An ancestral stomatal patterning module revealed in the non-vascular land plant Physcomitrella patens

The K/HDEL receptor does not recycle but instead acts as a Golgi-gatekeeper

Accurately measuring the ability of the K/HDEL receptor (ERD2) to retain the ER cargo Amy-HDEL has questioned earlier results on which the popular receptor recycling model is based upon. Here we demonstrate that ERD2 Golgi-retention, rather than fast ER export supports its function. Ligand-induced ERD2 redistribution is only observed when the C-terminus is masked or mutated, compromising the signal that prevents Golgi-to-ER transport of the receptor. Forcing COPI mediated retrograde transport destroys receptor function, but introducing ER-to-Golgi export or cis-Golgi retention signals re-activate ERD2 when its endogenous Golgi-retention signal is masked or deleted. We propose that ERD2 remains fixed as a Golgi gatekeeper, capturing K/HDEL proteins when they arrive and releasing them again into a subdomain for retrograde transport back to the ER. An in vivo ligand:receptor ratio far greater than 100 to 1 strongly supports this model, and the underlying mechanism appears to be extremely conserved across kingdoms.

Variations of stomata development in tea plant (Camellia sinensis) leaves in different light and temperature environments and genetic backgrounds

Stomata perform important functions in plant photosynthesis, respiration, gas exchange, and interactions with environments. However, tea plant stomata development and functions are not known. Here, we show morphological changes during stomata development and genetic dissection of stomata lineage genes regulating stomata formation in tea developing leaves. Different tea plant cultivars displayed clear variations in the stomata development rate, density and size, which are closely related to their tolerance against dehydration capabilities. Whole sets of stomata lineage genes were identified to display predicted functions in regulating stomatal development and formation. The stomata development and lineage genes were tightly regulated by light intensities and high or low temperature stresses, which affected stomata density and function. Furthermore, lower stomatal density and larger size were observed in triploid tea varieties as compared to those in diploid plant. Key stomata lineage genes such as CsSPCHs, CsSCRM, and CsFAMA showed much lower expression levels, whereas negative regulators CsEPF1 and CsYODAs had higher expression levels in triploid than in diploid tea varieties. Our study provides new insight into tea plant stomatal morphological development and the genetic regulatory mechanisms on stomata development under abiotic stresses and genetic backgrounds. The study lays a foundation for future exploring of the genetic improvement of water use efficiency in tea plants for living up to the challenge of global climate change.

Stu-miR827-Targeted StWRKY48 Transcription Factor Negatively Regulates Drought Tolerance of Potato by Increasing Leaf Stomatal Density

Stomata are specialized portals in plant leaves to modulate water loss from plants to the atmosphere by control of the transpiration, thereby determining the water-use efficiency and drought resistance of plants. Despite that the stomata developmental progression is well-understood at the molecular level, the experimental evidence that miRNA regulates stomata development is still lacking, and the underlying mechanism remains elusive. This study demonstrates the involvement of stu-miR827 in regulating the drought tolerance of potato due to its control over the leaf stomatal density. The expression analysis showed that stu-miR827 was obviously repressed by drought stresses and then rapidly increased after rewatering. Suppressing the expression of stu-miR827 transgenic potato lines showed an increase in stomatal density, correlating with a weaker drought resistance compared with wildtype potato lines. In addition, StWRKY48 was identified as the target gene of stu-miR827, and the expression of StWRKY48 was obviously induced by drought stresses and was greatly upregulated in stu-miR827 knockdown transgenic potato lines, suggesting its involvement in the drought stress response. Importantly, the expression of genes associated with stomata development, such as SDD (stomatal density and distribution) and TMM (too many mouths), was seriously suppressed in transgenic lines. Altogether, these observations demonstrated that suppression of stu-miR827 might lead to overexpression of StWRKY48, which may contribute to negatively regulating the drought adaptation of potato by increasing the stomatal density. The results may facilitate functional studies of miRNAs in the process of drought tolerance in plants.

Carbon monoxide promotes stomatal initiation by regulating the expression of two EPF genes in Arabidopsis cotyledons

The gaseous molecule carbon monoxide (CO) can freely pass through the cell membrane and participate in signal transduction in the cell to regulate physiological activities in plants. Here, we report that CO has a positive regulatory role in stomatal development. Exogenous CO donor CORM-2 [Tricarbonyldichlororuthenium (II) dimer] treatment resulted in an increase of stomatal index (SI) on the abaxial epidermis of cotyledons in wild-type, which can be reversed by the addition of the CO biosynthesis inhibitor ZnPPIX [Protoporphyrin IX zinc (II)]. Consistent with this result, mutation of the CO biosynthesis gene HY1 resulted in a decrease of SI in hy1-100 plants, while overexpression of HY1 led to an increase of SI. Further investigation revealed that CO acts upstream of SPCH and YDA in the stomatal development pathway, since the loss of function mutants spch-1 and yda-2 were insensitive to CORM-2. The expression of EPF2 was inhibited by CORM-2 treatment in wild type and is lower in hy1 than in wild-type plants. In contrast, the expression of STOMAGEN was promoted by CORM-2 treatment and is higher in HY1-overexpression lines. Loss of function mutants of both epf2 and stomagen are insensitive to CORM-2 treatment. These results indicated that CO positively regulates stomatal initiation and distribution by modulating the expression of EPF2 and STOMAGEN.

Small EPIDERMAL PATTERNING FACTOR-LIKE2 peptides regulate awn development in rice

The EPIDERMAL PATTERNING FACTOR (EPF) and EPF-LIKE (EPFL) family of small secreted peptides act to regulate many aspects of plant growth and development; however, their functions are not widely characterized in rice (Oryza sativa). Here, we used clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) technology to individually knockout each of 11 EPF/EPFL genes in the rice cultivar Kasalath. Loss of function of most OsEPF/EPFL genes generated no obvious phenotype alteration, while disruption of OsEPFL2 in Kasalath caused a short or no awn phenotype and reduced grain size. OsEPFL2 is strongly expressed in the young panicle, consistent with a role in regulating awn and grain development. Haplotype analysis indicated that OsEPFL2 can be classified into six major haplotypes. Nucleotide diversity and genetic differentiation analyses suggested that OsEPFL2 was positively selected during the domestication of rice. Our work to systematically investigate the function of EPF/EPFL peptides demonstrates that different members of the same gene family have been independently selected for their ability to regulate a similar biological function and provides perspective on rice domestication.

A Locus Controlling Leaf Rolling Degree in Wheat under Drought Stress Identified by Bulked Segregant Analysis

Drought stress frequently occurs, which seriously restricts the production of wheat (Triticum aestivum L.). Leaf rolling is a typical physiological phenomenon of plants during drought stress. To understand the genetic mechanism of wheat leaf rolling, we constructed an F₂ segregating population by crossing the slight-rolling wheat cultivar “Aikang 58” (AK58) with the serious-rolling wheat cultivar ″Zhongmai 36″ (ZM36). A combination of bulked segregant analysis (BSA) with Wheat 660K SNP Array was used to identify molecular markers linked to leaf rolling degree. A major locus for leaf rolling degree under drought stress was detected on chromosome 7A. We named this locus LEAF ROLLING DEGREE 1 (LERD1), which was ultimately mapped to a region between 717.82 and 720.18 Mb. Twenty-one genes were predicted in this region, among which the basic helix-loop-helix (bHLH) transcription factor TraesCS7A01G543300 was considered to be the most likely candidate gene for LERD1. The TraesCS7A01G543300 is highly homologous to the Arabidopsis ICE1 family proteins ICE/SCREAM, SCREAM2 and bHLH093, which control stomatal initiation and development. Two nucleotide variation sites were detected in the promoter region of TraesCS7A01G543300 between the two wheat cultivars. Gene expression assays indicated that TraesCS7A01G543300 was higher expressed in AK58 seedlings than that of ZM36. This research discovered a candidate gene related to wheat leaf rolling under drought stress, which may be helpful for understanding the leaf rolling mechanism and molecular breeding in wheat.

Pole position: How plant cells polarize along the axes

Having a sense of direction is a fundamental cellular trait that can determine cell shape, division orientation, or function, and ultimately the formation of a functional, multicellular body. Cells acquire and integrate directional information by establishing discrete subcellular domains along an axis with distinct molecular profiles, a process known as cell polarization. Insight into the principles and mechanisms underlying cell polarity has been propelled by decades of extensive research mostly in yeast and animal models. Our understanding of cell polarity establishment in plants, which lack most of the regulatory molecules identified in other eukaryotes, is more limited, but significant progress has been made in recent years. In this review, we explore how plant cells coordinately establish stable polarity axes aligned with the organ axes, highlighting similarities in the molecular logic used to polarize both plant and animal cells. We propose a classification system for plant cell polarity events and nomenclature guidelines. Finally, we provide a deep phylogenetic analysis of polar proteins and discuss the evolution of polarity machineries in plants.

Moss stomata do not respond to light and CO2 concentration but facilitate carbon uptake by sporophytes: a gas exchange, stomatal aperture, and 13 C-labelling study

Stomata exert control on fluxes of CO₂ and water (H₂ O) in the majority of vascular plants and thus are pivotal for planetary fluxes of carbon and H₂ O. However, in mosses, the significance and possible function of the sporophytic stomata are not well understood, hindering understanding of the ancestral function and evolution of these key structures of land plants. Infrared gas analysis and ¹³ CO₂ labelling, with supporting data from gravimetry and optical and scanning electron microscopy, were used to measure CO₂ assimilation and water exchange on young, green, ± fully expanded capsules of 11 moss species with a range of stomatal numbers, distributions, and aperture sizes. Moss sporophytes are effectively homoiohydric. In line with their open fixed apertures, moss stomata, contrary to those in tracheophytes, do not respond to light and CO₂ concentration. Whereas the sporophyte cuticle is highly impermeable to gases, stomata are the predominant sites of ¹³ CO₂ entry and H₂ O loss in moss sporophytes, and CO₂ assimilation is closely linked to total stomatal surface areas. Higher photosynthetic autonomy of moss sporophytes, consequent on the presence of numerous stomata, may have been the key to our understanding of evolution of large, gametophyte-independent sporophytes at the onset of plant terrestrialization.

Shouting out loud: signaling modules in the regulation of stomatal development

Stomata are small pores on the surface of land plants that facilitate gas exchange for photosynthesis while minimizing water loss. The function of stomata is pivotal for plant growth and survival. Intensive research on the model plant Arabidopsis (Arabidopsis thaliana) has discovered key peptide signaling pathways, transcription factors, and polarity components that together drive proper stomatal development and patterning. In this review, we focus on recent findings that have revealed co-option of peptide-receptor kinase signaling modules-utilized for diverse developmental processes and immune response. We further discuss an emerging connection between extrinsic signaling and intrinsic polarity modules. These findings have further enlightened our understanding of this fascinating developmental process.

Stomatal Lineage Control by Developmental Program and Environmental Cues

Stomata are micropores that allow plants to breathe and play a critical role in photosynthesis and nutrient uptake by regulating gas exchange and transpiration. Stomatal development, therefore, is optimized for survival and growth of the plant despite variable environmental conditions. Signaling cascades and transcriptional networks that determine the birth, proliferation, and differentiation of a stomate have been identified. These networks ensure proper stomatal patterning, density, and polarity. Environmental cues also influence stomatal development. In this review, we highlight recent findings regarding the developmental program governing cell fate and dynamics of stomatal lineage cells at the cell state- or single-cell level. We also overview the control of stomatal development by environmental cues as well as developmental plasticity associated with stomatal function and physiology. Recent advances in our understanding of stomatal development will provide a route to improving photosynthesis and water-stress resilience of crop plants in the climate change we currently face.

Stomata and Sporophytes of the Model Moss Physcomitrium patens

Mosses are an ancient land plant lineage and are therefore important in studying the evolution of plant developmental processes. Here, we describe stomatal development in the model moss species Physcomitrium patens (previously known as Physcomitrella patens) over the duration of sporophyte development. We dissect the molecular mechanisms guiding cell division and fate and highlight how stomatal function might vary under different environmental conditions. In contrast to the asymmetric entry divisions described in Arabidopsis thaliana, moss protodermal cells can enter the stomatal lineage directly by expanding into an oval shaped guard mother cell (GMC). We observed that when two early stage P. patens GMCs form adjacently, a spacing division can occur, leading to separation of the GMCs by an intervening epidermal spacer cell. We investigated whether orthologs of Arabidopsis stomatal development regulators are required for this spacing division. Our results indicated that bHLH transcription factors PpSMF1 and PpSCRM1 are required for GMC formation. Moreover, the ligand and receptor components PpEPF1 and PpTMM are also required for orientating cell divisions and preventing single or clustered early GMCs from developing adjacent to one another. The identification of GMC spacing divisions in P. patens raises the possibility that the ability to space stomatal lineage cells could have evolved before mosses diverged from the ancestral lineage. This would have enabled plants to integrate stomatal development with sporophyte growth and could underpin the adoption of multiple bHLH transcription factors and EPF ligands to more precisely control stomatal patterning in later diverging plant lineages. We also observed that when P. patens sporophyte capsules mature in wet conditions, stomata are typically plugged whereas under drier conditions this is not the case; instead, mucilage drying leads to hollow sub-stomatal cavities. This appears to aid capsule drying and provides further evidence for early land plant stomata contributing to capsule rupture and spore release.

A functionally informed evolutionary framework for the study of LRR-RLKs during stem cell maintenance

Plants maintain populations of stem cells to generate new organs throughout the course of their lives. The pathways that regulate plant stem cell maintenance have garnered great interest over the past decades, as variation in these pathways contributes plant morphological diversity and can be harnessed for crop improvement. In order to facilitate cross-species comparisons of gene function and better understand how these stem cell regulatory pathways evolved, we undertook a functionally informed phylogenetic analysis of leucine-rich receptor-like kinases (LRR-RLK) and related proteins across diverse land plant model systems. Based on our phylogenetic analysis and on functional data, we propose a naming scheme for these stem cell signaling genes. We discovered evidence for frequent loss of protein domains in angiosperms but not in bryophytes. In addition, several clades of stem cell signaling genes are closely related to genes that function in immunity, although these distinct developmental and immune functions likely separated or after the divergence of lycophytes and angiosperms. Overall, the phylogenetic framework and evolutionary hypotheses we provide here will empower future research on cross-species comparisons of stem cell signaling pathways.

Phylogenomic Evidence for the Monophyly of Bryophytes and the Reductive Evolution of Stomata

The origin of land plants was accompanied by new adaptations to life on land, including the evolution of stomata-pores on the surface of plants that regulate gas exchange. The genes that underpin the development and function of stomata have been extensively studied in model angiosperms, such as Arabidopsis. However, little is known about stomata in bryophytes, and their evolutionary origins and ancestral function remain poorly understood. Here, we resolve the position of bryophytes in the land plant tree and investigate the evolutionary origins of genes that specify stomatal development and function. Our analyses recover bryophyte monophyly and demonstrate that the guard cell toolkit is more ancient than has been appreciated previously. We show that a range of core guard cell genes, including SPCH/MUTE, SMF, and FAMA, map back to the common ancestor of embryophytes or even earlier. These analyses suggest that the first embryophytes possessed stomata that were more sophisticated than previously envisioned and that the stomata of bryophytes have undergone reductive evolution, including their complete loss from liverworts.

Proteome Analysis of the Gametophytes of a Western Himalayan Fern Diplazium maximum Reveals Their Adaptive Responses to Changes in Their Micro-Environment

Ferns have survived changing habitats and environmental extremes of different eras, wherein, the exploratory haploid gametophytes are believed to have played a major role. Therefore, the proteome of in vitro grown gametophytes of a temperate Himalayan fern, Diplazium maximum in response to 0 (G0), 1 (G1), and 3% (G3) sucrose was studied. A total of 110 differentially abundant protein spots (DAPs) were obtained. Among these, only 67 could be functionally categorized as unique proteins involved in various metabolic processes. Calcium dependent proteins, receptor like kinases, G proteins, proteins related to hormonal signaling and their interaction with other pathways, and regulatory proteins were recorded indicating the involvement of five different signaling pathways. DAPs involved in the activation of genes and transcription factors of signaling and transduction pathways, transport and ion channels, cell-wall and structural proteins, defense, chaperons, energy metabolism, protein synthesis, modification, and turnover were identified. The gametophytes responded to changes in their micro-environment. There was also significant increase in prothallus biomass and conversion of two-dimensional prothalli into three-dimensional prothallus clumps at 3% sucrose. The three-D clumps had higher photosynthetic surface area and also closer proximity for sexual reproduction and sporophyte formation. Highest accumulation of proline, enhanced scavenging of reactive oxygen species (ROS) and DAPs of mostly, abiotic stress tolerance, secondary metabolite synthesis, and detoxification at 3% sucrose indicated an adaptive response of gametophytes. Protein Protein Interaction network and Principal Component analyses, and qRT-PCR validation of genes encoding 12 proteins of various metabolic processes indicated differential adjustment of gametophytes to different levels of sucrose in the culture medium. Therefore, a hypothetical mechanism was proposed to show that even slight changes in the micro-environment of D. maximum gametophytes triggered multiple mechanisms of adaptation. Many DAPs identified in the study have potential use in crop improvement and metabolic engineering programs, phytoremediation and environmental protection.

The Tomato Genome Encodes SPCH, MUTE, and FAMA Candidates That Can Replace the Endogenous Functions of Their Arabidopsis Orthologs

Stomatal abundance determines the maximum potential for gas exchange between the plant and the atmosphere. In Arabidopsis, it is set during organ development through complex genetic networks linking epidermal differentiation programs with environmental response circuits. Three related bHLH transcription factors, SPCH, MUTE, and FAMA, act as positive drivers of stomata differentiation. Mutant alleles of some of these genes sustain different stomatal numbers in the mature organs and have potential to modify plant performance under different environmental conditions. However, knowledge about stomatal genes in dicotyledoneous crops is scarce. In this work, we identified the Solanum lycopersicum putative orthologs of these three master regulators and assessed their functional orthology by their ability to complement Arabidopsis loss-of-function mutants, the epidermal phenotypes elicited by their conditional overexpression, and the expression patterns of their promoter regions in Arabidopsis. Our results indicate that the tomato proteins are functionally equivalent to their Arabidopsis counterparts and that the tomato putative promoter regions display temporal and spatial expression domains similar to those reported for the Arabidopsis genes. In vivo tracking of tomato stomatal lineages in developing cotyledons revealed cell division and differentiation histories similar to those of Arabidopsis. Interestingly, the S. lycopersicum genome harbors a FAMA-like gene, expressed in leaves but functionally distinct from the true FAMA orthologue. Thus, the basic program for stomatal development in S. lycopersicum uses key conserved genetic determinants. This opens the possibility of modifying stomatal abundance in tomato through previously tested Arabidopsis alleles conferring altered stomata abundance phenotypes that correlate with physiological traits related to water status, leaf cooling, or photosynthesis.

Reduced stomatal density in bread wheat leads to increased water-use efficiency

Wheat is a staple crop, frequently cultivated in water-restricted environments. Improving crop water-use efficiency would be desirable if grain yield can be maintained. We investigated whether a decrease in wheat stomatal density via the manipulation of epidermal patterning factor (EPF) gene expression could improve water-use efficiency. Our results show that severe reductions in stomatal density in EPF-overexpressing wheat plants have a detrimental outcome on yields. However, wheat plants with a more moderate reduction in stomatal density (i.e. <50% reduction in stomatal density on leaves prior to tillering) had yields indistinguishable from controls, coupled with an increase in intrinsic water-use efficiency. Yields of these moderately reduced stomatal density plants were also comparable with those of control plants under conditions of drought and elevated CO2. Our data demonstrate that EPF-mediated control of wheat stomatal development follows that observed in other grasses, and we identify the potential of stomatal density as a tool for breeding wheat plants that are better able to withstand water-restricted environments without yield loss.

Agrobacterium-mediated Tnt1 mutagenesis of moss protonemal filaments and generation of stable mutants with impaired gametophyte

The gametophyte of moss exhibits a simple body plan, yet its growth is regulated by complex developmental phenomena similar to angiosperms. Because moss can be easily maintained under laboratory conditions, amenable for gene targeting and the availability of genome sequence, P. patens has become an attractive model system for studying evolutionary traits. Until date, there has been no Agrobacterium-mediated Tnt1 mutagenesis protocol for haploid protonemal filaments of moss. Hence, we attempted to use the intact tobacco Tnt1 retrotransposon as a mutagen for P. patens. Bioinformatic analysis of initiator methionyl-tRNA (Met-tRNAi), a critical host factor for Tnt1 transposition process, suggested that it can be explored as a mutagen for bryophytes. Using protonemal filaments and Agrobacterium-mediated transformation, 75 Tnt1 mutants have been generated and cryopreserved. SSAP analysis and TAIL-PCR revealed that Tnt1 is functional in P. patens and has a high-preference for gene and GC-rich regions. In addition, LTR::GUS lines exhibited a basal but tissue-specific inducible expression pattern. Forward genetic screen resulted in 5 novel phenotypes related to hormonal and gravity response, phyllid, and gamete development. SSAP analysis suggests that the Tnt1 insertion pattern is stable under normal growth conditions and the high-frequency phenotypic deviations are possibly due to the combination of haploid explant (protonema) and the choice of mutagen (Tnt1). We demonstrate that Agrobacterium-mediated Tnt1 insertional mutagenesis could generate stable P. patens mutant populations for future forward genetic studies.

The plant stomatal lineage at a glance

Stomata are structures on the surfaces of most land plants that are required for gas exchange between plants and their environment. In Arabidopsis thaliana, stomata comprise two kidney bean-shaped epidermal guard cells that flank a central pore overlying a cavity in the mesophyll. These guard cells can adjust their shape to occlude or facilitate access to this pore, and in so doing regulate the release of water vapor and oxygen from the plant, in exchange for the intake of carbon dioxide from the atmosphere. Stomatal guard cells are the end product of a specialized lineage whose cell divisions and fate transitions ensure both the production and pattern of cells in aerial epidermal tissues. The stomatal lineage is dynamic and flexible, altering stomatal production in response to environmental change. As such, the stomatal lineage is an excellent system to study how flexible developmental transitions are regulated in plants. In this Cell Science at a Glance article and accompanying poster, we will summarize current knowledge of the divisions and fate decisions during stomatal development, discussing the role of transcriptional regulators, cell-cell signaling and polarity proteins. We will highlight recent work that links the core regulators to systemic or environmental information and provide an evolutionary perspective on stomata lineage regulators in plants.

Impact of Stomatal Density and Morphology on Water-Use Efficiency in a Changing World

Global warming and associated precipitation changes will negatively impact on many agricultural ecosystems. Major food production areas are expected to experience reduced water availability and increased frequency of drought over the coming decades. In affected areas, this is expected to reduce the production of important food crops including wheat, rice, and maize. The development of crop varieties able to sustain or improve yields with less water input is, therefore, a priority for crop research. Almost all water used for plant growth is lost to the atmosphere by transpiration through stomatal pores on the leaf epidermis. By altering stomatal pore apertures, plants are able to optimize their CO2 uptake for photosynthesis while minimizing water loss. Over longer periods, stomatal development may also be adjusted, with stomatal size and density being adapted to suit the prevailing conditions. Several approaches to improve drought tolerance and water-use efficiency through the modification of stomatal traits have been tested in the model plant Arabidopsis thaliana. However, there is surprisingly little known about the stomata of crop species. Here, we review the current understanding of how stomatal number and morphology are involved in regulating water-use efficiency. Moreover, we discuss the potential and limitations of manipulating stomatal development to increase drought tolerance and to reduce water loss in crops as the climate changes.

Homologous genes of epidermal patterning factor regulate stomatal development in rice

Stomata are microscopic pores on the surface of leaves through which water as vapor passes to the atmosphere and CO₂ uptake for the photosynthesis. The signaling peptides of the epidermal patterning factor (EPF) family regulate stomatal development and density in Arabidopsis. Several putative homologs of EPF/EPFL exist in rice genome. To understand their possible involvement in stomatal formation, in this study we generated a series of transgenic lines including reporter promoter fusions, down-regulation and overexpression and demonstrated drastic differences in stomatal densities between different genotypes, as elevated expression of OsEPF1 or OsEPF2 greatly reduced stomatal density in rice, whereas ectopic overexpression of either OsEPF1 or OsEPF2 significantly decreased the high stomatal frequency of both mutant lines of epf2 and epf1epf2 Arabidopsis. Conversely, knocking down OsEPFL9 transcription conferred transgenic plants with fewer stomata than WT in rice, whereas overexpressing rice OsEPFL9 gene could cause excessive production of stomata in Arabidopsis. In conclusion, homologs of EPF/EPFL regulate stomatal development in a generally highly conserved way yet there exist function distinctions between dicot and monocot plants.

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.

EPFL Signals in the Boundary Region of the SAM Restrict Its Size and Promote Leaf Initiation

The shoot apical meristem (SAM) enables the formation of new organs throughout the life of a plant. ERECTA family (ERf) receptors restrict SAM size and promote initiation of leaves while simultaneously supporting establishment of correct phyllotaxy. In the epidermis and during organ elongation ERf activity is regulated by a family of Epidermal Patterning Factor-Like (EPFL) secreted Cys-rich small proteins. Here we show that ERfs play a critical role in communication between the SAM leaf boundary and the central zone in Arabidopsis (Arabidopsis thaliana). Ectopic expression of ERECTA in the central zone using the CLAVATA3 promoter is sufficient to restrict meristem size and promote leaf initiation. Genetic analysis demonstrated that four putative ligands: EPFL1, EPFL2, EPFL4, and EPFL6 function redundantly in the SAM. These genes are expressed at the SAM-leaf boundary and in the peripheral zone. Previously EPFL4 and EPFL6 have been linked with elongation of aboveground organs. Here we demonstrate that EPFL1 and EPFL2 promote organ elongation as well. In addition, we show that expression of ERECTA in the central zone of the SAM has a strong impact on elongation of internodes and pedicels and growth of leaves. These results suggest that ERfs can stimulate organ growth cell nonautonomously.

Hornwort stomata do not respond actively to exogenous and environmental cues

Backgrounds and Aims

Because stomata in bryophytes occur on sporangia, they are subject to different developmental and evolutionary constraints from those on leaves of tracheophytes. No conclusive experimental evidence exists on the responses of hornwort stomata to exogenous stimulation.

Methods

Responses of hornwort stomata to abscisic acid (ABA), desiccation, darkness and plasmolysis were compared with those in tracheophyte leaves. Potassium ion concentrations in the guard cells and adjacent cells were analysed by X-ray microanalysis, and the ontogeny of the sporophytic intercellular spaces was compared with those of tracheophytes by cryo-scanning electron microscopy.

Key Results

The apertures in hornwort stomata open early in development and thereafter remain open. In hornworts, the experimental treatments, based on measurements of >9000 stomata, produced only a slight reduction in aperture dimensions after desiccation and plasmolysis, and no changes following ABA treatments and darkness. In tracheophytes, all these treatments resulted in complete stomatal closure. Potassium concentrations are similar in hornwort guard cells and epidermal cells under all treatments at all times. The small changes in hornwort stomatal dimensions in response to desiccation and plasmolysis are probably mechanical and/or stress responses of all the epidermal and spongy chlorophyllose cells, affecting the guard cells. In contrast to their nascent gas-filled counterparts across tracheophytes, sporophytic intercellular spaces in hornworts are initially liquid filled.

Conclusions

Our experiments demonstrate a lack of physiological regulation of opening and closing of stomata in hornworts compared with tracheophytes, and support accumulating developmental and structural evidence that stomata in hornworts are primarily involved in sporophyte desiccation and spore discharge rather than the regulation of photosynthesis-related gaseous exchange. Our results run counter to the notion of the early acquisition of active control of stomatal movements in bryophytes as proposed from previous experiments on mosses.

The evolution of the stomatal apparatus: intercellular spaces and sporophyte water relations in bryophytes-two ignored dimensions

Cryo-scanning electron microscopy shows that nascent intercellular spaces (ICSs) in bryophytes are liquid-filled, whereas these are gas-filled from the outset in tracheophytes except in the gametophytes of Lycopodiales. ICSs are absent in moss gametophytes and remain liquid-filled in hornwort gametophytes and in both generations in liverworts. Liquid is replaced by gas following stomatal opening in hornworts and is ubiquitous in moss sporophytes even in astomate taxa. New data on moss water relations and sporophyte weights indicate that the latter are homiohydric while X-ray microanalysis reveals an absence of potassium pumps in the stomatal apparatus. The distribution of ICSs in bryophytes is strongly indicative of very ancient multiple origins. Inherent in this scenario is either the dual or triple evolution of stomata. The absence, in mosses, of any relationship between increases in sporophyte biomass and stomata numbers and absences, suggests that CO2 entry through the stomata, possible only after fluid replacement by gas in the ICSs, makes but a minor contribution to sporophyte nutrition. Save for a single claim of active regulation of aperture dimensions in mosses, all other functional and structural data point to the sporophyte desiccation, leading to spore discharge, as the primeval role of the stomatal apparatus.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.

Stomatal development: focusing on the grasses

The development and patterning of stomata in the plant epidermis has emerged as an ideal system for studying fundamental plant developmental processes. Over the past twenty years most studies of stomata have used the model dicotyledonous plant Arabidopsis thaliana. However, cultivated monocotyledonous grass (or Gramineae) varieties provide the majority of human nutrition, and future research into grass stomata could be of critical importance for improving food security. Recent studies using Brachypodium distachyon, Hordeum vulgare (barley) and Oryza sativa (rice) have led to the identification of the core transcriptional regulators essential for stomatal initiation and progression in grasses, and begun to unravel the role of secretory signaling peptides in controlling stomatal developmental. This review revisits how stomatal developmental unfolds in grasses, and identifies key ontogenetic steps for which knowledge of the underpinning molecular mechanisms remains outstanding.

Environmental sensing and morphological plasticity in plants

All creatures on earth are affected by their surrounding environment. Animals can move and escape unfavorable environmental changes, whereas plants must respond to environmental stimuli. Plants adapt to changes with cellular-level responses to short-term environmental changes, but may adapt to changes in the environment by regulating their development and growth. In this review, we considered changes in atmospheric CO₂ concentrations, dry/wet moisture conditions, flooding, and temperature as examples of environmental stimuli. We mainly focused on leaf morphology and stomatal density as examples of developmental and growth patterns of plants in response to the environment.

The desert plant Phoenix dactylifera closes stomata via nitrate-regulated SLAC1 anion channel

Date palm Phoenix dactylifera is a desert crop well adapted to survive and produce fruits under extreme drought and heat. How are palms under such harsh environmental conditions able to limit transpirational water loss? Here, we analysed the cuticular waxes, stomata structure and function, and molecular biology of guard cells from P. dactylifera. To understand the stomatal response to the water stress phytohormone of the desert plant, we cloned the major elements necessary for guard cell fast abscisic acid (ABA) signalling and reconstituted this ABA signalosome in Xenopus oocytes. The PhoenixSLAC1-type anion channel is regulated by ABA kinase PdOST1. Energy-dispersive X-ray analysis (EDXA) demonstrated that date palm guard cells release chloride during stomatal closure. However, in Cl^- medium, PdOST1 did not activate the desert plant anion channel PdSLAC1 per se. Only when nitrate was present at the extracellular face of the anion channel did the OST1-gated PdSLAC1 open, thus enabling chloride release. In the presence of nitrate, ABA enhanced and accelerated stomatal closure. Our findings indicate that, in date palm, the guard cell osmotic motor driving stomatal closure uses nitrate as the signal to open the major anion channel SLAC1. This initiates guard cell depolarization and the release of anions together with potassium.

Biology of SLAC1-type anion channels - from nutrient uptake to stomatal closure

Contents 46 I. 46 II. 47 III. 50 IV. 53 V. 56 VI. 57 58 58 References 58 SUMMARY: Stomatal guard cells control leaf CO₂ intake and concomitant water loss to the atmosphere. When photosynthetic CO₂ assimilation is limited and the ratio of CO₂ intake to transpiration becomes suboptimal, guard cells, sensing the rise in CO₂ concentration in the substomatal cavity, deflate and the stomata close. Screens for mutants that do not close in response to experimentally imposed high CO₂ atmospheres identified the guard cell-expressed Slowly activating anion channel, SLAC1, as the key player in the regulation of stomatal closure. SLAC1 evolved, though, before the emergence of guard cells. In Arabidopsis, SLAC1 is the founder member of a family of anion channels, which comprises four homologues. SLAC1 and SLAH3 mediate chloride and nitrate transport in guard cells, while SLAH1, SLAH2 and SLAH3 are engaged in root nitrate and chloride acquisition, and anion translocation to the shoot. The signal transduction pathways involved in CO₂ , water stress and nutrient-sensing activate SLAC/SLAH via distinct protein kinase/phosphatase pairs. In this review, we discuss the role that SLAC/SLAH channels play in guard cell closure, on the one hand, and in the root-shoot continuum on the other, along with the molecular basis of the channels' anion selectivity and gating.

Reducing Stomatal Density in Barley Improves Drought Tolerance without Impacting on Yield

The epidermal patterning factor (EPF) family of secreted signaling peptides regulate the frequency of stomatal development in model dicot and basal land plant species. Here, we identify and manipulate the expression of a barley (Hordeum vulgare) ortholog and demonstrate that when overexpressed HvEPF1 limits entry to, and progression through, the stomatal development pathway. Despite substantial reductions in leaf gas exchange, barley plants with significantly reduced stomatal density show no reductions in grain yield. In addition, HvEPF1OE barley lines exhibit significantly enhanced water use efficiency, drought tolerance, and soil water conservation properties. Our results demonstrate the potential of manipulating stomatal frequency for the protection and optimization of cereal crop yields under future drier environments.

Origins and Evolution of Stomatal Development

The fossil record suggests stomata-like pores were present on the surfaces of land plants over 400 million years ago. Whether stomata arose once or whether they arose independently across newly evolving land plant lineages has long been a matter of debate. In Arabidopsis, a genetic toolbox has been identified that tightly controls stomatal development and patterning. This includes the basic helix-loop-helix (bHLH) transcription factors SPEECHLESS (SPCH), MUTE, FAMA, and ICE/SCREAMs (SCRMs), which promote stomatal formation. These factors are regulated via a signaling cascade, which includes mobile EPIDERMAL PATTERNING FACTOR (EPF) peptides to enforce stomatal spacing. Mosses and hornworts, the most ancient extant lineages to possess stomata, possess orthologs of these Arabidopsis (Arabidopsis thaliana) stomatal toolbox genes, and manipulation in the model bryophyte Physcomitrella patens has shown that the bHLH and EPF components are also required for moss stomatal development and patterning. This supports an ancient and tightly conserved genetic origin of stomata. Here, we review recent discoveries and, by interrogating newly available plant genomes, we advance the story of stomatal development and patterning across land plant evolution. Furthermore, we identify potential orthologs of the key toolbox genes in a hornwort, further supporting a single ancient genetic origin of stomata in the ancestor to all stomatous land plants.

Evolution of the Stomatal Regulation of Plant Water Content

Changes in the function of stomata from the earliest bryophytes to derived angiosperms are examined.

A receptor-like protein acts as a specificity switch for the regulation of stomatal development

Stomata are microscopic openings that allow for the exchange of gases between plants and the environment. In Arabidopsis, stomatal patterning is specified by the ERECTA family (ERf) receptor kinases (RKs), the receptor-like protein (RLP) TOO MANY MOUTHS (TMM), and EPIDERMAL PATTERNING FACTOR (EPF) peptides. Here we show that TMM and ER or ER-LIKE1 (ERL1) form constitutive complexes, which recognize EPF1 and EPF2, but the single ERfs do not. TMM interaction with ERL1 creates a binding pocket for recognition of EPF1 and EPF2, indicating that the constitutive TMM-ERf complexes function as the receptors of EPF1 and EPF2. EPFL9 competes with EPF1 and EPF2 for binding to the ERf-TMM complex. EPFL4 and EPFL6, however, are recognized by the single ERfs without the requirement of TMM. In contrast to EPF1,2, the interaction of EPFL4,6 with an ERf is greatly reduced in the presence of TMM. Taken together, our data demonstrate that TMM dictates the specificity of ERfs for the perception of different EPFs, thus functioning as a specificity switch for the regulation of the activities of ERfs.

Different adaptation strategies of two citrus scion/rootstock combinations in response to drought stress

Scion/rootstock interaction is important for plant development and for breeding programs. In this context, polyploid rootstocks presented several advantages, mainly in relation to biotic and abiotic stresses. Here we analyzed the response to drought of two different scion/rootstock combinations presenting different polyploidy: the diploid (2x) and autotetraploid (4x) Rangpur lime (Citrus limonia, Osbeck) rootstocks grafted with 2x Valencia Delta sweet orange (Citrus sinensis) scions, named V/2xRL and V/4xRL, respectively. Based on previous gene expression data, we developed an interactomic approach to identify proteins involved in V/2xRL and V/4xRL response to drought. A main interactomic network containing 3,830 nodes and 97,652 edges was built from V/2xRL and V/4xRL data. Exclusive proteins of the V/2xRL and V/4xRL networks (2,056 and 1,001, respectively), as well as common to both networks (773) were identified. Functional clusters were obtained and two models of drought stress response for the V/2xRL and V/4xRL genotypes were designed. Even if the V/2xRL plant implement some tolerance mechanisms, the global plant response to drought was rapid and quickly exhaustive resulting in a general tendency to dehydration avoidance, which presented some advantage in short and strong drought stress conditions, but which, in long terms, does not allow the plant survival. At the contrary, the V/4xRL plants presented a response which strong impacts on development but that present some advantages in case of prolonged drought. Finally, some specific proteins, which presented high centrality on interactomic analysis were identified as good candidates for subsequent functional analysis of citrus genes related to drought response, as well as be good markers of one or another physiological mechanism implemented by the plants.

Autocrine regulation of stomatal differentiation potential by EPF1 and ERECTA-LIKE1 ligand-receptor signaling

Development of stomata, valves on the plant epidermis for optimal gas exchange and water control, is fine-tuned by multiple signaling peptides with unique, overlapping, or antagonistic activities. EPIDERMAL PATTERNING FACTOR1 (EPF1) is a founding member of the secreted peptide ligands enforcing stomatal patterning. Yet, its exact role remains unclear. Here, we report that EPF1 and its primary receptor ERECTA-LIKE1 (ERL1) target MUTE, a transcription factor specifying the proliferation-to-differentiation switch within the stomatal cell lineages. In turn, MUTE directly induces ERL1. The absolute co-expression of ERL1 and MUTE, with the co-presence of EPF1, triggers autocrine inhibition of stomatal fate. During normal stomatal development, this autocrine inhibition prevents extra symmetric divisions of stomatal precursors likely owing to excessive MUTE activity. Our study reveals the unexpected role of self-inhibition as a mechanism for ensuring proper stomatal development and suggests an intricate signal buffering mechanism underlying plant tissue patterning.

DOI:http://dx.doi.org/10.7554/eLife.24102.001

Stomatal development in time: the past and the future

Stomata have significantly diversified in nature since their first appearance around 400 million years ago. The diversification suggests the active reprogramming of molecular machineries of stomatal development during evolution. This review focuses on recent progress that sheds light on how this rewiring occurred in different organisms. Three specific aspects are discussed in this review: (i) the evolution of the transcriptional complex that governs stomatal state transitions; (ii) the evolution of receptor-ligand pairs that mediate extrinsic signaling; and (iii) the loss of stomatal development genes in an astomatous angiosperm.

Secondary development in the stem: when Arabidopsis and trees are closer than it seems

Secondary growth, the increase in girth of plant organs, is primarily driven by the vascular and cork cambium. In perennial dicotyledons and gymnosperms, it represents a major source of biomass accumulation in the form of wood. However, the molecular framework underlying secondary growth is largely based on studies in the annual herbaceous plant Arabidopsis thaliana. In this review, we will focus on a selection of major regulators of stem secondary growth, which have recently been shown to play a role in woody species. In particular, we will focus on thermospermine and its bivalent role in controlling xylem differentiation and cell proliferation and we will highlight the contributions of the different LRR-Receptor-Like Kinase signaling hubs.