Novel and Expanded Roles for MAPK Signaling inArabidopsisStomatal Cell Fate Revealed by Cell Type–Specific Manipulations  

Abstract

Mitogen-activated protein kinase (MAPK) signaling networks regulate numerous eukaryotic biological processes. In Arabidopsis thaliana, signaling networks that contain MAPK kinases MKK4/5 and MAPKs MPK3/6 function in abiotic and biotic stress responses and regulate embryonic and stomatal development. However, how single MAPK modules direct specific output signals without cross-activating additional downstream processes is largely unknown. Studying relationships between MAPK components and downstream signaling outcomes is difficult because broad experimental manipulation of these networks is often lethal or associated with multiple phenotypes. Stomatal development in Arabidopsis follows a series of discrete, stereotyped divisions and cell state transitions. By expressing a panel of constitutively active MAPK kinase (MAPKK) variants in discrete stomatal lineage cell types, we identified a new inhibitory function of MKK4 and MKK5 in meristemoid self-renewal divisions. Furthermore, we established roles for MKK7 and MKK9 as both negative and (unexpectedly) positive regulators during the major stages of stomatal development. This has expanded the number of known MAPKKs that regulate stomatal development and allowed us to build plausible and testable subnetworks of signals. This in vivo cell type-specific assay can be adapted to study other protein families and thus may reveal insights into other complex signal transduction pathways in plants.

Stomatal development: a plant's perspective on cell polarity, cell fate transitions and intercellular communication

“…At least seven kinases can modulate stomatal development: the MAPKKK YODA (YDA), MKK4, MKK5, MKK7, MKK9, MPK3 and MPK6 (Fig. 3) (Bergmann et al, 2004;Wang et al, 2007;Lampard et al, 2009). Although, thus far, only genetic evidence exists to place the MAPK cascade downstream of ERf signaling, a biochemical connection was established between MAPKs and the fate-promoting bHLH transcription factors when MPK3 and MPK6 were shown to directly phosphorylate and repress SPCH (Lampard et al, 2008).…”

“…Intracellular transduction of the signaling initiated by the EPFLERf-TMM module is likely to be through a MAPK pathway (Bergmann et al, 2004;Wang et al, 2007;Lampard et al, 2009). Plant MAPK pathways, like their yeast and animal counterparts, are used to mediate developmental, stress and defense responses, and involve the sequential phosphorylation and activation of a three kinase module: the MAPK kinase kinases (MAPKKKs or MEKKs), the MAPK kinases (MKKs or MEKs) and the MAPKs (Rodriguez et al, 2010).…”

Auxin represses stomatal development in dark-grown seedlings via Aux/IAA proteins

“…Upstream of these bHLH transcription factors, a MAP kinase cascade, comprising the MAP kinase kinase kinase YDA, the MAP kinase kinases MKK4 and MKK5 and the MAP kinases MPK4, MPK5, MPK7 and MPK9, suppresses stomatal development (Bergmann et al, 2004;Lampard et al, 2008Lampard et al, , 2009Wang et al, 2007). Additional negative regulators act upstream of this cascade: leucine-rich repeat (LRR) receptor-like kinases of the ERECTA (ER) family (ERf), consisting of ER, ERECTA-LIKE1 (ERL1) and ERL2, control stomatal patterning and differentiation (Shpak et al, 2005).…”

Auxin inhibits stomatal development through MONOPTEROS repression of a mobile peptide gene STOMAGEN in mesophyll

“…As a gas and water passage between external environment and internal plant tissue, stomata play important roles in photosynthesis and global carbon and water circulation (15). Stomatal generation undergoes several stages, including meristemoid mother cell, meristemoid, guard mother cell, and guard cells, which is modulated by an intrinsic program (14) mainly involving putative peptide ligands [EPI-DERMAL PATTERNING FACTOR (EPF) family] (16)(17)(18)(19)(20), membrane proteins (receptor-like protein TMM and receptorlike kinase ERECTA family) (21)(22)(23), MAPK cascades (protein kinase YDA, MKK4/5/7/9, and MPK3/6) (24)(25)(26), and transcription factors (bHLH and MYB type) (23,(27)(28)(29)(30)(31). EPF factors, the small secretory peptides, are proposed to act at the top of this hierarchical signaling pathway (16)(17)(18)(19)(20).…”