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Plant Signal Behav. 2010 March; 5(3): 278–280.
PMCID: PMC2881277

BASL and EPF2 act independently to regulate asymmetric divisions during stomatal development

Abstract

The initiation of stomatal development in the developing Arabidopsis epidermis is characterized by an asymmetric ‘entry’ division in which a small cell, known as a meristemoid, and a larger daughter cell is formed. The meristemoid may undergo further asymmetric divisions, regenerating a meristemoid each time, before differentiating into a guard mother cell which divides symmetrically to form a pair of guard cells surrounding a stomatal pore. Recently EPF2 and BASL have emerged as regulators of these asymmetric divisions and here we present results indicating that these two factors operate independently to control stomatal development

Key words: stomata, development, meristemoids, asymmetric cell division, leaf epidermis, cell polarity, peptide signal

EPF Peptides Regulate Asymmetric Divisions of Stomatal Precursors

Several factors that regulate asymmetric cell divisions involved in the differentiation of specialized plant cell types have been identified from molecular genetic studies, including putative transcription factors, receptors, phosphatases, kinases and proteases.1 Recently, we and others reported putative peptide ligands for the receptor-like protein TMM which regulates both the number and spacing of stomata during epidermal development. 2,3 EPIDERMAL PATTERNING FACTORs 1 and 2 (EPF1 and EPF2) are members of a family of secretory peptides, all with previously unknown function.46 EPF1 and EPF2 are both specifically expressed in stomatal precursor cells. Knockdown mutants of epf1 exhibit pairs of adjacent stomata in their epidermis, and epf2 mutants are characterized by increased stomatal density and the presence of an abnormally large proliferation of arrested meristemoids. Thus EPF1 functions to orientate asymmetric divisions of meristemoids and prevent stomata from forming in contact with one another, whereas EPF2 acts to restrict the number of ‘entry’ and asymmetric divisions of meristemoids and to promote the differentiation of daughter cells into pavement cells.

Polarized Cellular Expression of BASL Occurs Prior to Asymmetric Division

A novel protein, BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL), which accumulates in stomatal precursor cells has also recently been reported.7 Interestingly, BASL accumulates in a distinct polarized pattern inside stomatal precursor cells, marking the periphery of those cells about to undergo an asymmetric division. Knockdown basl mutants show extra symmetric divisions of meristemoids and BASL is therefore proposed to be necessary for promoting correctly orientated asymmetric divisions of stomatal precursors. Epigenetic studies have shown that double mutants epf1;basl and tmm;basl have additive phenotypes relative to the single mutants suggesting that BASL acts independently of these previously characterised stomatal development regulators. As EPF2 and BASL are both expressed in stomatal precursor cells and are both involved in regulating their asymmetric divisions, we investigated whether EPF2 and BASL could act together to regulate this step in stomatal development.

Comparison of Single basl and epf2 Mutants with the Double Mutant, basl;epf2

The basl-4;epf2-1 leaf epidermis contained large numbers of additional small cells which appeared to be the result of increases in both symmetric and asymmetric divisions of stomatal lineage cells. This resulted in a reduced density of mature tessellated epidermal pavement cells (Fig 1A–D). Stomatal density was not altered in fully expanded leaves of either basl-4 or basl-4;epf2-1 in comparison to the wild-type control (Col-0) but was significantly higher in epf2-1 (p < 0.003) than wild-type or basl-4;epf2-1 (p < 0.03) (Fig. 1E). In contrast to basl-1 and basl-2,7 basl-4 did not show increased stomatal density. This may be explained by the different tissue examined as Dong et al. examined cotyledons, whereas we used mature leaves. There were however clearly more clustered stomata in basl-4 than wild-type and the epf2-1 mutation caused a significant reduction in the number of paired or clustered stomata in basl-4;epf2-1 (p > 0.016) (Fig. 1F). Neither basl-4 nor epf2-1 showed any obvious effect on growth rate or leaf shape, but basl-4;epf2-1 plants developed narrow twisted leaves, perhaps because of their considerably reduced number of epidermal pavement cells.

Figure 1
BASL & EPF2 act independently. Cleared Normarski images of abaxial epidermis of (A) Col-0, (B) basl-4, (C) epf2-1, (D) basl-4;epf2-1 leaves approximately 2 cm in length. gc, pair of guard cells; pc, pavement cell; m, meristemoids. *indicates meristemoids ...

To confirm that the additional small epidermal cells that we found in our mutants were the result of excessive asymmetric or symmetric divisions of meristemoids we expressed the pTMM:GUS-GFP gene construct which is specifically expressed in stomatal lineage cells, in each of our genetic backgrounds and observed GUS and GFP cellular expression patterns (Fig. 2C). GUS staining and GFP fluorescence directed by the TMM promoter indicted that the additional small epidermal cells that we observed in our single and double mutants were indeed arrested stomatal lineage cells.

Figure 2
Additional small epidermal cells in basl-4;epf2-1 are stomatal lineage cells. Col-0, epf2-1, basl-4 and basl-4;epf2-1 mutants expressing the pTMM:GUS-GFP transcriptional reporter histochemically stained for GUS (A–D) or examined under UV fluorescence ...

Conclusions

The recently reported Arabidopsis gene products BASL and EPF2 are both involved in regulating the asymmetric cell division programthat leads to stomatal development. Lack of BASL expression leads to a large number of symmetric rather than asymmetric divisions of stomatal precursors, and lack of EPF2 leads to excessive numbers of asymmetric divisions. Our analysis of the double mutant basl-4;epf2-1 suggests however, that BASL and EPF2 operate independently to regulate asymmetric divisions during stomatal development.

Footnotes

References

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4. Hara K, Kajita R, Torii KU, Bergmann DC, Kakimoto T. The secretory peptide gene EPF1 enforces the stomatal one-cell-spacing rule. Genes Dev. 2007;21:1720–1725. [PubMed]
5. Hara K, Yokoo T, Kajita R, Onishi T, Yahata S, Peterson KM, et al. Epidermal cell density is autoregulated via a secretory peptide, EPIDERMAL PATTERNING FACTOR 2 in Arabidopsis leaves. Plant Cell Physiol. 2009;50:1019–1031. [PubMed]
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7. Dong J, MacAlister CA, Bergmann DC. BASL controls asymmetric cell division in Arabidopsis. Cell. 2009;137:1320–1330. [PMC free article] [PubMed]
8. Nadeau JA, Sack FD. Control of stomatal distribution on the Arabidopsis leaf surface. Science. 2002;296:1697–1700. [PubMed]

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