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Other Publications (34)

Articles by Keiko U. Torii in JoVE

 JoVE Biology

Long-term, High-resolution Confocal Time Lapse Imaging of Arabidopsis Cotyledon Epidermis during Germination

1Department of Biology, University of Washington, 2Howard Hughes Medical Institute, University of Washington, 3PRESTO, Japan Science and Technology Agency


JoVE 4426

We describe a protocol using chamber slides and media to immobilize plant cotyledons for confocal imaging of the epidermis over several days of development, documenting stomatal differentiation. Fluorophore-tagged proteins can be tracked dynamically by expression and subcellular localization, increasing understanding of their possible roles during cell division and cell-type differentiation.

Other articles by Keiko U. Torii on PubMed

Dominant-negative Receptor Uncovers Redundancy in the Arabidopsis ERECTA Leucine-rich Repeat Receptor-like Kinase Signaling Pathway That Regulates Organ Shape

Arabidopsis ERECTA, a Leu-rich repeat receptor-like Ser/Thr kinase (LRR-RLK), regulates organ shape and inflorescence architecture. Here, we show that a truncated ERECTA protein that lacks the cytoplasmic kinase domain (DeltaKinase) confers dominant-negative effects when expressed under the control of the native ERECTA promoter and terminator. Transgenic plants expressing DeltaKinase displayed phenotypes, including compact inflorescence and short, blunt siliques, that are characteristic of loss-of-function erecta mutant plants. The DeltaKinase fragment migrated as a stable approximately 400-kD protein complex in the complete absence of the endogenous ERECTA protein and significantly exaggerated the growth defects of the null erecta plants. A functional LRR domain of DeltaKinase was required for dominant-negative effects. Accumulation of DeltaKinase did not interfere with another LRR-RLK signaling pathway (CLAVATA1), which operates in the same cells as ERECTA but has a distinct biological function. Both the erecta mutation and DeltaKinase expression conferred a lesser number of large, disorganized, and expanded cortex cells, which are associated with an increased level of somatic endoploidy. These findings suggest that functionally redundant RLK signaling pathways, including ERECTA, are required to fine-tune the proliferation and growth of cells in the same tissue type during Arabidopsis organogenesis.

ERECTA, an LRR Receptor-like Kinase Protein Controlling Development Pleiotropically Affects Resistance to Bacterial Wilt

Bacterial wilt, one of the most devastating bacterial diseases of plants worldwide, is caused by Ralstonia solanacearum and affects many important crop species. We show that several strains isolated from solanaceous crops in Europe are pathogenic in different accessions of Arabidopsis thaliana. One of these strains, 14.25, causes wilting symptoms in A. thaliana accession Landsberg erecta (Ler) and no apparent symptoms in accession Columbia (Col-0). Disease development and bacterial multiplication in the susceptible Ler accession depend on functional hypersensitive response and pathogenicity (hrp) genes, key elements for bacterial pathogenicity. Genetic analysis using Ler x Col-0 recombinant inbred lines showed that resistance is governed by at least three loci: QRS1 (Quantitative Resistance to R. solanacearum) and QRS2 on chromosome 2, and QRS3 on chromosome 5. These loci explain about 90% of the resistance carried by the Col-0 accession. The ERECTA gene, which encodes a leucine-rich repeat receptor-like kinase (LRR-RLK) and affects development of aerial organs, is dimorphic in our population and lies close to QRS1. Susceptible Ler plants transformed with a wild-type ERECTA gene, and the LER line showed increased disease resistance to R. solanacearum as indicated by reduced wilt symptoms and impaired bacterial growth, suggesting unexpected cross-talk between resistance and developmental pathways.

Synergistic Interaction of Three ERECTA-family Receptor-like Kinases Controls Arabidopsis Organ Growth and Flower Development by Promoting Cell Proliferation

Growth of plant organs relies on coordinated cell proliferation followed by cell growth, but the nature of the cell-cell signal that specifies organ size remains elusive. The Arabidopsis receptor-like kinase (RLK) ERECTA regulates inflorescence architecture. Our previous study using a dominant-negative fragment of ERECTA revealed the presence of redundancy in the ERECTA-mediated signal transduction pathway. Here, we report that Arabidopsis ERL1 and ERL2, two functional paralogs of ERECTA, play redundant but unique roles in a part of the ERECTA signaling pathway, and that synergistic interaction of three ERECTA-family RLKs define aerial organ size. Although erl1 and erl2 mutations conferred no detectable phenotype, they enhanced erecta defects in a unique manner. Overlapping but distinct roles of ERL1 and ERL2 can be ascribed largely to their intricate expression patterns rather than their functions as receptor kinases. Loss of the entire ERECTA family genes led to striking dwarfism, reduced lateral organ size and abnormal flower development, including defects in petal polar expansion, carpel elongation, and anther and ovule differentiation. These defects are due to severely reduced cell proliferation. Our findings place ERECTA-family RLKs as redundant receptors that link cell proliferation to organ growth and patterning.

Leucine-rich Repeat Receptor Kinases in Plants: Structure, Function, and Signal Transduction Pathways

Leucine-rich repeat receptor kinases (LRR-RKs) comprise the largest subfamily of transmembrane receptor-like kinases in plants, with over 200 members in Arabidopsis. LRR-RKs regulate a wide variety of developmental and defense-related processes including cell proliferation, stem cell maintenance, hormone perception, host-specific as well as non-host-specific defense response, wounding response, and symbiosis. Several studies indicate that LRR-RKs act as dimers, and some may form a receptor complex with leucine-rich repeat receptor-like proteins (LRR-RPs) that lack a cytoplasmic kinase domain. Despite the fact that structural features of LRR-RKs are fairy similar, five available ligand molecules for LRR-RKs are structurally diverse, from steroids (brassinolides) to peptides (phytosulfokine and systemin) and secreted proteins (CLV3). Precise ligand-binding sites of LRR-RKs are not understood. However, the extracellular "island" domain that intercepts the LRR domain in some LRR-RKs may play an important role in ligand binding. Advances in unveiling components of three LRR-RK signaling pathways, namely BRI1 in steroid signaling, CLV1 in meristem maintenance, and FLS2 in bacterial elicitor perception, revealed an intriguing link between plant LRR-RK and animal receptor signaling pathways. Finally, rapid progress made in LRR-RK research beyond the model system Arabidopsis has provided exciting, novel insights into the evolution of the LRR-RK signaling system in plants, such as BRI1 utilized in the wound-responsive signaling pathway in Solanaceae plants and recruitment of CLV1 in nodule development in leguminous plants.

Stomatal Patterning and Differentiation by Synergistic Interactions of Receptor Kinases

Coordinated spacing and patterning of stomata allow efficient gas exchange between plants and the atmosphere. Here we report that three ERECTA (ER)-family leucine-rich repeat-receptor-like kinases (LRR-RLKs) together control stomatal patterning, with specific family members regulating the specification of stomatal stem cell fate and the differentiation of guard cells. Loss-of-function mutations in all three ER-family genes cause stomatal clustering. Genetic interactions with a known stomatal patterning mutant too many mouths (tmm) revealed stoichiometric epistasis and combination-specific neomorphism. Our findings suggest that the negative regulation of ER-family RLKs by TMM, which is an LRR receptor-like protein, is critical for proper stomatal differentiation.

Two Callose Synthases, GSL1 and GSL5, Play an Essential and Redundant Role in Plant and Pollen Development and in Fertility

Callose, a beta-1,3-glucan that is widespread in plants, is synthesized by callose synthase. Arabidopsis thaliana contains a family of 12 putative callose synthase genes (GSL1-12). The role of callose and of the individual genes in plant development is still largely uncertain. We have now used TILLING and T-DNA insertion mutants (gsl1-1, gsl5-2 and gsl5-3) to study the role of two closely related and linked genes, GSL1 and GSL5, in sporophytic development and in reproduction. Both genes are expressed in all parts of the plant. Sporophytic development was nearly normal in gsl1-1 homozygotes and only moderately defective in homozygotes for either of the two gsl5 alleles. On the other hand, plants that were gsl1-1/+ gsl5/gsl5 were severely defective, with smaller leaves, shorter roots and bolts and smaller flowers. Plants were fertile when the sporophytes had either two wild-type GSL1 alleles, or one GSL5 allele in a gsl1-1 background, but gsl1-1/+ gsl5/gsl5 plants produced an extremely reduced number of viable seeds. A chromosome with mutations in both GSL1 and GSL5 rendered pollen infertile, although such a chromosome could be transmitted via the egg. As a result, it was not possible to obtain plants that were homozygous for mutations in both the GSL genes. Pollen grain development was severely affected in double mutant plants. Many pollen grains were collapsed and inviable in the gsl1-1/gsl1-1 gsl5/+ and gsl1-1/+ gsl5/gsl5 plants. In addition, gsl1-1/+ gsl5/gsl5 plants produced abnormally large pollen with unusual pore structures, and had problems with tetrad dissociation. In this particular genotype, while the callose wall formed around the pollen mother cells, no callose wall separated the resulting tetrads. We conclude that GSL1 and GSL5 play important, but at least partially redundant roles in both sporophytic development and in the development of pollen. They are responsible for the formation of the callose wall that separates the microspores of the tetrad, and also play a gametophytic role later in pollen grain maturation. Other GSL genes may control callose formation at different steps during pollen development.

Interaction of Auxin and ERECTA in Elaborating Arabidopsis Inflorescence Architecture Revealed by the Activation Tagging of a New Member of the YUCCA Family Putative Flavin Monooxygenases

The aboveground body of higher plants has a modular structure of repeating units, or phytomers. As such, the position, size, and shape of the individual phytomer dictate the plant architecture. The Arabidopsis (Arabidopsis thaliana) ERECTA (ER) gene regulates the inflorescence architecture by affecting elongation of the internode and pedicels, as well as the shape of lateral organs. A large-scale activation-tagging genetic screen was conducted in Arabidopsis to identify novel genes and pathways that interact with the ER locus. A dominant mutant, super1-D, was isolated as a nearly complete suppressor of a partial loss-of-function allele er-103. We found that SUPER1 encodes YUCCA5, a novel member of the YUCCA family of flavin monooxygenases. The activation tagging of YUCCA5 conferred increased levels of free indole acetic acid, increased auxin response, and mild phenotypic characteristics of auxin overproducers, such as elongated hypocotyls, epinastic cotyledons, and narrow leaves. Both genetic and cellular analyses indicate that auxin and the ER pathway regulate cell division and cell expansion in a largely independent but overlapping manner during elaboration of inflorescence architecture.

[Stomatal Patterning and Differentiation: Emerging Role of Cell-cell Signaling]

Termination of Asymmetric Cell Division and Differentiation of Stomata

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.

Autonomy of Cell Proliferation and Developmental Programs During Arabidopsis Aboveground Organ Morphogenesis

Elaboration of size and shape in multicellular organisms involves coordinated cell division and cell growth. In higher plants, continuity of cell layer structures exists from the shoot apical meristem (SAM), where organ primordia arise, to mature aboveground organs. To unravel the extent of inter-cell layer coordination during SAM and aboveground organ development, cell division in the epidermis was selectively restricted by expressing two cyclin-dependent kinase inhibitor genes, KRP1/ICK1 and KRP4, driven by the L1 layer-specific AtML1 promoter. The transgenes conferred reduced plant size with striking, distorted lateral organ shape. While epidermal cell division was severely inhibited with compensatory cell size enlargement, the underlying mesophyll/cortex layer kept normal cell numbers and resulted in small, packed cells with disrupted cell files. Our results demonstrate the autonomy of cell number checkpoint in the underlying tissues when epidermal cell division is restricted. Finally, the L1 layer-specific expression of both KRP1/ICK1 and KRP4 showed no effects on the structure and function of the SAM, suggesting that the effects of these cyclin-dependent kinase inhibitors are context dependent.

The Secretory Peptide Gene EPF1 Enforces the Stomatal One-cell-spacing Rule

Stomata are innovations of land plants that allow regulated gas exchange. Stomatal precursor cells are produced by asymmetric cell division, and once formed, signal their neighbors to inhibit the formation of stomatal precursors in direct contact. We report a gene of Arabidopsis thaliana, EPIDERMAL PATTERNING FACTOR 1 (EPF1) that encodes a small secretory peptide expressed in stomatal cells and precursors and that controls stomatal patterning through regulation of asymmetric cell division. EPF1 activity is dependent on the TOO MANY MOUTHS receptor-like protein and ERECTA family receptor kinases, suggesting that EPF1 may provide a positional cue interpreted by these receptors.

Haploinsufficiency After Successive Loss of Signaling Reveals a Role for ERECTA-family Genes in Arabidopsis Ovule Development

The Arabidopsis genome contains three ERECTA-family genes, ERECTA (ER), ERECTA-LIKE 1 (ERL1) and ERL2 that encode leucine-rich repeat receptor-like kinases. This gene family acts synergistically to coordinate cell proliferation and growth during above-ground organogenesis with the major player, ER, masking the loss-of-function phenotypes of the other two members. To uncover the specific developmental consequence and minimum threshold requirement for signaling, ER-family gene function was successively eliminated. We report here that ERL2 is haploinsufficient for maintaining female fertility in the absence of ER and ERL1. Ovules of the haploinsufficient er-105 erl1-2 erl2-1/+ mutant exhibit abnormal development with reduced cell proliferation in the integuments and gametophyte abortion. Our analysis indicates that progression of integument growth requires ER-family signaling in a dosage-dependent manner and that transcriptional compensation among ER-family members occurs to maintain the required signaling threshold. The specific misregulation of cyclin A genes in the er-105 erl1-2 erl2-1/+ mutant suggests that downstream targets of the ER-signaling pathway might include these core cell-cycle regulators. Finally, genetic interaction of the ER family and the WOX-family gene, PFS2, reveals their contribution to integument development through interrelated mechanisms.

Breaking the Silence: Three BHLH Proteins Direct Cell-fate Decisions During Stomatal Development

Stomata are microscopic pores on the surface of land plants used for gas and water vapor exchange. A pair of highly specialized guard cells surround the pore and adjust pore size. Studies in Arabidopsis have revealed that cell-cell communication is essential to coordinate the asymmetric cell divisions required for proper stomatal patterning. Initial research in this area identified signaling molecules that negatively regulate stomatal differentiation. However, genes promoting cell-fate transition leading to mature guard cells remained elusive. Now, three closely related basic helix-loop-helix (bHLH) proteins, SPEECHLESS, MUTE and FAMA have been identified as positive regulators that direct three consecutive cell-fate decisions during stomatal development. The identification of these genes opens a new direction to investigate the evolution of stomatal development and the conserved functions of bHLH proteins in cell type differentiation adopted by plants and animals.

Stomatal Development: Three Steps for Cell-type Differentiation

Stomata are microscopic pores on the plant epidermis that act as a major passage for the gas and water vapor exchange between a plant and the atmosphere. A pair of specialized guard cells work in concert to adjust pore size to maintain gas exchange while minimizing the water loss. The formation of stomata requires a series of cell-fate transitions from an initial meristemoid mother cell (MMC), to a stem-cell-like precursor meristemoid, to a guard mother cell (GMC), and finally to terminally-differentiated guard cells. Three closely-related Arabidopsis basic helix-loop-helix (bHLH) genes SPEECHLESS (SPCH), MUTE, and FAMA act sequentially at each key step to direct cell-fate transitions during stomatal development. In this addendum, we propose that a three-step relay of the three bHLHs establishes the molecular framework for stomatal differentiation. Specific expression patterns as well as protein domain structure and dimerization partners of each stomatal bHLH protein may determine the specific function as a key switch in each regulatory node.

The BHLH Protein, MUTE, Controls Differentiation of Stomata and the Hydathode Pore in Arabidopsis

Stomata are turgor-driven epidermal valves on the surface of plants that allow for efficient gas and water exchange between the plant and its environment. The Arabidopsis thaliana basic helix-loop-helix (bHLH) protein, MUTE, is a master regulator of stomatal differentiation where it is required for progression through the stomatal lineage and the differentiation of stomata. The genetic control of stomatal spacing across the epidermal surface is variable in different organs. For instance, a distinct suite of genes from those in leaves regulates stomatal patterning in hypocotyls. Here we report that regardless of organ type, MUTE controls downstream events directing stomatal differentiation, specifically the transition from meristemoid to guard mother cell. Ectopic MUTE expression is sufficient to over-ride cell fate specification in cell types that do not normally differentiate stomata. Furthermore, MUTE is required for the production of the structure evolutionarily related to stomata, the hydathode pore. Consistently, MUTE displays expression at the tip of cotyledons and leaves, thus co-localizing with the auxin maxima. However, MUTE itself was not regulated by the auxin, and the absence of hydathode pores in mute did not affect the auxin maxima. Surprisingly, our analysis revealed that the requirement for MUTE could be partially circumvented under conditions of compromised inhibitory signaling.

SCREAM/ICE1 and SCREAM2 Specify Three Cell-state Transitional Steps Leading to Arabidopsis Stomatal Differentiation

Differentiation of specialized cell types in multicellular organisms requires orchestrated actions of cell fate determinants. Stomata, valves on the plant epidermis, are formed through a series of differentiation events mediated by three closely related basic-helix-loop-helix proteins: SPEECHLESS (SPCH), MUTE, and FAMA. However, it is not known what mechanism coordinates their actions. Here, we identify two paralogous proteins, SCREAM (SCRM) and SCRM2, which directly interact with and specify the sequential actions of SPCH, MUTE, and FAMA. The gain-of-function mutation in SCRM exhibited constitutive stomatal differentiation in the epidermis. Conversely, successive loss of SCRM and SCRM2 recapitulated the phenotypes of fama, mute, and spch, indicating that SCRM and SCRM2 together determined successive initiation, proliferation, and terminal differentiation of stomatal cell lineages. Our findings identify the core regulatory units of stomatal differentiation and suggest a model strikingly similar to cell-type differentiation in animals. Surprisingly, map-based cloning revealed that SCRM is INDUCER OF CBF EXPRESSION1, a master regulator of freezing tolerance, thus implicating a potential link between the transcriptional regulation of environmental adaptation and development in plants.

Regulation of Arabidopsis Early Anther Development by the Mitogen-activated Protein Kinases, MPK3 and MPK6, and the ERECTA and Related Receptor-like Kinases

Mitogen-activated protein kinase (MAPK) and leucine-rich repeat receptor-like kinase (LRR-RLK) signaling pathways have been shown to regulate diverse aspects of plant growth and development. In Arabidopsis, proper anther development relies on intercellular communication to coordinate cell proliferation and differentiation. Two closely related genes encoding MAPKs, MPK3 and MPK6, function redundantly in regulating stomatal patterning. Although the mpk6 mutant has reduced fertility, the function of MPK3 and MPK6 in anther development has not been characterized. Similarly, the ERECTA (ER), ERECTA-LIKE1 (ERL1) and ERL2 genes encoding LRR-RLKs function together to direct stomatal cell fate specification and the er-105 erl1-2 erl2-1 triple mutant is sterile. Because the mpk3 mpk6 double null mutant is embryo lethal, anther development was characterized in the viable mpk3/+ mpk6/- and er-105 erl1-2 erl2-1 mutants. We found that both mutant anthers usually fail to form one or more of the four anther lobes, with the er-105 erl1-2 erl2-1 triple mutant exhibiting more severe phenotypes than those of the mpk3/+ mpk6/- mutant. The somatic cell layers of the differentiated mutant lobes appeared larger and more disorganized than that of wild-type. In addition, the er-105 erl1-2 erl2-1 triple mutant has a reduced number of stamens, the majority of which possess completely undifferentiated or under-differentiated anthers. Furthermore, sometimes, the mpk3/+ mpk6/- mutant anthers do not dehisce, and the er-105 erl1-2 erl2-1 anthers were not observed to dehisce. Therefore, our results indicate that both ER/ERL1/ERL2 and MPK3/MPK6 play important roles in normal anther lobe formation and anther cell differentiation. The close functional relationship between these genes in other developmental processes and the similarities in anther developmental phenotypes of the two types of mutants reported here further suggest the possibility that these genes might also function in the same pathway to regulate anther cell division and differentiation.

Epidermal Cell Density is Autoregulated Via a Secretory Peptide, EPIDERMAL PATTERNING FACTOR 2 in Arabidopsis Leaves

Regulation of the number of cells is critical for development of multicellular organisms. During plant epidermal development, a protodermal cell first makes a fate decision of whether or not to be the meristemoid mother cell (MMC), which undergoes asymmetric cell division forming a meristemoid and its sister cell. The MMC-derived lineage produces all stomatal guard cells and a large proportion of non-guard cells. We demonstrate that a small secretory peptide, EPIDERMAL PATTERING FACTOR 2 (EPF2), is produced by the MMC and its early descendants, and negatively regulates the density of guard and non-guard epidermal cells. Our results suggest that EPF2 inhibits cells from adopting the MMC fate in a non-cell-autonomous manner, thus limiting the number of MMCs. This feedback loop is critical for regulation of epidermal cell density. The amino acid sequence of EPF2 resembles that of EPF1, which is known to control stomatal positioning. Over-expression of EPF1 also inhibits stomatal development, but EPF1 can act only on a later developmental process than EPF2. Overexpression and promoter swapping experiments suggested that the protein functions of EPF1 and EPF2, rather than the expression patterns of the genes, are responsible for the specific functions. Although targets of EPF1 and EPF2 are different, both EPF1 and EPF2 require common putative receptor components TOO MANY MOUTHS (TMM), ERECTA (ER), ERECTA LIKE 1 (ERL1) and ERL2 in order to function.

Ethylene-induced Hyponastic Growth in Arabidopsis Thaliana is Controlled by ERECTA

Plants can respond quickly and profoundly to detrimental changes in their environment. For example, Arabidopsis thaliana can induce an upward leaf movement response through differential petiole growth (hyponastic growth) to outgrow complete submergence. This response is induced by accumulation of the phytohormone ethylene in the plant. Currently, only limited information is available on how this response is molecularly controlled. In this study, we utilized quantitative trait loci (QTL) analysis of natural genetic variation among Arabidopsis accessions to isolate novel factors controlling constitutive petiole angles and ethylene-induced hyponastic growth. Analysis of mutants in various backgrounds and complementation analysis of naturally occurring mutant accessions provided evidence that the leucin-rich repeat receptor-like Ser/Thr kinase gene, ERECTA, controls ethylene-induced hyponastic growth. Moreover, ERECTA controls leaf positioning in the absence of ethylene treatment. Our data demonstrate that this is not due to altered ethylene production or sensitivity.

ERECTA Controls Low Light Intensity-induced Differential Petiole Growth Independent of Phytochrome B and Cryptochrome 2 Action in Arabidopsis Thaliana

Plants can respond quickly and profoundly to changes in their environment. Several species, including Arabidopsis thaliana, are capable of differential petiole growth driven upward leaf movement (hyponastic growth) to escape from detrimental environmental conditions. Recently, we demonstrated that the leucine-rich repeat receptor-like Ser/Thr kinase gene ERECTA, explains a major effect Quantitative Trait Locus (QTL) for ethylene-induced hyponastic growth in Arabidopsis. Here, we demonstrate that ERECTA controls the hyponastic growth response to low light intensity treatment in a genetic background dependent manner. Moreover, we show that ERECTA affects low light-induced hyponastic growth independent of Phytochrome B and Cryptochrome 2 signaling, despite that these photoreceptors are positive regulators of low light-induced hyponastic growth.

Out of the Mouths of Plants: the Molecular Basis of the Evolution and Diversity of Stomatal Development

Stomata are microscopic valves on the plant epidermis that played a critical role in the evolution of land plants. Studies in the model dicot Arabidopsis thaliana have identified key transcription factors and signaling pathways controlling stomatal patterning and differentiation. Three paralogous Arabidopsis basic helix-loop-helix proteins, SPEECHLESS (SPCH), MUTE, and FAMA, mediate sequential steps of cell-state transitions together with their heterodimeric partners SCREAM (SCRM) and SCRM2. Cell-cell signaling components, including putative ligands, putative receptors, and mitogen-activated protein kinase cascades, orient asymmetric cell divisions and prevent overproduction and clustering of stomata. The recent availability of genome sequence and reverse genetics tools for model monocots and basal land plants allows for the examination of the conservation of genes important in stomatal patterning and differentiation. Studies in grasses have revealed that divergence of SPCH-MUTE-FAMA predates the evolutionary split of monocots and dicots and that these proteins show conserved and novel roles in stomatal differentiation. By contrast, specific asymmetric cell divisions in Arabidopsis and grasses require unique molecular components. Molecular phylogenetic analysis implies potential conservation of signaling pathways and prototypical functions of the transcription factors specifying stomatal differentiation.

Plant Twitter: Ligands Under 140 Amino Acids Enforcing Stomatal Patterning

Stomata are an essential land plant innovation whose patterning and density are under genetic and environmental control. Recently, several putative ligands have been discovered that influence stomatal density, and they all belong to the epidermal patterning factor-like family of secreted cysteine-rich peptides. Two of these putative ligands, EPF1 and EPF2, are expressed exclusively in the stomatal lineage cells and negatively regulate stomatal density. A third, EPFL6 or CHALLAH, is also a negative regulator of density, but is expressed subepidermally in the hypocotyl. A fourth, EPFL9 or STOMAGEN, is expressed in the mesophyll tissues and is a positive regulator of density. Genetic evidence suggests that these ligands may compete for the same receptor complex. Proper stomatal patterning is likely to be an intricate process involving ligand competition, regional specificity, and communication between tissue layers. EPFL-family genes exist in the moss Physcomitrella patens, the lycophyte Selaginella moellendorffii, and rice, Oryza sativa, and their sequence analysis yields several genes some of which are related to EPF1, EPF2, EPFL6, and EPFL9. Presence of these EPFL family members in the basal land plants suggests an exciting hypothesis that the genetic components for stomatal patterning originated early in land plant evolution.

Dysregulation of Cell-to-cell Connectivity and Stomatal Patterning by Loss-of-function Mutation in Arabidopsis Chorus (glucan Synthase-like 8)

Patterning of stomata, valves on the plant epidermis, requires the orchestrated actions of signaling components and cell-fate determinants. To understand the regulation of stomatal patterning, we performed a genetic screen using a background that partially lacks stomatal signaling receptors. Here, we report the isolation and characterization of chorus (chor), which confers excessive proliferation of stomatal-lineage cells mediated by SPEECHLESS (SPCH). chor breaks redundancy among three ERECTA family genes and strongly enhances stomatal patterning defects caused by loss-of-function in TOO MANY MOUTHS. chor seedlings also exhibit incomplete cytokinesis and growth defects, including disruptions in root tissue patterning and root hair cell morphogenesis. CHOR encodes a putative callose synthase, GLUCAN SYNTHASE-LIKE 8 (GSL8), that is required for callose deposition at the cell plate, cell wall and plasmodesmata. Consistently, symplastic macromolecular diffusion between epidermal cells is significantly increased in chor, and proteins that do not normally move cell-to-cell, including a fluorescent protein-tagged SPCH, diffuse to neighboring cells. Such a phenotype is not a general trait caused by cytokinesis defects. Our findings suggest that the restriction of symplastic movement might be an essential step for the proper segregation of cell-fate determinants during stomatal development.

FERONIA As an Upstream Receptor Kinase for Polar Cell Growth in Plants

Arabidopsis ERECTA-family Receptor Kinases Mediate Morphological Alterations Stimulated by Activation of NB-LRR-type UNI Proteins

Shoot apical meristems (SAMs), which maintain stem cells at the tips of stems, and axillary meristems (AMs), which arise at leaf axils for branch formation, play significant roles in the establishment of plant architecture. Previously, we showed that, in Arabidopsis thaliana, activation of NB-LRR (nucleotide-binding site-leucine-rich repeat)-type UNI proteins affects plant morphology through modulation of the regulation of meristems. However, information about genes involved in the processes was still lacking. Here, we report that ERECTA (ER) receptor kinase family members cooperatively mediate the morphological alterations that are stimulated by activation of UNI proteins. uni-1D is a gain-of-function mutation in the UNI gene and uni-1D mutants exhibit early termination of inflorescence stem growth and also formation of extra AMs at leaf axils. The former defect involves modulation of the SAM activity and is suppressed by er mutation. Though the AM phenotype is not affected by a single er mutation, it is suppressed by simultaneous mutations of ER-family members. It was previously shown that trans-zeatin (tZ)-type cytokinins were involved in the morphological phenotypes of uni-1D mutants and that expression of CYP735A2, which is essential for biosynthesis of tZ-type cytokinins, was modulated in uni-1D mutants. We show that this modulation of CYP735A2 expression requires activities of ER-family members. Moreover, the ER activity in UNI-expressing cells contributes to all morphological phenotypes of uni-1D mutants, suggesting that a cross-talk between ER-family-dependent and UNI-triggered signaling pathways plays a significant role in the morphological alterations observed in uni-1D mutants.

The Presence of Multiple Introns is Essential for ERECTA Expression in Arabidopsis

Gene expression in eukaryotes is often enhanced by the presence of introns. Depending on the specific gene, this enhancement can be minor or very large and occurs at both the transcriptional and post-transcriptional levels. The Arabidopsis ERECTA gene contains 27 exons encoding a receptor-like kinase that promotes cell proliferation and inhibits cell differentiation in above-ground plant organs. The expression of ERECTA very strongly depends on the presence of introns. The intronless ERECTA gene does not rescue the phenotype of erecta mutant plants and produces about 500-900 times less protein compared with the identical construct containing introns. This result is somewhat surprising as the region upstream of the ERECTA coding sequence effectively promotes the expression of extraneous genes. Here, we demonstrate that introns are essential for ERECTA mRNA accumulation and, to a lesser extent, for mRNA utilization in translation. Since mRNA produced by intronless ERECTA is degraded at the 3' end, we speculate that introns increase mRNA accumulation through increasing its stability at least in part. No individual intron is absolutely necessary for ERECTA expression, but rather multiple introns in specific locations increase ERECTA expression in an additive manner. The ability of introns to promote ERECTA expression might be linked to the process of splicing and not to a particular intron sequence.

Molecular Profiling of Stomatal Meristemoids Reveals New Component of Asymmetric Cell Division and Commonalities Among Stem Cell Populations in Arabidopsis

The balance between maintenance and differentiation of stem cells is a central question in developmental biology. Development of stomata in Arabidopsis thaliana begins with de novo asymmetric divisions producing meristemoids, proliferating precursor cells with stem cell-like properties. The transient and asynchronous nature of the meristemoid has made it difficult to study its molecular characteristics. Synthetic combination of stomatal differentiation mutants due to loss- or gain-of-function mutations in SPEECHLESS, MUTE, and SCREAM create seedlings with an epidermis overwhelmingly composed of pavement cells, meristemoids, or stomata, respectively. Through transcriptome analysis, we define and characterize the molecular signatures of meristemoids. The reporter localization studies of meristemoid-enriched proteins reveals pathways not previously associated with stomatal development. We identified a novel protein, POLAR, and demonstrate through time-lapse live imaging that it exhibits transient polar localization and segregates unevenly during meristemoid asymmetric divisions. The polar localization of POLAR requires BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE. Comparative bioinformatic analysis of the transcriptional profiles of a meristemoid with shoot and root apical meristems highlighted cytokinin signaling and the ERECTA family receptor-like kinases in the broad regulation of stem cell populations. Our work reveals molecular constituents of stomatal stem cells and illuminates a common theme among stem cell populations in plants.

Direct Interaction of Ligand-receptor Pairs Specifying Stomatal Patterning

Valves on the plant epidermis called stomata develop according to positional cues, which likely involve putative ligands (EPIDERMAL PATTERNING FACTORS [EPFs]) and putative receptors (ERECTA family receptor kinases and TOO MANY MOUTHS [TMM]) in Arabidopsis. Here we report the direct, robust, and saturable binding of bioactive EPF peptides to the ERECTA family. In contrast, TMM exhibits negligible binding to EPF1 but binding to EPF2. The ERECTA family forms receptor homomers in vivo. On the other hand, TMM associates with the ERECTA family but not with itself. While ERECTA family receptor kinases exhibit complex redundancy, blocking ERECTA and ERECTA-LIKE1 (ERL1) signaling confers specific insensitivity to EPF2 and EPF1, respectively. Our results place the ERECTA family as the primary receptors for EPFs with TMM as a signal modulator and establish EPF2-ERECTA and EPF1-ERL1 as ligand-receptor pairs specifying two steps of stomatal development: initiation and spacing divisions.

Mechanisms of Stomatal Development

The main route for CO(2) and water vapor exchange between a plant and the environment is through small pores called stomata. The accessibility of stomata and predictable division series that characterize their development provides an excellent system to address fundamental questions in biology. Stomatal cell-state transition and specification are regulated by a suite of transcription factors controlled by positional signaling via peptide ligands and transmembrane receptors. Downstream effectors include several members of the core cell-cycle genes. Environmentally induced signals are integrated into this essential developmental program to modulate stomatal development or function in response to changes in the abiotic environment. In addition, the recent identification of premitotic polarly localized proteins from both Arabidopsis and maize has laid a foundation for the future understanding of intrinsic cell polarity in plants. This review highlights the mechanisms of stomatal development through characterization of genes controlling cell-fate specification, cell polarity, cell division, and cell-cell communication during stomatal development and discusses the genetic framework linking these molecular processes with the correct spacing, density, and differentiation of stomata.

Regulation of Inflorescence Architecture by Intertissue Layer Ligand-receptor Communication Between Endodermis and Phloem

Multicellular organisms achieve final body shape and size by coordinating cell proliferation, expansion, and differentiation. Loss of function in the Arabidopsis ERECTA (ER) receptor-kinase gene confers characteristic compact inflorescence architecture, but its underlying signaling pathways remain unknown. Here we report that the expression of ER in the phloem is sufficient to rescue compact er inflorescences. We further identified two Epidermal Patterning Factor-like (EPFL) secreted peptide genes, EPFL4 and EPFL6/CHALLAH (CHAL), as redundant, upstream components of ER-mediated inflorescence growth. The expression of EPFL4 or EPFL6 in the endodermis, a layer adjacent to phloem, is sufficient to rescue the er-like inflorescence of epfl4 epfl6 plants. EPFL4 and EPFL6 physically associate with ER in planta. Finally, transcriptome analysis of er and epfl4 epfl6 revealed a potential downstream component as well as a role for plant hormones in EPFL4/6- and ER-mediated inflorescence growth. Our results suggest that intercell layer communication between the endodermis and phloem mediated by peptide ligands and a receptor kinase coordinates proper inflorescence architecture in Arabidopsis.

Two-dimensional Spatial Patterning in Developmental Systems

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.

Mix-and-match: Ligand-receptor Pairs in Stomatal Development and Beyond

Stomata are small valves on the plant epidermis balancing gas exchange and water loss. Stomata are formed according to positional cues. In Arabidopsis, two EPIDERMAL PATTERNING FACTOR (EPF) peptides, EPF1 and EPF2, are secreted from stomatal precursors enforcing proper stomatal patterning. Here, I review recent studies revealing the ligand-receptor pairs and revising the previously predicted relations between receptors specifying stomatal patterning: ERECTA-family and TOO MANY MOUTHS (TMM). Furthermore, EPF-LIKE9 (EPFL9/Stomagen) promotes stomatal differentiation from internal tissues. Two EPFL peptides specify inflorescence architecture, a process beyond stomatal development, as ligands for ERECTA. Thus, broadly expressed receptor kinases may regulate multiple developmental processes through perceiving different peptide ligands, each with a specialized expression pattern. TMM in the epidermis may fine-tune multiple EPF/EPFL signals to prevent signal interference.

Cell Biology - Building Blocks for Dynamic Development and Behaviors

A MAPK Cascade Downstream of ERECTA Receptor-Like Protein Kinase Regulates Arabidopsis Inflorescence Architecture by Promoting Localized Cell Proliferation

Spatiotemporal-specific cell proliferation and cell differentiation are critical to the formation of normal tissues, organs, and organisms. The highly coordinated cell differentiation and proliferation events illustrate the importance of cell-cell communication during growth and development. In Arabidopsis thaliana, ERECTA (ER), a receptor-like protein kinase, plays important roles in promoting localized cell proliferation, which determines inflorescence architecture, organ shape, and size. However, the downstream signaling components remain unidentified. Here, we report a mitogen-activated protein kinase (MAPK; or MPK) cascade that functions downstream of ER in regulating localized cell proliferation. Similar to an er mutant, loss of function of MPK3/MPK6 or their upstream MAPK kinases (MAPKKs; or MKKs), MKK4/MKK5, resulted in shortened pedicels and clustered inflorescences. Epistasis analysis demonstrated that the gain of function of MKK4 and MKK5 transgenes could rescue the loss-of-function er mutant phenotype at both morphological and cellular levels, suggesting that the MPK3/MPK6 cascade functions downstream of the ER receptor. Furthermore, YODA (YDA), a MAPKK kinase, was shown to be upstream of MKK4/MKK5 and downstream of ER in regulating inflorescence architecture based on both gain- and loss-of-function data. Taken together, these results suggest that the YDA-MKK4/MKK5-MPK3/MPK6 cascade functions downstream of the ER receptor in regulating localized cell proliferation, which further shapes the morphology of plant organs.

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