Mechanisms of stomatal development: an evolutionary view

Genome-wide analyses of member identification, expression pattern, and protein-protein interaction of EPF/EPFL gene family in Gossypium

BACKGROUND

Epidermal patterning factor / -like (EPF/EPFL) gene family encodes a class of cysteine-rich secretory peptides, which are widelyfound in terrestrial plants.Multiple studies has indicated that EPF/EPFLs might play significant roles in coordinating plant development and growth, especially as the morphogenesis processes of stoma, awn, stamen, and fruit skin. However, few research on EPF/EPFL gene family was reported in Gossypium.

RESULTS

We separately identified 20 G. raimondii, 24 G. arboreum, 44 G. hirsutum, and 44 G. barbadense EPF/EPFL genes in the 4 representative cotton species, which were divided into four clades together with 11 Arabidopsis thaliana, 13 Oryza sativa, and 17 Selaginella moellendorffii ones based on their evolutionary relationships. The similar gene structure and common motifs indicated the high conservation among the EPF/EPFL members, while the uneven distribution in chromosomes implied the variability during the long-term evolutionary process. Hundreds of collinearity relationships were identified from the pairwise comparisons of intraspecifc and interspecific genomes, which illustrated gene duplication might contribute to the expansion of cotton EPF/EPFL gene family. A total of 15 kinds of cis-regulatory elements were predicted in the promoter regions, and divided into three major categories relevant to the biological processes of development and growth, plant hormone response, and abiotic stress response. Having performing the expression pattern analyses with the basic of the published RNA-seq data, we found most of GhEPF/EPFL and GbEPF/EPFL genes presented the relatively low expression levels among the 9 tissues or organs, while showed more dramatically different responses to high/low temperature and salt or drought stresses. Combined with transcriptome data of developing ovules and fibers and quantitative Real-time PCR results (qRT-PCR) of 15 highly expressed GhEPF/EPFL genes, it could be deduced that the cotton EPF/EPFL genes were closely related with fiber development. Additionally, the networks of protein-protein interacting among EPF/EPFLs concentrated on the cores of GhEPF1 and GhEPF7, and thosefunctional enrichment analyses indicated that most of EPF/EPFLs participate in the GO (Gene Ontology) terms of stomatal development and plant epidermis development, and the KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways of DNA or base excision repair.

CONCLUSION

Totally, 132 EPF/EPFL genes were identified for the first time in cotton, whose bioinformatic analyses of cis-regulatory elements and expression patterns combined with qRT-PCR experiments to prove the potential functions in the biological processes of plant growth and responding to abiotic stresses, specifically in the fiber development. These results not only provide comprehensive and valuable information for cotton EPF/EPFL gene family, but also lay solid foundation for screening candidate EPF/EPFL genes in further cotton breeding.

Genome-wide identification and phylogenetic and expression pattern analyses of EPF/EPFL family genes in the Rye (Secale cereale L.)

Rye (Secale cereale L.) is one of the major cereal crop species in the Triticeae family and is known to be most tolerant to diverse abiotic stresses, such as cold, heat, osmotic, and salt stress. The EPIDERMAL PATTERNING FACTOR (EPF) and EPF-LIKE (EPFL) families of small secreted peptides act to regulate many aspects of plant growth and development; however, their functions are not widely characterized in rye. In this study, we identified 12 ScEPF/EPFL genes, which can be divided into six groups and are evenly distributed on six rye chromosomes. Further examination of the gene structure and protein conservation motifs of EPF/EPFL family members demonstrated the high conservation of the ScEPF/EPFL sequence. Interactions between ScEPF/EPFL proteins and promoters containing hormone- and stress-responsive cis-acting elements suggest that the regulation of ScEPF/EPFL expression is complex. Expression profiling analyses revealed that ScEPF/EPFL genes exhibited tissue-specific expression patterns. Notably, ScEPFL1,ScEPFL7, ScEPFL9, and ScEPFL10 displayed significantly higher expression levels in spikelets compared to other tissues. Moreover, fluorescence quantification experiments demonstrated that these genes exhibited distinct expression patterns in response to various stress conditions, suggesting that each gene plays a unique role in stress signaling pathways. Our research findings provide a solid basis for further investigation into the functions of ScEPF/EPFLs. Furthermore, these genes can serve as potential candidates for breeding stress-resistant rye varieties and improving production yields.

Stomatal evolution and plant adaptation to future climate

Global climate change is affecting plant photosynthesis and transpiration processes, as well as increasing weather extremes impacting socio-political and environmental events and decisions for decades to come. One major research challenge in plant biology and ecology is the interaction of photosynthesis with the environment. Stomata control plant gas exchange and their evolution was a crucial innovation that facilitated the earliest land plants to colonize terrestrial environments. Stomata couple homoiohydry, together with cuticles, intercellular gas space, with the endohydric water-conducting system, enabling plants to adapt and diversify across the planet. Plants control stomatal movement in response to environmental change through regulating guard cell turgor mediated by membrane transporters and signaling transduction. However, the origin, evolution, and active control of stomata remain controversial topics. We first review stomatal evolution and diversity, providing fossil and phylogenetic evidence of their origins. We summarize functional evolution of guard cell membrane transporters in the context of climate changes and environmental stresses. Our analyses show that the core signaling elements of stomatal movement are more ancient than stomata, while genes involved in stomatal development co-evolved de novo with the earliest stomata. These results suggest that novel stomatal development-specific genes were acquired during plant evolution, whereas genes regulating stomatal movement, especially cell signaling pathways, were inherited ancestrally and co-opted by dynamic functional differentiation. These two processes reflect the different adaptation strategies during land plant evolution.

Maize LOST SUBSIDIARY CELL encoding a large subunit of ribonucleotide reductase is required for subsidiary cell development and plant growth

The four-celled stomatal complex consists of a pair of guard cells (GCs) and two subsidiary cells (SCs) in grasses, which supports a fast adjustment of stomatal aperture. The formation and development of SCs are thus important for stomatal functionality. Here, we report a maize lost subsidiary cells (lsc) mutant, with many stomata lacking one or two SCs. The loss of SCs is supposed to have resulted from impeded subsidiary mother cell (SMC) polarization and asymmetrical division. Besides the defect in SCs, the lsc mutant also displays a dwarf morphology and pale and striped newly-grown leaves. LSC encodes a large subunit of ribonucleotide reductase (RNR), an enzyme involved in deoxyribonucleotides (dNTPs) synthesis. Consistently, the concentration of dNTPs and expression of genes involved in DNA replication, cell cycle progression, and SC development were significantly reduced in the lsc mutant compared with the wild-type B73 inbred line. Conversely, overexpression of maize LSC increased dNTP synthesis and promoted plant growth in both maize and Arabidopsis. Our data indicate that LSC regulates dNTP production and is required for SMC polarization, SC differentiation, and growth of maize.

Anatomical drivers of stomatal conductance in sorghum lines with different leaf widths grown under different temperatures

Sustaining crop productivity and resilience in water-limited environments and under rising temperatures are matters of concern worldwide. We investigated the leaf anatomical traits that underpin our recently identified link between leaf width (LW) and intrinsic water use efficiency (iWUE), as traits of interest in plant breeding. Ten sorghum lines with varying LW were grown under three temperatures to expand the range of variation of both LW and gas exchange rates. Leaf gas exchange, surface morphology and cross-sectional anatomy were measured and analysed using structural equations modelling. Narrower leaves had lower stomatal conductance (g_s ) and higher iWUE across growth temperatures. They also had smaller intercellular airspaces, stomatal size, percentage of open stomatal aperture relative to maximum, hydraulic pathway, mesophyll thickness, and leaf mass per area. Structural modelling revealed a developmental association among leaf anatomical traits that underpinned g_s variation in sorghum. Growing temperature and LW both impacted leaf gas exchange rates, but only LW directly impacted leaf anatomy. Wider leaves may be more productive under well-watered conditions, but consume more water for growth and development, which is detrimental under water stress.

Effects of Heavy Metals on Stomata in Plants: A Review

Stomata are one of the important structures for plants to alleviate metal stress and improve plant resistance. Therefore, a study on the effects and mechanisms of heavy metal toxicity to stomata is indispensable in clarifying the adaptation mechanism of plants to heavy metals. With the rapid pace of industrialization and urbanization, heavy metal pollution has been an environmental issue of global concern. Stomata, a special physiological structure of plants, play an important role in maintaining plant physiological and ecological functions. Recent studies have shown that heavy metals can affect the structure and function of stomata, leading to changes in plant physiology and ecology. However, although the scientific community has accumulated some data on the effects of heavy metals on plant stomata, the systematic understanding of the effects of heavy metals on plant stomata remains limited. Therefore, in this review, we present the sources and migration pathways of heavy metals in plant stomata, analyze systematically the physiological and ecological responses of stomata on heavy metal exposure, and summarize the current mechanisms of heavy metal toxicity on stomata. Finally, the future research perspectives of the effects of heavy metals on plant stomata are identified. This paper can serve as a reference for the ecological assessment of heavy metals and the protection of plant resources.

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.

Characterization of anatomical features, developmental roadmaps, and key genes of bamboo leaf epidermis

The exact developmental roadmaps of bamboo leaf epidermis and the regulating genes are largely unknown. In this study, we comprehensively investigated the morphological features of the leaf epidermis of bamboo, Pseudosasa japonica. We also established the developmental roadmaps of the abaxial epidermis along the linearly growing leaf. A variant of P. japonica, P. japonica var. tsutsumiana, with smaller stomata and higher stomata density, was identified. Further analysis revealed that the higher stomata density of the variant was due to the abnormal increase in stomata columns within the single stomata band. This abnormal development of stomata bands was observed as early as the guard mother cell stage in the leaf division zone (DZ). Interestingly, the developmental pattern of the single stomata was similar in P. japonica and the variant. Molecular data showed that PjDLT (Dwarf and Low Tillering) was significantly downregulated in leaves DZ of the variant. Overexpression of PjDLT in Arabidopsis and rice results in smaller plants with lower stomata density, whereas downregulation or mutation of OsDLT results in increased stomata density. Our results highlight the morphological features and developmental schedule of the leaf epidermis of bamboo and provide evidence that DLT plays an important role in regulating stomata in bamboo and rice.

Rice STOMATAL CYTOKINESIS DEFECTIVE2 regulates cell expansion by affecting vesicular trafficking in rice

Vesicular trafficking plays critical roles in cell expansion in yeast and mammals, but information linking vesicular trafficking and cell expansion in plants is limited. Here, we isolated and characterized a rice (Oryza sativa) mutant, decreased plant height 1-1 (dph1-1), which exhibited a wide spectrum of developmental phenotypes, including reduced plant height and smaller panicles and grains. Cytological analysis revealed that limited cell expansion was responsible for the dph1-1 mutant phenotype compared to the wild-type. Map-based cloning revealed that DPH1 encodes a plant-specific protein, OsSCD2, which is homologous to Arabidopsis (Arabidopsis thaliana) STOMATAL CYTOKINESIS DEFECTIVE2 (SCD2). Subcellular localization revealed that OsSCD2 is associated with clathrin. Confocal microscopy showed that the dph1-1 mutant has defective endocytosis and post-Golgi trafficking. Biochemical and confocal data indicated that OsSCD2 physically interacts with OsSCD1 and that they are associated with intracellular structures that colocalize with microtubules. Furthermore, we found that cellulose synthesis was affected in the dph1-1 mutant, evidenced by reduced cellulose synthase gene accumulation at the transcript and protein levels, most likely resulting from an impaired localization pattern. Our results suggest that OsSCD2 is involved in clathrin-related vesicular trafficking with an important role in maintaining plant growth in rice.

Water stress resilient cereal crops: Lessons from wild relatives

Cereal crops are significant contributors to global diets. As climate change disrupts weather patterns and wreaks havoc on crops, the need for generating stress-resilient, high-yielding varieties is more urgent than ever. One extremely promising avenue in this regard is to exploit the tremendous genetic diversity expressed by the wild ancestors of current day crop species. These crop wild relatives thrive in a range of environments and accordingly often harbor an array of traits that allow them to do so. The identification and introgression of these traits into our staple cereal crops can lessen yield losses in stressful environments. In the last decades, a surge in extreme drought and flooding events have severely impacted cereal crop production. Climate models predict a persistence of this trend, thus reinforcing the need for research on water stress resilience. Here we review: (i) how water stress (drought and flooding) impacts crop performance; and (ii) how identification of tolerance traits and mechanisms from wild relatives of the main cereal crops, that is, rice, maize, wheat, and barley, can lead to improved survival and sustained yields in these crops under water stress conditions.

SlKIX8 and SlKIX9 are negative regulators of leaf and fruit growth in tomato

Plant organ size and shape are major agronomic traits that depend on cell division and expansion, which are both regulated by complex gene networks. In several eudicot species belonging to the rosid clade, organ growth is controlled by a repressor complex consisting of PEAPOD (PPD) and KINASE-INDUCIBLE DOMAIN INTERACTING (KIX) proteins. The role of these proteins in asterids, which together with the rosids constitute most of the core eudicot species, is unknown. We used Clustered Regularly Interspaced Short Palindromic Repeats-CRISPR-associated protein 9 genome editing to target SlKIX8 and SlKIX9 in the asterid model species tomato (Solanum lycopersicum) and analyzed loss-of-function phenotypes. Loss-of-function of SlKIX8 and SlKIX9 led to the production of enlarged, dome-shaped leaves and these leaves exhibited increased expression of putative Solanum lycopersicum PPD (SlPPD target genes. Unexpectedly, kix8 kix9 mutants carried enlarged fruits with increased pericarp thickness due to cell expansion. At the molecular level, protein interaction assays indicated that SlKIX8 and SlKIX9 act as adaptors between the SlPPD and SlTOPLESS co-repressor proteins. Our results show that KIX8 and KIX9 are regulators of organ growth in asterids and can be used in strategies to improve important traits in produce such as thickness of the fruit flesh.

Evolution of polarity protein BASL and the capacity for stomatal lineage asymmetric divisions

Asymmetric and oriented stem cell divisions enable the continued production of patterned tissues. The molecules that guide these divisions include several "polarity proteins" that are localized to discrete plasma membrane domains, are differentially inherited during asymmetric divisions, and whose scaffolding activities can guide division plane orientation and subsequent cell fates. In the stomatal lineages on the surfaces of plant leaves, asymmetric and oriented divisions create distinct cell types in physiologically optimized patterns. The polarity protein BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) is a major regulator of stomatal lineage division and cell fate asymmetries in Arabidopsis, but its role in the stomatal lineages of other plants is unclear. Here, using phylogenetic and functional assays, we demonstrate that BASL is a eudicot-specific polarity protein. Dicot BASL orthologs can polarize in heterologous systems and rescue the Arabidopsis BASL mutant. The more widely distributed BASL-like proteins, although they share BASL's conserved C-terminal domain, are neither polarized nor do they function in asymmetric divisions of the stomatal lineage. Comparison of BASL protein localization and loss of function BASL phenotypes in Arabidopsis and tomato revealed previously unappreciated differences in how asymmetric cell divisions are employed for pattern formation in different species. This multi-species analysis therefore provides insight into the evolution of a unique polarity regulator and into the developmental choices available to cells as they build and pattern tissues.

Barley Genotypes Vary in Stomatal Responsiveness to Light and CO2 Conditions

Changes in stomatal conductance and density allow plants to acclimate to changing environmental conditions. In the present paper, the influence of atmospheric CO₂ concentration and light intensity on stomata were investigated for two barley genotypes-Barke and Bojos, differing in their sensitivity to oxidative stress and phenolic acid profiles. A novel approach for stomatal density analysis was used-a pair of convolution neural networks were developed to automatically identify and count stomata on epidermal micrographs. Stomatal density in barley was influenced by genotype, as well as by light and CO₂ conditions. Low CO₂ conditions resulted in increased stomatal density, although differences between ambient and elevated CO₂ were not significant. High light intensity increased stomatal density compared to low light intensity in both barley varieties and all CO₂ treatments. Changes in stomatal conductance were also measured alongside the accumulation of pentoses, hexoses, disaccharides, and abscisic acid detected by liquid chromatography coupled with mass spectrometry. High light increased the accumulation of all sugars and reduced abscisic acid levels. Abscisic acid was influenced by all factors-light, CO₂, and genotype-in combination. Differences were discovered between the two barley varieties: oxidative stress sensitive Barke demonstrated higher stomatal density, but lower conductance and better water use efficiency (WUE) than oxidative stress resistant Bojos at saturating light intensity. Barke also showed greater variability between treatments in measurements of stomatal density, sugar accumulation, and abscisic levels, implying that it may be more responsive to environmental drivers influencing water relations in the plant.

When vegetation indicates reproduction: The affinity between leaf morphology and flowering commitment in the lily meristem

At the reproductive stage, lily plants bear two morphological types of mature leaves, one at the lower and one at the upper part of the stem. At the vegetative stage, all the leaves are similar to each other and to the reproductive plant's lower leaves. This heterophylly has not yet been explored. In this study, we show that it is not a result of the plant's age but rather an outcome of floral induction. The induction appears as an on-going process, during which the meristem still produces leaves but progressively becomes committed to reproduction. This intermediate period lasts until the ultimate switch to flower primordia occurs. The leaves produced during floral induction, termed here as "inductive," appear at the upper part of the stem. Besides their typical higher stomata density, these leaves have a poly-layered palisade mesophyll, whose cells exhibit a unique morphology and contain more chlorophyll than leaves of vegetative plants. These leaves display higher carbon assimilation, soluble sugar production, and chloroplast-lipid accumulation. Accordingly, genes associated with stomata, chloroplast, and photosynthesis are upregulated in these leaves. Our results were obtained when floral induction was achieved either by vernalization or photoperiod signals, ruling out a mere environmental effect. We suggest that lily plants prepare themselves for the high-energy-demanding bloom by producing leaves with enhanced photosynthetic capacity, leading to an increase in soluble sugars. These novel findings introduce an adjacent affinity between photosynthesis and flowering and provide a nondestructive tool for identifying the plant's developmental stage-vegetative or reproductive.

The Role of Grass MUTE Orthologs in GMC Progression and GC Morphogenesis

Stomata arose about 400 million years ago when plants left their aquatic environment. The last step of stomatal development is shared by all plant groups, and it implies a symmetrical cell division from the guard mother cell (GMC) to produce two guard cells (GCs) flanking a pore. In Arabidopsis, the basic helix-loop-helix transcription factor MUTE controls this step, upregulating cell-cycle regulators of the GMC division, and immediately afterward, repressors of theses regulators like FAMA and FOUR LIPS. Recently, three grass MUTE orthologs (BdMUTE from Brachypodium distachyon, OsMUTE from rice, and ZmMUTE from maize) have been identified and characterized. Mutations in these genes disrupt GMC fate, with bdmute also blocking GC morphogenesis. However, because these genes also regulate subsidiary cell recruitment, which takes place before GMC division, their functions regulating GMC division and GC morphogenesis could be an indirect consequence of that inducing the recruitment of subsidiary cells. Comprehensive data evaluation indicates that BdMUTE, and probably grass MUTE orthologs, directly controls GMC fate. Although grass MUTE proteins, whose genes are expressed in the GMC, move between cells, they regulate GMC fate from the cells where they are transcribed. Grass MUTE genes also regulate GC morphogenesis. Specifically, OsMUTE controls GC shape by inducing OsFAMA expression. In addition, while SCs are not required for GMC fate progression, they are for GC maturation.

Light regulates stomatal development by modulating paracrine signaling from inner tissues

Developmental outcomes are shaped by the interplay between intrinsic and external factors. The production of stomata—essential pores for gas exchange in plants—is extremely plastic and offers an excellent system to study this interplay at the cell lineage level. For plants, light is a key external cue, and it promotes stomatal development and the accumulation of the master stomatal regulator SPEECHLESS (SPCH). However, how light signals are relayed to influence SPCH remains unknown. Here, we show that the light-regulated transcription factor ELONGATED HYPOCOTYL 5 (HY5), a critical regulator for photomorphogenic growth, is present in inner mesophyll cells and directly binds and activates STOMAGEN. STOMAGEN, the mesophyll-derived secreted peptide, in turn stabilizes SPCH in the epidermis, leading to enhanced stomatal production. Our work identifies a molecular link between light signaling and stomatal development that spans two tissue layers and highlights how an environmental signaling factor may coordinate growth across tissue types.

Light promotes stomatal development in plants. Here Wang et al. show that light stimulates stomatal development via the HY5 transcription factor which induces expression of STOMAGEN, a mesophyll-derived secreted peptide, that in turn leads to stabilization of a master regulator of stomatal development in the epidermis.

The diversity of stomatal development regulation in Callitriche is related to the intrageneric diversity in lifestyles

Plant stomata are produced through divisions and differentiation of stem cells, termed meristemoids. During stomatal development, we see diverse patterns of meristemoid behavior among land plant lineages. However, both the ecological significance and the diversification processes of this diversity remain mostly unknown. Here we report that the ecologically diverse genus Callitriche shows unprecedented intrageneric diversity in meristemoid behavior. While meristemoids in terrestrial species of Callitriche undergo a series of asymmetric divisions before differentiation, those in amphibious species skip the divisions and directly differentiate into stomata. The simple shift in the expression times of two key transcription factors underlies these different patterns. This study provides important insights into the evolution and ecological significance of stomatal patterning.

Stomata, the gas exchange structures of plants, are formed by the division and differentiation of stem cells, or meristemoids. Although diverse patterns of meristemoid behavior have been observed among different lineages of land plants, the ecological significance and diversification processes of these different patterns are not well understood. Here we describe an intrageneric diversity in the patterns of meristemoid division within the ecologically diverse genus Callitriche (Plantaginaceae). Meristemoids underwent a series of divisions before differentiating into stomata in the terrestrial species of Callitriche, but these divisions did not occur in amphibious species, which can grow in both air and water, in which meristemoids differentiated directly into stomata. These findings imply the adaptive significance of diversity in meristemoid division. Molecular genetic analyses showed that the different expression times of the stomatal key transcription factors SPEECHLESS and MUTE, which maintain and terminate the meristemoid division, respectively, underlie the different division patterns of meristemoids. Unlike terrestrial species, amphibious species prematurely expressed MUTE immediately after expressing SPEECHLESS, which corresponded to their early termination of stomatal division. By linking morphological, ecological, and genetic elements of stomatal development, this study provides significant insight that should aid ecological evolutionary developmental biology investigations of stomata.

The PEAPOD Pathway and Its Potential To Improve Crop Yield

A key strategy to increase plant productivity is to improve intrinsic organ growth. Some of the regulatory networks underlying organ growth and development, as well as the interconnections between these networks, are highly conserved. An example of such a growth-regulatory module with a highly conserved role in final organ size and shape determination in eudicot species is the PEAPOD (PPD)/KINASE-INDUCIBLE DOMAIN INTERACTING (KIX)/STERILE APETALA (SAP) module. We review the proteins constituting the PPD pathway and their roles in different plant developmental processes, and explore options for future research. We also speculate on strategies to exploit knowledge about the PPD pathway for targeted yield improvement to engineer crop traits of agronomic interest, such as leaf, fruit, and seed size.

Intrinsic Cell Polarity Coupled to Growth Axis Formation in Tobacco BY-2 Cells

Several plant proteins are preferentially localized to one end of a cell, allowing a polarity to be assigned to the cell. These cell polarity proteins often exhibit coordinated patterns between neighboring cells, termed tissue cell polarity. Tissue cell polarity is widespread in plants and can influence how cells grow, divide, and differentiate [1-5]. However, it is unclear whether cell polarity is established through cell-intrinsic or -extrinsic mechanisms and how polarity is coupled to growth. To address these issues, we analyzed the behavior of a tissue cell polarity protein BASL (BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE) in the simplifying context of cultured cell filaments and in protoplasts before and during regeneration. We show that BASL is polarly localized when ectopically expressed in tobacco BY-2 cell cultures. Ectopic BASL is found preferentially at the developing tips of cell filaments, likely marking a polarized molecular address. Polarity can shift during the cell cycle and is resistant to treatment with microtubule, actin or auxin transport inhibitors. BASL also exhibits polar localization in spherical protoplasts, in contrast to other polarity proteins so far tested. BASL polarity within protoplasts is dynamic and resistant to auxin transport inhibitors. As protoplasts regenerate, polarity remains dynamic in isotropically growing cells but becomes fixed in anisotropic cells and aligns with the axis of cell growth. Our findings suggest that plant cells have an intrinsic ability to polarize and that environmental or developmental cues may act by biasing the direction of this polarity and thus the orientation of anisotropic growth.

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.

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.

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.

A Ca2+/CaM-regulated transcriptional switch modulates stomatal development in response to water deficit

Calcium (Ca²⁺) signals are decoded by the Ca²⁺-sensor protein calmodulin (CaM) and are transduced to Ca²⁺/CaM-binding transcription factors to directly regulate gene expression necessary for acclimation responses in plants. The molecular mechanisms of Ca²⁺/CaM signal transduction processes and their functional significance remains enigmatic. Here we report a novel Ca²⁺/CaM signal transduction mechanism that allosterically regulates DNA-binding activity of GT2-LIKE 1 (GTL1), a transrepressor of STOMATAL DENSITY AND DISTRIBUTION 1 (SDD1), to repress stomatal development in response to water stress. We demonstrated that Ca²⁺/CaM interaction with the 2^nd helix of the GTL1 N-terminal trihelix DNA-binding domain (GTL1N) destabilizes a hydrophobic core of GTL1N and allosterically inhibits 3^rd helix docking to the SDD1 promoter, leading to osmotic stress-induced Ca²⁺/CaM-dependent activation (de-repression) of SDD1 expression. This resulted in GTL1-dependent repression of stomatal development in response to water-deficit stress. Together, our results demonstrate that a Ca²⁺/CaM-regulated transcriptional switch on a trihelix transrepressor directly transduces osmotic stress to repress stomatal development to improve plant water-use efficiency as an acclimation response.

BZU2/ZmMUTE controls symmetrical division of guard mother cell and specifies neighbor cell fate in maize

Intercellular communication in adjacent cell layers determines cell fate and polarity, thus orchestrating tissue specification and differentiation. Here we use the maize stomatal apparatus as a model to investigate cell fate determination. Mutations in ZmBZU2 (bizui2, bzu2) confer a complete absence of subsidiary cells (SCs) and normal guard cells (GCs), leading to failure of formation of mature stomatal complexes. Nuclear polarization and actin accumulation at the interface between subsidiary mother cells (SMCs) and guard mother cells (GMCs), an essential pre-requisite for asymmetric cell division, did not occur in Zmbzu2 mutants. ZmBZU2 encodes a basic helix-loop-helix (bHLH) transcription factor, which is an ortholog of AtMUTE in Arabidopsis (BZU2/ZmMUTE). We found that a number of genes implicated in stomatal development are transcriptionally regulated by BZU2/ZmMUTE. In particular, BZU2/ZmMUTE directly binds to the promoters of PAN1 and PAN2, two early regulators of protodermal cell fate and SMC polarization, consistent with the low levels of transcription of these genes observed in bzu2-1 mutants. BZU2/ZmMUTE has the cell-to-cell mobility characteristic similar to that of BdMUTE in Brachypodium distachyon. Unexpectedly, BZU2/ZmMUTE is expressed in GMC from the asymmetric division stage to the GMC division stage, and especially in the SMC establishment stage. Taken together, these data imply that BZU2/ZmMUTE is required for early events in SMC polarization and differentiation as well as for the last symmetrical division of GMCs to produce the two GCs, and is a master determinant of the cell fate of its neighbors through cell-to-cell communication.

Differential responses of stomatal kinetics and steady-state conductance to abscisic acid in a fern: comparison with a gymnosperm and an angiosperm

Origins of abscisic acid (ABA)-mediated metabolic control of stomatal conductance have been suggested to be recent, based on a gradualistic model of stomatal evolution. In ferns, steady-state stomatal conductance (g_s ) was unresponsive to ABA in some studies, supporting this model. Stomatal kinetic responses to ABA have not been considered. We used dynamic gas exchange methods to characterise half times of stomatal opening and closing in response to step changes in light, across a range of ABA exposures in three diverse taxa. All taxa had asymmetric kinetics, with closure slower than opening in fern and cedar, but faster than opening in soybean. Closing was fastest in soybean but opening was slowest. Stomatal kinetics, particularly for closure, responded to ABA in all three taxa. Steady-state g_s did not respond significantly to ABA in fern or cedar but responded strongly in soybean. Stomatal kinetics were responsive to ABA in fern. This finding supports a contrasting, single origin model, with ABA-mediated regulation of stomata arising early, in conjunction with stomata themselves. Stomatal kinetics are underutilised. Differential responses of opening and closing rates to environmental and hormonal stimuli may provide insights into phylogeny and stomatal regulatory strategies with potential application to selection for crop improvement.

Rice plants overexpressing OsEPF1 show reduced stomatal density and increased root cortical aerenchyma formation

Stomata are adjustable pores in the aerial epidermis of plants. The role of stomata is usually described in terms of the trade-off between CO₂ uptake and water loss. Little consideration has been given to their interaction with below-ground development or diffusion of other gases. We overexpressed the rice EPIDERMAL PATTERNING FACTOR1 (OsEPF1) to produce rice plants with reduced stomatal densities, resulting in lowered leaf stomatal conductance and enhanced water use efficiency. Surprisingly, we found that root cortical aerenchyma (RCA) is formed constitutively in OsEPF1OE lines regardless of tissue age and position. Aerenchyma is tissue containing air-spaces that can develop in the plant root during stressful conditions, e.g. oxygen deficiency when it functions to increase O₂ diffusion from shoot to root. The relationship with stomata is unknown. We conclude that RCA development and stomatal development are linked by two possible mechanisms: first that reduced stomatal conductance inhibits the diffusion of oxygen to the root, creating an oxygen deficit and stimulating the formation of RCA, second that an unknown EPF signalling pathway may be involved. Our observations have fundamental implications for the understanding of whole plant gas diffusion and root-to-shoot signalling events.

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.

Automatic segmentation and measurement methods of living stomata of plants based on the CV model

BACKGROUND

The stomata of plants mainly regulate gas exchange and water dispersion between the interior and external environments of plants and play a major role in the plants' health. The existing methods of stomata segmentation and measurement are mostly for specialized plants. The purpose of this research is to develop a generic method for the fully automated segmentation and measurement of the living stomata of different plants. The proposed method utilizes level set theory and image processing technology and can outperform the existing stomata segmentation and measurement methods based on threshold and skeleton in terms of its versatility.

RESULTS

The single stomata images of different plants were the input of the method and a level set based on the Chan-Vese model was used for stomatal segmentation. This allowed the morphological features of the stomata to be measured. Contrary to existing methods, the proposed segmentation method does not need any prior information about the stomata and is independent of the plant types. The segmentation results of 692 living stomata of black poplars show that the average measurement accuracies of the major and minor axes, area, eccentricity and opening degree are 95.68%, 95.53%, 93.04%, 99.46% and 94.32%, respectively. A segmentation test on dayflower (Commelina benghalensis) stomata data available in the literature was completed. The results show that the proposed method can effectively segment the stomata images (181 stomata) of dayflowers using bright-field microscopy. The fitted slope of the manually and automatically measured aperture is 0.993, and the R² value is 0.9828, which slightly outperforms the segmentation results that are given in the literature.

CONCLUSIONS

The proposed automated segmentation and measurement method for living stomata is superior to the existing methods based on the threshold and skeletonization in terms of versatility. The method does not need any prior information about the stomata. It is an unconstrained segmentation method, which can accurately segment and measure the stomata for different types of plants (woody or herbs). The method can automatically discriminate whether the pore region is independent or not and perform pore region extraction. In addition, the segmentation accuracy of the method is positively correlated with the stomata's opening degree.

On the mechanisms of development in monocot and eudicot leaves

Contents Summary 706 I. Introduction 707 II. Leaf zones in monocot and eudicot leaves 707 III. Monocot and eudicot leaf initiation: differences in degree and timing, but not kind 710 IV. Reticulate and parallel venation: extending the model? 711 V. Flat laminar growth: patterning and coordination of adaxial-abaxial and mediolateral axes 713 VI. Stipules and ligules: ontogeny of primordial elaborations 715 VII. Leaf architecture 716 VIII. Stomatal development: shared and diverged mechanisms for making epidermal pores 717 IX. Conclusion 719 Acknowledgements 720 References 720 SUMMARY: Comparisons of concepts in monocot and eudicot leaf development are presented, with attention to the morphologies and mechanisms separating these angiosperm lineages. Monocot and eudicot leaves are distinguished by the differential elaborations of upper and lower leaf zones, the formation of sheathing/nonsheathing leaf bases and vasculature patterning. We propose that monocot and eudicot leaves undergo expansion of mediolateral domains at different times in ontogeny, directly impacting features such as venation and leaf bases. Furthermore, lineage-specific mechanisms in compound leaf development are discussed. Although models for the homologies of enigmatic tissues, such as ligules and stipules, are proposed, tests of these hypotheses are rare. Likewise, comparisons of stomatal development are limited to Arabidopsis and a few grasses. Future studies may investigate correlations in the ontogenies of parallel venation and linear stomatal files in monocots, and the reticulate patterning of veins and dispersed stoma in eudicots. Although many fundamental mechanisms of leaf development are shared in eudicots and monocots, variations in the timing, degree and duration of these ontogenetic events may contribute to key differences in morphology. We anticipate that the incorporation of an ever-expanding number of sequenced genomes will enrich our understanding of the developmental mechanisms generating eudicot and monocot leaves.

Ectopic BASL Reveals Tissue Cell Polarity throughout Leaf Development in Arabidopsis thaliana

Tissue-wide polarity fields, in which cell polarity is coordinated across the tissue, have been described for planar organs such as the Drosophila wing and are considered important for coordinating growth and differentiation [1]. In planar plant organs, such as leaves, polarity fields have been identified for subgroups of cells, such as stomatal lineages [2], trichomes [3, 4], serrations [5], or early developmental stages [6]. Here, we show that ectopic induction of the stomatal protein BASL (BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE) reveals a tissue-wide epidermal polarity field in leaves throughout development. Ectopic GFP-BASL is typically localized toward the proximal end of cells and to one lobe of mature pavement cells, revealing a polarity field that aligns with the proximodistal axis of the leaf (base to tip). The polarity field is largely parallel to the midline of the leaf but diverges in more lateral positions, particularly at later stages in development, suggesting it may be deformed during growth. The polarity field is observed in the speechless mutant, showing that it is independent of stomatal lineages, and is observed in isotropic cells, showing that cell shape anisotropy is not required for orienting polarity. Ectopic BASL forms convergence and divergence points at serrations, mirroring epidermal PIN polarity patterns, suggesting a common underlying polarity mechanism. Thus, we show that similar to the situation in animals, planar plant organs have a tissue-wide cell polarity field, and this may provide a general cellular mechanism for guiding growth and differentiation.

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Ectopic expression of BASL in Arabidopsis leaves reveals coordinated polarity

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The ectopic BASL polarity field is independent of the stomatal lineage

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The polarity field reorients around serrations, mirroring PIN1 polarity

Loss or duplication of key regulatory genes coincides with environmental adaptation of the stomatal complex in Nymphaea colorata and Kalanchoe laxiflora

The stomatal complex is critical for gas and water exchange between plants and the atmosphere. Originating over 400 million years ago, the structure of the stomata has evolved to facilitate the adaptation of plants to various environments. Although the molecular mechanism of stomatal development in Arabidopsis has been widely studied, the evolution of stomatal structure and its molecular regulators in different species remains to be answered. In this study, we examined stomatal development and the orthologues of Arabidopsis stomatal genes in a basal angiosperm plant, Nymphaea colorata, and a member of the eudicot CAM family, Kalanchoe laxiflora, which represent the adaptation to aquatic and drought environments, respectively. Our results showed that despite the conservation of core stomatal regulators, a number of critical genes were lost in the N. colorata genome, including EPF2, MPK6, and AP2C3 and the polarity regulators BASL and POLAR. Interestingly, this is coincident with the loss of asymmetric divisions during the stomatal development of N. colorata. In addition, we found that the guard cell in K. laxiflora is surrounded by three or four small subsidiary cells in adaxial leaf surfaces. This type of stomatal complex is formed via repeated asymmetric cell divisions and cell state transitions. This may result from the doubled or quadrupled key genes controlling stomatal development in K. laxiflora. Our results show that loss or duplication of key regulatory genes is associated with environmental adaptation of the stomatal complex.

A2-type cyclin is required for the asymmetric entry division in rice stomatal development

In rice, and other major cereal grass crops, stomata are arranged in linear files parallel to the long growth axis of leaves. Each stomatal unit comprises two dumbbell-shaped guard cells flanked by two subsidiary cells. These morphological and developmental characteristics enable grass stomata to respond to environmental changes more efficiently. Cyclin-dependent kinases (CDKs) and their cyclin partners co-ordinate cell proliferation and differentiation during the development of multicellular organisms. In contrast to animals, plants have many more types and members of cyclins. In Arabidopsis, four A2-type cyclins (CYCA2s) function redundantly in regulating CDKB1 activity to promote the asymmetric division for stomatal initiation and the symmetric division of guard mother cells (GMCs). In this study, we examine the function of the single A2-type cyclin in rice, OsCYCA2;1, as well the single B1-type CDK, OsCDKB1;1. Cross-species complementation tests demonstrated that OsCYCA2;1 and OsCDKB1;1 could complement the defective stomatal phenotypes of Arabidopsis cyca2 and cdkb1 mutants, but also could suppress DNA endoduplication and cell enlargement. The early asymmetric divisions that establish the stomatal lineages are often missing within the stomatal cell files of OsCYCA2;1-RNAi rice transgenic lines, leading to a significantly reduced stomatal production. However, GMC divisions are not disrupted either in OsCYCA2;1-RNAi or in OsCDKB1;1-RNAi rice transgenic lines as expected. Our results demonstrate a conserved but diverged function and behavior of rice A2-type cyclins, which might be associated with the distinct stomatal development pathways between rice and Arabidopsis.

LITTLELEAF (LL) encodes a WD40 repeat domain-containing protein associated with organ size variation in cucumber

Plants employ tight genetic control to integrate intrinsic growth signals and environmental cues to enable organs to grow to a defined size. Many genes contributing to cell proliferation and/or cell expansion, and consequently organ size control, have been identified, but the regulatory pathways are poorly understood. Here we have characterized a cucumber littleleaf (ll) mutant which exhibits smaller organ sizes but more lateral branches than the wild type. The small organ size in ll was due to a reduction of both cell number and cell size. Quantitative trait locus (QTL) analyses revealed co-localization of major-effect QTLs for fruit size, fruit and seed weight, as well as number of lateral branches, with the LL locus indicating pleiotropic effects of the ll mutation. We demonstrate that LL is an ortholog of Arabidopsis STERILE APETALA (SAP) encoding a WD40 repeat domain-containing protein; the mutant protein differed from the wild type by a single amino acid substitution (W264G) in the second WD40 repeat. W264 was conserved in 34 vascular plant genomes examined. Phylogenetic analysis suggested that LL originated before the emergence of flowering plants but was lost in the grass genome lineage. The function of LL in organ size control was confirmed by its overexpression in transgenic cucumbers and ectopic expression in Arabidopsis. Transcriptome profiling in LL and ll bulks revealed a complex regulatory network for LL-mediated organ size variation that involves several known organ size regulators and associated pathways. The data support LL as an important player in organ size control and lateral branch development in cucumber.

Stomatal function, density and pattern, and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1-1 mutants

Stomata modulate the exchange of water and CO₂ between plant and atmosphere. Although stomatal density is known to affect CO₂ diffusion into the leaf and thus photosynthetic rate, the effect of stomatal density and patterning on CO₂ assimilation is not fully understood. We used wild types Col-0 and C24 and stomatal mutants sdd1-1 and tmm1 of Arabidopsis thaliana, differing in stomatal density and pattern, to study the effects of these variations on both stomatal and mesophyll conductance and CO₂ assimilation rate. Anatomical parameters of stomata, leaf temperature and carbon isotope discrimination were also assessed. Our results indicate that increased stomatal density enhanced stomatal conductance in sdd1-1 plants, with no effect on photosynthesis, due to both unchanged photosynthetic capacity and decreased mesophyll conductance. Clustering (abnormal patterning formed by clusters of two or more stomata) and a highly unequal distribution of stomata between the adaxial and abaxial leaf sides in tmm1 mutants also had no effect on photosynthesis. Except at very high stomatal densities, stomatal conductance and water loss were proportional to stomatal density. Stomatal formation in clusters reduced stomatal dynamics and their operational range as well as the efficiency of CO₂ transport.

Evolution of the Stomatal Regulation of Plant Water Content

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

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.

Molecular Evolution of Grass Stomata

Grasses began to diversify in the late Cretaceous Period and now dominate more than one third of global land area, including three-quarters of agricultural land. We hypothesize that their success is likely attributed to the evolution of highly responsive stomata capable of maximizing productivity in rapidly changing environments. Grass stomata harness the active turgor control mechanisms present in stomata of more ancient plant lineages, maximizing several morphological and developmental features to ensure rapid responses to environmental inputs. The evolutionary development of grass stomata appears to have been a gradual progression. Therefore, understanding the complex structures, developmental events, regulatory networks, and combinations of ion transporters necessary to drive rapid stomatal movement may inform future efforts towards breeding new crop varieties.

Origin and function of stomata in the moss Physcomitrella patens

Stomata are microscopic valves on plant surfaces that originated over 400 million years (Myr) ago and facilitated the greening of Earth's continents by permitting efficient shoot-atmosphere gas exchange and plant hydration1. However, the core genetic machinery regulating stomatal development in non-vascular land plants is poorly understood2-4 and their function has remained a matter of debate for a century5. Here, we show that genes encoding the two basic helix-loop-helix proteins PpSMF1 (SPEECH, MUTE and FAMA-like) and PpSCREAM1 (SCRM1) in the moss Physcomitrella patens are orthologous to transcriptional regulators of stomatal development in the flowering plant Arabidopsis thaliana and essential for stomata formation in moss. Targeted P. patens knockout mutants lacking either PpSMF1 or PpSCRM1 develop gametophytes indistinguishable from wild-type plants but mutant sporophytes lack stomata. Protein-protein interaction assays reveal heterodimerization between PpSMF1 and PpSCRM1, which, together with moss-angiosperm gene complementations6, suggests deep functional conservation of the heterodimeric SMF1 and SCRM1 unit is required to activate transcription for moss stomatal development, as in A. thaliana7. Moreover, stomata-less sporophytes of ΔPpSMF1 and ΔPpSCRM1 mutants exhibited delayed dehiscence, implying stomata might have promoted dehiscence in the first complex land-plant sporophytes.

Plant adaptation or acclimation to rising CO2 ? Insight from first multigenerational RNA-Seq transcriptome

Atmospheric carbon dioxide (CO₂ ) directly determines the rate of plant photosynthesis and indirectly effects plant productivity and fitness and may therefore act as a selective pressure driving evolution, but evidence to support this contention is sparse. Using Plantago lanceolata L. seed collected from a naturally high CO₂ spring and adjacent ambient CO₂ control site, we investigated multigenerational response to future, elevated atmospheric CO₂ . Plants were grown in either ambient or elevated CO₂ (700 μmol mol^-1 ), enabling for the first time, characterization of the functional and population genomics of plant acclimation and adaptation to elevated CO₂ . This revealed that spring and control plants differed significantly in phenotypic plasticity for traits underpinning fitness including above-ground biomass, leaf size, epidermal cell size and number and stomatal density and index. Gene expression responses to elevated CO₂ (acclimation) were modest [33-131 genes differentially expressed (DE)], whilst those between control and spring plants (adaptation) were considerably larger (689-853 DE genes). In contrast, population genomic analysis showed that genetic differentiation between spring and control plants was close to zero, with no fixed differences, suggesting that plants are adapted to their native CO₂ environment at the level of gene expression. An unusual phenotype of increased stomatal index in spring but not control plants in elevated CO₂ correlated with altered expression of stomatal patterning genes between spring and control plants for three loci (YODA, CDKB1;1 and SCRM2) and between ambient and elevated CO₂ for four loci (ER, YODA, MYB88 and BCA1). We propose that the two positive regulators of stomatal number (SCRM2) and CDKB1;1 when upregulated act as key controllers of stomatal adaptation to elevated CO₂ . Combined with significant transcriptome reprogramming of photosynthetic and dark respiration and enhanced growth in spring plants, we have identified the potential basis of plant adaptation to high CO₂ likely to occur over coming decades.

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

The patterning of stomata plays a vital role in plant development and has emerged as a paradigm for the role of peptide signals in the spatial control of cellular differentiation. Research in Arabidopsis has identified a series of epidermal patterning factors (EPFs), which interact with an array of membrane-localised receptors and associated proteins (encoded by ERECTA and TMM genes) to control stomatal density and distribution. However, although it is well-established that stomata arose very early in the evolution of land plants, until now it has been unclear whether the established angiosperm stomatal patterning system represented by the EPF/TMM/ERECTA module reflects a conserved, universal mechanism in the plant kingdom. Here, we use molecular genetics to show that the moss Physcomitrella patens has conserved homologues of angiosperm EPF, TMM and at least one ERECTA gene that function together to permit the correct patterning of stomata and that, moreover, elements of the module retain function when transferred to Arabidopsis Our data characterise the stomatal patterning system in an evolutionarily distinct branch of plants and support the hypothesis that the EPF/TMM/ERECTA module represents an ancient patterning system.

Optimal allocation of leaf epidermal area for gas exchange

A long-standing research focus in phytology has been to understand how plants allocate leaf epidermal space to stomata in order to achieve an economic balance between the plant's carbon needs and water use. Here, we present a quantitative theoretical framework to predict allometric relationships between morphological stomatal traits in relation to leaf gas exchange and the required allocation of epidermal area to stomata. Our theoretical framework was derived from first principles of diffusion and geometry based on the hypothesis that selection for higher anatomical maximum stomatal conductance (gsmax ) involves a trade-off to minimize the fraction of the epidermis that is allocated to stomata. Predicted allometric relationships between stomatal traits were tested with a comprehensive compilation of published and unpublished data on 1057 species from all major clades. In support of our theoretical framework, stomatal traits of this phylogenetically diverse sample reflect spatially optimal allometry that minimizes investment in the allocation of epidermal area when plants evolve towards higher gsmax . Our results specifically highlight that the stomatal morphology of angiosperms evolved along spatially optimal allometric relationships. We propose that the resulting wide range of viable stomatal trait combinations equips angiosperms with developmental and evolutionary flexibility in leaf gas exchange unrivalled by gymnosperms and pteridophytes.

Patterning of stomata in the moss Funaria: a simple way to space guard cells

BACKGROUND AND AIMS

Studies on stomatal development and the molecular mechanisms controlling patterning have provided new insights into cell signalling, cell fate determination and the evolution of these processes in plants. To fill a major gap in knowledge of stomatal patterning, this study describes the pattern of cell divisions that give rise to stomata and the underlying anatomical changes that occur during sporophyte development in the moss Funaria.

METHODS

Developing sporophytes at different stages were examined using light, fluorescence and electron microscopy; immunogold labelling was used to investigate the presence of pectin in the newly formed cavities.

KEY RESULTS

Substomatal cavities are liquid-filled when formed and drying of spaces is synchronous with pore opening and capsule expansion. Stomata in mosses do not develop from a self-generating meristemoid as in Arabidopsis, but instead they originate from a protodermal cell that differentiates directly into a guard mother cell. Epidermal cells develop from protodermal or other epidermal cells, i.e. there are no stomatal lineage ground cells.

CONCLUSIONS

Development of stomata in moss occurs by differentiation of guard mother cells arranged in files and spaced away from each other, and epidermal cells that continue to divide after stomata are formed. This research provides evidence for a less elaborated but effective mechanism for stomata spacing in plants, and we hypothesize that this operates by using some of the same core molecular signalling mechanism as angiosperms.

Functional analysis of the Arabidopsis thaliana MUTE promoter reveals a regulatory region sufficient for stomatal-lineage expression

The MUTE promoter contains a 175-bp region rich in Dof regulatory elements (AAAG) that is necessary and sufficient for initiation of transcription in meristemoids and the stomatal lineage. The molecular mechanism underlying the decision to divide or differentiate is a central question in developmental biology. During stomatal development, expression of the master regulator MUTE triggers the differentiation of meristemoids into stomata. In this study, we carried out MUTE promoter deletion analysis to define a regulatory region that promotes the initiation of expression in meristemoids. Expression constructs with truncated promoter fragments fused to β-glucuronidase (GUS) were developed. The full-length promoter and promoter truncations of at least 500 bp from the translational start site exhibited normal spatiotemporal expression patterns. Further truncation revealed a 175-bp promoter fragment that was necessary and sufficient for stomatal-lineage expression. Known cis-elements were identified and tested for functional relevance. Comparison of orthologous MUTE promoters suggested DNA binding with one finger (Dof) regulatory elements and novel motifs may be important for regulation. Our data highlight the complexity and combinatorial control of gene regulation and provides tools to further investigate the genetic control of stomatal development.

Light-induced STOMAGEN-mediated stomatal development in Arabidopsis leaves

The initiation of stomata, microscopic valves in the epidermis of higher plants that control of gas exchange, requires a co-ordinated sequence of asymmetric and symmetric divisions, which is under tight environmental and developmental control. Arabidopsis leaves grown under elevated photosynthetic photon flux density have a higher density of stomata. STOMAGEN encodes an epidermal patterning factor produced in the mesophyll, and our observations indicated that elevated photosynthetic irradiation stimulates STOMAGEN expression. Our analysis of gain and loss of function of STOMAGEN further detailed its function as a positive regulator of stomatal formation on both sides of the leaf, not only in terms of stomatal density across the leaf surface but also in terms of their stomatal index. STOMAGEN function was rate limiting for the light response of the stomatal lineage in the adaxial epidermis. Mutants in pathways that regulate stomatal spacing in the epidermis and have elevated stomatal density, such as stomatal density and distribution (sdd1) and too many mouth alleles, displayed elevated STOMAGEN expression, suggesting that STOMAGEN is either under the direct control of these pathways or is indirectly affected by stomatal patterning, suggestive of a feedback mechanism. These observations support a model in which changes in levels of light irradiation are perceived in the mesophyll and control the production of stomata in the epidermis by mesophyll-produced STOMAGEN, and whereby, conversely, stomatal patterning, either directly or indirectly, influences STOMAGEN levels.

Cell-specific transcriptomic analyses of three-dimensional shoot development in the moss Physcomitrella patens

Haploid moss gametophytes harbor distinct stem cell types, including tip cells that divide in single planes to generate filamentous protonemata, and bud cells that divide in three planes to yield axial gametophore shoots. This transition from filamentous to triplanar growth occurs progressively during the moss life cycle, and is thought to mirror evolution of the first terrestrial plants from Charophycean green algal ancestors. The innovation of morphologically complex plant body plans facilitated colonization of the vertical landscape, and enabled development of complex vegetative and reproductive plant morphologies. Despite its profound evolutionary significance, the molecular programs involved in this transition from filamentous to triplanar meristematic plant growth are poorly understood. In this study, we used single-cell type transcriptomics to identify more than 4000 differentially expressed genes that distinguish uniplanar protonematal tip cells from multiplanar gametophore bud cells in the moss Physcomitrella patens. While the transcriptomes of both tip and bud cells show molecular signatures of proliferative cells, the bud cell transcriptome exhibits a wider variety of genes with significantly increased transcript abundances. Our data suggest that combined expression of genes involved in shoot patterning and asymmetric cell division accompanies the transition from uniplanar to triplanar meristematic growth in moss.

A Repressor Protein Complex Regulates Leaf Growth in Arabidopsis

Cell number is an important determinant of final organ size. In the leaf, a large proportion of cells are derived from the stomatal lineage. Meristemoids, which are stem cell-like precursor cells, undergo asymmetric divisions, generating several pavement cells adjacent to the two guard cells. However, the mechanism controlling the asymmetric divisions of these stem cells prior to differentiation is not well understood. Here, we characterized PEAPOD (PPD) proteins, the only transcriptional regulators known to negatively regulate meristemoid division. PPD proteins interact with KIX8 and KIX9, which act as adaptor proteins for the corepressor TOPLESS. D3-type cyclin encoding genes were identified among direct targets of PPD2, being negatively regulated by PPDs and KIX8/9. Accordingly, kix8 kix9 mutants phenocopied PPD loss-of-function producing larger leaves resulting from increased meristemoid amplifying divisions. The identified conserved complex might be specific for leaf growth in the second dimension, since it is not present in Poaceae (grasses), which also lack the developmental program it controls.

Patterning and processes: how stomatal development defines physiological potential

Stomata present an excellent opportunity for connecting scientific disciplines: they are governed by complex genetic controls and unique cell biology, while also possessing a large influence over plant productivity and relationships with the environment. For this reason, stomata have engaged scientists for many centuries and continue to be a central interest for many fields of research. Recent technological advances have enabled interdisciplinary studies of stomata that were previously out of reach, and as a result, we are beginning to realize new insights about stomatal biology that place them at the intersection of our changing world. This review is intended to describe these interdisciplinary connections, discuss the relevant scales at which they are having an influence, and highlight ways we can capitalize on such novel approaches. While we incorporate knowledge about molecular advances, this is not intended to be an extensive review of that field, but rather, we focus on how those systems inform plant physiology and are connected to global scales.

Endosperm breakdown in Arabidopsis requires heterodimers of the basic helix-loop-helix proteins ZHOUPI and INDUCER OF CBP EXPRESSION 1

In Arabidopsis seeds, embryo growth is coordinated with endosperm breakdown. Mutants in the endosperm-specific gene ZHOUPI (ZOU), which encodes a unique basic helix-loop-helix (bHLH) transcription factor, have an abnormal endosperm that persists throughout seed development, significantly impeding embryo growth. Here we show that loss of function of the bHLH-encoding gene INDUCER OF CBP EXPRESSION 1 (ICE1) causes an identical endosperm persistence phenotype. We show that ZOU and ICE1 are co-expressed in the endosperm and interact in yeast via their bHLH domains. We show both genetically and in a heterologous plant system that, despite the fact that both ZOU and ICE1 can form homodimers in yeast, their role in endosperm breakdown requires their heterodimerization. Consistent with this conclusion, we confirm that ZOU and ICE1 regulate the expression of common target genes in the developing endosperm. Finally, we show that heterodimerization of ZOU and ICE1 is likely to be necessary for their binding to specific targets, rather than for their nuclear localization in the endosperm. By comparing our results with paradigms of bHLH function and evolution in animal systems we propose that the ZOU/ICE1 complex might have ancient origins, acquiring novel megagametophyte-specific functions in heterosporous land plants that were conserved in the angiosperm endosperm.