Dissection of MAPK signaling specificity through protein engineering in a developmental context.

BACKGROUND: Mitogen-activated protein kinases (MAPK) signaling affects many processes, some of which have different outcomes in the same cell. In Arabidopsis, activation of a MAPK cascade consisting of YODA, MKK4/5 and MPK3/6 inhibits early stages of stomatal developmental, but the ability to halt stomatal progression is lost at the later stage when guard mother cells (GMCs) transition to guard cells (GCs). Rather than downregulating cascade components, stomatal precursors must have a mechanism to prevent late stage inhibition because the same MKKs and MPKs mediate other physiological responses. RESULTS: We artificially activated the MAPK cascade using MKK7, another MKK that can modulate stomatal development, and found that inhibition of stomatal development is still possible in GMCs. This suggests that MKK4/5, but not MKK7, are specifically prevented from inhibiting stomatal development. To identify regions of MKKs responsible for cell-type specific regulation, we used a domain swap approach with MKK7 and a battery of in vitro and in vivo kinase assays. We found that N-terminal regions of MKK5 and MKK7 establish specific signal-to-output connections like they do in other organisms, but they do so in combination with previously undescribed modules in the C-terminus. One of these modules encoding the GMC-specific regulation of MKK5, when swapped with sequences from the equivalent region of MKK7, allows MKK5 to mediate robust inhibition of late stomatal development. CONCLUSIONS: Because MKK structure is conserved across species, the identification of new MKK specificity modules and signaling rules furthers our understanding of how eukaryotes create specificity in complex biological systems.

Manipulation of mitogen-activated protein kinase kinase signaling in the Arabidopsis stomatal lineage reveals motifs that contribute to protein localization and signaling specificity.

When multiple mitogen-activated protein kinase (MAPK) components are recruited recurrently to transduce signals of different origins, and often opposing outcomes, mechanisms to enforce signaling specificity are of utmost importance. These mechanisms are largely uncharacterized in plant MAPK signaling networks. The Arabidopsis thaliana stomatal lineage was previously used to show that when rendered constitutively active, four MAPK kinases (MKKs), MKK4/5/7/9, are capable of perturbing stomatal development and that these kinases comprise two pairs, MKK4/5 and MKK7/9, with both overlapping and divergent functions. We characterized the contributions of specific structural domains of these four "stomatal" MKKs to MAPK signaling output and specificity both in vitro and in vivo within the three discrete cell types of the stomatal lineage. These results verify the influence of functional docking (D) domains of MKKs on MAPK signal output and identify novel regulatory functions for previously uncharacterized structures within the N termini of MKK4/5. Beyond this, we present a novel function of the D-domains of MKK7/9 in regulating the subcellular localization of these kinases. These results provide tools to broadly assess the extent to which these and additional motifs within MKKs function to regulate MAPK signal output throughout the plant.

Novel and expanded roles for MAPK signaling in Arabidopsis stomatal cell fate revealed by cell type-specific manipulations.

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.

Plasticity in stomatal development: what role does MAPK signaling play?

Stomata are specialized pores found on the epidermal surface of many aerial tissues of plants, where they function to regulate the exchange of gases such as carbon dioxide and water vapor between the plant and its environment. This makes stomatal complexes essential for the survival of the plant; a complete loss of stomata is lethal. On a global level, stomatal regulation of gas exchange makes stomata critical regulators of carbon and water cycles, while on an organismal level, stomatal development is flexible in that the ultimate distribution of stomata can be controlled by environmental stimuli. (1) While several environmental factors capable of influencing stomatal development have been identified, the molecular mechanisms mediating this flexibility have remained elusive. Recent studies suggest that this plasticity involves an expanding collection of mitogen activated protein kinase (MAPK) signaling components and putative upstream extracellular ligands. (2,3) Furthermore, it appears that stomatal development and distribution may not be the result of a simple "on/off" switch regulating lineage entry. Rather, stomatal precursors in Arabidopsis can be influenced at multiple points in the well-characterized stomatal development pathway by modulation of a core MAPK signaling module. (3.)

The missing link?: Arabidopsis SPCH is a MAPK specificity factor that controls entry into the stomatal lineage.

Mitogen activated protein kinase (MAPK) signaling modules that incorporate AtMPK3 and AtMPK6 control critical aspects of Arabidopsis biology including stress responses, development, cell division and cell death. Arabidopsis stomatal development is negatively regulated by the YDA-MKK4/5-MPK3/6 MAPK module and follows a three step pathway of asymmetric and symmetric divisions followed by terminal differentiation. We have identified the bHLH transcription factor SPCH, which controls entry into the stomatal lineage as a substrate of AtMPK3 and AtMPK6. These findings suggest that SPCH activity may be directly affected by environmental conditions to enable the plant to modify stomatal development in response to suboptimal climates.

Arabidopsis stomatal initiation is controlled by MAPK-mediated regulation of the bHLH SPEECHLESS.

Stomata, epidermal structures that modulate gas exchange between plants and the atmosphere, play critical roles in primary productivity and the global climate. Positively acting transcription factors and negatively acting mitogen-activated protein kinase (MAPK) signaling control stomatal development in Arabidopsis; however, it is not known how the opposing activities of these regulators are integrated. We found that a unique domain in a basic helix-loop-helix (bHLH) stomatal initiating factor, SPEECHLESS, renders it a MAPK phosphorylation target in vitro and modulates its function in vivo. MAPK cascades modulate a diverse set of activities including development, cell proliferation, and response to external stresses. The coupling of MAPK signaling to SPEECHLESS activity provides cell type specificity for MAPK output while allowing the integration of multiple developmental and environmental signals into the production and spacing of stomata.

A Shout-Out to Stomatal Development: How the bHLH Proteins SPEECHLESS, MUTE and FAMA Regulate Cell Division and Cell Fate.

Arabidopsis guard cell development requires a three step series of asymmetric and symmetric divisions followed by terminal differentiation. We have recently identified three paralogous bHLH transcription factors, SPEECHLESS, MUTE and FAMA, that each function as a master regulator of a specific stage of stomatal development. These findings provide the expected counterbalance to the previously described negative regulatory signaling network and raise intriguing new questions about relationships among the regulators that ultimately enable proper stomatal development and pattern.

Secretion of foreign proteins mediated by chicken lysozyme gene regulatory sequences.

Exploitation of the insulating properties of the complete chicken lysozyme gene domain may facilitate the production of transgenic chicken bioreactors with the capacity to deposit valuable proteins in the egg white. Chimeric genes consisting of the chicken lysozyme gene regulatory sequences and sequences encoding foreign proteins could be inserted randomly into the chicken genome and retain appropriate expression levels. The research reported here established that chicken lysozyme gene regulatory sequences can be used to direct the production and secretion of green fluorescent protein (used as a reporter protein) in transiently transfected chicken blastodermal cells. Attempts to verify these findings in transgenic hens are currently in progress. To provide a rapid means of generating constructs encoding other foreign proteins under the control of lysozyme gene regulatory sequences that can facilitate the secretion of heterologous proteins in vivo, a generic lysozyme gene regulatory scaffold was created using a poxvirus-mediated gene targeting system.