The science of the stomata of plants: a continuously growing list of references, abstracts and illustrations, helping researchers to data on publications.
Multicellular organisms produce complex tissues with specialized cell types. During animal development, numerous cell–cell interactions shape tissue patterning through mechanisms involving contact-dependent cell migration and ligand–receptor-mediated lateral inhibition.
Owing to the presence of cell walls, plant cells neither migrate nor undergo apoptosis as a means to correct for mis-specified cells. How can plants generate functional tissue patterns?
This review aims to deduce fundamental principles of pattern formation through examining two-dimensional (2-D) spatial tissue patterning in plants and animals.
Turing’s mathematical framework will be introduced and applied to classic examples of de novo 2-D patterning in both animal and plant systems. By comparing their regulatory circuits, new insights into the similarities and differences of the basic principles governing tissue patterning will be discussed.
Stomata are normally evenly spaced for photosynthetic gas exchange, as in the normal Arabidopsis seedling shown at the top left. In a plant lacking all ERECTA-family receptor kinases (top right), excessive stomatal clusters result.
Chemical signal helps plants control their “breathing”
For most plants, staying alive means adapting quickly to a constantly changing environment. In a drought, staving off water loss is vital. On a sunny day, absorbing carbon dioxide to generate energy through photosynthesis is key. Now, researchers have discovered how plants regulate the development of stomata, the pores through which these critical exchanges with the environment occur.
“Stomata are really vital for plant growth and survival,” says Keiko Torii, a Howard Hughes Medical Institute – Gordon and Betty Moore Foundation investigator at the University of Washington who led the new work, which was published online January 12, 2012, in the journal Genes & Development.
“It will be interesting to study this in crop plants and see the relationship is between stomatal density and plant productivity,” said Keiko U. Torii
When their stomata are open, plants can absorb carbon dioxide and oxygen from the air and release oxygen, the byproduct of energy-generating photosynthesis. But open stomata also allow water to escape from the plant. So most plants open stomata during the day to allow photosynthesis to occur and close stomata during the night to prevent water loss.
Stomata function best when they are evenly spaced over a leaf’s surface, says Torii, whose lab focuses on uncovering the genetics that underlie plant development. Different plants have different patterns of spacing, and until now, scientists have not understood how this pattern is established during development.
Torii and her colleagues knew that two related proteins, called epidermal patterning factor 1 and 2 (EPF1 and EPF2), control the fate of cells in developing leaves.
Fig. 5: Stomatal development defects in hdg2 mutants and higher-order mutants in closely relatedHD-ZIP IV genes. (A) Stomatal index (SI; white bars), meristemoid index (MI; grey bars) and stomatal-lineage index (SLI; black bars) of 10-day-old abaxial cotyledons from wild-type (wt) and three independent hdg2 T-DNA insertion alleles: hdg2-2, hdg2-3 and hdg2-4. SLI is defined here as the sum of SI and MI. See supplementary material Fig. S1 for RT-PCR analysis. Data are means (n=8); error bars indicate s.e.m. *P<0.0005; **P<0.0001 (two-tailed Student’s t-test of each hdg2 allele against wild type). Tukey’s HSD did not reveal statistical difference of SI, MI and SLI among three hdg2 alleles. (B) SI of 10-day-old abaxial cotyledons from wild type, atml1, pdf2, hdg2, hdg2 atml1 and hdg2 pdf2. hdg2-2 and atml1-3 alleles were used for the analysis. Data are means (n=8); error bars indicate s.e.m. Total numbers of stomata counted: 680 (wild type), 639 (atml1), 673 (pdf2), 630 (hdg2), 550 (hdg2 atml1) and 620 (hdg2 pdf2). (C) MI of the six genotypes described above. Data are means; error bars indicate s.e.m. Total numbers of meristemoids counted: 25 (wild type), 34 (atml1), 35 (pdf2), 93 (hdg2), 243 (hdg2 atml1) and 94 (hdg2 pdf2). (D) SLI of the six genotypes described above. Data are mean; error bars indicate s.e.m. Total numbers of cells counted: 2516 (wild type), 2542 (atml1), 2661 (pdf2), 2786 (hdg2), 3331 (hdg2 atml1) and 2661 (hdg2 pdf2). For B,C, genotypes with non-significant phenotypes were grouped together with a letter (Tukey’s HSD test after one-way ANOVA). For D, only one genotype was significantly different from others (Tukey’s HSD test; P<0.01). (E-J) Representative DIC images of 10-day-old abaxial cotyledons from wild type (E), hdg2 (F), atml1(G), pdf2 (H), hdg2 atml1 (I) and hdg2 pdf2 (J). Asterisks indicate meristemoids. Images were taken under the same magnification. Scale bar: 100 μm. (K-N) Aberrant stomatal complexes found in hdg2. (K) Arrested stomatal precursor after extensive asymmetric amplifying divisions from 30-day-old cotyledon. (L) Confocal image of a stomatal complex with a single GC from 10-day-old abaxial cotyledon. (M,N) Singular GCs from 30-day-old cotyledon. Asterisks indicate singular GCs; + indicates arrested stomatal precursor. Scale bars: in L, 20 μm; in K,M,N, 25 μm.
Summary
The shoot epidermis of land plants serves as a crucial interface between plants and the atmosphere: pavement cells protect plants from desiccation and other environmental stresses, while stomata facilitate gas exchange and transpiration.
Advances have been made in our understanding of stomatal patterning and differentiation, and a set of ‘master regulatory’ transcription factors of stomatal development have been identified. However, they are limited to specifying stomatal differentiation within the epidermis.
Here, we report the identification of an Arabidopsis homeodomain-leucine zipper IV (HD-ZIP IV) protein, HOMEODOMAIN GLABROUS2 (HDG2), as a key epidermal component promoting stomatal differentiation. HDG2 is highly enriched in meristemoids, which are transient-amplifying populations of stomatal-cell lineages.
Ectopic expression of HDG2 confers differentiation of stomata in internal mesophyll tissues and occasional multiple epidermal layers. Conversely, a loss-of-function hdg2 mutation delays stomatal differentiation and, rarely but consistently, results in aberrant stomata.
A closely related HD-ZIP IV gene, Arabidopsis thaliana MERISTEM LAYER1 (AtML1), shares overlapping function with HDG2: AtML1 overexpression also triggers ectopic stomatal differentiation in the mesophyll layer and atml1 mutation enhances the stomatal differentiation defects of hdg2.
Consistently, HDG2 and AtML1 bind the same DNA elements, and activate transcription in yeast. Furthermore, HDG2 transactivates expression of genes that regulate stomatal development in planta.
Our study highlights the similarities and uniqueness of these two HD-ZIP IV genes in the specification of protodermal identity and stomatal differentiation beyond predetermined tissue layers.
PLANT STOMATA ENCYCLOPEDIA 
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