A MAPKK Kinase Gene Regulates Extra-Embryonic Cell Fate in Arabidopsis

Identification and Validation of a Novel Major Quantitative Trait Locus for Plant Height in Common Wheat (Triticum aestivum L.)

Plant height (PH) plays a pivotal role in plant morphological architecture and is associated with yield potential in wheat. For the quantitative trait locus (QTL) analysis, a recombinant inbred line population was developed between varieties differing significantly in PH. Two major QTL were identified on chromosomes 4B (QPh.sicau-4B) and 6D (QPh.sicau-6D) in multiple environments, which were then validated in two different backgrounds by using closely linked markers. QPh.sicau-4B explained 10.1-21.3% of the phenotypic variance, and the location corresponded to the dwarfing gene Rht-B1. QPh.sicau-6D might be a novel QTL for PH, explaining 6.6-13.6% of the phenotypic variance and affecting spike length, thousand-kernel weight, and spikelet compactness. Three candidate genes associated with plant growth and development were identified in the physical interval of QPh.sicau-6D. Collectively, we identified a novel stable and major PH QTL, QPh.sicau-6D, which could aid in the development of closely linked markers for marker-assisted breeding and cloning genes underlying this QTL.

Genetic activity during early plant embryogenesis

Seeds are essential for human civilization, so understanding the molecular events underpinning seed development and the zygotic embryo it contains is important. In addition, the approach of somatic embryogenesis is a critical propagation and regeneration strategy to increase desirable genotypes, to develop new genetically modified plants to meet agricultural challenges, and at a basic science level, to test gene function. We briefly review some of the transcription factors (TFs) involved in establishing primary and apical meristems during zygotic embryogenesis, as well as TFs necessary and/or sufficient to drive somatic embryo programs. We focus on the model plant Arabidopsis for which many tools are available, and review as well as speculate about comparisons and contrasts between zygotic and somatic embryo processes.

Receptor-Like Protein Kinases Function Upstream of MAPKs in Regulating Plant Development

Mitogen-activated protein kinases (MAPKs) are a group of protein kinase broadly involved in various signal pathways in eukaryotes. In plants, MAPK cascades regulate growth, development, stress responses and immunity by perceiving signals from the upstream regulators and transmitting the phosphorylation signals to the downstream signaling components. To reveal the interactions between MAPK cascades and their upstream regulators is important for understanding the functional mechanisms of MAPKs in the life span of higher plants. Typical receptor-like protein kinases (RLKs) are plasma membrane-located to perceive endogenous or exogenous signal molecules in regulating plant growth, development and immunity. MAPK cascades bridge the extracellular signals and intracellular transcription factors in many RLK-mediated signaling pathways. This review focuses on the current findings that RLKs regulate plant development through MAPK cascades and discusses questions that are worth investigating in the near future.

YODA Kinase Controls a Novel Immune Pathway of Tomato Conferring Enhanced Disease Resistance to the Bacterium Pseudomonas syringae

Mitogen-activated protein kinases (MAPK) play pivotal roles in transducing developmental cues and environmental signals into cellular responses through pathways initiated by MAPK kinase kinases (MAP3K). AtYODA is a MAP3K of Arabidopsis thaliana that controls stomatal development and non-canonical immune responses. Arabidopsis plants overexpressing a constitutively active YODA protein (AtCA-YDA) show broad-spectrum disease resistance and constitutive expression of defensive genes. We tested YDA function in crops immunity by heterologously overexpressing AtCA-YDA in Solanum lycopersicum. We found that these tomato AtCA-YDA plants do not show developmental phenotypes and fitness alterations, except a reduction in stomatal index, as reported in Arabidopsis AtCA-YDA plants. Notably, AtCA-YDA tomato plants show enhanced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 and constitutive upregulation of defense-associated genes, corroborating the functionality of YDA in tomato immunity. This function was further supported by generating CRISPR/Cas9-edited tomato mutants impaired in the closest orthologs of AtYDA [Solyc08g081210 (SlYDA1) and Solyc03g025360 (SlYDA2)]. Slyda1 and Slyda2 mutants are highly susceptible to P. syringae pv. tomato DC3000 in comparison to wild-type plants but only Slyda2 shows altered stomatal index. These results indicate that tomato orthologs have specialized functions and support that YDA also regulates immune responses in tomato and may be a trait for breeding disease resistance.

Stomatal development in the grasses: lessons from models and crops (and crop models)

When plants emerged from their aquatic origins to colonise land, they needed to avoid desiccation while still enabling gas and water exchange with the environment. The solution was the development of a waxy cuticle interrupted by epidermal pores, known as stomata. Despite the importance of stomata in plant physiology and their contribution to global water and carbon cycles, our knowledge of the genetic basis of stomatal development is limited mostly to the model dicot, Arabidopsis thaliana. This limitation is particularly troublesome when evaluating grasses, whose members represent our most agriculturally significant crops. Grass stomatal development follows a trajectory strikingly different from Arabidopsis and their uniquely shaped four-celled stomatal complexes are especially responsive to environmental inputs. Thus, understanding the development and regulation of these efficient complexes is of particular interest for the purposes of crop engineering. This review focuses on genetic regulation of grass stomatal development and prospects for the future, highlighting discoveries enabled by parallel comparative investigations in cereal crops and related genetic model species such as Brachypodium distachyon.

The integral spliceosomal component CWC15 is required for development in Arabidopsis

Efficient mRNA splicing is a prerequisite for protein biosynthesis and the eukaryotic splicing machinery is evolutionarily conserved among species of various phyla. At its catalytic core resides the activated splicing complex Bact consisting of the three small nuclear ribonucleoprotein complexes (snRNPs) U2, U5 and U6 and the so-called NineTeen complex (NTC) which is important for spliceosomal activation. CWC15 is an integral part of the NTC in humans and it is associated with the NTC in other species. Here we show the ubiquitous expression and developmental importance of the Arabidopsis ortholog of yeast CWC15. CWC15 associates with core components of the Arabidopsis NTC and its loss leads to inefficient splicing. Consistent with the central role of CWC15 in RNA splicing, cwc15 mutants are embryo lethal and additionally display strong defects in the female haploid phase. Interestingly, the haploid male gametophyte or pollen in Arabidopsis, on the other hand, can cope without functional CWC15, suggesting that developing pollen might be more tolerant to CWC15-mediated defects in splicing than either embryo or female gametophyte.

Cellular dynamics of double fertilization and early embryogenesis in flowering plants

Flowering plants (angiosperms) perform a unique double fertilization in which two sperm cells fuse with two female gamete cells in the embryo sac to develop a seed. Furthermore, during land plant evolution, the mode of sexual reproduction has been modified dramatically from motile sperm in the early-diverging land plants, such as mosses and ferns as well as some gymnosperms (Ginkgo and cycads) to nonmotile sperm that are delivered to female gametes by the pollen tube in flowering plants. Recent studies have revealed the cellular dynamics and molecular mechanisms for the complex series of double fertilization processes and elucidated differences and similarities between animals and plants. Here, together with a brief comparison with animals, we review the current understanding of flowering plant zygote dynamics, covering from gamete nuclear migration, karyogamy, and polyspermy block, to zygotic genome activation as well as asymmetrical division of the zygote. Further analyses of the detailed molecular and cellular mechanisms of flowering plant fertilization should shed light on the evolution of the unique sexual reproduction of flowering plants.

Multifaceted roles of HEAT SHOCK PROTEIN 90 molecular chaperones in plant development

HEAT SHOCK PROTEINS 90 (HSP90s) are molecular chaperones that mediate correct folding and stability of many client proteins. These chaperones act as master molecular hubs involved in multiple aspects of cellular and developmental signalling in diverse organisms. Moreover, environmental and genetic perturbations affect both HSP90s and their clients, leading to alterations of molecular networks determining respectively plant phenotypes and genotypes and contributing to a broad phenotypic plasticity. Although HSP90 interaction networks affecting the genetic basis of phenotypic variation and diversity have been thoroughly studied in animals, such studies are just starting to emerge in plants. Here, we summarize current knowledge and discuss HSP90 network functions in plant development and cellular homeostasis.

Mitochondrial ribosomal protein S9M is involved in male gametogenesis and seed development in Arabidopsis

Mitochondrial function is critical for cell vitality in all eukaryotes including plants. Although plant mitochondria contain many proteins, few have been studied in the context of plant development and physiology. We used knock-down mutant RPS9M to study its important role in male gametogenesis and seed development in Arabidopsis thaliana. Knock-down of RPS9M in the rps9m-3 mutant led to abnormal pollen development and impaired pollen tube growth. In addition, both embryo and endosperm development were affected. Phenotype analysis revealed that the rps9m-3 mutant contained a lower amount of endosperm and nuclear proteins, and both embryo cell division and embryo pattern were affected, resulting in an abnormal and defective embryo. Lowering the level of RPS9M in rps9m-3 affects mitochondrial ribosome biogenesis, energy metabolism and production of ROS. Our data revealed that RPS9M plays important roles in normal gametophyte development and seed formation, possibly by sustaining mitochondrial function.

Plasmogamic Paternal Contributions to Early Zygotic Development in Flowering Plants

Flowering plant zygotes possess complete developmental potency, and the mixture of male and female genetic and cytosolic materials in the zygote is a trigger to initiate embryo development. Plasmogamy, the fusion of the gamete cytoplasms, facilitates the cellular dynamics of the zygote. In the last decade, mutant analyses, live cell imaging-based observations, and direct observations of fertilized egg cells by in vitro fusion of isolated gametes have accelerated our understanding of the post-plasmogamic events in flowering plants including cell wall formation, gamete nuclear migration and fusion, and zygotic cell elongation and asymmetric division. Especially, it has become more evident that paternal parent-of-origin effects, via sperm cytoplasm contents, not only control canonical early zygotic development, but also activate a biparental signaling pathway critical for cell fate determination after the first cell division. Here, we summarize the plasmogamic paternal contributions via the entry of sperm contents during/after fertilization in flowering plants.

Intracellular dynamics and transcriptional regulations in plant zygotes: a case study of Arabidopsis

Recent understandings ofArabidopsiszygote. Body axis formation is essential for the proper development of multicellular organisms. The apical-basal axis in Arabidopsis thaliana is determined by the asymmetric division of the zygote, following its cellular polarization. However, the regulatory mechanism of zygote polarization is unclear due to technical issues. The zygote is located deep in the seed (ovule) in flowers, which prevents the living dynamics of zygotes from being observed. In addition, elucidation of molecular pathways by conventional forward genetic screens was not enough because of high gene redundancy in early development. Here, we present a review introducing two new methods, which have been developed to overcome these problems. Method 1: the two-photon live-cell imaging method provides a new system to visualize the dynamics of intracellular structures in Arabidopsis zygotes, such as cytoskeletons and vacuoles. Microtubules form transverse rings and control zygote elongation, while vacuoles dynamically change their shapes along longitudinal actin filaments and support polar nuclear migration. Method 2: the transcriptome method uses isolated Arabidopsis zygotes and egg cells to reveal the gene expression profiles before and after fertilization. This approach revealed that de novo transcription occurs extensively and immediately after fertilization. Moreover, inhibition of the de novo transcription was shown to sufficiently block the zygotic division, thus indicating a strong possibility that yet unidentified zygote regulators can be found using this transcriptome approach. These new strategies in Arabidopsis will help to further our understanding of the fundamental principles regarding the proper formation of plant bodies from unicellular zygotes.

The short and intricate life of the suspensor

The suspensor is a short-lived tissue critical for proper embryonic development in many higher plants. While the tissue was initially thought to simply suspend the embryo in the endosperm, it has been found through decades of research that it serves multiple important purposes. The suspensor has been found to be vital for proper embryo patterning and numerous studies have been undertaken into the complex transcriptional cross-talk between the suspensor and the embryo proper. Indeed, many suspensor mutants also display abnormalities in the embryo. The suspensor's role as a nutrient conduit has been shown using ultrastructural and histochemical techniques. Biochemical approaches have found that the suspensor is a centre of early embryonic hormone production in several species. The suspensor has also been frequently used as a model for programmed cell death as it shows signs of termination almost immediately upon developing. This review covers the essential functions of the suspensor throughout its short existence from multiple disciplines including structural, genetic and biochemical perspectives.

Does Molecular and Structural Evolution Shape the Speedy Grass Stomata?

It has been increasingly important for breeding programs to be aimed at crops that are capable of coping with a changing climate, especially with regards to higher frequency and intensity of drought events. Grass stomatal complex has been proposed as an important factor that may enable grasses to adapt to water stress and variable climate conditions. There are many studies focusing on the stomatal morphology and development in the eudicot model plant Arabidopsis and monocot model plant Brachypodium. However, the comprehensive understanding of the distinction of stomatal structure and development between monocots and eudicots, especially between grasses and eudicots, are still less known at evolutionary and comparative genetic levels. Therefore, we employed the newly released version of the One Thousand Plant Transcriptome (OneKP) database and existing databases of green plant genome assemblies to explore the evolution of gene families that contributed to the formation of the unique structure and development of grass stomata. This review emphasizes the differential stomatal morphology, developmental mechanisms, and guard cell signaling in monocots and eudicots. We provide a summary of useful molecular evidences for the high water use efficiency of grass stomata that may offer new horizons for future success in breeding climate resilient crops.

YODA-HSP90 Module Regulates Phosphorylation-Dependent Inactivation of SPEECHLESS to Control Stomatal Development under Acute Heat Stress in Arabidopsis

Stomatal ontogenesis, patterning, and function are hallmarks of environmental plant adaptation, especially to conditions limiting plant growth, such as elevated temperatures and reduced water availability. The specification and distribution of a stomatal cell lineage and its terminal differentiation into guard cells require a master regulatory protein phosphorylation cascade involving the YODA mitogen-activated protein kinase kinase kinase. YODA signaling results in the activation of MITOGEN-ACTIVATED PROTEIN KINASEs (MPK3 and MPK6), which regulate transcription factors, including SPEECHLESS (SPCH). Here, we report that acute heat stress affects the phosphorylation and deactivation of SPCH and modulates stomatal density. By using complementary molecular, genetic, biochemical, and cell biology approaches, we provide solid evidence that HEAT SHOCK PROTEINS 90 (HSP90s) play a crucial role in transducing heat-stress response through the YODA cascade. Genetic studies revealed that YODA and HSP90.1 are epistatic, and they likely function linearly in the same developmental pathway regulating stomata formation. HSP90s interact with YODA, affect its cellular polarization, and modulate the phosphorylation of downstream targets, such as MPK6 and SPCH, under both normal and heat-stress conditions. Thus, HSP90-mediated specification and differentiation of the stomatal cell lineage couples stomatal development to environmental cues, providing an adaptive heat stress response mechanism in plants.

Cell lineage-specific transcriptome analysis for interpreting cell fate specification of proembryos

In Arabidopsis, a zygote undergoes asymmetrical cell division that establishes the first two distinct cell types of early proembryos, apical and basal cells. However, the genome-wide transcriptional activities that guide divergence of apical and basal cell development remain unknown. Here, we present a comprehensive transcriptome analysis of apical and basal cell lineages, uncovering distinct molecular pathways during cell lineage specification. Selective deletion of inherited transcripts and specific de novo transcription contribute to the establishment of cell lineage-specific pathways for cell fate specification. Embryo-related pathways have been specifically activated in apical cell lineage since 1-cell embryo stage, but quick transcriptome remodeling toward suspensor-specific pathways are found in basal cell lineage. Furthermore, long noncoding RNAs and alternative splicing isoforms may be involved in cell lineage specification. This work also provides a valuable lineage-specific transcriptome resource to elucidate the molecular pathways for divergence of apical and basal cell lineages at genome-wide scale.

Asymmetric division of the Arabidopsis zygote produces apical and basal cells that mainly develop into embryo and suspensor, respectively. Here, Zhou et al. show that de novo transcription and selective RNA turnover establish distinct apical and basal transcriptomes as early as the 1-cell stage of embryo development.

Square one: zygote polarity and early embryogenesis in flowering plants

In the last two decades, work on auxin signaling has helped to understand many aspects of the fundamental process underlying the specification of tissue types in the plant embryo. However, the immediate steps after fertilization including the polarization of the zygote and the initial body axis formation remained poorly understood. Valuable insight into these enigmatic processes has been gained by studying fertilization in grasses. Recent technical advances in transcriptomics of developing embryos with high spatial and temporal resolution give an emerging picture of the rapid changes of the zygotic developmental program. Together with the use of live imaging of novel fluorescent marker lines, these data are now the basis of unraveling the very first steps of the embryonic patterning process.

Molecular Mechanism Underlying the Effect of the Intraspecific Alternation of Seed Size on Plant Drought Tolerance

In crop plants, the yield loss caused by drought exceeds the losses resulting from other adverse environment stresses. In numerous plant species, seedling establishment is positively correlated with the initial seed size under drought stress conditions. In intra- and interspecies, plants with large seeds can withstand water deficiency stresses, whereas those with small seeds are efficient colonizers as a result of their ability to produce more seeds. Therefore, larger initial seeds confer more drought resistance on germinating seedlings. Although this phenomenon has been observed by evolutionary biologists and ecologists, the correlation of initial seed size with the drought resistance of seedlings/plants is not well-reviewed and characterized. Furthermore, the related molecular mechanisms are unknown. Understanding these mechanisms will benefit future breeding or design strategies to increase crop yields. In the present review, we focus on recent research to analyze the genetic factors of plants/crops involved in the regulation of seed size and drought tolerance and their corresponding signal transduction pathways. Several signaling pathways that determine plant drought tolerance through influencing the initial seed size are identified. Such pathways include those that are involved in mitogen-activated protein kinase, abscisic acid, brassinosteroids, and several transcription factors and sugar signaling pathways.

Comparative population genomics dissects the genetic basis of seven domestication traits in jujube

Jujube (Ziziphus jujuba Mill.) is an important perennial fruit tree with a range of interesting horticultural traits. It was domesticated from wild jujube (Ziziphus acidojujuba), but the genomic variation dynamics and genetic changes underlying its horticultural traits during domestication are poorly understood. Here, we report a comprehensive genome variation map based on the resequencing of 350 accessions, including wild, semi-wild and cultivated jujube plants, at a >15× depth. Using the combination of a genome-wide association study (GWAS) and selective sweep analysis, we identified several candidate genes potentially involved in regulating seven domestication traits in jujube. For fruit shape and kernel shape, we integrated the GWAS approach with transcriptome profiling data, expression analysis and the transgenic validation of a candidate gene to identify a causal gene, ZjFS3, which encodes an ethylene-responsive transcription factor. Similarly, we identified a candidate gene for bearing-shoot length and the number of leaves per bearing shoot and two candidate genes for the seed-setting rate using GWAS. In the selective sweep analysis, we also discovered several putative genes for the presence of prickles on bearing shoots and the postharvest shelf life of fleshy fruits. This study outlines the genetic basis of jujube domestication and evolution and provides a rich genomic resource for mining other horticulturally important genes in jujube.

CRISPR-TSKO: A Technique for Efficient Mutagenesis in Specific Cell Types, Tissues, or Organs in Arabidopsis

Detailed functional analyses of many fundamentally important plant genes via conventional loss-of-function approaches are impeded by the severe pleiotropic phenotypes resulting from these losses. In particular, mutations in genes that are required for basic cellular functions and/or reproduction often interfere with the generation of homozygous mutant plants, precluding further functional studies. To overcome this limitation, we devised a clustered regularly interspaced short palindromic repeats (CRISPR)-based tissue-specific knockout system, CRISPR-TSKO, enabling the generation of somatic mutations in particular plant cell types, tissues, and organs. In Arabidopsis (Arabidopsis thaliana), CRISPR-TSKO mutations in essential genes caused well-defined, localized phenotypes in the root cap, stomatal lineage, or entire lateral roots. The modular cloning system developed in this study allows for the efficient selection, identification, and functional analysis of mutant lines directly in the first transgenic generation. The efficacy of CRISPR-TSKO opens avenues for discovering and analyzing gene functions in the spatial and temporal contexts of plant life while avoiding the pleiotropic effects of system-wide losses of gene function.

Transcriptional Activation of Arabidopsis Zygotes Is Required for Initial Cell Divisions

Commonly referred to as the maternal-to-zygotic transition, the shift of developmental control from maternal-to-zygotic genomes is a key event during animal and plant embryogenesis. Together with the degradation of parental gene products, the increased transcriptional activities of the zygotic genome remodels the early embryonic transcriptome during this transition. Although evidence from multiple flowering plants suggests that zygotes become transcriptionally active soon after fertilization, the timing and developmental requirements of zygotic genome activation in Arabidopsis thaliana (Arabidopsis) remained a matter of debate until recently. In this report, we optimized an expansion microscopy technique for robust immunostaining of Arabidopsis ovules and seeds. This enabled the detection of marks indicative of active transcription in zygotes before the first cell division. Moreover, we employed a live-imaging culture system together with transcriptional inhibitors to demonstrate that such active transcription is physiologically required in zygotes and early embryos. Our results indicate that zygotic genome activation occurs soon after fertilization and is required for the initial zygotic divisions in Arabidopsis.

Maternal regulation of seed growth and patterning in flowering plants

The plant haploid generation is specified late in higher plant development, and post-meiotic haploid plant cells divide mitotically to produce a haploid gametophyte, in which a subset of cells differentiates into the gametes. The immediate mother of the angiosperm seed is the female gametophyte, also called the embryo sac. In most flowering plants the embryo sac is comprised of two kinds of gametes (egg and central cell) and two kinds of subsidiary cells (antipodals and synergids) all of which descend from a single haploid spore produced by meiosis. The embryo sac develops within a specialized organ of the flower called the ovule, which supports and controls many steps in the development of both the embryo sac and the seed. Double fertilization of the central cell and egg cell by the two sperm cells of a pollen grain produce the endosperm and embryo of the seed, respectively. The endosperm and embryo develop under the influence of their precursor gametes and the surrounding tissues of the ovule and the gametophyte. The final size and pattern of the angiosperm seed then is the result of complex interactions across multiple tissues of three different generations (maternal sporophyte, maternal gametophyte, and the fertilization products) and three different ploidies (haploid gametophyte, diploid parental sporophyte and embryo, and triploid endosperm).

Peptide/receptor-like kinase-mediated signaling involved in male-female interactions

In flowering plants, extensive male-female interactions during pollen germination on the stigma, pollen tube growth and guidance in the transmitting tract, and pollen tube reception by the female gametophyte are required for successful double fertilization in which various signaling cascades are involved. Peptide/receptor-like kinase-mediated signaling has been found playing important roles in these male-female interactions. Here, we mainly summarized the progress made on the regulatory roles of peptide/receptor-like kinase-mediated signaling pathways in four critical stages during reproduction in higher plants.

Noncanonical auxin signaling regulates cell division pattern during lateral root development

In both plants and animals, multiple cellular processes must be orchestrated to ensure proper organogenesis. The cell division patterns control the shape of growing organs, yet how they are precisely determined and coordinated is poorly understood. In plants, the distribution of the phytohormone auxin is tightly linked to organogenesis, including lateral root (LR) development. Nevertheless, how auxin regulates cell division pattern during lateral root development remains elusive. Here, we report that auxin activates Mitogen-Activated Protein Kinase (MAPK) signaling via transmembrane kinases (TMKs) to control cell division pattern during lateral root development. Both TMK1/4 and MKK4/5-MPK3/6 pathways are required to properly orient cell divisions, which ultimately determine lateral root development in response to auxin. We show that TMKs directly and specifically interact with and phosphorylate MKK4/5, which is required for auxin to activate MKK4/5-MPK3/6 signaling. Our data suggest that TMK-mediated noncanonical auxin signaling is required to regulate cell division pattern and connect auxin signaling to MAPK signaling, which are both essential for plant development.

New Insights into the Molecular Mechanism Underlying Seed Size Control under Drought Stress

In higher plants, seed size is an important parameter and agricultural trait in many aspects of evolutionary fitness. The loss of water-deficiency-induced crop yield is the largest among all natural hazards. Under water-deficient stress, the most prevalent response to terminal stress is to accelerate the early arrest of floral development and, thereby, to accelerate fruit/seed production, which consequently reduces seed size. This phenomenon is well-known, but its molecular mechanism is not well-reviewed and characterized. However, increasing evidence have indicated that water-deficient stress is always coordinated with three genetic signals (i.e., seed size regulators, initial seed size, and fruit number) that decide the final seed size. Here, our review presents new insights into the mechanism underlying cross-talk water-deficient stress signaling with three genetic signals controlling final seed size. These new insights may aid in preliminary screening, identifying novel genetic factors and future design strategies, or breeding to increase crop yield.

Cystathionine beta-lyase is crucial for embryo patterning and the maintenance of root stem cell niche in Arabidopsis

The growth and development of roots in plants depends on the specification and maintenance of the root apical meristem. Here, we report the identification of CBL, a gene required for embryo and root development in Arabidopsis, and encodes cystathionine beta-lyase (CBL), which catalyzes the penultimate step in methionine (Met) biosynthesis, and which also led to the discovery of a previous unknown, but crucial, metabolic contribution by the Met biosynthesis pathway. CBL is expressed in embryos and shows quiescent center (QC)-enriched expression pattern in the root. cbl mutant has impaired embryo patterning, defective root stem cell niche, stunted root growth, and reduces accumulation of the root master regulators PLETHORA1 (PLT1) and PLT2. Furthermore, mutation in CBL severely decreases abundance of several PIN-FORMED (PIN) proteins and impairs auxin-responsive gene expression in the root tip. cbl seedlings also exhibit global reduction in histone H3 Lys-4 trimethylation (H3K4me3) and DNA methylation. Importantly, mutation in CBL reduces the abundance of H3K4me3 modification in PLT1/2 genes and downregulates their expression. Overexpression of PLT2 partially rescues cbl root meristem defect, suggesting that CBL acts in part through PLT1/2. Moreover, exogenous supplementation of Met also restores the impaired QC activity and the root growth defects of cbl. Taken together, our results highlight the unique role of CBL to maintain the root stem cell niche by cooperative actions between Met biosynthesis and epigenetic modification of key developmental regulators.

SERK Receptor-like Kinases Control Division Patterns of Vascular Precursors and Ground Tissue Stem Cells during Embryo Development in Arabidopsis

During embryo development, the vascular precursors and ground tissue stem cells divide to renew themselves and produce the vascular tissue, endodermal cells, and cortical cells. However, the molecular mechanisms regulating division of these stem cells have remained largely elusive. In this study, we show that loss of function of SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) genes results in aberrant embryo development. Fewer cortical, endodermal, and vascular cells are generated in the embryos of serk1 serk2 bak1 triple mutants. WUSCHEL-RELATED HOMEOBOX 5 (WOX5) is ectopically expressed in vascular cells of serk1 serk2 bak1 embryos. The first transverse division of vascular precursors in mid-globular embryos and second asymmetric division of ground tissue stem cells in early-heart embryos are abnormally altered to a longitudinal division. The embryo defects can be partially rescued by constitutively activated mitogen-activated protein kinase (MAPK) kinase kinase YODA (YDA) and MAPK kinase MKK5. Taken together, our results reveal that SERK-mediated signals regulate division patterns of vascular precursors and ground tissue stem cells, likely via the YDA-MKK4/5 cascade, during embryo development.

Specification of basal region identity after asymmetric zygotic division requires mitogen-activated protein kinase 6 in rice

Asymmetric cell division is a key step in cellular differentiation in multicellular organisms. In plants, asymmetric zygotic division produces the apical and basal cells. The mitogen-activated protein kinase (MPK) cascade in Arabidopsis acts in asymmetric divisions such as zygotic division and stomatal development, but whether the effect on cellular differentiation of this cascade is direct or indirect following asymmetric division is not clear. Here, we report the analysis of a rice mutant, globular embryo 4 (gle4). In two- and four-cell-stage embryos, asymmetric zygotic division and subsequent cell division patterns were indistinguishable between the wild type and gle4 mutants. However, marker gene expression and transcriptome analyses showed that specification of the basal region was compromised in gle4 We found that GLE4 encodes MPK6 and that GLE4/MPK6 is essential in cellular differentiation rather than in asymmetric zygotic division. Our findings provide a new insight into the role of MPK in plant development. We propose that the regulation of asymmetric zygotic division is separate from the regulation of cellular differentiation that leads to apical-basal polarity.

Constitutive signaling activity of a receptor-associated protein links fertilization with embryonic patterning in Arabidopsis thaliana

In flowering plants, the asymmetrical division of the zygote is the first hallmark of apical-basal polarity of the embryo and is controlled by a MAP kinase pathway that includes the MAPKKK YODA (YDA). In Arabidopsis, YDA is activated by the membrane-associated pseudokinase SHORT SUSPENSOR (SSP) through an unusual parent-of-origin effect: SSP transcripts accumulate specifically in sperm cells but are translationally silent. Only after fertilization is SSP protein transiently produced in the zygote, presumably from paternally inherited transcripts. SSP is a recently diverged, Brassicaceae-specific member of the BRASSINOSTEROID SIGNALING KINASE (BSK) family. BSK proteins typically play broadly overlapping roles as receptor-associated signaling partners in various receptor kinase pathways involved in growth and innate immunity. This raises two questions: How did a protein with generic function involved in signal relay acquire the property of a signal-like patterning cue, and how is the early patterning process activated in plants outside the Brassicaceae family, where SSP orthologs are absent? Here, we show that Arabidopsis BSK1 and BSK2, two close paralogs of SSP that are conserved in flowering plants, are involved in several YDA-dependent signaling events, including embryogenesis. However, the contribution of SSP to YDA activation in the early embryo does not overlap with the contributions of BSK1 and BSK2. The loss of an intramolecular regulatory interaction enables SSP to constitutively activate the YDA signaling pathway, and thus initiates apical-basal patterning as soon as SSP protein is translated after fertilization and without the necessity of invoking canonical receptor activation.

New cues for body axis formation in plant embryos

Plant embryogenesis initiates with the fusion of sperm and egg cell, and completes the generation of the basic outline of the future plant. Here, we summarize the recent findings about the signaling cascade triggering the zygotic transcription, and the intracellular events and regulatory factors involved in the formation of the two major body axes. We highlight the lack of systematic de novo transcriptional activation in the zygote, and emphasize the importance of cytoskeletal reorganization to polarize the zygote and control the first asymmetric division that establishes the apical-basal axis. Finally, the limited knowledge of mechanisms that control the cell divisions separating the inner and outer cell layers is summarized and we propose approaches to enhance our understanding of basic principles of plant embryogenesis.

Phosphorylation of Plant Microtubule-Associated Proteins During Cell Division

Progression of mitosis and cytokinesis depends on the reorganization of cytoskeleton, with microtubules driving the segregation of chromosomes and their partitioning to two daughter cells. In dividing plant cells, microtubules undergo global reorganization throughout mitosis and cytokinesis, and with the aid of various microtubule-associated proteins (MAPs), they form unique systems such as the preprophase band (PPB), the acentrosomal mitotic spindle, and the phragmoplast. Such proteins include nucleators of de novo microtubule formation, plus end binding proteins involved in the regulation of microtubule dynamics, crosslinking proteins underlying microtubule bundle formation and members of the kinesin superfamily with microtubule-dependent motor activities. The coordinated function of such proteins not only drives the continuous remodeling of microtubules during mitosis and cytokinesis but also assists the positioning of the PPB, the mitotic spindle, and the phragmoplast, affecting tissue patterning by controlling cell division plane (CDP) orientation. The affinity and the function of such proteins is variably regulated by reversible phosphorylation of serine and threonine residues within the microtubule binding domain through a number of protein kinases and phosphatases which are differentially involved throughout cell division. The purpose of the present review is to provide an overview of the function of protein kinases and protein phosphatases involved in cell division regulation and to identify cytoskeletal substrates relevant to the progression of mitosis and cytokinesis and the regulation of CDP orientation.

The RAF-like mitogen-activated protein kinase kinase kinases RAF22 and RAF28 are required for the regulation of embryogenesis in Arabidopsis

In Arabidopsis, embryonic development follows a stereotypical pattern of cell division. Although many factors have been reported to participate in establishment of the proper embryonic pattern, the molecular mechanisms underlying pattern formation are unclear. In this study we showed that RAF22 and RAF28, two RAF-like mitogen-activated protein kinase kinase kinases (MAPKKKs) in Arabidopsis, are involved in the regulation of embryogenesis. The double knockout mutant of RAF22 and RAF28 was embryo lethal. A large proportion of the raf22^-/- raf28^+/- mutant embryos exhibited various defects, including disordered proembryo cell divisions, disruption of the bilaterally symmetrical structure, abnormally formative divisions of hypophysis and exaggerated suspensor growth. Whereas the kinase active form of RAF22 could complement these embryonic aberrant phenotypes, the kinase inactive form could not. The restrictive expression of the basal cell fate marker WOX8 in the abnormally dividing suspensor cells and the apical cell linage marker WOX2 in the abnormal proembryos indicated that apical and basal cell fates were unchanged in the abnormal embryos. The polar distribution of the auxin maxima and the PIN1 and PIN7 auxin transporters was markedly altered in the abnormal embryos. Our results suggest that RAF22 and RAF28 are important components of embryogenesis and that auxin polar transport may be involved in this regulation.

The MAP3K-Coding QUI-GON JINN (QGJ) Gene Is Essential to the Formation of Unreduced Embryo Sacs in Paspalum

Apomixis is a clonal mode of reproduction via seeds, which results from the failure of meiosis and fertilization in the sexual female reproductive pathway. In previous transcriptomic surveys, we identified a mitogen-activated protein kinase kinase kinase (N46) displaying differential representation in florets of sexual and apomictic Paspalum notatum genotypes. Here, we retrieved and characterized the N46 full cDNA sequence from sexual and apomictic floral transcriptomes. Phylogenetic analyses showed that N46 was a member of the YODA family, which was re-named QUI-GON JINN (QGJ). Differential expression in florets of sexual and apomictic plants was confirmed by qPCR. In situ hybridization experiments revealed expression in the nucellus of aposporous plants' ovules, which was absent in sexual plants. RNAi inhibition of QGJ expression in two apomictic genotypes resulted in significantly reduced rates of aposporous embryo sac formation, with respect to the level detected in wild type aposporous plants and transformation controls. The QGJ locus segregated independently of apospory. However, a probe derived from a related long non-coding RNA sequence (PN_LNC_QGJ) revealed RFLP bands cosegregating with the Paspalum apospory-controlling region (ACR). PN_LNC_QGJ is expressed in florets of apomictic plants only. Our results indicate that the activity of QGJ in the nucellus of apomictic plants is necessary to form non-reduced embryo sacs and that a long non-coding sequence with regulatory potential is similar to sequences located within the ACR.

Sucrose Signaling Regulates Anthocyanin Biosynthesis Through a MAPK Cascade in Arabidopsis thaliana

Anthocyanin accumulation specifically depends on sucrose (Suc) signaling. However, the molecular basis of this process remains unknown. In this study, in vitro pull-down assays identified ETHYLENE-INSENSITIVE3 (EIN3), a component of both sugar signaling or/and metabolism. This protein interacted with YDA, and the physiological relevance of this interaction was confirmed by in planta co-immunoprecipitation, yeast two-hybrid (Y2H) assay, and bimolecular fluorescence complementation. Ethylene insensitive3-like 1 (eil1) ein3 double-mutant seedlings, but not ein3-1 seedlings, showed anthocyanin accumulation. Furthermore, ein3-1 suppressed anthocyanin accumulation in yda-1 plants. Thus, EMB71/YDA-EIN3-EIL1 may form a sugar-mediated gene cascade integral to the regulation of anthocyanin accumulation. Moreover, the EMB71/YDA-EIN3-EIL1 gene cascade module directly targeted the promoter of Transparent Testa 8 (TT8) by direct EIN3 binding. Collectively, our data inferred a molecular model where the signaling cascade of the YDA-EIN3-TT8 appeared to target TT8 via EIN3, thereby modulating Suc signaling-mediated anthocyanin accumulation.

Conservation and divergence of YODA MAPKKK function in regulation of grass epidermal patterning

All multicellular organisms must properly pattern cell types to generate functional tissues and organs. The organized and predictable cell lineages of the Brachypodium leaf enabled us to characterize the role of the MAPK kinase kinase gene BdYODA1 in regulating asymmetric cell divisions. We find that YODA genes promote normal stomatal spacing patterns in both Arabidopsis and Brachypodium, despite species-specific differences in those patterns. Using lineage tracing and cell fate markers, we show that, unexpectedly, patterning defects in bdyoda1 mutants do not arise from faulty physical asymmetry in cell divisions but rather from improper enforcement of alternative cellular fates after division. These cross-species comparisons allow us to refine our understanding of MAPK activities during plant asymmetric cell divisions.

Autonomous and non-autonomous functions of the maize Shohai1 gene, encoding a RWP-RK putative transcription factor, in regulation of embryo and endosperm development

In plants, establishment of the basic body plan during embryogenesis involves complex processes of axis formation, cell fate specification and organ differentiation. While molecular mechanisms of embryogenesis have been well studied in the eudicot Arabidopsis, only a small number of genes regulating embryogenesis has been identified in grass species. Here, we show that a RKD-type RWP-RK transcription factor encoded by Shohai1 (Shai1) is indispensable for embryo and endosperm development in maize. Loss of Shai1 function causes variable morphological defects in the embryo including small scutellum, shoot axis bifurcation and arrest during early organogenesis. Analysis of molecular markers in mutant embryos reveals disturbed patterning of gene expression and altered polar auxin transport. In contrast with typical embryo-defective (emb) mutants that expose a vacant embryo pocket in the endosperm, the endosperm of shai1 kernels conforms to the varied size and shape of the embryo. Furthermore, genetic analysis confirms that Shai1 is required for autonomous formation of the embryo pocket in endosperm of emb mutants. Analyses of genetic mosaic kernels generated by B-A translocation revealed that expression of Shai1 in the endosperm could partially rescue a shai1 mutant embryo and suggested that Shai1 is involved in non-cell autonomous signaling from endosperm that supports normal embryo growth. Taken together, we propose that the Shai1 gene functions in regulating embryonic patterning during grass embryogenesis partly by endosperm-to-embryo interaction.

Control of Grain Size and Weight by the OsMKKK10-OsMKK4-OsMAPK6 Signaling Pathway in Rice

Grain size is one of the key agronomic traits that determine grain yield in crops. However, the mechanisms underlying grain size control in crops remain elusive. Here we demonstrate that the OsMKKK10-OsMKK4-OsMAPK6 signaling pathway positively regulates grain size and weight in rice. In rice, loss of OsMKKK10 function results in small and light grains, short panicles, and semi-dwarf plants, while overexpression of constitutively active OsMKKK10 (CA-OsMKKK10) results in large and heavy grains, long panicles, and tall plants. OsMKKK10 interacts with and phosphorylates OsMKK4. We identified an OsMKK4 gain-of-function mutant (large11-1D) that produces large and heavy grains. OsMKK4^A227T encoded by the large11-1D allele has stronger kinase activity than OsMKK4. Plants overexpressing a constitutively active form of OsMKK4 (OsMKK4-DD) also produce large grains. Further biochemical and genetic analyses revealed that OsMKKK10, OsMKK4, and OsMAPK6 function in a common pathway to control grain size. Taken together, our study establishes an important genetic and molecular framework for OsMKKK10-OsMKK4-OsMAPK6 cascade-mediated control of grain size and weight in rice.

Antagonistic interactions between two MAP kinase cascades in plant development and immune signaling

Mitogen-activated protein kinase (MAPK) signaling plays important roles in diverse biological processes. In Arabidopsis, MPK3/MPK6, MKK4/MKK5, and the MAPKKK YODA (YDA) form a MAPK pathway that negatively regulates stomatal development. Brassinosteroid (BR) stimulates this pathway to inhibit stomata production. In addition, MPK3/MPK6 and MKK4/MKK5 also serve as critical signaling components in plant immunity. Here, we report that MAPKKK3/MAPKKK5 form a kinase cascade with MKK4/MKK5 and MPK3/MPK6 to transduce defense signals downstream of multiple plant receptor kinases. Loss of MAPKKK3/MAPKKK5 leads to reduced activation of MPK3/MPK6 in response to different pathogen-associated molecular patterns (PAMPs) and increased susceptibility to pathogens. Surprisingly, developmental defects caused by silencing of YDA are suppressed in the mapkkk3 mapkkk5 double mutant. On the other hand, loss of YDA or blocking BR signaling leads to increased PAMP-induced activation of MPK3/MPK6. These results reveal antagonistic interactions between a developmental MAPK pathway and an immune signaling MAPK pathway.

Receptor-Like Cytoplasmic Kinases: Central Players in Plant Receptor Kinase-Mediated Signaling

Receptor kinases (RKs) are of paramount importance in transmembrane signaling that governs plant reproduction, growth, development, and adaptation to diverse environmental conditions. Receptor-like cytoplasmic kinases (RLCKs), which lack extracellular ligand-binding domains, have emerged as a major class of signaling proteins that regulate plant cellular activities in response to biotic/abiotic stresses and endogenous extracellular signaling molecules. By associating with immune RKs, RLCKs regulate multiple downstream signaling nodes to orchestrate a complex array of defense responses against microbial pathogens. RLCKs also associate with RKs that perceive brassinosteroids and signaling peptides to coordinate growth, pollen tube guidance, embryonic and stomatal patterning, floral organ abscission, and abiotic stress responses. The activity and stability of RLCKs are dynamically regulated not only by RKs but also by other RLCK-associated proteins. Analyses of RLCK-associated components and substrates have suggested phosphorylation relays as a major mechanism underlying RK-mediated signaling.

Cell and Developmental Biology of Plant Mitogen-Activated Protein Kinases

Plant mitogen-activated protein kinases (MAPKs) constitute a network of signaling cascades responsible for transducing extracellular stimuli and decoding them to dedicated cellular and developmental responses that shape the plant body. Over the last decade, we have accumulated information about how MAPK modules control the development of reproductive tissues and gametes and the embryogenic and postembryonic development of vegetative organs such as roots, root nodules, shoots, and leaves. Of key importance to understanding how MAPKs participate in developmental and environmental signaling is the characterization of their subcellular localization, their interactions with upstream signal perception mechanisms, and the means by which they target their substrates. In this review, we summarize the roles of MAPK signaling in the regulation of key plant developmental processes, and we survey what is known about the mechanisms guiding the subcellular compartmentalization of MAPK modules.

Advanced microscopy methods for bioimaging of mitotic microtubules in plants

Mitotic cell division in plants is a dynamic process playing a key role in plant morphogenesis, growth, and development. Since progress of mitosis is highly sensitive to external stresses, documentation of mitotic cell division in living plants requires fast and gentle live-cell imaging microscopy methods and suitable sample preparation procedures. This chapter describes, both theoretically and practically, currently used advanced microscopy methods for the live-cell visualization of the entire process of plant mitosis. These methods include microscopy modalities based on spinning disk, Airyscan confocal laser scanning, structured illumination, and light-sheet bioimaging of tissues or whole plant organs with diverse spatiotemporal resolution. Examples are provided from studies of mitotic cell division using microtubule molecular markers in the model plant Arabidopsis thaliana, and from deep imaging of mitotic microtubules in robust plant samples, such as legume crop species Medicago sativa.

O11 is multi-functional regulator in maize endosperm

As a highly developed tissue, maize endosperm accumulates nutrients abundantly and supports embryo development. In a recent study, we constructed a regulatory network centered around Opaque11 (O11). This network unified cellular development, nutrient metabolism and stress responses during endosperm development. Here we discuss the evidences that O11 might have a regulatory role in cold stress response during seed development. Furthermore, we discuss the functional divergence between maize O11 and its Arabidopsis orthologue ZHOUPI, which might explain some of the differences in endosperm development between monocotyledonous and dicotyledonous seeds.

YODA MAP3K kinase regulates plant immune responses conferring broad-spectrum disease resistance

Mitogen-activated protein kinases (MAPKs) cascades play essential roles in plants by transducing developmental cues and environmental signals into cellular responses. Among the latter are microbe-associated molecular patterns perceived by pattern recognition receptors (PRRs), which trigger immunity. We found that YODA (YDA) - a MAPK kinase kinase regulating several Arabidopsis developmental processes, like stomatal patterning - also modulates immune responses. Resistance to pathogens is compromised in yda alleles, whereas plants expressing the constitutively active YDA (CA-YDA) protein show broad-spectrum resistance to fungi, bacteria, and oomycetes with different colonization modes. YDA functions in the same pathway as ERECTA (ER) Receptor-Like Kinase, regulating both immunity and stomatal patterning. ER-YDA-mediated immune responses act in parallel to canonical disease resistance pathways regulated by phytohormones and PRRs. CA-YDA plants exhibit altered cell-wall integrity and constitutively express defense-associated genes, including some encoding putative small secreted peptides and PRRs whose impairment resulted in enhanced susceptibility phenotypes. CA-YDA plants show strong reprogramming of their phosphoproteome, which contains protein targets distinct from described MAPKs substrates. Our results suggest that, in addition to stomata development, the ER-YDA pathway regulates an immune surveillance system conferring broad-spectrum disease resistance that is distinct from the canonical pathways mediated by described PRRs and defense hormones.

The suspensor as a model system to study the mechanism of cell fate specification during early embryogenesis

The advances in the suspensor. During early embryogenesis, the proembryo consists of two domains, the embryo proper and the suspensor. Unlike the embryo proper, which has been investigated extensively, research on the suspensor has been limited in past decades. Recent studies have revealed that the suspensor plays an important role in early embryogenesis and the process of suspensor formation and degeneration may provide a unique model for studies on cell division pattern, cell fate determination, and cell death. In this review, we briefly summarize the advances in research on the suspensor, which provide new insight in our understanding of the mechanism of early embryogenesis and show great potential for a unique model for future investigations.

Plant cell surface receptor-mediated signaling - a common theme amid diversity

Sessile plants employ a diverse array of plasma membrane-bound receptors to perceive endogenous and exogenous signals for regulation of plant growth, development and immunity. These cell surface receptors include receptor-like kinases (RLKs) and receptor-like proteins (RLPs) that harbor different extracellular domains for perception of distinct ligands. Several RLK and RLP signaling pathways converge at the somatic embryogenesis receptor kinases (SERKs), which function as shared co-receptors. A repertoire of receptor-like cytoplasmic kinases (RLCKs) associate with the receptor complexes to relay intracellular signaling. Downstream of the receptor complexes, mitogen-activated protein kinase (MAPK) cascades are among the key signaling modules at which the signals converge, and these cascades regulate diverse cellular and physiological responses through phosphorylation of different downstream substrates. In this Review, we summarize the emerging common theme that underlies cell surface receptor-mediated signaling pathways in Arabidopsisthaliana: the dynamic association of RLKs and RLPs with specific co-receptors and RLCKs for signal transduction. We further discuss how signaling specificities are maintained through modules at which signals converge, with a focus on SERK-mediated receptor signaling.

Mitogen-Activated Protein Kinase Cascades in Plant Hormone Signaling

Mitogen-activated protein kinase (MAPK) modules play key roles in the transduction of environmental and developmental signals through phosphorylation of downstream signaling targets, including other kinases, enzymes, cytoskeletal proteins or transcription factors, in all eukaryotic cells. A typical MAPK cascade consists of at least three sequentially acting serine/threonine kinases, a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK) and finally, the MAP kinase (MAPK) itself, with each phosphorylating, and hence activating, the next kinase in the cascade. Recent advances in our understanding of hormone signaling pathways have led to the discovery of new regulatory systems. In particular, this research has revealed the emerging role of crosstalk between the protein components of various signaling pathways and the involvement of this crosstalk in multiple cellular processes. Here we provide an overview of current models and mechanisms of hormone signaling with a special emphasis on the role of MAPKs in cell signaling networks. One-sentence summary: In this review we highlight the mechanisms of crosstalk between MAPK cascades and plant hormone signaling pathways and summarize recent findings on MAPK regulation and function in various cellular processes.

Maternal control of embryogenesis by MPK6 and its upstream MKK4/MKK5 in Arabidopsis

In flowering plants, developing embryos reside in maternal sporophytes. It is known that maternal generation influences the development of next-generation embryos; however, little is known about the signaling components in the process. Previously, we demonstrated that Arabidopsis mitogen-activated protein kinase 6 (MPK6) and MPK3 play critical roles in plant reproduction. In addition, we noticed that a large fraction of seeds from mpk6 single-mutant plants showed a wrinkled seed coat or a burst-out embryo phenotype. Here, we report that these seed phenotypes can be traced back to defective embryogenesis. The defective embryos have shorter suspensors and reduced growth along the longitudinal axis. Furthermore, the cotyledons fail to bend over to progress to the bent-cotyledon stage. As a result of the uneven circumference along the axis, the seed coat wrinkles to develop raisin-like morphology after dehydration. In more severe cases, the embryo can be pushed out from the micropylar end, resulting in the burst-out embryo seed phenotype. Genetic analyses demonstrated that the defective embryogenesis of the mpk6 mutant is a maternal effect. Heterozygous or homozygous mpk6 embryos have defects only in mpk6 homozygous maternal plants, but not in wild-type or heterozygous maternal plants. The loss of function of MKK4/MKK5 also results in the same phenotypes, suggesting that MKK4/MKK5 might act upstream of MPK6 in this pathway. The maternal-mediated embryo defects are associated with changes in auxin activity maxima and PIN localization. In summary, this research demonstrates that the Arabidopsis MKK4/MKK5-MPK6 cascade is an important player in the maternal control of embryogenesis.