Termination of asymmetric cell division and differentiation of stomata

Abstract

Stomata consist of a pair of guard cells that mediate gas and water-vapour exchange between plants and the atmosphere. Stomatal precursor cells-meristemoids-possess a transient stem-cell-like property and undergo several rounds of asymmetric divisions before further differentiation. Here we report that the Arabidopsis thaliana basic helix-loop-helix (bHLH) protein MUTE is a key switch for meristemoid fate transition. In the absence of MUTE, meristemoids abort after excessive asymmetric divisions and fail to differentiate stomata. Constitutive overexpression of MUTE directs the entire epidermis to adopt guard cell identity. MUTE has two paralogues: FAMA, a regulator of guard cell morphogenesis, and SPEECHLESS (SPCH). We show that SPCH directs the first asymmetric division that initiates stomatal lineage. Together, SPCH, MUTE and FAMA bHLH proteins control stomatal development at three consecutive steps: initiation, meristemoid differentiation and guard cell morphogenesis. Our findings highlight the roles of closely related bHLHs in cell type differentiation in plants and animals.

Arabidopsishomeodomain-leucine zipper IV proteins promote stomatal development and ectopically induce stomata beyond the epidermis

“…Mutants used in the study were in the Col background. The following mutants and reporter lines have been reported previously: TMMpro::GUS-GFP (Nadeau and Sack, 2002), TMMpro::GUS (Shpak et al, 2005), EPF2pro::erGFP (Hara et al, 2009), AtML1pro::GUS (Uchida et al, 2012), SPCHpro::GUS and E994 (Pillitteri et al, 2007), GL2pro::GUS (Masucci et al, 1996) and At5g17710pro::nuc-3xVENUS (Roeder et al, 2012). T-DNA insertion lines: hdg2-2 (SALK_127828C); hdg2-3 (SALK_138646C); hdg2-4 (SALK_120064); atml1-3 (SALK_128172); atml1-4 (SALK_033408) and pdf2-2 (SALK_109425C) were obtained from the Arabidopsis Biological Resource Center (ABRC), and loss or disruption of their transcripts was confirmed (supplementary material Fig.…”

“…Plasmid pER8 (Zuo et al, 2000) was used for estradiol-inducible constructs. Generation and selection of transgenic plants were performed as described previously (Pillitteri et al, 2007). For all transgenic Arabidopsis lines, more than 20 T1 plants per construct were isolated and characterized for phenotypes.…”

“…The stomatal cell-state transitions are specified by combinatorial activities of five basic helix-loop-helix (bHLH) proteins, SPEECHLESS (SPCH), MUTE, FAMA, SCREAM (SCRM, also known as ICE1) and SCRM2 (Kanaoka et al, 2008;MacAlister et al, 2007;Ohashi-Ito and Bergmann, 2006;Pillitteri et al, 2007). These bHLH proteins are necessary and sufficient for specifying each stomatal precursor state.…”

“…Among them, MUTE and SCRM are capable of converting the aerial shoot epidermis into stomata when ectopically overexpressed or stabilized (Kanaoka et al, 2008;Pillitteri et al, 2008). By contrast, ectopic overexpressions of SPCH and FAMA confer excessive asymmetric entry divisions and the formation of guard cell-like cells, respectively (MacAlister et al, 2007;Ohashi-Ito and Bergmann, 2006;Pillitteri et al, 2007). Importantly, the ability of MUTE and SCRM to trigger stomatal differentiation is limited to the epidermis.…”

A Tale of Two Systems: Peptide Ligand-Receptor Pairs in Plant Development

“…It is expressed in most protodermal cells and later its expression levels in pavement cells are gradually decreased, although its expression is maintained in stomatal lineage cells for these cells to undergo stomatal differentiation (MacAlister et al 2007;Pillitteri et al 2007). Interestingly, expression patterns of EPF2, an inhibitor of entry asymmetric division, are overlapped with SPCH, and SPCH is required for EPF2 expression (Hara et al 2009).…”