Linking cell cycle to stomatal differentiation

Regulation of stomatal development by epidermal, subepidermal and long-distance signals

Plant leaves consist of three layers, including epidermis, mesophyll and vascular tissues. Their development is meticulously orchestrated. Stomata are the specified structures on the epidermis for uptake of carbon dioxide (CO2) while release of water vapour and oxygen (O2), and thus play essential roles in regulation of plant photosynthesis and water use efficiency. To function efficiently, stomatal formation must coordinate with the development of other epidermal cell types, such as pavement cell and trichome, and tissues of other layers, such as mesophyll and leaf vein. This review summarizes the regulation of stomatal development in three dimensions (3D). In the epidermis, specific stomatal transcription factors determine cell fate transitions and also activate a ligand-receptor- MITOGEN-ACTIVATED PROTEIN KINASE (MAPK) signaling for ensuring proper stomatal density and patterning. This forms the core regulation network of stomatal development, which integrates various environmental cues and phytohormone signals to modulate stomatal production. Under the epidermis, mesophyll, endodermis of hypocotyl and inflorescence stem, and veins in grasses secrete mobile signals to influence stomatal formation in the epidermis. In addition, long-distance signals which may include phytohormones, RNAs, peptides and proteins originated from other plant organs modulate stomatal development, enabling plants to systematically adapt to the ever changing environment.

A cell size threshold triggers commitment to stomatal fate in Arabidopsis

How flexible developmental programs integrate information from internal and external factors to modulate stem cell behavior is a fundamental question in developmental biology. Cells of the Arabidopsis stomatal lineage modify the balance of stem cell proliferation and differentiation to adjust the size and cell type composition of mature leaves. Here, we report that meristemoids, one type of stomatal lineage stem cell, trigger the transition from asymmetric self-renewing divisions to commitment and terminal differentiation by crossing a critical cell size threshold. Through computational simulation, we demonstrate that this cell size-mediated transition allows robust, yet flexible termination of stem cell proliferation, and we observe adjustments in the number of divisions before the differentiation threshold under several genetic manipulations. We experimentally evaluate several mechanisms for cell size sensing, and our data suggest that this stomatal lineage transition is dependent on a nuclear factor that is sensitive to DNA content.

Stem Cell Basis of Shoot Branching

During their postembryonic development, plants continuously form branches to conquer more space and adapt to changing environments. In seed plants, this is achieved by lateral branching, in which axillary meristems (AMs) initiate at the leaf axils to form axillary buds. The developmental potential of AMs to form shoot branches is the same as that of embryonic shoot apical meristems (SAMs). Recent studies in Arabidopsis thaliana have revealed the cellular origin of AMs and have identified transcription factors and phytohormones that regulate sequential steps leading to AM initiation. In particular, a group of meristematic cells detached from the SAM are key to AM initiation, which constitutes an excellent system for understanding stem cell fate and de novo meristem formation.

Cell Cycle Dynamics during Stomatal Development: Window of MUTE Action and Ramification of Its Loss-of-Function on an Uncommitted Precursor

Plants develop in the absence of cell migration. As such, cell division and differentiation need to be coordinated for functional tissue formation. Cellular valves on the plant epidermis, stomata, are generated through a stereotypical sequence of cell division and differentiation events. In Arabidopsis, three master regulatory transcription factors, SPEECHLESS (SPCH), MUTE and FAMA, sequentially drive initiation, proliferation and differentiation of stomata. Among them, MUTE switches the cell cycle mode from proliferative asymmetric division to terminal symmetric division and orchestrates the execution of the single symmetric division event. However, it remains unclear to what extent MUTE regulates the expression of cell cycle genes through the symmetric division and whether MUTE accumulation itself is gated by the cell cycle. Here, we show that MUTE directly upregulates the expression of cell cycle components throughout the terminal cell cycle phases of a stomatal precursor, not only core cell cycle engines but also check-point regulators. Time-lapse live imaging using the multicolor Plant Cell Cycle Indicator revealed that MUTE accumulates up to the early G2 phase, whereas its successor and direct target, FAMA, accumulate at late G2 through terminal mitosis. In the absence of MUTE, meristemoids fail to differentiate and their G1 phase elongates as they reiterate asymmetric divisions. Together, our work provides the framework of cell cycle and master regulatory transcription factors to coordinate a single symmetric cell division and suggests a mechanism for the eventual cell cycle arrest of an uncommitted stem-cell-like precursor at the G1 phase.

Emerging roles of protein phosphorylation in regulation of stomatal development

Stomata, tiny epidermal spores, control gas exchange between plants and their external environment, thereby playing essential roles in plant development and physiology. Stomatal development requires rapid regulation of components in signaling pathways to respond flexibly to numerous intrinsic and extrinsic signals. In support of this, reversible phosphorylation, which is particularly suitable for rapid signal transduction, has been implicated in this process. This review highlights the current understanding of the essential roles of reversible phosphorylation in the regulation of stomatal development, most of which comes from the dicot Arabidopsis thaliana. Protein phosphorylation tightly controls the activity of SPEECHLESS (SPCH)-SCREAM (SCRM), the stomatal lineage switch, and the activity of several mitogen-activated protein kinases and receptor kinases upstream of SPCH-SCRM, thereby regulating stomatal cell differentiation and patterning. In addition, protein phosphorylation is involved in the establishment of cell polarity during stomatal asymmetric cell division. Finally, cyclin-dependent kinase-mediated protein phosphorylation plays essential roles in cell cycle control during stomatal development.

Protein polarization: Spatiotemporal precisions in cell division and differentiation

Specification of cell polarity is vital to normal cell growth, morphogenesis, and function. As other eukaryotes, plants generate cellular polarity that is coordinated with tissue polarity and organ axes. In development, new cell types are generated by stem-cell division and differentiation, a process often involving proteins that are polarized to cortical domains at the plasma membrane. In the past decade, pioneering work using the model plant Arabidopsis identified multiple proteins that are polarized in dividing cells to instruct divisional behaviors and/or specify cell fates. In this review, we use these polarized cell-division regulators as example to summarize key mechanisms underlying protein polarization in plant cells. Recent progress underscores that self-organizing amplification processes are commonly involved in establishing cell polarity, and cellular polarity is influenced by both tissue-level and local mechanochemical cues. In addition, protein polarization during asymmetric cell division shows a distinct feature of temporal control in the stomatal lineage. We further discuss possible coordination between protein polarization and the progression of cell cycle in this developmental context.

Mining Fiskeby III and Mandarin (Ottawa) Expression Profiles to Understand Iron Stress Tolerant Responses in Soybean

The soybean (Glycine max L. merr) genotype Fiskeby III is highly resistant to a multitude of abiotic stresses, including iron deficiency, incurring only mild yield loss during stress conditions. Conversely, Mandarin (Ottawa) is highly susceptible to disease and suffers severe phenotypic damage and yield loss when exposed to abiotic stresses such as iron deficiency, a major challenge to soybean production in the northern Midwestern United States. Using RNA-seq, we characterize the transcriptional response to iron deficiency in both Fiskeby III and Mandarin (Ottawa) to better understand abiotic stress tolerance. Previous work by our group identified a quantitative trait locus (QTL) on chromosome 5 associated with Fiskeby III iron efficiency, indicating Fiskeby III utilizes iron deficiency stress mechanisms not previously characterized in soybean. We targeted 10 of the potential candidate genes in the Williams 82 genome sequence associated with the QTL using virus-induced gene silencing. Coupling virus-induced gene silencing with RNA-seq, we identified a single high priority candidate gene with a significant impact on iron deficiency response pathways. Characterization of the Fiskeby III responses to iron stress and the genes underlying the chromosome 5 QTL provides novel targets for improved abiotic stress tolerance in soybean.

Morphology made for movement: formation of diverse stomatal guard cells

Stomata constantly open and close to optimize gas exchange. While the genetic programme guiding early development is well described, the formation of functional guard cells remains enigmatic. This review highlights recent findings on the developmental and morphogenetic processes shaping this essential and morphologically diverse cell type in Arabidopsis and grasses.

Appearance of microtubules at the cytokinesis to interphase transition in Arabidopsis thaliana

Microtubule arrays drastically reorganize during the cell cycle to facilitate specific events. Many cells contain a centrosome that dictates the assembly and organization of microtubule arrays. However, plant cells and many others do not contain centrosomes or discrete microtubule organizing centers. In plants, microtubules nucleate and polymerize from gamma-tubulin-containing complexes in the interphase cell cortex. During plant cell division, microtubules nucleate near nuclei to form the mitotic spindle and plant-specific phragmoplast required for cytokinesis. Therefore, during the plant cell cycle, microtubule nucleation shifts from cell cortex to the perinuclear region. While it is unclear how this shift occurs, previous studies observed microtubules that appeared to extend from nuclei into the cortex as cells transitioned into interphase in small cells. These data led to the hypothesis that microtubule nucleation complexes move from the nuclear surface to the cortex at the transition from cytokinesis into interphase. Here we document GFP labeled microtubules in living plant cells during the transition from cytokinesis to interphase. We observed apparent groups of microtubules spanning between the nucleus and cell cortex in large, vacuolated epidermal leaf cells. We also observed microtubules in the cell cortex that appeared separate from perinuclear-associated microtubules. While these cortical microtubules were not always seen, when present they were apparent before cytokinesis was complete and/or before nuclear-associated microtubules were obvious. These data add to and deepen the knowledge of microtubule reorganization at this cell cycle transition.

Plant CDKs-Driving the Cell Cycle through Climate Change

In a growing population, producing enough food has become a challenge in the face of the dramatic increase in climate change. Plants, during their evolution as sessile organisms, developed countless mechanisms to better adapt to the environment and its fluctuations. One important way is through the plasticity of their body and their forms, which are modulated during plant growth by accurate control of cell divisions. A family of serine/threonine kinases called cyclin-dependent kinases (CDK) is a key regulator of cell divisions by controlling cell cycle progression. In this review, we compile information on the primary response of plants in the regulation of the cell cycle in response to environmental stresses and show how the cell cycle proteins (mainly the cyclin-dependent kinases) involved in this regulation can act as components of environmental response signaling cascades, triggering adaptive responses to drive the cycle through climate fluctuations. Understanding the roles of CDKs and their regulators in the face of adversity may be crucial to meeting the challenge of increasing agricultural productivity in a new climate.

Transcriptome and Metabolomic Analyses Reveal Regulatory Networks Controlling Maize Stomatal Development in Response to Blue Light

(1) Background: Blue light is important for the formation of maize stomata, but the signal network remains unclear. (2) Methods: We replaced red light with blue light in an experiment and provided a complementary regulatory network for the stomatal development of maize by using transcriptome and metabolomics analysis. (3) Results: Exposure to blue light led to 1296 differentially expressed genes and 419 differential metabolites. Transcriptome comparisons and correlation signaling network analysis detected 55 genes, and identified 6 genes that work in the regulation of the HY5 module and MAPK cascade, that interact with PTI1, COI1, MPK2, and MPK3, in response to the substitution of blue light in environmental adaptation and signaling transduction pathways. Metabolomics analysis showed that two genes involved in carotenoid biosynthesis and starch and sucrose metabolism participate in stomatal development. Their signaling sites located on the PHI1 and MPK2 sites of the MAPK cascade respond to blue light signaling. (4) Conclusions: Blue light remarkably changed the transcriptional signal transduction and metabolism of metabolites, and eight obtained genes worked in the HY5 module and MAPK cascade.

Transcriptome and Metabolomic Analyses Reveal Regulatory Networks Controlling Maize Stomatal Development in Response to Blue Light

(1) Background: Blue light is important for the formation of maize stomata, but the signal network remains unclear. (2) Methods: We replaced red light with blue light in an experiment and provided a complementary regulatory network for the stomatal development of maize by using transcriptome and metabolomics analysis. (3) Results: Exposure to blue light led to 1296 differentially expressed genes and 419 differential metabolites. Transcriptome comparisons and correlation signaling network analysis detected 55 genes, and identified 6 genes that work in the regulation of the HY5 module and MAPK cascade, that interact with PTI1, COI1, MPK2, and MPK3, in response to the substitution of blue light in environmental adaptation and signaling transduction pathways. Metabolomics analysis showed that two genes involved in carotenoid biosynthesis and starch and sucrose metabolism participate in stomatal development. Their signaling sites located on the PHI1 and MPK2 sites of the MAPK cascade respond to blue light signaling. (4) Conclusions: Blue light remarkably changed the transcriptional signal transduction and metabolism of metabolites, and eight obtained genes worked in the HY5 module and MAPK cascade.

RNA polymerase II associated proteins regulate stomatal development through direct interaction with stomatal transcription factors in Arabidopsis thaliana

RNA polymerase II (Pol II) associated proteins (RPAPs) have been ascribed diverse functions at the cellular level; however, their roles in developmental processes in yeasts, animals and plants are very poorly understood. Through screening for interactors of NRPB3, which encodes the third largest subunit of Pol II, we identified RIMA, the orthologue of mammalian RPAP2. A combination of genetic and biochemical assays revealed the role of RIMA and other RPAPs in stomatal development in Arabidopsis thaliana. We show that RIMA is involved in nuclear import of NRPB3 and other Pol II subunits, and is essential for restraining division and for establishing cell identity in the stomatal cell lineage. Moreover, plant RPAPs IYO/RPAP1 and QQT1/RPAP4, which interact with RIMA, are also crucial for stomatal development. Importantly, RIMA and QQT1 bind physically to stomatal transcription factors SPEECHLESS, MUTE, FAMA and SCREAMs. The RIMA-QQT1-IYO complex could work together with key stomatal transcription factors and Pol II to drive cell fate transitions in the stomatal cell lineage. Direct interactions with stomatal transcription factors provide a novel mechanism by which RPAP proteins may control differentiation of cell types and tissues in eukaryotes.

Plant stem cell research is uncovering the secrets of longevity and persistent growth

Plant stem cells have several extraordinary features: they are generated de novo during development and regeneration, maintain their pluripotency, and produce another stem cell niche in an orderly manner. This enables plants to survive for an extended period and to continuously make new organs, representing a clear difference in their developmental program from animals. To uncover regulatory principles governing plant stem cell characteristics, our research project 'Principles of pluripotent stem cells underlying plant vitality' was launched in 2017, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Japanese government. Through a collaboration involving 28 research groups, we aim to identify key factors that trigger epigenetic reprogramming and global changes in gene networks, and thereby contribute to stem cell generation. Pluripotent stem cells in the shoot apical meristem are controlled by cytokinin and auxin, which also play a crucial role in terminating stem cell activity in the floral meristem; therefore, we are focusing on biosynthesis, metabolism, transport, perception, and signaling of these hormones. Besides, we are uncovering the mechanisms of asymmetric cell division and of stem cell death and replenishment under DNA stress, which will illuminate plant-specific features in preserving stemness. Our technology support groups expand single-cell omics to describe stem cell behavior in a spatiotemporal context, and provide correlative light and electron microscopic technology to enable live imaging of cell and subcellular dynamics at high spatiotemporal resolution. In this perspective, we discuss future directions of our ongoing projects and related research fields.

A single-cell analysis of the Arabidopsis vegetative shoot apex

The shoot apical meristem allows for reiterative formation of new aerial structures throughout the life cycle of a plant. We use single-cell RNA sequencing to define the cellular taxonomy of the Arabidopsis vegetative shoot apex at the transcriptome level. We find that the shoot apex is composed of highly heterogeneous cells, which can be partitioned into 7 broad populations with 23 transcriptionally distinct cell clusters. We delineate cell-cycle continuums and developmental trajectories of epidermal cells, vascular tissue, and leaf mesophyll cells and infer transcription factors and gene expression signatures associated with cell fate decisions. Integrative analysis of shoot and root apical cell populations further reveals common and distinct features of epidermal and vascular tissues. Our results, thus, offer a valuable resource for investigating the basic principles underlying cell division and differentiation in plants at single-cell resolution.

SPEECHLESS and MUTE Mediate Feedback Regulation of Signal Transduction during Stomatal Development

Stomatal density, spacing, and patterning greatly influence the efficiency of gas exchange, photosynthesis, and water economy. They are regulated by a complex of extracellular and intracellular factors through the signaling pathways. After binding the extracellular epidermal patterning factor 1 (EPF1) and 2 (EPF2) as ligands, the receptor-ligand complexes activate by phosphorylation through the MAP-kinase cascades, regulating basic helix-loop-helix (bHLH) transcription factors SPEECHLESS (SPCH), MUTE, and FAMA. In this review, we summarize the molecular mechanisms and signal transduction pathways running within the transition of the protodermal cell into a pair of guard cells with a space (aperture) between them, called a stoma, comprising asymmetric and symmetric cell divisions and draw several functional models. The feedback mechanisms involving the bHLH factors SPCH and MUTE are not fully recognized yet. We show the feedback mechanisms driven by SPCH and MUTE in the regulation of EPF2 and the ERECTA family. Intersections of the molecular mechanisms for fate determination of stomatal lineage cells with the role of core cell cycle-related genes and stabilization of SPCH and MUTE are also reported.

The Natural Product Fucoidan Inhibits Proliferation and Induces Apoptosis of Human Ovarian Cancer Cells: Focus on the PI3K/Akt Signaling Pathway

OBJECTIVE

Ovarian cancer (OC) is the leading cause of death among gynecological tumors; however, no effective treatment is currently available. Fucoidan, which is extracted from marine algae, has significant anti-cancer effects. The aim of this study was to determine the effects of fucoidan on the proliferation and apoptosis of OC cells through inhibition of the PI3K/Akt signaling pathway.

METHODS

Human ovarian normal epithelial cells (IOSE80) and human OC cells (SKOV-3, A2780, OVCAR-3, TOV-112D, and Caov-3) were selected to verify the safety of fucoidan at various doses in SKOV-3 and Caov-3 cells as well as a xenograft mouse model using various molecular biology techniques.

RESULTS

Fucoidan had no significant effect on normal ovarian epithelial cells, but had significantly inhibited the proliferation of OC cells, induced cell cycle arrest at the G0/G1 phase, increased the proportion of apoptotic cells and expression of pro-apoptotic proteins, and inhibited the expression of PI3K and phosphorylation of Akt, which could be partly rescued by IGF-1.

CONCLUSION

Fucoidan had anti-tumor effects both in vivo and in vitro via a mechanism that is related to the inhibition of the PI3K/Akt signaling pathway.

Beyond What Your Retina Can See: Similarities of Retinoblastoma Function between Plants and Animals, from Developmental Processes to Epigenetic Regulation

The Retinoblastoma protein (pRb) is a key cell cycle regulator conserved in a wide variety of organisms. Experimental analysis of pRb's functions in animals and plants has revealed that this protein participates in cell proliferation and differentiation processes. In addition, pRb in animals and its orthologs in plants (RBR), are part of highly conserved protein complexes which suggest the possibility that analogies exist not only between functions carried out by pRb orthologs themselves, but also in the structure and roles of the protein networks where these proteins are involved. Here, we present examples of pRb/RBR participation in cell cycle control, cell differentiation, and in the regulation of epigenetic changes and chromatin remodeling machinery, highlighting the similarities that exist between the composition of such networks in plants and animals.

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.

Pores for Thought: Can Genetic Manipulation of Stomatal Density Protect Future Rice Yields?

Rice (Oryza sativa L.) contributes to the diets of around 3.5 billion people every day and is consumed more than any other plant. Alarmingly, climate predictions suggest that the frequency of severe drought and high-temperature events will increase, and this is set to threaten the global rice supply. In this review, we consider whether water or heat stresses in crops - especially rice - could be mitigated through alterations to stomata; minute pores on the plant epidermis that permit carbon acquisition and regulate water loss. In the short-term, water loss is controlled via alterations to the degree of stomatal "openness", or, in the longer-term, by altering the number (or density) of stomata that form. A range of molecular components contribute to the regulation of stomatal density, including transcription factors, plasma membrane-associated proteins and intercellular and extracellular signaling molecules. Much of our existing knowledge relating to stomatal development comes from research conducted on the model plant, Arabidopsis thaliana. However, due to the importance of cereal crops to global food supply, studies on grass stomata have expanded in recent years, with molecular-level discoveries underscoring several divergent developmental and morphological features. Cultivation of rice is particularly water-intensive, and there is interest in developing varieties that require less water yet still maintain grain yields. This could be achieved by manipulating stomatal development; a crop with fewer stomata might be more conservative in its water use and therefore more capable of surviving periods of water stress. However, decreasing stomatal density might restrict the rate of CO₂ uptake and reduce the extent of evaporative cooling, potentially leading to detrimental effects on yields. Thus, the extent to which crop yields in the future climate will be affected by increasing or decreasing stomatal density should be determined. Here, our current understanding of the regulation of stomatal development is summarised, focusing particularly on the genetic mechanisms that have recently been described for rice and other grasses. Application of this knowledge toward the creation of "climate-ready" rice is discussed, with attention drawn to the lesser-studied molecular elements whose contributions to the complexity of grass stomatal development must be understood to advance efforts.