity to productivity (8).
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We thank the Western Australia Department of Parks and Wildlife
for access to field sites and facilities. This research was supported
by a Discovery Early Career Research Award (DE120100352) from
the Australian Research Council and research grants from the
University of Western Australia to E.L. and a Ph.D. scholarship to
G.Z. from the Paul Hasluck Bequest, administered by the Kwongan
Foundation. We thank P. Hayes for collection of leaf nutrient data
and J. Grace and H. Lambers for providing comments on the
manuscript. The vegetation and soil data are available on Dryad
Materials and Methods
Figs. S1 to S5
21 May 2014; accepted 2 September 2014
Direct roles of SPEECHLESS
in the specification of stomatal
On Sun Lau,1 Kelli A. Davies,1 Jessica Chang,1 Jessika Adrian,1 Matthew H. Rowe,1†
Catherine E. Ballenger,2 Dominique C. Bergmann1,2,3‡
Lineage-specific stem cells are critical for the production and maintenance of specific cell
types and tissues in multicellular organisms. In Arabidopsis, the initiation and proliferation
of stomatal lineage cells is controlled by the basic helix-loop-helix transcription factor
SPEECHLESS (SPCH). SPCH-driven asymmetric and self-renewing divisions allow flexibility
in stomatal production and overall organ growth. How SPCH directs stomatal lineage cell
behaviors, however, is unclear. Here, we improved the chromatin immunoprecipitation (ChIP)
assay and profiled the genome-wide targets of Arabidopsis SPCH in vivo. We found that SPCH
controls key regulators of cell fate and asymmetric cell divisions and modulates responsiveness
to peptide and phytohormone-mediated intercellular communication. Our results delineate
the molecular pathways that regulate an essential adult stem cell lineage in plants.
In multicellular organisms, the need to gen- erate and maintain diverse cell types and tis- sues is fulfilled by lineage-specific stem cells (1). These stem cell lineages, active postem- bryonically, produceadefinedsetof celltypes.
Although the origins of these lineage-specific stem
cells during development are largely obscure,
master transcription factors are implicated in
their specification in both animals and plants (1–3).
However, low expression levels and/or presence
in a limited number of cells makes genome-wide
study of these transcriptional regulators by standard chromatin immunoprecipitation (ChIP) assays, the most common technique for studying
protein-DNA interactions, technically challenging.
Stomata are epidermal valves that mediate
gas exchange between the plant and atmosphere.
In Arabidopsis, stomatal guard cells are derived
Fig. 3. Selected pairwise relationships between chronosequence stage, soil pH,
species pool size, and local
(rarefied) plant species richness.
Increasing soil age (i.e., chrono-
sequence stage) leads to an
increase in plant species richness
(A) and a decrease in soil pH
(B). Declining soil pH leads to
greater local plant species
richness (C) and species pool
sizes (D). Local plant species richness is greater with larger species pools (E). In (D), there are only six data
points (i.e., one per chronosequence stage) because species pool size was estimated for each stage.